Molecular potentials and relaxation dynamics
International Nuclear Information System (INIS)
Karo, A.M.
1981-01-01
The use of empirical pseudopotentials, in evaluating interatomic potentials, provides an inexpensive and convenient method for obtaining highly accurate potential curves and permits the modeling of core-valence correlation, and the inclusion of relativistic effects when these are significant. Recent calculations of the X 1 Σ + and a 3 Σ + states of LiH, NaH, KH, RbH, and CsH and the X 2 Σ + states of their anions are discussed. Pseudopotentials, including core polarization terms, have been used to replace the core electrons, and this has been coupled with the development of compact, higly-optimized basis sets for the corresponding one- and two-electron atoms. Comparisons of the neutral potential curves with experiment and other ab initio calculations show good agreement (within 1000 cm -1 over most of the potential curves) with the difference curves being considerably more accurate. In the method of computer molecular dynamics, the force acting on each particle is the resultant of all interactions with other atoms in the neighborhood and is obtained as the derivative of an effective many-body potential. Exploiting the pseudopotential approach, in obtaining the appropriate potentials may be very fruitful in the future. In the molecular dynamics example considered here, the conventional sum-of-pairwise-interatomic-potentials (SPP) approximation is used with the potentials derived either from experimental spectroscopic data or from Hartree-Fock calculations. The problem is the collisional de-excitation of vibrationally excited molecular hydrogen at an Fe surface. The calculations have been carried out for an initial vibrotational state v = 8, J = 1 and a translational temperature corresponding to a gas temperature of 500 0 K. Different angles of approach and different initial random impact points on the surface have been selected. For any given collision with the wall, the molecule may pick up or lose vibrotatonal and translational energy
Accelerating convergence of molecular dynamics-based structural relaxation
DEFF Research Database (Denmark)
Christensen, Asbjørn
2005-01-01
We describe strategies to accelerate the terminal stage of molecular dynamics (MD)based relaxation algorithms, where a large fraction of the computational resources are used. First, we analyze the qualitative and quantitative behavior of the QuickMin family of MD relaxation algorithms and explore...
Molecular dynamics study on the relaxation properties of bilayered ...
Indian Academy of Sciences (India)
2017-08-31
Aug 31, 2017 ... Abstract. The influence of defects on the relaxation properties of bilayered graphene (BLG) has been studied by molecular dynamics simulation in nanometre sizes. Type and position of defects were taken into account in the calculated model. The results show that great changes begin to occur in the ...
Excitation dynamics and relaxation in a molecular heterodimer
International Nuclear Information System (INIS)
Balevičius, V.; Gelzinis, A.; Abramavicius, D.; Mančal, T.; Valkunas, L.
2012-01-01
Highlights: ► Dynamics of excitation within a heterogenous molecular dimer. ► Excited states can be swapped due to different reorganization energies of monomers. ► Conventional excitonic basis becomes renormalized due to interaction with the bath. ► Relaxation is independent of mutual positioning of monomeric excited states. -- Abstract: The exciton dynamics in a molecular heterodimer is studied as a function of differences in excitation and reorganization energies, asymmetry in transition dipole moments and excited state lifetimes. The heterodimer is composed of two molecules modeled as two-level systems coupled by the resonance interaction. The system-bath coupling is taken into account as a modulating factor of the molecular excitation energy gap, while the relaxation to the ground state is treated phenomenologically. Comparison of the description of the excitation dynamics modeled using either the Redfield equations (secular and full forms) or the Hierarchical quantum master equation (HQME) is demonstrated and discussed. Possible role of the dimer as an excitation quenching center in photosynthesis self-regulation is discussed. It is concluded that the system-bath interaction rather than the excitonic effect determines the excitation quenching ability of such a dimer.
Reliable Approximation of Long Relaxation Timescales in Molecular Dynamics
Directory of Open Access Journals (Sweden)
Wei Zhang
2017-07-01
Full Text Available Many interesting rare events in molecular systems, like ligand association, protein folding or conformational changes, occur on timescales that often are not accessible by direct numerical simulation. Therefore, rare event approximation approaches like interface sampling, Markov state model building, or advanced reaction coordinate-based free energy estimation have attracted huge attention recently. In this article we analyze the reliability of such approaches. How precise is an estimate of long relaxation timescales of molecular systems resulting from various forms of rare event approximation methods? Our results give a theoretical answer to this question by relating it with the transfer operator approach to molecular dynamics. By doing so we also allow for understanding deep connections between the different approaches.
Relaxation Estimation of RMSD in Molecular Dynamics Immunosimulations
Directory of Open Access Journals (Sweden)
Wolfgang Schreiner
2012-01-01
Full Text Available Molecular dynamics simulations have to be sufficiently long to draw reliable conclusions. However, no method exists to prove that a simulation has converged. We suggest the method of “lagged RMSD-analysis” as a tool to judge if an MD simulation has not yet run long enough. The analysis is based on RMSD values between pairs of configurations separated by variable time intervals Δt. Unless RMSD(Δt has reached a stationary shape, the simulation has not yet converged.
Molecular excitation dynamics and relaxation quantum theory and spectroscopy
Valkunas, Leonas; Mancal, Tomas
2013-01-01
Meeting the need for a work that brings together quantum theory and spectroscopy to convey excitation processes to advanced students and specialists wishing to conduct research and understand the entire field rather than just single aspects.Written by an experienced author and recognized authority in the field, this text covers numerous applications and offers examples taken from different disciplines. As a result, spectroscopists, molecular physicists, physical chemists, and biophysicists will all find this a must-have for their research. Also suitable as supplementary reading in graduate
Dynamics of relaxation to a stationary state for interacting molecular motors
Gomes, Luiza V. F.; Kolomeisky, Anatoly B.
2018-01-01
Motor proteins are active enzymatic molecules that drive a variety of biological processes, including transfer of genetic information, cellular transport, cell motility and muscle contraction. It is known that these biological molecular motors usually perform their cellular tasks by acting collectively, and there are interactions between individual motors that specify the overall collective behavior. One of the fundamental issues related to the collective dynamics of motor proteins is the question if they function at stationary-state conditions. To investigate this problem, we analyze a relaxation to the stationary state for the system of interacting molecular motors. Our approach utilizes a recently developed theoretical framework, which views the collective dynamics of motor proteins as a totally asymmetric simple exclusion process of interacting particles, where interactions are taken into account via a thermodynamically consistent approach. The dynamics of relaxation to the stationary state is analyzed using a domain-wall method that relies on a mean-field description, which takes into account some correlations. It is found that the system quickly relaxes for repulsive interactions, while attractive interactions always slow down reaching the stationary state. It is also predicted that for some range of parameters the fastest relaxation might be achieved for a weak repulsive interaction. Our theoretical predictions are tested with Monte Carlo computer simulations. The implications of our findings for biological systems are briefly discussed.
A study of internal energy relaxation in shocks using molecular dynamics based models
International Nuclear Information System (INIS)
Li, Zheng; Parsons, Neal; Levin, Deborah A.
2015-01-01
Recent potential energy surfaces (PESs) for the N 2 + N and N 2 + N 2 systems are used in molecular dynamics (MD) to simulate rates of vibrational and rotational relaxations for conditions that occur in hypersonic flows. For both chemical systems, it is found that the rotational relaxation number increases with the translational temperature and decreases as the rotational temperature approaches the translational temperature. The vibrational relaxation number is observed to decrease with translational temperature and approaches the rotational relaxation number in the high temperature region. The rotational and vibrational relaxation numbers are generally larger in the N 2 + N 2 system. MD-quasi-classical trajectory (QCT) with the PESs is also used to calculate the V-T transition cross sections, the collision cross section, and the dissociation cross section for each collision pair. Direct simulation Monte Carlo (DSMC) results for hypersonic flow over a blunt body with the total collision cross section from MD/QCT simulations, Larsen-Borgnakke with new relaxation numbers, and the N 2 dissociation rate from MD/QCT show a profile with a decreased translational temperature and a rotational temperature close to vibrational temperature. The results demonstrate that many of the physical models employed in DSMC should be revised as fundamental potential energy surfaces suitable for high temperature conditions become available
Fuson, Michael M.
2017-01-01
Laboratories studying the anisotropic rotational diffusion of bromobenzene using nuclear spin relaxation and molecular dynamics simulations are described. For many undergraduates, visualizing molecular motion is challenging. Undergraduates rarely encounter laboratories that directly assess molecular motion, and so the concept remains an…
Rotational dynamics in supercooled water from nuclear spin relaxation and molecular simulations.
Qvist, Johan; Mattea, Carlos; Sunde, Erik P; Halle, Bertil
2012-05-28
Structural dynamics in liquid water slow down dramatically in the supercooled regime. To shed further light on the origin of this super-Arrhenius temperature dependence, we report high-precision (17)O and (2)H NMR relaxation data for H(2)O and D(2)O, respectively, down to 37 K below the equilibrium freezing point. With the aid of molecular dynamics (MD) simulations, we provide a detailed analysis of the rotational motions probed by the NMR experiments. The NMR-derived rotational correlation time τ(R) is the integral of a time correlation function (TCF) that, after a subpicosecond librational decay, can be described as a sum of two exponentials. Using a coarse-graining algorithm to map the MD trajectory on a continuous-time random walk (CTRW) in angular space, we show that the slowest TCF component can be attributed to large-angle molecular jumps. The mean jump angle is ∼48° at all temperatures and the waiting time distribution is non-exponential, implying dynamical heterogeneity. We have previously used an analogous CTRW model to analyze quasielastic neutron scattering data from supercooled water. Although the translational and rotational waiting times are of similar magnitude, most translational jumps are not synchronized with a rotational jump of the same molecule. The rotational waiting time has a stronger temperature dependence than the translation one, consistent with the strong increase of the experimentally derived product τ(R) D(T) at low temperatures. The present CTRW jump model is related to, but differs in essential ways from the extended jump model proposed by Laage and co-workers. Our analysis traces the super-Arrhenius temperature dependence of τ(R) to the rotational waiting time. We present arguments against interpreting this temperature dependence in terms of mode-coupling theory or in terms of mixture models of water structure.
Energy Technology Data Exchange (ETDEWEB)
Tacke, Christian
2015-07-01
Multi spin systems with spin 1/2 nuclei and dipolar coupled quadrupolar nuclei can show so called ''quadrupolar dips''. There are two main reasons for this behavior: polarization transfer and relaxation. They look quite alike and without additional research cannot be differentiated easily in most cases. These two phenomena have quite different physical and theoretical backgrounds. For no or very slow dynamics, polarization transfer will take place, which is energy conserving inside the spin system. This effect can entirely be described using quantum mechanics on the spin system. Detailed knowledge about the crystallography is needed, because this affects the relevant hamiltonians directly. For systems with fast enough dynamics, relaxation takes over, and the energy flows from the spin system to the lattice; thus a more complex theoretical description is needed. This description has to include a dynamic model, usually in the form of a spectral density function. Both models should include detailed modelling of the complete spin system. A software library was developed to be able to model complex spin systems. It allows to simulate polarization transfer or relaxation effects. NMR measurements were performed on the protonic conductor K{sub 3}H(SO{sub 4}){sub 2}. A single crystal shows sharp quadrupolar dips at room temperature. Dynamics could be excluded using relaxation measurements and literature values. Thus, a polarization transfer analysis was used to describe those dips with good agreement. As a second system, imidazolium based molecular crystals were analyzed. The quadrupolar dips were expected to be caused by polarization transfer; this was carefully analyzed and found not to be true. A relaxation based analysis shows good agreement with the measured data in the high temperature area. It leverages a two step spectral density function, which indicates two distinct dynamic processes happening in this system.
Arosio, Paolo; Corti, Maurizio; Mariani, Manuel; Orsini, Francesco; Bogani, Lapo; Caneschi, Andrea; Lago, Jorge; Lascialfari, Alessandro
2015-05-01
The spin dynamics of the molecular magnetic chain [Dy(hfac)3{NIT(C6H4OPh)}] were investigated by means of the Muon Spin Relaxation (μ+SR) technique. This system consists of a magnetic lattice of alternating Dy(III) ions and radical spins, and exhibits single-chain-magnet behavior. The magnetic properties of [Dy(hfac)3{NIT(C6H4OPh)}] have been studied by measuring the magnetization vs. temperature at different applied magnetic fields (H = 5, 3500, and 16500 Oe) and by performing μ+SR experiments vs. temperature in zero field and in a longitudinal applied magnetic field H = 3500 Oe. The muon asymmetry P(t) was fitted by the sum of three components, two stretched-exponential decays with fast and intermediate relaxation times, and a third slow exponential decay. The temperature dependence of the spin dynamics has been determined by analyzing the muon longitudinal relaxation rate λinterm(T), associated with the intermediate relaxing component. The experimental λinterm(T) data were fitted with a corrected phenomenological Bloembergen-Purcell-Pound law by using a distribution of thermally activated correlation times, which average to τ = τ0 exp(Δ/kBT), corresponding to a distribution of energy barriers Δ. The correlation times can be associated with the spin freezing that occurs when the system condenses in the ground state.
International Nuclear Information System (INIS)
Arosio, Paolo; Orsini, Francesco; Corti, Maurizio; Mariani, Manuel; Bogani, Lapo; Caneschi, Andrea; Lago, Jorge; Lascialfari, Alessandro
2015-01-01
The spin dynamics of the molecular magnetic chain [Dy(hfac) 3 (NIT(C 6 H 4 OPh))] were investigated by means of the Muon Spin Relaxation (μ + SR) technique. This system consists of a magnetic lattice of alternating Dy(III) ions and radical spins, and exhibits single-chain-magnet behavior. The magnetic properties of [Dy(hfac) 3 (NIT(C 6 H 4 OPh))] have been studied by measuring the magnetization vs. temperature at different applied magnetic fields (H = 5, 3500, and 16500 Oe) and by performing μ + SR experiments vs. temperature in zero field and in a longitudinal applied magnetic field H = 3500 Oe. The muon asymmetry P(t) was fitted by the sum of three components, two stretched-exponential decays with fast and intermediate relaxation times, and a third slow exponential decay. The temperature dependence of the spin dynamics has been determined by analyzing the muon longitudinal relaxation rate λ interm (T), associated with the intermediate relaxing component. The experimental λ interm (T) data were fitted with a corrected phenomenological Bloembergen-Purcell-Pound law by using a distribution of thermally activated correlation times, which average to τ = τ 0 exp(Δ/k B T), corresponding to a distribution of energy barriers Δ. The correlation times can be associated with the spin freezing that occurs when the system condenses in the ground state
International Nuclear Information System (INIS)
Micotti, E.; Lascialfari, A.; Rigamonti, A.; Aldrovandi, S.; Caneschi, A.; Gatteschi, D.; Bogani, L.
2004-01-01
The spin dynamics in the helical chain Co(hfac) 2 NITPhOMe has been investigated by 1 H NMR and μSR relaxation. In the temperature range 15< T<60 K, the results are consistent with the relaxation of the homogeneous magnetization. For T≤15 K, NMR and μSR evidence a second spin relaxation mechanism, undetected by the magnetization measurements. From the analysis of these data, insights on this novel relaxation process are derived
Micotti, E.; Lascialfari, A.; Rigamonti, A.; Aldrovandi, S.; Caneschi, A.; Gatteschi, D.; Bogani, L.
2004-05-01
The spin dynamics in the helical chain Co(hfac) 2NITPhOMe has been investigated by 1H NMR and μSR relaxation. In the temperature range 15
International Nuclear Information System (INIS)
Azevedo, Eduardo R. de; Becker-Guedes, Fabio; Bonagamba, Tito J.; Schmidt-Rohr, Klaus; Iowa State University, Ames, IA
2001-01-01
The dynamics in the amorphous regions of semicrystalline polymers exert important influences on mechanical properties, but have been notoriously difficult to characterize. Two new solid-state NMR techniques, PUREX (pure exchange) and CODEX (center band-only detection of exchange) NMR, make it possible to analyze the molecular motions near the glass transition in the amorphous regions of semicrystalline polymers. This is achieved by selectively suppressing the otherwise dominant signals of the static segments in the crystallites. We have applied both NMR techniques to study the slow motions near the glass transition in semicrystalline polymers (β relaxation) and in fully amorphous samples for reference. The studied polymers were isotactic poly(1-butene) (iPB1) (form I), syndiotactic and atactic polypropylenes (sPP, and aPP, respectively), as well as polyisobutylene (PIB). We have analyzed the geometry and time scale of the slow molecular motion for all samples and determined the apparent activation energies. (author)
Zhao, Yinjian
2017-09-01
Aiming at a high simulation accuracy, a Particle-Particle (PP) Coulombic molecular dynamics model is implemented to study the electron-ion temperature relaxation. In this model, the Coulomb's law is directly applied in a bounded system with two cutoffs at both short and long length scales. By increasing the range between the two cutoffs, it is found that the relaxation rate deviates from the BPS theory and approaches the LS theory and the GMS theory. Also, the effective minimum and maximum impact parameters (bmin* and bmax*) are obtained. For the simulated plasma condition, bmin* is about 6.352 times smaller than the Landau length (bC), and bmax* is about 2 times larger than the Debye length (λD), where bC and λD are used in the LS theory. Surprisingly, the effective relaxation time obtained from the PP model is very close to the LS theory and the GMS theory, even though the effective Coulomb logarithm is two times greater than the one used in the LS theory. Besides, this work shows that the PP model (commonly known as computationally expensive) is becoming practicable via GPU parallel computing techniques.
Tangarife, Walter; Tobioka, Kohsaku; Ubaldi, Lorenzo; Volansky, Tomer
2018-02-01
The cosmological relaxation of the electroweak scale has been proposed as a mechanism to address the hierarchy problem of the Standard Model. A field, the relaxion, rolls down its potential and, in doing so, scans the squared mass parameter of the Higgs, relaxing it to a parametrically small value. In this work, we promote the relaxion to an inflaton. We couple it to Abelian gauge bosons, thereby introducing the necessary dissipation mechanism which slows down the field in the last stages. We describe a novel reheating mechanism, which relies on the gauge-boson production leading to strong electro-magnetic fields, and proceeds via the vacuum production of electron-positron pairs through the Schwinger effect. We refer to this mechanism as Schwinger reheating. We discuss the cosmological dynamics of the model and the phenomenological constraints from CMB and other experiments. We find that a cutoff close to the Planck scale may be achieved. In its minimal form, the model does not generate sufficient curvature perturbations and additional ingredients, such as a curvaton field, are needed.
Strate, Anne; Neumann, Jan; Overbeck, Viviane; Bonsa, Anne-Marie; Michalik, Dirk; Paschek, Dietmar; Ludwig, Ralf
2018-05-01
We report a concerted theoretical and experimental effort to determine the reorientational dynamics as well as hydrogen bond lifetimes for the doubly ionic hydrogen bond +OH⋯O- in the ionic liquid (2-hydroxyethyl)trimethylammonium bis(trifluoromethylsulfonyl)imide [Ch][NTf2] by using a combination of NMR relaxation time experiments, density functional theory (DFT) calculations, and molecular dynamics (MD) simulations. Due to fast proton exchange, the determination of rotational correlation times is challenging. For molecular liquids, 17O-enhanced proton relaxation time experiments have been used to determine the rotational correlation times for the OH vectors in water or alcohols. As an alternative to those expensive isotopic substitution experiments, we employed a recently introduced approach which is providing access to the rotational dynamics from a single NMR deuteron quadrupolar relaxation time experiment. Here, the deuteron quadrupole coupling constants (DQCCs) are obtained from a relation between the DQCC and the δ1H proton chemical shifts determined from a set of DFT calculated clusters in combination with experimentally determined proton chemical shifts. The NMR-obtained rotational correlation times were compared to those obtained from MD simulations and then related to viscosities for testing the applicability of popular hydrodynamic models. In addition, hydrogen bond lifetimes were derived, using hydrogen bond population correlation functions computed from MD simulations. Here, two different time domains were observed: The short-time contributions to the hydrogen lifetimes and the reorientational correlation times have roughly the same size and are located in the picosecond range, whereas the long-time contributions decay with relaxation times in the nanosecond regime and are related to rather slow diffusion processes. The computed average hydrogen bond lifetime is dominated by the long-time process, highlighting the importance and longevity of
Relaxation dynamics following transition of solvated electrons
International Nuclear Information System (INIS)
Barnett, R.B.; Landman, U.; Nitzan, A.
1989-01-01
Relaxation dynamics following an electronic transition of an excess solvated electron in clusters and in bulk water is studied using an adiabatic simulation method. In this method the solvent evolves classically and the electron is constrained to a specified state. The coupling between the solvent and the excess electron is evaluated via the quantum expectation value of the electron--water molecule interaction potential. The relaxation following excitation (or deexcitation) is characterized by two time scales: (i) a very fast (/similar to/20--30 fs) one associated with molecular rotations in the first solvation shell about the electron, and (ii) a slower stage (/similar to/200 fs), which is of the order of the longitudinal dielectric relaxation time. The fast relaxation stage exhibits an isotope effect. The spectroscopical consequences of the relaxation dynamics are discussed
Baranowski, M; Woźniak-Braszak, A; Jurga, K
2016-01-01
The paper presents the benefits of using fast adiabatic passage for the study of molecular dynamics in the solid state heteronuclear systems in the laboratory frame. A homemade pulse spectrometer operating at the frequency of 30.2MHz and 28.411MHz for protons and fluorines, respectively, has been enhanced with microcontroller direct digital synthesizer DDS controller [1-4]. This work briefly describes how to construct a low-cost and easy-to-assemble adiabatic extension set for homemade and commercial spectrometers based on recently very popular Arduino shields. The described set was designed for fast adiabatic generation. Timing and synchronization problems are discussed. The cross-relaxation experiments with different initial states of the two spin systems have been performed. Contrary to our previous work [5] where the steady-state NOE experiments were conducted now proton spins (1)H are polarized in the magnetic field B0 while fluorine spins (19)F are perturbed by selective saturation for a short time and then the system is allowed to evolve for a period in the absence of a saturating field. The adiabatic passage application leads to a reversal of magnetization of fluorine spins and increases the amplitude of the signal. Copyright © 2015 Elsevier Inc. All rights reserved.
Solomentsev, Gleb; Diehl, Carl; Akke, Mikael
2018-03-06
FKBP12 (FK506 binding protein 12 kDa) is an important drug target. Nuclear magnetic resonance (NMR) order parameters, describing amplitudes of motion on the pico- to nanosecond time scale, can provide estimates of changes in conformational entropy upon ligand binding. Here we report backbone and methyl-axis order parameters of the apo and FK506-bound forms of FKBP12, based on 15 N and 2 H NMR relaxation. Binding of FK506 to FKBP12 results in localized changes in order parameters, notably for the backbone of residues E54 and I56 and the side chains of I56, I90, and I91, all positioned in the binding site. The order parameters increase slightly upon FK506 binding, indicating an unfavorable entropic contribution to binding of TΔ S = -18 ± 2 kJ/mol at 293 K. Molecular dynamics simulations indicate a change in conformational entropy, associated with all dihedral angles, of TΔ S = -26 ± 9 kJ/mol. Both these values are significant compared to the total entropy of binding determined by isothermal titration calorimetry and referenced to a reactant concentration of 1 mM ( TΔ S = -29 ± 1 kJ/mol). Our results reveal subtle differences in the response to ligand binding compared to that of the previously studied rapamycin-FKBP12 complex, despite the high degree of structural homology between the two complexes and their nearly identical ligand-FKBP12 interactions. These results highlight the delicate dependence of protein dynamics on drug interactions, which goes beyond the view provided by static structures, and reinforce the notion that protein conformational entropy can make important contributions to the free energy of ligand binding.
Nonlinear Relaxation in Population Dynamics
Cirone, Markus A.; de Pasquale, Ferdinando; Spagnolo, Bernardo
We analyze the nonlinear relaxation of a complex ecosystem composed of many interacting species. The ecological system is described by generalized Lotka-Volterra equations with a multiplicative noise. The transient dynamics is studied in the framework of the mean field theory and with random interaction between the species. We focus on the statistical properties of the asymptotic behaviour of the time integral of the ith population and on the distribution of the population and of the local field.
Charge Relaxation Dynamics of an Electrolytic Nanocapacitor
2015-01-01
Understanding ion relaxation dynamics in overlapping electric double layers (EDLs) is critical for the development of efficient nanotechnology-based electrochemical energy storage, electrochemomechanical energy conversion, and bioelectrochemical sensing devices as well as the controlled synthesis of nanostructured materials. Here, a lattice Boltzmann (LB) method is employed to simulate an electrolytic nanocapacitor subjected to a step potential at t = 0 for various degrees of EDL overlap, solvent viscosities, ratios of cation-to-anion diffusivity, and electrode separations. The use of a novel continuously varying and Galilean-invariant molecular-speed-dependent relaxation time (MSDRT) with the LB equation recovers a correct microscopic description of the molecular-collision phenomena and enhances the stability of the LB algorithm. Results for large EDL overlaps indicated oscillatory behavior for the ionic current density, in contrast to monotonic relaxation to equilibrium for low EDL overlaps. Further, at low solvent viscosities and large EDL overlaps, anomalous plasmalike spatial oscillations of the electric field were observed that appeared to be purely an effect of nanoscale confinement. Employing MSDRT in our simulations enabled modeling of the fundamental physics of the transient charge relaxation dynamics in electrochemical systems operating away from equilibrium wherein Nernst–Einstein relation is known to be violated. PMID:25678941
Modern problems of relaxation gas dynamics
International Nuclear Information System (INIS)
Losev, S.A.; Osipov, A.I.
1985-01-01
Some of the dynamical characteristics of relaxation processes are studied. Unfortunately, many dynamical characteristics of relaxation processes, necessary for the solution of important scientific and applied problems, are not known. These problems require further development of experimental methods of the study of nonequilibrium gas. It is known, that gas systems are shifted from the equilibrium by different methods: by acoustic and shock wav es, by means of gas expansion in nozzles and jets, by powerful radiations (laser, first of all), by electric discharges, in burning and combustion devices, etc. Non-equilibrium gas is produced in installations of continuum, impulse and periodic regime. Molecular beams, shock tubes (especially with nozzles), flow and jet installations, aerodynamical tubes, plasmatrons, vessels with a gas, influenced by the strong radiation, burners and combustion devices, where the study of non-euilibrium gas is helpful to solve the problems of the determination of kinetic equations and constants of physico-chemical kinetics
Energy Technology Data Exchange (ETDEWEB)
Micotti, E. E-mail: micotti@fisicavolta.unipv.it; Lascialfari, A.; Rigamonti, A.; Aldrovandi, S.; Caneschi, A.; Gatteschi, D.; Bogani, L
2004-05-01
The spin dynamics in the helical chain Co(hfac){sub 2}NITPhOMe has been investigated by {sup 1}H NMR and {mu}SR relaxation. In the temperature range 15
Energy Technology Data Exchange (ETDEWEB)
Arosio, Paolo, E-mail: paolo.arosio@guest.unimi.it; Orsini, Francesco [Department of Physics, Università degli Studi di Milano, and INSTM, Milano (Italy); Corti, Maurizio [Department of Physics, Università degli Studi di Pavia and INSTM, Pavia (Italy); Mariani, Manuel [Department of Physics and Astronomy, Università degli Studi di Bologna, Bologna (Italy); Bogani, Lapo [Physikalisches Institut, Universität Stuttgart, Stuttgart (Germany); Caneschi, Andrea [INSTM and Department of Chemistry, University of Florence, Firenze (Italy); Lago, Jorge [Departamento de Quimica Inorganica, Universidad del Pais Vasco, Bilbao (Spain); Lascialfari, Alessandro [Department of Physics, Università degli Studi di Milano, and INSTM, Milano (Italy); Centro S3, Istituto Nanoscienze - CNR, Modena (Italy)
2015-05-07
The spin dynamics of the molecular magnetic chain [Dy(hfac){sub 3}(NIT(C{sub 6}H{sub 4}OPh))] were investigated by means of the Muon Spin Relaxation (μ{sup +}SR) technique. This system consists of a magnetic lattice of alternating Dy(III) ions and radical spins, and exhibits single-chain-magnet behavior. The magnetic properties of [Dy(hfac){sub 3}(NIT(C{sub 6}H{sub 4}OPh))] have been studied by measuring the magnetization vs. temperature at different applied magnetic fields (H = 5, 3500, and 16500 Oe) and by performing μ{sup +}SR experiments vs. temperature in zero field and in a longitudinal applied magnetic field H = 3500 Oe. The muon asymmetry P(t) was fitted by the sum of three components, two stretched-exponential decays with fast and intermediate relaxation times, and a third slow exponential decay. The temperature dependence of the spin dynamics has been determined by analyzing the muon longitudinal relaxation rate λ{sub interm}(T), associated with the intermediate relaxing component. The experimental λ{sub interm}(T) data were fitted with a corrected phenomenological Bloembergen-Purcell-Pound law by using a distribution of thermally activated correlation times, which average to τ = τ{sub 0} exp(Δ/k{sub B}T), corresponding to a distribution of energy barriers Δ. The correlation times can be associated with the spin freezing that occurs when the system condenses in the ground state.
Energy Technology Data Exchange (ETDEWEB)
Teichler, Helmar [Inst. Materialphysik, Univ Goettingen (Germany)
2013-07-01
In glass-forming melts the decay of structural fluctuation shows the well known transition from beta-relaxation (von-Schweidler law with exponent b) to alpha-decay (KWW law with exponent beta). Here we present results from molecular dynamics simulations for a metallic glass forming Ni0.5Zr0.5 model aimed at giving an understanding of this transition on the atomistic scale. At the considered temperature below mode coupling Tc, the dynamics of the system can be interpreted by residence of the particles in their neighbour cages and escape from the cages as rare processes. Our analysis yields that the fraction of residing particles is characterized by a hierarchical law in time, with von-Schweidler b explicitly related to the exponent of this law. In the alpha-decay regime the stretching exponent reflects, in addition, floating of the cages due to strain effects of escaped particles. Accordingly, the change from beta-relaxation to alpha-decay indicates the transition from low to large fraction of escaped particles.
International Nuclear Information System (INIS)
Colmenero, Juan; Arbe, Arantxa; Alvarez, Fernando; Monkenbusch, Michael; Richter, Dieter; Farago, Bela; Frick, Bernhard
2003-01-01
The momentum transfer dependence of the self-motion of main chain hydrogens in the α-relaxation regime of a glass forming polymer, polyisoprene, has been thoroughly investigated by a combined effort involving fully atomistic molecular dynamic simulations and quasielastic neutron scattering measurements. In this way, we have established the existence of a crossover from a Gaussian regime of sublinear diffusion to a strongly non-Gaussian regime at short distances. We show that an anomalous jump diffusion model with a distribution of jump lengths gives rise to such a crossover. This model leads to a time-dependent non-Gaussian parameter exhibiting all features revealed so far from various simulations of different glass forming systems
International Nuclear Information System (INIS)
Powers, R.; Jones, C.R.; Gorenstein, D.G.
1990-01-01
Assignment of the 1H and 31P resonances of a decamer DNA duplex, d(CGCTTAAGCG)2 was determined by two-dimensional COSY, NOESY and 1H-31P Pure Absorption phase Constant time (PAC) heteronuclear correlation spectroscopy. The solution structure of the decamer was calculated by an iterative hybrid relaxation matrix method combined with NOESY-distance restrained molecular dynamics. The distances from the 2D NOESY spectra were calculated from the relaxation rate matrix which were evaluated from a hybrid NOESY volume matrix comprising elements from the experiment and those calculated from an initial structure. The hybrid matrix-derived distances were then used in a restrained molecular dynamics procedure to obtain a new structure that better approximates the NOESY spectra. The resulting partially refined structure was then used to calculate an improved theoretical NOESY volume matrix which is once again merged with the experimental matrix until refinement is complete. JH3'-P coupling constants for each of the phosphates of the decamer were obtained from 1H-31P J-resolved selective proton flip 2D spectra. By using a modified Karplus relationship the C4'-C3'-O3'-P torsional angles were obtained. Comparison of the 31P chemical shifts and JH3'-P coupling constants of this sequence has allowed a greater insight into the various factors responsible for 31P chemical shift variations in oligonucleotides. It also provides an important probe of the sequence-dependent structural variation of the deoxyribose phosphate backbone of DNA in solution. These correlations are consistent with the hypothesis that changes in local helical structure perturb the deoxyribose phosphate backbone. The variation of the 31P chemical shift, and the degree of this variation from one base step to the next is proposed as a potential probe of local helical conformation within the DNA double helix
Generalized extended Navier-Stokes theory: Multiscale spin relaxation in molecular fluids
DEFF Research Database (Denmark)
Hansen, Jesper Schmidt
2013-01-01
This paper studies the relaxation of the molecular spin angular velocity in the framework of generalized extended Navier-Stokes theory. Using molecular dynamics simulations, it is shown that for uncharged diatomic molecules the relaxation time decreases with increasing molecular moment of inertia...
da Silva, Maria Cristina Rodrigues
1998-11-01
Molecular orbital methods and laser ionization mass spectrometry measurements are used to investigate the spectroscopy and relaxation dynamics of benzaldehyde following excitation to its S2(/pi/pi/sp/*) state. Energies, equilibrium geometries and vibrational frequencies of ground and low-lying excited states of benzaldehyde neutral and cation determined by ab initio calculations provide a theoretical description of the electronic spectroscopy of benzaldehyde and of the changes occurring on excitation and ionization. The S2(/pi/pi/sp/*)[/gets]S0 excitation spectrum of jet-cooled benzaldehyde acquired using two-color laser ionization mass spectrometry techniques is interpreted with the aid of these calculations. The spectrum is dominated by the origin band and by transitions involving some of the ring modes consistent with the results of the molecular orbital calculations that indicate that the major geometric changes on excitation to S2 are located in the aromatic ring. Ten fundamental vibrations of the S2(/pi/pi/sp/*) state are assigned. The dissociation dynamics of benzaldehyde into benzene and carbon monoxide following excitation to its S2(/pi/pi/sp/*) state are investigated under jet- cooled conditions by two-color laser ionization mass spectrometry using a pump-probe technique. This experimental arrangement allows monitoring the benzaldehyde reactant and the benzene product ion signals as a function of the time delay between the excitation and ionization steps. A kinetic model is proposed to explain the observed biexponential decay of the benzaldehyde signal and the single exponential growth of the benzene product signal in terms of a sequential decay of two excited states of benzaldehyde, one of which leads to formation of benzene molecules in its lowest triplet state. Reactant disappearance and product appearance rates are determined for a number of vibronic transitions of the S2 state. They are found to increase with excitation energy without any indication
Zhu, Ming; Liu, Tingting; Wang, Shu; Zhang, Kesheng
2017-08-01
Existing two-frequency reconstructive methods can only capture primary (single) molecular relaxation processes in excitable gases. In this paper, we present a reconstructive method based on the novel decomposition of frequency-dependent acoustic relaxation spectra to capture the entire molecular multimode relaxation process. This decomposition of acoustic relaxation spectra is developed from the frequency-dependent effective specific heat, indicating that a multi-relaxation process is the sum of the interior single-relaxation processes. Based on this decomposition, we can reconstruct the entire multi-relaxation process by capturing the relaxation times and relaxation strengths of N interior single-relaxation processes, using the measurements of acoustic absorption and sound speed at 2N frequencies. Experimental data for the gas mixtures CO2-N2 and CO2-O2 validate our decomposition and reconstruction approach.
Dynamics of helicity transport and Taylor relaxation
International Nuclear Information System (INIS)
Diamond, P.H.; Malkov, M.
2003-01-01
A simple model of the dynamics of Taylor relaxation is derived using symmetry principles alone. No statistical closure approximations are invoked or detailed plasma model properties assumed. Notably, the model predicts several classes of nondiffusive helicity transport phenomena, including traveling nonlinear waves and superdiffusive turbulent pulses. A universal expression for the scaling of the effective magnetic Reynolds number of a system undergoing Taylor relaxation is derived. Some basic properties of intermittency in helicity transport are examined
Solar, M; Binder, K; Paul, W
2017-05-28
Molecular dynamics simulations of a chemically realistic model for 1,4-polybutadiene in a thin film geometry confined by two graphite walls are presented. Previous work on melts in the bulk has shown that the model faithfully reproduces static and dynamic properties of the real material over a wide temperature range. The present work studies how these properties change due to nano-confinement. The focus is on orientational correlations observable in nuclear magnetic resonance experiments and on the local intermediate incoherent neutron scattering function, F s (q z , z, t), for distances z from the graphite walls in the range of a few nanometers. Temperatures from about 2T g down to about 1.15T g , where T g is the glass transition temperature in the bulk, are studied. It is shown that weakly attractive forces between the wall atoms and the monomers suffice to effectively bind a polymer coil that is near the wall. For a wide regime of temperatures, the Arrhenius-like adsorption/desorption kinetics of the monomers is the slowest process, while very close to T g the Vogel-Fulcher-Tammann-like α-relaxation takes over. The α-process is modified only for z≤1.2 nm due to the density changes near the walls, less than expected from studies of coarse-grained (bead-spring-type) models. The weakness of the surface effects on the glass transition in this case is attributed to the interplay of density changes near the wall with the torsional potential. A brief discussion of pertinent experiments is given.
The effect of the Magnus force on skyrmion relaxation dynamics
Brown, Barton L.; Täuber, Uwe C.; Pleimling, Michel
2018-01-01
We perform systematic Langevin molecular dynamics simulations of interacting skyrmions in thin films. The interplay between Magnus force, repulsive skyrmion-skyrmion interaction and thermal noise yields different regimes during non-equilibrium relaxation. In the noise-dominated regime the Magnus force enhances the disordering effects of the thermal noise. In the Magnus-force-dominated regime, the Magnus force cooperates with the skyrmion-skyrmion interaction to yield a dynamic regime with slo...
TURBULENCE DECAY AND CLOUD CORE RELAXATION IN MOLECULAR CLOUDS
International Nuclear Information System (INIS)
Gao, Yang; Law, Chung K.; Xu, Haitao
2015-01-01
The turbulent motion within molecular clouds is a key factor controlling star formation. Turbulence supports molecular cloud cores from evolving to gravitational collapse and hence sets a lower bound on the size of molecular cloud cores in which star formation can occur. On the other hand, without a continuous external energy source maintaining the turbulence, such as in molecular clouds, the turbulence decays with an energy dissipation time comparable to the dynamic timescale of clouds, which could change the size limits obtained from Jean's criterion by assuming constant turbulence intensities. Here we adopt scaling relations of physical variables in decaying turbulence to analyze its specific effects on the formation of stars. We find that the decay of turbulence provides an additional approach for Jeans' criterion to be achieved, after which gravitational infall governs the motion of the cloud core. This epoch of turbulence decay is defined as cloud core relaxation. The existence of cloud core relaxation provides a more complete understanding of the effect of the competition between turbulence and gravity on the dynamics of molecular cloud cores and star formation
Stretched exponential relaxation in molecular and electronic glasses
Phillips, J. C.
1996-09-01
der Waals supercooled liquids and glasses, orientational glasses, water, fused salts, and heme proteins. In the intrinsic cases the theory of 0034-4885/59/9/003/img5 is often accurate to 2%, which is often better than the quoted experimental accuracies 0034-4885/59/9/003/img12. The extrinsic cases are identified by explicit structural signatures which are discussed at length. The discussion also includes recent molecular dynamical simulations for metallic glasses, spin glasses, quasicrystals and polymers which have achieved the intermediate relaxed Kohlrausch state and which have obtained values of 0034-4885/59/9/003/img2 in excellent agreement with the prediction of the microscopic theory.
Stretched exponential relaxation in molecular and electronic glasses
International Nuclear Information System (INIS)
Phillips, J.C.
1996-01-01
is often better than the quoted experimental accuracies ∼5%. The extrinsic cases are identified by explicit structural signatures which are discussed at length. The discussion also includes recent molecular dynamical simulations for metallic glasses, spin glasses, quasicrystals and polymers which have achieved the intermediate relaxed Kohlrausch state and which have obtained values of β in excellent agreement with the prediction of the microscopic theory. (author)
Directory of Open Access Journals (Sweden)
Nima Kasraie
2011-01-01
Full Text Available The aims of this study were to determine whether standard extracellular contrast agents of Gd(III ions in combination with a polymeric entity susceptible to hydrolytic degradation over a finite period of time, such as Hyaluronic Acid (HA, have sufficient vascular residence time to obtain comparable vascular imaging to current conventional compounds and to obtain sufficient data to show proof of concept that HA with Gd-DTPA ligands could be useful as vascular imaging agents. We assessed the dynamic relaxivity of the HA bound DTPA compounds using a custom-made phantom, as well as relaxation rates at 10.72 MHz with concentrations ranging between 0.09 and 7.96 mM in phosphate-buffered saline. Linear dependences of static longitudinal relaxation rate (R1 on concentration were found for most measured samples, and the HA samples continued to produce high signal strength after 24 hours after injection into a dialysis cassette at 3T, showing superior dynamic relaxivity values compared to conventional contrast media such as Gd-DTPA-BMA.
Relaxation Dynamics of Nanoparticle-Tethered Polymer Chains
Kim, Sung A
2015-09-08
© 2015 American Chemical Society. Relaxation dynamics of nanoparticle-tethered cis-1,4-polyisoprene (PI) are investigated using dielectric spectroscopy and rheometry. A model system composed of polymer chains densely grafted to spherical SiO2 nanoparticles to form self-suspended suspensions facilitates detailed studies of slow global chain and fast segmental mode dynamics under surface and geometrical confinement-from experiments performed in bulk materials. We report that unentangled polymer molecules tethered to nanoparticles relax far more slowly than their tethered entangled counterparts. Specifically, at fixed grafting density we find, counterintuitively, that increasing the tethered polymer molecular weight up to values close to the entanglement molecular weight speeds up chain relaxation dynamics. Decreasing the polymer grafting density for a fixed molecular weight has the opposite effect: it dramatically slows down chain relaxation, increases interchain coupling, and leads to a transition in rheological response from simple fluid behavior to viscoelastic fluid behavior for tethered PI chains that are unentangled by conventional measures. Increasing the measurement temperature produces an even stronger elastic response and speeds up molecular relaxation at a rate that decreases with grafting density and molecular weight. These observations are discussed in terms of chain confinement driven by crowding between particles and by the existence of an entropic attractive force produced by the space-filling constraint on individual chains in a self-suspended material. Our results indicate that the entropic force between densely grafted polymer molecules couples motions of individual chains in an analogous manner to reversible cross-links in associating polymers.
Molecular potentials and relaxation dynamics
International Nuclear Information System (INIS)
Karo, A.M.
1981-01-01
The use of empirical pseudopotentials, in evaluating interatomic potentials, provides an inexpensive and convenient method for obtaining highly accurate potential curves and permits the modeling of core-valence correlation, and the inclusion of relativistic effects when these are significant. As an example, recent calculations of the chi 1 Σ + and a 3 Σ + states of LiH, NaH, KH, RbH, and CsH and the chi 2 Σ + states of their anions are discussed. Pseudopotentials, including core polarization terms, have been used to replace the core electrons, and this has been coupled with the development of compact, highly-optimized basis sets for the corresponding one- and two-electron atoms. Comparisons of the neutral potential curves with experiment and other ab initio calculations show good agreement (within 1000 cm -1 over most of the potential curves) with the difference curves being considerably more accurate
Non-exponential dynamic relaxation in strongly nonequilibrium nonideal plasmas
International Nuclear Information System (INIS)
Morozov, I V; Norman, G E
2003-01-01
Relaxation of kinetic energy to the equilibrium state is simulated by the molecular dynamics method for nonideal two-component non-degenerate plasmas. Three limiting examples of initial states of strongly nonequilibrium plasma are considered: zero electron velocities, zero ion velocities and zero velocities of both electrons and ions. The initial non-exponential stage, its duration τ nB and subsequent exponential stages of the relaxation process are studied for a wide range of the nonideality parameter and the ion mass
Relaxation dynamics of multilayer triangular Husimi cacti
Galiceanu, Mircea; Jurjiu, Aurel
2016-09-01
We focus on the relaxation dynamics of multilayer polymer structures having, as underlying topology, the Husimi cactus. The relaxation dynamics of the multilayer structures is investigated in the framework of generalized Gaussian structures model using both Rouse and Zimm approaches. In the Rouse type-approach, we determine analytically the complete eigenvalues spectrum and based on it we calculate the mechanical relaxation moduli (storage and loss modulus) and the average monomer displacement. First, we monitor these physical quantities for structures with a fixed generation number and we increase the number of layers, such that the linear topology will smoothly come into play. Second, we keep constant the size of the structures, varying simultaneously two parameters: the generation number of the main layer, G, and the number of layers, c. This fact allows us to study in detail the crossover from a pure Husimi cactus behavior to a predominately linear chain behavior. The most interesting situation is found when the two limiting topologies cancel each other. For this case, we encounter in the intermediate frequency/time domain regions of constant slope for different values of the parameter set (G, c) and we show that the number of layers follows an exponential-law of G. In the Zimm-type approach, which includes the hydrodynamic interactions, the quantities that describe the mechanical relaxation dynamics do not show scaling behavior as in the Rouse model, except the limiting case, namely, a very high number of layers and low generation number.
Dynamical relaxation in 2HDM models
Lalak, Zygmunt; Markiewicz, Adam
2018-03-01
Dynamical relaxation provides an interesting solution to the hierarchy problem in face of the missing signatures of any new physics in recent experiments. Through a dynamical process taking place in the inflationary phase of the Universe it manages to achieve a small electroweak scale without introducing new states observable in current experiments. Appropriate approximation makes it possible to derive an explicit formula for the final vevs in the double-scanning scenario extended to a model with two Higgs doublets (2HDM). Analysis of the relaxation in the 2HDM confirms that in a general case it is impossible to keep vevs of both scalars small, unless fine-tuning is present or additional symmetries are cast upon the Lagrangian. Within the slightly constrained variant of the 2HDM, where odd powers of the fields’ expectation values are not present (which can be easily enforced by requiring that the doublets have different gauge transformations or by imposing a global symmetry) it is shown that the difference between the vevs of two scalars tends to be proportional to the cutoff. The analysis of the relaxation in 2HDM indicates that in a general case the relaxation would be stopped by the first doublet that gains a vev, with the other one remaining vevless with a mass of the order of the cutoff. This happens to conform with the inert doublet model.
Simulations of vibrational relaxation in dense molecular fluids
International Nuclear Information System (INIS)
Holian, B.L.
1985-07-01
In the understanding of high-temperatre and -pressure chemistry in explosives, first step is the study of the transfer of energy from translational degrees of freedom into internal vibrations of the molecules. We present new methods using nonequilibrium molecular dynamics (NEMD) for measuring vibrational relaxation in a diatomic fluid, where we expect a classical treatment of many-body collisions to be relevant because of the high densities (2 to 3 times compressed compared to the normal fluid) and high temperatures (2000 to 4000 K) involved behind detonation waves. NEMD techniques are discussed, including their limitations, and qualitative results presented
Hydrodynamic relaxations in dissipative particle dynamics
Hansen, J. S.; Greenfield, Michael L.; Dyre, Jeppe C.
2018-01-01
This paper studies the dynamics of relaxation phenomena in the standard dissipative particle dynamics (DPD) model [R. D. Groot and P. B. Warren, J. Chem. Phys. 107, 4423 (1997)]. Using fluctuating hydrodynamics as the framework of the investigation, we focus on the collective transverse and longitudinal dynamics. It is shown that classical hydrodynamic theory predicts the transverse dynamics at relatively low temperatures very well when compared to simulation data; however, the theory predictions are, on the same length scale, less accurate for higher temperatures. The agreement with hydrodynamics depends on the definition of the viscosity, and here we find that the transverse dynamics are independent of the dissipative and random shear force contributions to the stress. For high temperatures, the spectrum for the longitudinal dynamics is dominated by the Brillouin peak for large length scales and the relaxation is therefore governed by sound wave propagation and is athermal. This contrasts the results at lower temperatures and small length scale, where the thermal process is clearly present in the spectra. The DPD model, at least qualitatively, re-captures the underlying hydrodynamical mechanisms, and quantitative agreement is excellent at intermediate temperatures for the transverse dynamics.
Selective excitation, relaxation, and energy channeling in molecular systems
International Nuclear Information System (INIS)
Rhodes, W.C.
1993-08-01
Research involves theoretical studies of response, relaxation, and correlated motion in time-dependent behavior of large molecular systems ranging from polyatomic molecules to protein molecules in their natural environment. Underlying theme is subsystem modulation dynamics. Main idea is that quantum mechanical correlations between components of a system develop with time, playing a major role in determining the balance between coherent and dissipative forces. Central theme is interplay of coherence and dissipation in determining the nature of dynamic structuring and energy flow in molecular transformation mechanisms. Subsystem equations of motion are being developed to show how nonlinear, dissipative dynamics of a particular subsystem arise from correlated interactions with the rest of the system (substituent groups, solvent, lattice modes, etc.); one consequence is resonance structures and networks. Quantum dynamics and thermodynamics are being applied to understand control and energy transfer mechanisms in biological functions of protein molecules; these mechanisms are both global and local. Besides the above theory, the research deals with phenomenological aspects of molecular systems
Effect of the Magnus force on skyrmion relaxation dynamics
Brown, Barton L.; Täuber, Uwe C.; Pleimling, Michel
2018-01-01
We perform systematic Langevin molecular dynamics simulations of interacting skyrmions in thin films. The interplay between the Magnus force, the repulsive skyrmion-skyrmion interaction, and the thermal noise yields different regimes during nonequilibrium relaxation. In the noise-dominated regime, the Magnus force enhances the disordering effects of the thermal noise. In the Magnus-force-dominated regime, the Magnus force cooperates with the skyrmion-skyrmion interaction to yield a dynamic regime with slow decaying correlations. These two regimes are characterized by different values of the aging exponent. In general, the Magnus force accelerates the approach to the steady state.
Generalized extended Navier-Stokes theory: multiscale spin relaxation in molecular fluids.
Hansen, J S
2013-09-01
This paper studies the relaxation of the molecular spin angular velocity in the framework of generalized extended Navier-Stokes theory. Using molecular dynamics simulations, it is shown that for uncharged diatomic molecules the relaxation time decreases with increasing molecular moment of inertia per unit mass. In the regime of large moment of inertia the fast relaxation is wave-vector independent and dominated by the coupling between spin and the fluid streaming velocity, whereas for small inertia the relaxation is slow and spin diffusion plays a significant role. The fast wave-vector-independent relaxation is also observed for highly packed systems. The transverse and longitudinal spin modes have, to a good approximation, identical relaxation, indicating that the longitudinal and transverse spin viscosities have same value. The relaxation is also shown to be isomorphic invariant. Finally, the effect of the coupling in the zero frequency and wave-vector limit is quantified by a characteristic length scale; if the system dimension is comparable to this length the coupling must be included into the fluid dynamical description. It is found that the length scale is independent of moment of inertia but dependent on the state point.
Structural relaxation dynamics and annealing effects of sodium silicate glass.
Naji, Mohamed; Piazza, Francesco; Guimbretière, Guillaume; Canizarès, Aurélien; Vaills, Yann
2013-05-09
Here we report high-precision measurements of structural relaxation dynamics in the glass transition range at the intermediate and short length scale for a strong sodium silicate glass during long annealing times. We evidence for the first time the heterogeneous dynamics at the intermediate range order by probing the acoustic longitudinal frequency in the GHz region by Brillouin light scattering spectroscopy. Or, from in-situ Raman measurements, we show that relaxation is indeed homogeneous at the interatomic length scale. Our results show that the dynamics at the intermediate range order contains two distinct relaxation time scales, a fast and a slow component, differing by about a 10-fold factor below Tg and approaching to one another past the glass transition. The slow relaxation time agrees with the shear relaxation time, proving that Si-O bond breaking constitutes the primary control of structural relaxation at the intermediate range order.
NMR investigations of molecular dynamics
Palmer, Arthur
2011-03-01
NMR spectroscopy is a powerful experimental approach for characterizing protein conformational dynamics on multiple time scales. The insights obtained from NMR studies are complemented and by molecular dynamics (MD) simulations, which provide full atomistic details of protein dynamics. Homologous mesophilic (E. coli) and thermophilic (T. thermophilus) ribonuclease H (RNase H) enzymes serve to illustrate how changes in protein sequence and structure that affect conformational dynamic processes can be monitored and characterized by joint analysis of NMR spectroscopy and MD simulations. A Gly residue inserted within a putative hinge between helices B and C is conserved among thermophilic RNases H, but absent in mesophilic RNases H. Experimental spin relaxation measurements show that the dynamic properties of T. thermophilus RNase H are recapitulated in E. coli RNase H by insertion of a Gly residue between helices B and C. Additional specific intramolecular interactions that modulate backbone and sidechain dynamical properties of the Gly-rich loop and of the conserved Trp residue flanking the Gly insertion site have been identified using MD simulations and subsequently confirmed by NMR spin relaxation measurements. These results emphasize the importance of hydrogen bonds and local steric interactions in restricting conformational fluctuations, and the absence of such interactions in allowing conformational adaptation to substrate binding.
Markov state modeling and dynamical coarse-graining via discrete relaxation path sampling.
Fačkovec, B; Vanden-Eijnden, E; Wales, D J
2015-07-28
A method is derived to coarse-grain the dynamics of complex molecular systems to a Markov jump process (MJP) describing how the system jumps between cells that fully partition its state space. The main inputs are relaxation times for each pair of cells, which are shown to be robust with respect to positioning of the cell boundaries. These relaxation times can be calculated via molecular dynamics simulations performed in each cell separately and are used in an efficient estimator for the rate matrix of the MJP. The method is illustrated through applications to Sinai billiards and a cluster of Lennard-Jones discs.
Thermal relaxation of molecular oxygen in collisions with nitrogen atoms
Energy Technology Data Exchange (ETDEWEB)
Andrienko, Daniil A., E-mail: daniila@umich.edu; Boyd, Iain D. [Department of Aerospace Engineering, University of Michigan, 1320 Beal Ave., Ann Arbor, Michigan 48108 (United States)
2016-07-07
Investigation of O{sub 2}–N collisions is performed by means of the quasi-classical trajectory method on the two lowest ab initio potential energy surfaces at temperatures relevant to hypersonic flows. A complete set of bound–bound and bound–free transition rates is obtained for each precollisional rovibrational state. Special attention is paid to the vibrational and rotational relaxations of oxygen as a result of chemically non-reactive interaction with nitrogen atoms. The vibrational relaxation of oxygen partially occurs via the formation of an intermediate NO{sub 2} complex. The efficient energy randomization results in rapid vibrational relaxation at low temperatures, compared to other molecular systems with a purely repulsive potential. The vibrational relaxation time, computed by means of master equation studies, is nearly an order of magnitude lower than the relaxation time in N{sub 2}–O collisions. The rotational nonequilibrium starts to play a significant effect at translational temperatures above 8000 K. The present work provides convenient relations for the vibrational and rotational relaxation times as well as for the quasi-steady dissociation rate coefficient and thus fills a gap in data due to a lack of experimental measurements for this system.
Hydrogen Bond Dynamics in Aqueous Solutions: Ab initio Molecular ...
Indian Academy of Sciences (India)
Rate equation for the decay of CHB(t) · Definition of Hydrogen Bonds · Results of Molecular Dynamics · Dynamics of anion-water and water-water hydrogen bonds · Structural relaxation of anion-water & water-water H-bonds · Ab initio Molecular Dynamics : · Slide 14 · Dynamics of hydrogen bonds : CPMD results · Slide 16.
Density dependence of relaxation dynamics in glass formers, and ...
Indian Academy of Sciences (India)
Anshul D S Parmar
formers, we study the variation of relaxation dynamics with density, rather than temperature, as a control ... stronger behaviour, the use of scaled variables involving temperature and ... of the temperature dependence of B as written defines.
Slow logarithmic relaxation in models with hierarchically constrained dynamics
Brey, J. J.; Prados, A.
2000-01-01
A general kind of models with hierarchically constrained dynamics is shown to exhibit logarithmic anomalous relaxation, similarly to a variety of complex strongly interacting materials. The logarithmic behavior describes most of the decay of the response function.
Relaxation Dynamics of Nanoparticle-Tethered Polymer Chains
Kim, Sung A; Mangal, Rahul; Archer, Lynden A.
2015-01-01
© 2015 American Chemical Society. Relaxation dynamics of nanoparticle-tethered cis-1,4-polyisoprene (PI) are investigated using dielectric spectroscopy and rheometry. A model system composed of polymer chains densely grafted to spherical SiO2
Exciton-relaxation dynamics in lead halides
International Nuclear Information System (INIS)
Iwanaga, Masanobu; Hayashi, Tetsusuke
2003-01-01
We survey recent comprehensive studies of exciton relaxation in the crystals of lead halides. The luminescence and electron-spin-resonance studies have revealed that excitons in lead bromide spontaneously dissociate and both electrons and holes get self-trapped individually. Similar relaxation has been also clarified in lead chloride. The electron-hole separation is ascribed to repulsive correlation via acoustic phonons. Besides, on the basis of the temperature profiles of self-trapped states, we discuss the origin of luminescence components which are mainly induced under one-photon excitation into the exciton band in lead fluoride, lead chloride, and lead bromide
Non-monotonic behaviour in relaxation dynamics of image restoration
International Nuclear Information System (INIS)
Ozeki, Tomoko; Okada, Masato
2003-01-01
We have investigated the relaxation dynamics of image restoration through a Bayesian approach. The relaxation dynamics is much faster at zero temperature than at the Nishimori temperature where the pixel-wise error rate is minimized in equilibrium. At low temperature, we observed non-monotonic development of the overlap. We suggest that the optimal performance is realized through premature termination in the relaxation processes in the case of the infinite-range model. We also performed Markov chain Monte Carlo simulations to clarify the underlying mechanism of non-trivial behaviour at low temperature by checking the local field distributions of each pixel
Stability and suppression of turbulence in relaxing molecular gas flows
Grigoryev, Yurii N
2017-01-01
This book presents an in-depth systematic investigation of a dissipative effect which manifests itself as the growth of hydrodynamic stability and suppression of turbulence in relaxing molecular gas flows. The work describes the theoretical foundations of a new way to control stability and laminar turbulent transitions in aerodynamic flows. It develops hydrodynamic models for describing thermal nonequilibrium gas flows which allow the consideration of suppression of inviscid acoustic waves in 2D shear flows. Then, nonlinear evolution of large-scale vortices and Kelvin-Helmholtz waves in relaxing shear flows are studied. Critical Reynolds numbers in supersonic Couette flows are calculated analytically and numerically within the framework of both linear and nonlinear classical energy hydrodynamic stability theories. The calculations clearly show that the relaxation process can appreciably delay the laminar-turbulent transition. The aim of the book is to show the new dissipative effect, which can be used for flo...
Stability investigations of relaxing molecular gas flows. Results and perspectives
Grigor'ev, Yurii N.; Ershov, Igor V.
2017-10-01
This article presents results of systematic investigations of a dissipative effect which manifests itself as the growth of hydrodynamic stability and suppression of turbulence in relaxing molecular gas flows. The effect can be a new way for control stability and laminar turbulent transition in aerodynamic flows. The consideration of suppression of inviscid acoustic waves in 2D shear flows is presented. Nonlinear evolution of large-scale vortices and Kelvin — Helmholtz waves in relaxing shear flows are studied. Critical Reynolds numbers in supersonic Couette flows are calculated analytically and numerically within the framework of both classical linear and nonlinear energy hydrodynamic stability theories. The calculations clearly show that the relaxation process can appreciably delay the laminar-turbulent transition. The aim of this article is to show the new dissipative effect, which can be used for flow control and laminarization.
Relaxation towards phase-locked dynamics in long Josephson junctions
DEFF Research Database (Denmark)
Salerno, M.; Grønbech-Jensen, Niels; Samuelsen, Mogens Rugholm
1995-01-01
We study the relaxation phenomenon towards phase-locked dynamics in long Josephson junctions. In particular the dependence of the relaxation frequency for the equal time of flight solution on the junction parameters is derived. The analysis is based on a phase-locked map and is compared with direct...... numerical experiments performed both on the map and on the perturbed sine-Gordon equation. As an interesting result we find that very close to a bifurcation the relaxation frequency is exactly equal to the half of the step frequency, i.e., the frequency characterizing the period-one solution....
Hotspot relaxation dynamics in a current-carrying superconductor
Marsili, F.; Stevens, M. J.; Kozorezov, A.; Verma, V. B.; Lambert, Colin; Stern, J. A.; Horansky, R. D.; Dyer, S.; Duff, S.; Pappas, D. P.; Lita, A. E.; Shaw, M. D.; Mirin, R. P.; Nam, S. W.
2016-03-01
We experimentally studied the dynamics of optically excited hotspots in current-carrying WSi superconducting nanowires as a function of bias current, bath temperature, and excitation wavelength. We observed that the hotspot relaxation time depends on bias current, temperature, and wavelength. We explained this effect with a model based on quasiparticle recombination, which provides insight into the quasiparticle dynamics of superconductors.
Glass transition and relaxation dynamics of propylene glycol-water solutions confined in clay
Elamin, Khalid; Björklund, Jimmy; Nyhlén, Fredrik; Yttergren, Madeleine; Mârtensson, Lena; Swenson, Jan
2014-07-01
The molecular dynamics of aqueous solutions of propylene glycol (PG) and propylene glycol methylether (PGME) confined in a two-dimensional layer-structured Na-vermiculite clay has been studied by broadband dielectric spectroscopy and differential scanning calorimetry. As typical for liquids in confined geometries the intensity of the cooperative α-relaxation becomes considerably more suppressed than the more local β-like relaxation processes. In fact, at high water contents the calorimetric glass transition and related structural α-relaxation cannot even be observed, due to the confinement. Thus, the intensity of the viscosity related α-relaxation is dramatically reduced, but its time scale as well as the related glass transition temperature Tg are for both systems only weakly influenced by the confinement. In the case of the PGME-water solutions it is an important finding since in the corresponding bulk system a pronounced non-monotonic concentration dependence of the glass transition related dynamics has been observed due to the growth of hydrogen bonded relaxing entities of water bridging between PGME molecules [J. Sjöström, J. Mattsson, R. Bergman, and J. Swenson, Phys. Chem. B 115, 10013 (2011)]. The present results suggest that the same type of structural entities are formed in the quasi-two-dimensional space between the clay platelets. It is also observed that the main water relaxation cannot be distinguished from the β-relaxation of PG or PGME in the concentration range up to intermediate water contents. This suggests that these two processes are coupled and that the water molecules affect the time scale of the β-relaxation. However, this is most likely true also for the corresponding bulk solutions, which exhibit similar time scales of this combined relaxation process below Tg. Finally, it is found that at higher water contents the water relaxation does not merge with, or follow, the α-relaxation above Tg, but instead crosses the α-relaxation
AC relaxation in the iron(8) molecular magnet
Rose, Geordie
2000-11-01
We investigate the low energy magnetic relaxation characteristics of the ``iron eight'' (Fe8) molecular magnet. Each molecule in this material contains a cluster of eight Fe 3+ ions surrounded by organic ligands. The molecules arrange themselves into a regular lattice with triclinic symmetry. At sufficiently low energies, the electronic spins of the Fe3+ ions lock together into a ``quantum rotator'' with spin S = 10. We derive a low energy effective Hamiltonian for this system, valid for temperatures less than Tc ~ 360 mK , where Tc is the temperature at which the Fe8 system crosses over into a ``quantum regime'' where relaxation characteristics become temperature independent. We show that in this regime the dominant environmental coupling is to the environmental spin bath in the molecule. We show how to explicitly calculate these couplings, given crystallographic information about the molecule, and do this for Fe8. We use this information to calculate the linewidth, topological decoherence and orthogonality blocking parameters. All of these quantities are shown to exhibit an isotope effect. We demonstrate that orthogonality blocking in Fe8 is significant and suppresses coherent tunneling. We then use our low energy effective Hamiltonian to calculate the single-molecule relaxation rate in the presence of an external magnetic field with both AC and DC components by solving the Landau-Zener problem in the presence of a nuclear spin bath. Both sawtooth and sinusoidal AC fields are analyzed. This single-molecule relaxation rate is then used as input into a master equation in order to take into account the many-molecule nature of the full system. Our results are then compared to quantum regime relaxation experiments performed on the Fe8 system.
Nonequilibrium thermodynamics and information theory: basic concepts and relaxing dynamics
International Nuclear Information System (INIS)
Altaner, Bernhard
2017-01-01
Thermodynamics is based on the notions of energy and entropy. While energy is the elementary quantity governing physical dynamics, entropy is the fundamental concept in information theory. In this work, starting from first principles, we give a detailed didactic account on the relations between energy and entropy and thus physics and information theory. We show that thermodynamic process inequalities, like the second law, are equivalent to the requirement that an effective description for physical dynamics is strongly relaxing. From the perspective of information theory, strongly relaxing dynamics govern the irreversible convergence of a statistical ensemble towards the maximally non-commital probability distribution that is compatible with thermodynamic equilibrium parameters. In particular, Markov processes that converge to a thermodynamic equilibrium state are strongly relaxing. Our framework generalizes previous results to arbitrary open and driven systems, yielding novel thermodynamic bounds for idealized and real processes. (paper)
Polymer friction Molecular Dynamics
DEFF Research Database (Denmark)
Sivebæk, Ion Marius; Samoilov, Vladimir N.; Persson, Bo N. J.
We present molecular dynamics friction calculations for confined hydrocarbon solids with molecular lengths from 20 to 1400 carbon atoms. Two cases are considered: a) polymer sliding against a hard substrate, and b) polymer sliding on polymer. In the first setup the shear stresses are relatively...... independent of molecular length. For polymer sliding on polymer the friction is significantly larger, and dependent on the molecular chain length. In both cases, the shear stresses are proportional to the squeezing pressure and finite at zero load, indicating an adhesional contribution to the friction force....
Exploring the dynamics about the glass transition by muon spin relaxation and muon spin rotation
International Nuclear Information System (INIS)
Bermejo, F J; Bustinduy, I; Cox, S F J; Lord, J S; Cabrillo, C; Gonzalez, M A
2006-01-01
The capability of muon spin rotation and muon spin relaxation to explore dynamics in the vicinity of the glass transition is illustrated by results pertaining to three materials exhibiting two different glass-forming abilities. Measurements under transverse magnetic fields enable us to monitor the dynamics of muonium-labelled closed-shell molecules within the microsecond range. The results display the onset of stochastic molecular motions taking place upon crossing from below the glass-transition temperature. In turn, the molecular dynamics of radicals formed by addition of atomic muonium to unsaturated organic molecules can also be explored up to far shorter times by means of relaxation measurements under longitudinal fields. The technique is then shown to be capable of singling out stochastic reorientational motions from others, which usually are strongly coupled to them and usually dominate the material response when measured using higher-frequency probes such as neutron and light scattering
Substructured multibody molecular dynamics.
Energy Technology Data Exchange (ETDEWEB)
Grest, Gary Stephen; Stevens, Mark Jackson; Plimpton, Steven James; Woolf, Thomas B. (Johns Hopkins University, Baltimore, MD); Lehoucq, Richard B.; Crozier, Paul Stewart; Ismail, Ahmed E.; Mukherjee, Rudranarayan M. (Rensselaer Polytechnic Institute, Troy, NY); Draganescu, Andrei I.
2006-11-01
We have enhanced our parallel molecular dynamics (MD) simulation software LAMMPS (Large-scale Atomic/Molecular Massively Parallel Simulator, lammps.sandia.gov) to include many new features for accelerated simulation including articulated rigid body dynamics via coupling to the Rensselaer Polytechnic Institute code POEMS (Parallelizable Open-source Efficient Multibody Software). We use new features of the LAMMPS software package to investigate rhodopsin photoisomerization, and water model surface tension and capillary waves at the vapor-liquid interface. Finally, we motivate the recipes of MD for practitioners and researchers in numerical analysis and computational mechanics.
Hydration dynamics in water clusters via quantum molecular dynamics simulations
Energy Technology Data Exchange (ETDEWEB)
Turi, László, E-mail: turi@chem.elte.hu [Department of Physical Chemistry, Eötvös Loránd University, Budapest 112, P. O. Box 32, H-1518 (Hungary)
2014-05-28
We have investigated the hydration dynamics in size selected water clusters with n = 66, 104, 200, 500, and 1000 water molecules using molecular dynamics simulations. To study the most fundamental aspects of relaxation phenomena in clusters, we choose one of the simplest, still realistic, quantum mechanically treated test solute, an excess electron. The project focuses on the time evolution of the clusters following two processes, electron attachment to neutral equilibrated water clusters and electron detachment from an equilibrated water cluster anion. The relaxation dynamics is significantly different in the two processes, most notably restoring the equilibrium final state is less effective after electron attachment. Nevertheless, in both scenarios only minor cluster size dependence is observed. Significantly different relaxation patterns characterize electron detachment for interior and surface state clusters, interior state clusters relaxing significantly faster. This observation may indicate a potential way to distinguish surface state and interior state water cluster anion isomers experimentally. A comparison of equilibrium and non-equilibrium trajectories suggests that linear response theory breaks down for electron attachment at 200 K, but the results converge to reasonable agreement at higher temperatures. Relaxation following electron detachment clearly belongs to the linear regime. Cluster relaxation was also investigated using two different computational models, one preferring cavity type interior states for the excess electron in bulk water, while the other simulating non-cavity structure. While the cavity model predicts appearance of several different hydrated electron isomers in agreement with experiment, the non-cavity model locates only cluster anions with interior excess electron distribution. The present simulations show that surface isomers computed with the cavity predicting potential show similar dynamical behavior to the interior clusters of
Free volume and relaxation dynamics of polymeric systems
International Nuclear Information System (INIS)
Bartos, J.; Kristiak, J.
1999-01-01
In this contribution use of positron annihilation spectroscopy (PALS) for the study of free volume and relaxation dynamics of some polymeric systems (1,4-polybutadiene, cis-1,4-polyisoprene, polyisobutylene, trans-1,4-polychloropropene, atactic polypropylene and 1,2-polybutadiene) is discussed
Molecular order and T1-relaxation, cross-relaxation in nitroxide spin labels
Marsh, Derek
2018-05-01
Interpretation of saturation-recovery EPR experiments on nitroxide spin labels whose angular rotation is restricted by the orienting potential of the environment (e.g., membranes) currently concentrates on the influence of rotational rates and not of molecular order. Here, I consider the dependence on molecular ordering of contributions to the rates of electron spin-lattice relaxation and cross relaxation from modulation of N-hyperfine and Zeeman anisotropies. These are determined by the averages and , where θ is the angle between the nitroxide z-axis and the static magnetic field, which in turn depends on the angles that these two directions make with the director of uniaxial ordering. For saturation-recovery EPR at 9 GHz, the recovery rate constant is predicted to decrease with increasing order for the magnetic field oriented parallel to the director, and to increase slightly for the perpendicular field orientation. The latter situation corresponds to the usual experimental protocol and is consistent with the dependence on chain-labelling position in lipid bilayer membranes. An altered dependence on order parameter is predicted for saturation-recovery EPR at high field (94 GHz) that is not entirely consistent with observation. Comparisons with experiment are complicated by contributions from slow-motional components, and an unexplained background recovery rate that most probably is independent of order parameter. In general, this analysis supports the interpretation that recovery rates are determined principally by rotational diffusion rates, but experiments at other spectral positions/field orientations could increase the sensitivity to order parameter.
Dynamics and relaxation in confined medium. Application to 129Xe magnetic relaxation in Vycor
International Nuclear Information System (INIS)
Pasquier, Virginie
1995-01-01
Porous media morphology and topology drive the exploration of pore space by fluid. So, analysis of transport process, associated with relaxation mechanism, allows indirect study of pore geometry. The purpose of this work is to understand better the relation between geometry and transport. This study involves two parts: a modelization and prediction step is followed by an experimental application of magnetic relaxation. Numerical simulations and analytical models allow to quantify the influence on the solid interface of the dynamical behavior of confined gas in disordered porous media (granular structure and porous network) or in common geometry (cylindrical and lamellar interfaces). The formalism of diffusion propagator is a powerful tool to quantify the influence of the pore geometry on the diffusion of confined gas. The propagator holds all dynamical information on the system; it also predicts the temporal evolution of the autocorrelation functions of the Hamiltonian describing local coupling. In an intermediate time scale, magnetic relaxation shows complex diffusional regime: the autocorrelation functions decrease in a power law with a exponent smaller than d/2 (where d is the Euclidian dimension of the system). This behavior is analogous to dynamic in low-dimensional space, but here arises from surface correlations of the porous media. The long-time behavior of the autocorrelation functions retrieves the asymptotic decrease t -d/2 . Moreover, atypical behavior is observed for the Knudsen diffusion between infinite planes. It turns out that 129 Xe NMR is a appropriate technique to characterize organization and diffusion of gas confined in Vycor. Systematic studies of temperature and pressure effect on the 129 Xe chemical shift allow to specify the Xe/solid interaction. The analysis of the relaxation measurements, thanks to the numerical development, confirms conclusions arising from the study of diffusion propagator. (author) [fr
Molecular dynamics for fermions
International Nuclear Information System (INIS)
Feldmeier, H.; Schnack, J.
2000-02-01
The time-dependent variational principle for many-body trial states is used to discuss the relation between the approaches of different molecular dynamics models to describe indistinguishable fermions. Early attempts to include effects of the Pauli principle by means of nonlocal potentials as well as more recent models which work with antisymmetrized many-body states are reviewed under these premises. (orig.)
Tracking ultrafast relaxation dynamics of furan by femtosecond photoelectron imaging
International Nuclear Information System (INIS)
Liu, Yuzhu; Knopp, Gregor; Qin, Chaochao; Gerber, Thomas
2015-01-01
Graphical abstract: - Highlights: • Relaxation dynamics of furan are tracked by femtosecond photoelectron imaging. • The mechanism for ultrafast formation of α-carbene and β-carbene is proposed. • Ultrafast internal conversion from S 2 to S 1 is observed. • The transient characteristics of the fragment ions are obtained. • Single-color multi-photon ionization dynamics at 800 nm are also studied. - Abstract: Ultrafast internal conversion dynamics of furan has been studied by femtosecond photoelectron imaging (PEI) coupled with photofragmentation (PF) spectroscopy. Photoelectron imaging of single-color multi-photon ionization and two-color pump–probe ionization are obtained and analyzed. Photoelectron bands are assigned to the related states. The time evolution of the photoelectron signal by pump–probe ionization can be well described by a biexponential decay: two rapid relaxation pathways with time constants of ∼15 fs and 85 (±11) fs. The rapid relaxation is ascribed to the ultrafast internal conversion (IC) from the S 2 state to the vibrationally hot S 1 state. The second relaxation process is attributed to the redistributions and depopulation of secondarily populated high vibronic S 1 state and the formation of α-carbene and β-carbene by H immigration. Additionally, the transient characteristics of the fragment ions are also measured and discussed as a complementary understanding
Tracking ultrafast relaxation dynamics of furan by femtosecond photoelectron imaging
Energy Technology Data Exchange (ETDEWEB)
Liu, Yuzhu, E-mail: yuzhu.liu@gmail.com [School of Physics and Optoelectronic Engineering, Nanjing University of Information Science and Technology, Nanjing 210044 (China); Knopp, Gregor [Paul Scherrer Institute, Villigen 5232 (Switzerland); Qin, Chaochao [Department of Physics, Henan Normal University, Xinxiang 453007 (China); Gerber, Thomas [Paul Scherrer Institute, Villigen 5232 (Switzerland)
2015-01-13
Graphical abstract: - Highlights: • Relaxation dynamics of furan are tracked by femtosecond photoelectron imaging. • The mechanism for ultrafast formation of α-carbene and β-carbene is proposed. • Ultrafast internal conversion from S{sub 2} to S{sub 1} is observed. • The transient characteristics of the fragment ions are obtained. • Single-color multi-photon ionization dynamics at 800 nm are also studied. - Abstract: Ultrafast internal conversion dynamics of furan has been studied by femtosecond photoelectron imaging (PEI) coupled with photofragmentation (PF) spectroscopy. Photoelectron imaging of single-color multi-photon ionization and two-color pump–probe ionization are obtained and analyzed. Photoelectron bands are assigned to the related states. The time evolution of the photoelectron signal by pump–probe ionization can be well described by a biexponential decay: two rapid relaxation pathways with time constants of ∼15 fs and 85 (±11) fs. The rapid relaxation is ascribed to the ultrafast internal conversion (IC) from the S{sub 2} state to the vibrationally hot S{sub 1} state. The second relaxation process is attributed to the redistributions and depopulation of secondarily populated high vibronic S{sub 1} state and the formation of α-carbene and β-carbene by H immigration. Additionally, the transient characteristics of the fragment ions are also measured and discussed as a complementary understanding.
Relaxation oscillations and transport barrier dynamics in tokamak edge plasmas
International Nuclear Information System (INIS)
Benkadda, Sadruddin; Beyer, Peter; Fuhr-Chaudier, Guillaume; Garbet, Xavier; Ghendrih, Philippe; Sarazin, Yanick
2004-01-01
Oscillations of turbulent transport of particles and energy in magnetically confined plasmas can be easily observed in simulations of a variety of turbulence models. These oscillations typically involve a mechanism of energy exchange between fluctuations and a poloidal shear flow. This kind of ''predator-prey'' mechanism is found to be not relevant for transport barrier relaxations. In RBM simulations of resistive ballooning turbulence with transport barrier, relaxation oscillations of the latter are observed even in the case of frozen poloidal shear flow. These relaxations are due to a transitory growth of a mode localized at the barrier center. A one-dimensional model for the evolution of such a mode in the presence of a shear flow describes a transitory growth of an initial perturbation. Oscillations in the case of a finite steady-state shear flow are possible due to the coupling of the mode to the dynamics of the pressure profile. (author)
International Nuclear Information System (INIS)
Perly, Bruno
1980-01-01
The objective of this research thesis is to study local conformations and mobilities of some typical homo-polypeptides by using techniques of magnetic resonance. By using these techniques, it is possible to make highly local observations of molecular elements which allows very efficient analysis of structural and dynamic properties of several biologically important compounds to be performed, and the study of their interactions. After a presentation of the general properties of the studied polypeptides, of magnetic resonance and of magnetic relaxation, the author presents some elements of macromolecular dynamics and movement models. Then, he reports the study of local conformations and structural transitions, applications of spin marking to the dynamic study of polypeptides, a dynamic study of the polypeptide skeleton under the form of statistic balls, the study of local movements of side chains by using nuclear relaxation, the study of the coupling of movements of main and side chains, and of the nuclear relaxation induced by a radical spin marker
Vibrational relaxation of a triatomic molecular impurity: D2O in vitreous As2S3
International Nuclear Information System (INIS)
Rella, C.W.; Schwettman, H.A.; Engholm, J.R.
1995-01-01
Measurements of the relaxation of the D 2 O stretch mode in vitreous As 2 S 3 are presented. Because the bending mode of the molecule offers an intra-molecular decay channel for the stretch mode, the decay scheme of the D 2 O molecule is more complex than that of diatomic molecules. The asymmetric stretch mode of D 2 O has a frequency of 2680 cm -1 . To study the relaxation of this mode we applied a pump-probe technique, using intense psec; pulses of the Stanford Free Electron Laser. Due to the small cross-section of the vibrational mode, successful efforts were made to improve the signal to noise ratio by using a laser stabilization system and a tightly focused beam to increase the intensity, by averaging the signal with a kHz repetition rate and by using samples with an optimized D 2 O concentration. A rapid relaxation rate on the order of 5 x 10 9 sec -1 at low temperature is found that increases with temperature. Recalling that the bending mode of the D 2 O molecule has a frequency of 1170 cm -1 , one would expect a decay in a third order process, involving two quanta of the bending mode plus a vibrational host quanta with a frequency of 340 cm -1 , which coincides with a fundamental frequency of the pyramidal building blocks of the glassy As 2 S 3 host. Instead, we find from the temperature dependence of the relaxation rate that the D 2 O stretching mode relaxes in a higher order process. This indicates that the relaxation dynamics of small molecules is more complex than generally assumed
Dynamic relaxation method in analysis of reinforced concrete bent elements
Directory of Open Access Journals (Sweden)
Anna Szcześniak
2015-12-01
Full Text Available The paper presents a method for the analysis of nonlinear behaviour of reinforced concrete bent elements subjected to short-term static load. The considerations in the range of modelling of deformation processes of reinforced concrete element were carried out. The method of structure effort analysis was developed using the finite difference method. The Dynamic Relaxation Method, which — after introduction of critical damping — allows for description of the static behaviour of a structural element, was used to solve the system of nonlinear equilibrium equations. In order to increase the method effectiveness in the range of the post-critical analysis, the Arc Length Parameter on the equilibrium path was introduced into the computational procedure.[b]Keywords[/b]: reinforced concrete elements, physical nonlinearity, geometrical nonlinearity, dynamic relaxation method, arc-length method
The effect of host relaxation and dynamics on guest molecule dynamics in H2/tetrahydrofuranhydrate.
Peterson, Vanessa K; Shoko, Elvis; Kearley, Gordon J
2011-01-01
We use ab initio molecular dynamics simulations to obtain classically the effects of H2O cage motions on the potential-energy surface (PES) of encapsulated H2 in the H2/tetrahydrofuran-hydrate system. The significant differences between the PES for the H2 in rigid and flexible cages that we find will influence calculation of the quantum dynamics of the H2. Part of these differences arises from the relaxation of the H2O cage around the classical H2, with a second part arising from the coupling of both translational and rotational motions of H2 with the H20 cage. We find that isotopic substitution of 2H for 1H of the H2O cage affects the coupling, which has implications for experiments that require the use of 2H2O, including inelastic neutron scattering that uses 2H2O cages in order to focus on the H2 guest dynamics. Overall, this work emphasizes the importance of taking into account cage dynamics in any approach used to understand the dynamics of H2 guests in porous framework materials.
Nonequilibrium thermodynamics and information theory: basic concepts and relaxing dynamics
Altaner, Bernhard
2017-11-01
Thermodynamics is based on the notions of energy and entropy. While energy is the elementary quantity governing physical dynamics, entropy is the fundamental concept in information theory. In this work, starting from first principles, we give a detailed didactic account on the relations between energy and entropy and thus physics and information theory. We show that thermodynamic process inequalities, like the second law, are equivalent to the requirement that an effective description for physical dynamics is strongly relaxing. From the perspective of information theory, strongly relaxing dynamics govern the irreversible convergence of a statistical ensemble towards the maximally non-commital probability distribution that is compatible with thermodynamic equilibrium parameters. In particular, Markov processes that converge to a thermodynamic equilibrium state are strongly relaxing. Our framework generalizes previous results to arbitrary open and driven systems, yielding novel thermodynamic bounds for idealized and real processes. , which features invited work from the best early-career researchers working within the scope of J. Phys. A. This project is part of the Journal of Physics series’ 50th anniversary celebrations in 2017. Bernhard Altaner was selected by the Editorial Board of J. Phys. A as an Emerging Talent.
Dynamical renormalization group approach to relaxation in quantum field theory
International Nuclear Information System (INIS)
Boyanovsky, D.; Vega, H.J. de
2003-01-01
The real time evolution and relaxation of expectation values of quantum fields and of quantum states are computed as initial value problems by implementing the dynamical renormalization group (DRG). Linear response is invoked to set up the renormalized initial value problem to study the dynamics of the expectation value of quantum fields. The perturbative solution of the equations of motion for the field expectation values of quantum fields as well as the evolution of quantum states features secular terms, namely terms that grow in time and invalidate the perturbative expansion for late times. The DRG provides a consistent framework to resum these secular terms and yields a uniform asymptotic expansion at long times. Several relevant cases are studied in detail, including those of threshold infrared divergences which appear in gauge theories at finite temperature and lead to anomalous relaxation. In these cases the DRG is shown to provide a resummation akin to Bloch-Nordsieck but directly in real time and that goes beyond the scope of Bloch-Nordsieck and Dyson resummations. The nature of the resummation program is discussed in several examples. The DRG provides a framework that is consistent, systematic, and easy to implement to study the non-equilibrium relaxational dynamics directly in real time that does not rely on the concept of quasiparticle widths
Anomalous relaxation in binary mixtures: a dynamic facilitation picture
International Nuclear Information System (INIS)
Moreno, A J; Colmenero, J
2007-01-01
Recent computational investigations of polymeric and non-polymeric binary mixtures have reported anomalous relaxation features when both components exhibit very different mobilities. Anomalous relaxation is characterized by sublinear power-law behaviour for mean-squared displacements, logarithmic decay in dynamic correlators, and a striking concave-to-convex crossover in the latter by tuning the relevant control parameter, in analogy with predictions of the mode-coupling theory for state points close to higher-order transitions. We present Monte Carlo simulations on a coarse-grained model for relaxation in binary mixtures. The liquid structure is substituted by a three-dimensional array of cells. A spin variable is assigned to each cell, representing unexcited and excited local states of a mobility field. Changes in local mobility (spin flip) are permitted according to kinetic constraints determined by the mobilities of the neighbouring cells. We introduce two types of cell ('fast' and 'slow') with very different rates for spin flip. This coarse-grained model qualitatively reproduces the mentioned anomalous relaxation features observed for real binary mixtures
Dynamic signature of molecular association in methanol
International Nuclear Information System (INIS)
Bertrand, C. E.; Copley, J. R. D.; Faraone, A.; Self, J. L.
2016-01-01
Quasielastic neutron scattering measurements and molecular dynamics simulations were combined to investigate the collective dynamics of deuterated methanol, CD 3 OD. In the experimentally determined dynamic structure factor, a slow, non-Fickian mode was observed in addition to the standard density-fluctuation heat mode. The simulation results indicate that the slow dynamical process originates from the hydrogen bonding of methanol molecules. The qualitative behavior of this mode is similar to the previously observed α-relaxation in supercooled water [M. C. Bellissent-Funel et al., Phys. Rev. Lett. 85, 3644 (2000)] which also originates from the formation and dissolution of hydrogen-bonded associates (supramolecular clusters). In methanol, however, this mode is distinguishable well above the freezing transition. This finding indicates that an emergent slow mode is not unique to supercooled water, but may instead be a general feature of hydrogen-bonding liquids and associating molecular liquids.
Numerical modeling of bubble dynamics in viscoelastic media with relaxation
Warnez, M. T.; Johnsen, E.
2015-06-01
Cavitation occurs in a variety of non-Newtonian fluids and viscoelastic materials. The large-amplitude volumetric oscillations of cavitation bubbles give rise to high temperatures and pressures at collapse, as well as induce large and rapid deformation of the surroundings. In this work, we develop a comprehensive numerical framework for spherical bubble dynamics in isotropic media obeying a wide range of viscoelastic constitutive relationships. Our numerical approach solves the compressible Keller-Miksis equation with full thermal effects (inside and outside the bubble) when coupled to a highly generalized constitutive relationship (which allows Newtonian, Kelvin-Voigt, Zener, linear Maxwell, upper-convected Maxwell, Jeffreys, Oldroyd-B, Giesekus, and Phan-Thien-Tanner models). For the latter two models, partial differential equations (PDEs) must be solved in the surrounding medium; for the remaining models, we show that the PDEs can be reduced to ordinary differential equations. To solve the general constitutive PDEs, we present a Chebyshev spectral collocation method, which is robust even for violent collapse. Combining this numerical approach with theoretical analysis, we simulate bubble dynamics in various viscoelastic media to determine the impact of relaxation time, a constitutive parameter, on the associated physics. Relaxation time is found to increase bubble growth and permit rebounds driven purely by residual stresses in the surroundings. Different regimes of oscillations occur depending on the relaxation time.
Microscopic dynamics and relaxation processes in liquid hydrogen fluoride
International Nuclear Information System (INIS)
Angelini, R.; Giura, P.; Monaco, G.; Sette, F.; Fioretto, D.; Ruocco, G.
2004-01-01
Inelastic x-ray scattering and Brillouin light scattering measurements of the dynamic structure factor of liquid hydrogen fluoride have been performed in the temperature range T=214-283 K. The data, analyzed using a viscoelastic model with a two time-scale memory function, show a positive dispersion of the sound velocity c(Q) between the low frequency value c 0 (Q) and the high frequency value c ∞α (Q). This finding confirms the existence of a structural (α) relaxation directly related to the dynamical organization of the hydrogen bonds network of the system. The activation energy E a of the process has been extracted by the analysis of the temperature behavior of the relaxation time τ α (T) that follows an Arrhenius law. The obtained value for E a , when compared with that observed in another hydrogen bond liquid as water, suggests that the main parameter governing the α-relaxation process is the number of hydrogen bonds per molecule
The nonequilibrium molecular dynamics
International Nuclear Information System (INIS)
Hoover, W.G.
1992-03-01
MOLECULAR DYNAMICS has been generalized in order to simulate a variety of NONEQUILIBRIUM systems. This generalization has been achieved by adopting microscopic mechanical definitions of macroscopic thermodynamic and hydrodynamic variables, such as temperature and stress. Some of the problems already treated include rapid plastic deformation, intense heat conduction, strong shockwaves simulation, and far-from-equilibrium phase transformations. Continuing advances in technique and in the modeling of interatomic forces, coupled with qualitative improvements in computer hardware, are enabling such simulations to approximate real-world microscale and nanoscale experiments
Energy Technology Data Exchange (ETDEWEB)
Xie, Wen Jun; Yang, Yi Isaac; Gao, Yi Qin, E-mail: gaoyq@pku.edu.cn [Institute of Theoretical and Computational Chemistry, College of Chemistry and Molecular Engineering and Biodynamic Optical Imaging Center, Peking University, Beijing 100871 (China)
2015-12-14
In this study, we examine how complex ions such as oxyanions influence the dynamic properties of water and whether differences exist between simple halide anions and oxyanions. Nitrate anion is taken as an example to investigate the hydration properties of oxyanions. Reorientation relaxation of its hydration water can occur through two different routes: water can either break its hydrogen bond with the nitrate to form one with another water or switch between two oxygen atoms of the same nitrate. The latter molecular mechanism increases the residence time of oxyanion’s hydration water and thus nitrate anion slows down the translational motion of neighbouring water. But it is also a “structure breaker” in that it accelerates the reorientation relaxation of hydration water. Such a result illustrates that differences do exist between the hydration of oxyanions and simple halide anions as a result of different molecular geometries. Furthermore, the rotation of the nitrate solute is coupled with the hydrogen bond rearrangement of its hydration water. The nitrate anion can either tilt along the axis perpendicularly to the plane or rotate in the plane. We find that the two reorientation relaxation routes of the hydration water lead to different relaxation dynamics in each of the two above movements of the nitrate solute. The current study suggests that molecular geometry could play an important role in solute hydration and dynamics.
International Nuclear Information System (INIS)
Xie, Wen Jun; Yang, Yi Isaac; Gao, Yi Qin
2015-01-01
In this study, we examine how complex ions such as oxyanions influence the dynamic properties of water and whether differences exist between simple halide anions and oxyanions. Nitrate anion is taken as an example to investigate the hydration properties of oxyanions. Reorientation relaxation of its hydration water can occur through two different routes: water can either break its hydrogen bond with the nitrate to form one with another water or switch between two oxygen atoms of the same nitrate. The latter molecular mechanism increases the residence time of oxyanion’s hydration water and thus nitrate anion slows down the translational motion of neighbouring water. But it is also a “structure breaker” in that it accelerates the reorientation relaxation of hydration water. Such a result illustrates that differences do exist between the hydration of oxyanions and simple halide anions as a result of different molecular geometries. Furthermore, the rotation of the nitrate solute is coupled with the hydrogen bond rearrangement of its hydration water. The nitrate anion can either tilt along the axis perpendicularly to the plane or rotate in the plane. We find that the two reorientation relaxation routes of the hydration water lead to different relaxation dynamics in each of the two above movements of the nitrate solute. The current study suggests that molecular geometry could play an important role in solute hydration and dynamics
Dynamical X-ray scattering from the relaxed structures
International Nuclear Information System (INIS)
Benediktovitch, A.; Feranchuk, I.; Ulyanenkov, A.
2009-01-01
High-resolution X-ray diffraction is now widely used analytical tool for investigation of nano scale multilayered structures in semiconductor and optical technologies. The HRXRD method delivers unique information on the crystallographic lattice of the samples, concentration of solid solutions, lattice mismatches, layer thicknesses, defect distribution, and relaxation degree of the epitaxial layers. The evaluation of the experimental results, however, requires a robust and precise theory due to complex dynamical scattering of X-rays from near perfect crystallographic structure of the samples. Usually, the Takagi-Taupin approach [1] or the recurrent matrix methods [2] are used for the simulation of the X-ray diffraction profiles from the epitaxial multilayered structures. The use of these theories, however, becomes essentially difficult, when the lateral lattice mismatches are present in multilayers, for example, in the case of partially or fully relaxed epitaxially grown samples. In the present work, the general solution of this problem is found analytically. The angular divergence of the incident beam is also considered and the algorithm for the diffracted profile mapping in the reciprocal space is developed. The experimental reciprocal space mapping of typical AlGaN/GaN/AlN samples with partially relaxed layers is compared to the simulated maps, which describe well the location and character of the diffraction spots caused by different layers. (author)
Non-linear calculation of PCRV using dynamic relaxation
International Nuclear Information System (INIS)
Schnellenbach, G.
1979-01-01
A brief review is presented of a numerical method called the dynamic relaxation method for stress analysis of the concrete in prestressed concrete pressure vessels. By this method the three-dimensional elliptic differential equations of the continuum are changed into the four-dimensional hyperbolic differential equations known as wave equations. The boundary value problem of the static system is changed into an initial and boundary value problem for which a solution exists if the physical system is defined at time t=0. The effect of non-linear stress-strain behaviour of the material as well as creep and cracking are considered
DEFF Research Database (Denmark)
Adrjanowicz, Karolina; Kaminski, Kamil; Paluch, Marian
2015-01-01
In this paper, we study crystallization behavior and molecular dynamics in the supercooled liquid state of the pharmaceutically important compound ketoprofen at various thermodynamic conditions. Dielectric relaxation for a racemic mixture was investigated in a wide range of temperatures and press...
Molecular dynamics simulations
International Nuclear Information System (INIS)
Alder, B.J.
1985-07-01
The molecular dynamics computer simulation discovery of the slow decay of the velocity autocorrelation function in fluids is briefly reviewed in order to contrast that long time tail with those observed for the stress autocorrelation function in fluids and the velocity autocorrelation function in the Lorentz gas. For a non-localized particle in the Lorentz gas it is made plausible that even if it behaved quantum mechanically its long time tail would be the same as the classical one. The generalization of Fick's law for diffusion for the Lorentz gas, necessary to avoid divergences due to the slow decay of correlations, is presented. For fluids, that generalization has not yet been established, but the region of validity of generalized hydrodynamics is discussed. 20 refs., 5 figs
Zhu, Ming; Liu, Tingting; Zhang, Xiangqun; Li, Caiyun
2018-01-01
Recently, a decomposition method of acoustic relaxation absorption spectra was used to capture the entire molecular multimode relaxation process of gas. In this method, the acoustic attenuation and phase velocity were measured jointly based on the relaxation absorption spectra. However, fast and accurate measurements of the acoustic attenuation remain challenging. In this paper, we present a method of capturing the molecular relaxation process by only measuring acoustic velocity, without the necessity of obtaining acoustic absorption. The method is based on the fact that the frequency-dependent velocity dispersion of a multi-relaxation process in a gas is the serial connection of the dispersions of interior single-relaxation processes. Thus, one can capture the relaxation times and relaxation strengths of N decomposed single-relaxation dispersions to reconstruct the entire multi-relaxation dispersion using the measurements of acoustic velocity at 2N + 1 frequencies. The reconstructed dispersion spectra are in good agreement with experimental data for various gases and mixtures. The simulations also demonstrate the robustness of our reconstructive method.
Colmenero, J.
1993-12-01
The dynamics of the α-relaxation in three different polymeric systems, poly(vinyl methyl ether) (PVME), poly(vinyl chloride) (PVC) and poly(bisphenol A, 2-hydroxypropylether) (PH) has been studied by means of relaxation techniques and quasielastic neutron scattering (backscattering spectrometers IN10 and IN13 at the ILL-Grenoble). By using these techniques we have covered a wide time scale ranging from mesoscopic to macroscopic times (10 -10 -10 1 s). For analyzing the experimental data we have developed a phenomenological procedure in the frequency domain based on the Havriliak-Negami relaxation function, which in fact implies a Kohlrausch-Williams-Watts relaxation function in the time domain. The results obtained indicate that the dynamics of the α-relaxation in a wide time scale shows a clear non-Debye behaviour. The shape of the relaxation functions is found to be similar for the different techniques used and independent of temperature and momentum transfer ( Q). Moreover, the characteristic relaxation times deduced from the fitting of the experimental data can also be described using only one Vogel-Fulcher functional form. Besides we found that the Q-dependence of the relaxation times obtained by QENS is given by a power law, τ( Q) ∞ Q- n ( n>2), n being dependent on the system, and that the Q-behaviour and the non-Debye behaviour are directly correlated. In the case of PVC, time of flight (TOF) neutron scattering experiments confirm these results in a shorter time scale (2×10 -11 -2× 10 -12 s). Moreover, TOF results also suggest the possibility of interpreting the “fast process” usually detected in glass-forming systems as a Debye-like short regime of the α-relaxation.
Hydration water dynamics in biopolymers from NMR relaxation in the rotating frame.
Blicharska, Barbara; Peemoeller, Hartwig; Witek, Magdalena
2010-12-01
Assuming dipole-dipole interaction as the dominant relaxation mechanism of protons of water molecules adsorbed onto macromolecule (biopolymer) surfaces we have been able to model the dependences of relaxation rates on temperature and frequency. For adsorbed water molecules the correlation times are of the order of 10(-5)s, for which the dispersion region of spin-lattice relaxation rates in the rotating frame R(1)(ρ)=1/T(1)(ρ) appears over a range of easily accessible B(1) values. Measurements of T(1)(ρ) at constant temperature and different B(1) values then give the "dispersion profiles" for biopolymers. Fitting a theoretical relaxation model to these profiles allows for the estimation of correlation times. This way of obtaining the correlation time is easier and faster than approaches involving measurements of the temperature dependence of R(1)=1/T(1). The T(1)(ρ) dispersion approach, as a tool for molecular dynamics study, has been demonstrated for several hydrated biopolymer systems including crystalline cellulose, starch of different origins (potato, corn, oat, wheat), paper (modern, old) and lyophilized proteins (albumin, lysozyme). Copyright © 2010 Elsevier Inc. All rights reserved.
Ottochian, A; De Michele, C; Leporini, D
2009-12-14
On approaching the glass transition, the microscopic kinetic unit spends increasing time rattling in the cage of the first neighbors, whereas its average escape time, the structural relaxation time tau(alpha), increases from a few picoseconds up to thousands of seconds. A thorough study of the correlation between tau(alpha) and the rattling amplitude, expressed by the Debye-Waller factor, was carried out. Molecular-dynamics simulations of both a model polymer system and a binary mixture were performed by varying the temperature, the density rho, the potential and the polymer length to consider the structural relaxation as well as both the rotational and the translation diffusion. The present simulations, together with MD studies on other glassformers, evidence the scaling between the structural relaxation and the caged dynamics. An analytic model of the master curve is developed in terms of two characteristic length scales a(2) (1/2) and sigma(a(2) ) (1/2), pertaining to the distance to be covered by the kinetic unit to reach a transition state. The model does not imply tau(alpha) divergences. The comparison with the experiments supports the numerical evidence over a range of relaxation times as wide as about eighteen orders of magnitude. A comparison with other scaling and correlation procedures is presented. In particular, the density scaling of the length scales a(2) (1/2), sigma(a(2) ) (1/2) proportional to rho(-1/3) is shown to be not supported by the present simulations. The study suggests that the equilibrium and the moderately supercooled states of the glassformers possess key information on the huge slowing-down of their relaxation close to the glass transition. The latter, according to the present simulations, exhibits features consistent with the Lindemann melting criterion and the free-volume model.
Molecular dynamics of bacteriorhodopsin.
Lupo, J A; Pachter, R
1997-02-01
A model of bacteriorhodopsin (bR), with a retinal chromophore attached, has been derived for a molecular dynamics simulation. A method for determining atomic coordinates of several ill-defined strands was developed using a structure prediction algorithm based on a sequential Kalman filter technique. The completed structure was minimized using the GROMOS force field. The structure was then heated to 293 K and run for 500 ps at constant temperature. A comparison with the energy-minimized structure showed a slow increase in the all-atom RMS deviation over the first 200 ps, leveling off to approximately 2.4 A relative to the starting structure. The final structure yielded a backbone-atom RMS deviation from the crystallographic structure of 2.8 A. The residue neighbors of the chromophore atoms were followed as a function of time. The set of persistent near-residue neighbors supports the theory that differences in pKa values control access to the Schiff base proton, rather than formation of a counterion complex.
Relaxation dynamics and thermophysical properties of vegetable oils using time-domain reflectometry.
Sonkamble, Anil A; Sonsale, Rahul P; Kanshette, Mahesh S; Kabara, Komal B; Wananje, Kunal H; Kumbharkhane, Ashok C; Sarode, Arvind V
2017-04-01
Dielectric relaxation studies of vegetable oils are important for insights into their hydrogen bonding and intermolecular dynamics. The dielectric relaxation and thermo physical properties of triglycerides present in some vegetable oils have been measured over the frequency range of 10 MHz to 7 GHz in the temperature region 25 to 10 °C using a time-domain reflectometry approach. The frequency and temperature dependence of dielectric constants and dielectric loss factors were determined for coconut, peanut, soya bean, sunflower, palm, and olive oils. The dielectric permittivity spectra for each of the studied vegetable oils are explained using the Debye model with their complex dielectric permittivity analyzed using the Havriliak-Negami equation. The dielectric parameters static permittivity (ε 0 ), high-frequency limiting static permittivity (ε ∞ ), average relaxation time (τ 0 ), and thermodynamic parameters such as free energy (∆F τ ), enthalpy (∆H τ ), and entropy of activation (∆S τ ) were also measured. Calculation and analysis of these thermodynamic parameters agrees with the determined dielectric parameters, giving insights into the temperature dependence of the molecular dynamics of these systems.
International Nuclear Information System (INIS)
Nakano, Masayoshi; Kishi, Ryohei; Nitta, Tomoshige; Yamaguchi, Kizashi
2004-01-01
We investigate the relaxation effects on the quantum dynamics in a two-state molecular system interacting with a single-mode strongly amplitude-squeezed coherent field using the second-order Monte Carlo wave-function method. The molecular population inversion (collapse-revival behavior of Rabi oscillations) is known to show the echoes after each revival, which are referred to as ringing revivals, in the case of strongly squeezed coherent fields with oscillatory photon-number distributions due to the phase-space interference effect. Two types of relaxation effects, i.e., cavity relaxation (the dissipation of an internal single mode to outer mode) and molecular coherent (phase) relaxation caused by nuclear vibrations on ringing revivals are investigated from the viewpoint of the quantum-phase dynamics using the quasiprobability (Q function) distribution of a single-mode field and the off-diagonal molecular density matrix ρ elec1,2 (t). It turns out that the molecular phase relaxation attenuates both the entire revival-collapse behavior and the increase in ρ elec1,2 (t) during the quiescent region, whereas a very slight cavity relaxation particularly suppresses the echoes in ringing revivals more significantly than the first revival but hardly changes a primary variation in envelope of ρ elec1,2 (t) in the nonrelaxation case
International Nuclear Information System (INIS)
Sabass, Benedikt; Schwarz, Ulrich S
2010-01-01
In migrating cells, retrograde flow of the actin cytoskeleton is related to traction at adhesion sites located at the base of the lamellipodium. The coupling between the moving cytoskeleton and the stationary adhesions is mediated by the continuous association and dissociation of molecular bonds. We introduce a simple model for the competition between the stochastic dynamics of elastic bonds at the moving interface and relaxation within the moving actin cytoskeleton represented by an internal viscous friction coefficient. Using exact stochastic simulations and an analytical mean field theory, we show that the stochastic bond dynamics lead to biphasic friction laws as observed experimentally. At low internal dissipation, stochastic bond dynamics lead to a regime of irregular stick-and-slip motion. High internal dissipation effectively suppresses cooperative effects among bonds and hence stabilizes the adhesion.
Energy Technology Data Exchange (ETDEWEB)
Uranga-Piña, L. [Facultad de Física, Universidad de la Habana, San Lázaro y L, Vedado, 10400 Havana (Cuba); Institute for Chemistry and Biochemistry, Freie Universität Berlin, Takustr. 3, D-14195 Berlin (Germany); Tremblay, J. C., E-mail: jean.c.tremblay@gmail.com [Institute for Chemistry and Biochemistry, Freie Universität Berlin, Takustr. 3, D-14195 Berlin (Germany)
2014-08-21
We investigate the effect of inter-mode coupling on the vibrational relaxation dynamics of molecules in weak dissipative environments. The simulations are performed within the reduced density matrix formalism in the Markovian regime, assuming a Lindblad form for the system-bath interaction. The prototypical two-dimensional model system representing two CO molecules approaching a Cu(100) surface is adapted from an ab initio potential, while the diatom-diatom vibrational coupling strength is systematically varied. In the weak system-bath coupling limit and at low temperatures, only first order non-adiabatic uni-modal coupling terms contribute to surface-mediated vibrational relaxation. Since dissipative dynamics is non-unitary, the choice of representation will affect the evolution of the reduced density matrix. Two alternative representations for computing the relaxation rates and the associated operators are thus compared: the fully coupled spectral basis, and a factorizable ansatz. The former is well-established and serves as a benchmark for the solution of Liouville-von Neumann equation. In the latter, a contracted grid basis of potential-optimized discrete variable representation is tailored to incorporate most of the inter-mode coupling, while the Lindblad operators are represented as tensor products of one-dimensional operators, for consistency. This procedure results in a marked reduction of the grid size and in a much more advantageous scaling of the computational cost with respect to the increase of the dimensionality of the system. The factorizable method is found to provide an accurate description of the dissipative quantum dynamics of the model system, specifically of the time evolution of the state populations and of the probability density distribution of the molecular wave packet. The influence of intra-molecular vibrational energy redistribution appears to be properly taken into account by the new model on the whole range of coupling strengths. It
Vibrational relaxation dynamics of SD molecules in As2S3: Observation of an anomalous isotope effect
International Nuclear Information System (INIS)
Engholm, J.R.; Happek, U.; Rella, C.W.
1995-01-01
It is generally assumed that the vibrational relaxation of molecular impurities in crystals and glasses mainly depends on the order of the decay process, with lower order processes leading to more rapid relaxation (a behavior that is known under the term open-quotes gap-lawclose quotes). Here we present measurements that contradict this assumption. Using high intensity psec pulses of the Stanford FEL we measured the relaxation rate of the SD vibrational stretch mode (at a frequency of 1800 cm) by applying a pump-probe technique. We find relaxation rates on the order of 2x10 9 sec -1 , which are a factor of 2 lower than those found for the isotope molecule SH (at a frequency of about 2500 cm - 1 ) in the same host 1 . We recall that the relaxation of the SD vibrational stretch mode is controlled by a lower order process as compared to the SH molecule, which is due to the smaller number of host vibrational quanta to match the energy of the stretch mode; a fact we have confirmed experimentally by temperature dependent relaxation measurements. Thus our remits are in marked contrast to the so-called open-quotes Gap-Lawclose quotes and emphasize the importance of the molecule - host coupling in the relaxation dynamics
Molecular photoionization dynamics
International Nuclear Information System (INIS)
Dehmer, J.L.
1982-01-01
This program seeks to develop both physical insight and quantitative characterization of molecular photoionization processes. Progress is briefly described, and some publications resulting from the research are listed
Physical adsorption and molecular dynamics
International Nuclear Information System (INIS)
Cohan, N.V.
1981-01-01
Some aspects of noble gases adsorption (except He) on graphite substracts are reviewed. Experimental results from this adsorption are analyzed and compared with molecular dynamics calculations. (L.C.) [pt
Violent Relaxation, Dynamical Instabilities and the Formation of Elliptical Galaxies
Aguilar, L. A.
1990-11-01
RESUMEN: El problema de la formaci6n de galaxias elfpticas por medjo de colapso gravitacional sin disipaci6n de energfa es estudiado usando un gran numero de simulaciones numericas. Se muestra que este tipo de colapsos, partiendo de condiciones iniciales frfas donde la energfa cinetica inicial representa s6lo un 5%, 0 , de a potencial inicial, produce sistemas relajados de forma triaxial muy similares a las galaxias elfpticas reales en sus formas y perfiles de densidad en proyecci6i . La forina triaxial resulta de la acci6n de una inestabilidad dinamica que aparece en sistemas 'inicos dominados por movimientos radiales, mientras que el perfil de densidad final Cs debido al llamado relajamiento violento que tiende a producir una distribuci6n en espacio fase unica. Estos dos fen6menos tienden a borrar los detalles particulares sobre las condiciones iniciales y dan lugar a una evoluci6n convergente hacia sistemas realistas, esto innecesario el uso de condiciones iniciales especiales (excepto por Ia condici6i de que estas deben ser frfas). Las condiciones iniciales frfas producen los movimientos radiales y fluctuaciones de la energfa potencial requeridos por ambos fen6menos. ABSTRACT: The problem of formation of elliptical galaxies via dissipationless collapse is studied using a large set of numerical simulations. It is shown that dissipationless collapses from cold initial conditions, where the total initial kinetic energy is less than 5% ofthe initial potential energy, lead to relaxed triaxial systems ery similar to real elliptical galaxies ii projected shape and density profiles. The triaxial shape is due to the of a dynamical instability that appears on systems dominated by radial orbits, while final density profile is due to violent relaxation that tends to produce a unique distribution iii space. These two phenomena erase memory of the initial prodtice a convergent evolution toward realistic systems, thus making unnecessary use o[special initial conditions (other
Extended Lagrangian Excited State Molecular Dynamics.
Bjorgaard, J A; Sheppard, D; Tretiak, S; Niklasson, A M N
2018-02-13
An extended Lagrangian framework for excited state molecular dynamics (XL-ESMD) using time-dependent self-consistent field theory is proposed. The formulation is a generalization of the extended Lagrangian formulations for ground state Born-Oppenheimer molecular dynamics [Phys. Rev. Lett. 2008 100, 123004]. The theory is implemented, demonstrated, and evaluated using a time-dependent semiempirical model, though it should be generally applicable to ab initio theory. The simulations show enhanced energy stability and a significantly reduced computational cost associated with the iterative solutions of both the ground state and the electronically excited states. Relaxed convergence criteria can therefore be used both for the self-consistent ground state optimization and for the iterative subspace diagonalization of the random phase approximation matrix used to calculate the excited state transitions. The XL-ESMD approach is expected to enable numerically efficient excited state molecular dynamics for such methods as time-dependent Hartree-Fock (TD-HF), Configuration Interactions Singles (CIS), and time-dependent density functional theory (TD-DFT).
Stellar dynamics around a massive black hole - II. Resonant relaxation
Sridhar, S.; Touma, Jihad R.
2016-06-01
We present a first-principles theory of resonant relaxation (RR) of a low-mass stellar system orbiting a more massive black hole (MBH). We first extend the kinetic theory of Gilbert to include the Keplerian field of a black hole of mass M•. Specializing to a Keplerian stellar system of mass M ≪ M•, we use the orbit-averaging method of Sridhar & Touma to derive a kinetic equation for RR. This describes the collisional evolution of a system of N ≫ 1 Gaussian rings in a reduced 5-dim space, under the combined actions of self-gravity, 1 post-Newtonian (PN) and 1.5 PN relativistic effects of the MBH and an arbitrary external potential. In general geometries, RR is driven by both apsidal and nodal resonances, so the distinction between scalar RR and vector RR disappears. The system passes through a sequence of quasi-steady secular collisionless equilibria, driven by irreversible two-ring correlations that accrue through gravitational interactions, both direct and collective. This correlation function is related to a `wake function', which is the linear response of the system to the perturbation of a chosen ring. The wake function is easier to appreciate, and satisfies a simpler equation, than the correlation function. We discuss general implications for the interplay of secular dynamics and non-equilibrium statistical mechanics in the evolution of Keplerian stellar systems towards secular thermodynamic equilibria, and set the stage for applications to the RR of axisymmetric discs in Paper III.
Advances in dynamic relaxation techniques for nonlinear finite element analysis
International Nuclear Information System (INIS)
Sauve, R.G.; Metzger, D.R.
1995-01-01
Traditionally, the finite element technique has been applied to static and steady-state problems using implicit methods. When nonlinearities exist, equilibrium iterations must be performed using Newton-Raphson or quasi-Newton techniques at each load level. In the presence of complex geometry, nonlinear material behavior, and large relative sliding of material interfaces, solutions using implicit methods often become intractable. A dynamic relaxation algorithm is developed for inclusion in finite element codes. The explicit nature of the method avoids large computer memory requirements and makes possible the solution of large-scale problems. The method described approaches the steady-state solution with no overshoot, a problem which has plagued researchers in the past. The method is included in a general nonlinear finite element code. A description of the method along with a number of new applications involving geometric and material nonlinearities are presented. They include: (1) nonlinear geometric cantilever plate; (2) moment-loaded nonlinear beam; and (3) creep of nuclear fuel channel assemblies
Electronic excited states and relaxation dynamics in polymer heterojunction systems
Ramon, John Glenn Santos
The potential for using conducting polymers as the active material in optoelectronic devices has come to fruition in the past few years. Understanding the fundamental photophysics behind their operations points to the significant role played by the polymer interface in their performance. Current device architectures involve the use of bulk heterojunctions which intimately blend the donor and acceptor polymers to significantly increase not only their interfacial surface area but also the probability of exciton formation within the vicinity of the interface. In this dissertation, we detail the role played by the interface on the behavior and performance of bulk heterojunction systems. First, we explore the relation between the exciton binding energy to the band offset in determining device characteristics. As a general rule, when the exciton binding energy is greater than the band offset, the exciton remains the lowest energy excited state leading to efficient light-emitting properties. On the other hand, if the offset is greater than the binding energy, charge separation becomes favorable leading to better photovoltaic behavior. Here, we use a Wannier function, configuration interaction based approach to examine the essential excited states and predict the vibronic absorption and emission spectra of the PPV/BBL, TFB/F8BT and PFB/F8BT heterojunctions. Our results underscore the role of vibrational relaxation in the formation of charge-transfer states following photoexcitation. In addition, we look at the relaxation dynamics that occur upon photoexcitation. For this, we adopt the Marcus-Hush semiclassical method to account for lattice reorganization in the calculation of the interconversion rates in TFB/F8BT and PFB/F8BT. We find that, while a tightly bound charge-transfer state (exciplex) remains the lowest excited state, a regeneration pathway to the optically active lowest excitonic state in TFB/F8BT is possible via thermal repopulation from the exciplex. Finally
Quantifying protein dynamics in the ps–ns time regime by NMR relaxation
Energy Technology Data Exchange (ETDEWEB)
Hernández, Griselda; LeMaster, David M., E-mail: david.lemaster@health.ny.gov [University at Albany - SUNY, Wadsworth Center, New York State Department of Health and Department of Biomedical Sciences, School of Public Health (United States)
2016-11-15
Both {sup 15}N chemical shift anisotropy (CSA) and sufficiently rapid exchange linebroadening transitions exhibit relaxation contributions that are proportional to the square of the magnetic field. Deconvoluting these contributions is further complicated by residue-dependent variations in protein amide {sup 15}N CSA values which have proven difficult to accurately measure. Exploiting recently reported improvements for the implementation of T{sub 1} and T{sub 1ρ} experiments, field strength-dependent studies have been carried out on the B3 domain of protein G (GB3) as well as on the immunophilin FKBP12 and a H87V variant of that protein in which the major conformational exchange linebroadening transition is suppressed. By applying a zero frequency spectral density rescaling analysis to the relaxation data collected at magnetic fields from 500 to 900 MHz {sup 1}H, differential residue-specific {sup 15}N CSA values have been obtained for GB3 which correlate with those derived from solid state and liquid crystalline NMR measurements to a level similar to the correlation among those previously reported studies. Application of this analysis protocol to FKBP12 demonstrated an efficient quantitation of both weak exchange linebroadening contributions and differential residue-specific {sup 15}N CSA values. Experimental access to such differential residue-specific {sup 15}N CSA values should significantly facilitate more accurate comparisons with molecular dynamics simulations of protein motion that occurs within the timeframe of global molecular tumbling.
Ultrafast relaxation dynamics of electrons in Au clusters capped with dodecanethiol molecules
International Nuclear Information System (INIS)
Hamanaka, Y.; Fukagawa, K.; Tai, Y.; Murakami, J.; Nakamura, A.
2006-01-01
We have investigated electron relaxation dynamics of size-selected Au clusters capped by dodecanethiol molecules in the cluster sizes of 28-142 atoms using femtosecond pump-probe spectroscopy. Absorption spectra of 28-71-atom clusters show discrete peaks due to the optical transitions between quantized states, while an absorption band due to the surface plasmon is observed in 142-atom clusters. In the differential absorption spectra measured by the pump-probe experiments, a large redshift of 140 meV lasting over 10 ps and absorption bleaching decaying within 2 ps are observed at the absorption peaks of 28-atom clusters. The redshift is ascribed to a charge transfer between Au clusters and dodecanethiol molecules adsorbed on the cluster surface, and the bleaching is due to blocking of the optical transitions between the ground state and the occupied electronic states due to the Pauli's-exclusion principle. Such behavior is in contrast to the 142-atom clusters, where the cooling of hot electrons generated by photo-excitation determines the relaxation dynamics. These results indicate molecular properties of the 28-atom Au cluster-dodecanethiol system
International Nuclear Information System (INIS)
Schwerdtfeger, Christine A.; Soudackov, Alexander V.; Hammes-Schiffer, Sharon
2014-01-01
The development of efficient theoretical methods for describing electron transfer (ET) reactions in condensed phases is important for a variety of chemical and biological applications. Previously, dynamical dielectric continuum theory was used to derive Langevin equations for a single collective solvent coordinate describing ET in a polar solvent. In this theory, the parameters are directly related to the physical properties of the system and can be determined from experimental data or explicit molecular dynamics simulations. Herein, we combine these Langevin equations with surface hopping nonadiabatic dynamics methods to calculate the rate constants for thermal ET reactions in polar solvents for a wide range of electronic couplings and reaction free energies. Comparison of explicit and implicit solvent calculations illustrates that the mapping from explicit to implicit solvent models is valid even for solvents exhibiting complex relaxation behavior with multiple relaxation time scales and a short-time inertial response. The rate constants calculated for implicit solvent models with a single solvent relaxation time scale corresponding to water, acetonitrile, and methanol agree well with analytical theories in the Golden rule and solvent-controlled regimes, as well as in the intermediate regime. The implicit solvent models with two relaxation time scales are in qualitative agreement with the analytical theories but quantitatively overestimate the rate constants compared to these theories. Analysis of these simulations elucidates the importance of multiple relaxation time scales and the inertial component of the solvent response, as well as potential shortcomings of the analytical theories based on single time scale solvent relaxation models. This implicit solvent approach will enable the simulation of a wide range of ET reactions via the stochastic dynamics of a single collective solvent coordinate with parameters that are relevant to experimentally accessible
International Nuclear Information System (INIS)
Hall, G.E.; Prrese, J.M.; Sears, T.J.; Weston, R.E.
1999-01-01
The goal of this research is the understanding of elementary chemical and physical processes important in the combustion of fossil fuels. Interest centers on reactions involving short-lived chemical intermediates and their properties. High-resolution high-sensitivity laser absorption methods are augmented by high temperature flow-tube reaction kinetics studies with mass spectrometric sampling. These experiments provide information on the energy levels, structures and reactivity of molecular flee radical species and, in turn, provide new tools for the study of energy flow and chemical bond cleavage in the radicals in chemical systems. The experimental work is supported by theoretical and computational work using time-dependent quantum wave packet calculations that provide insights into energy flow between the vibrational modes of the molecule
Relaxation Dynamics of a Granular Pile on a Vertically Vibrating Plate
Tsuji, Daisuke; Otsuki, Michio; Katsuragi, Hiroaki
2018-03-01
Nonlinear relaxation dynamics of a vertically vibrated granular pile is experimentally studied. In the experiment, the flux and slope on the relaxing pile are measured by using a high-speed laser profiler. The relation of these quantities can be modeled by the nonlinear transport law assuming the uniform vibrofluidization of an entire pile. The fitting parameter in this model is only the relaxation efficiency, which characterizes the energy conversion rate from vertical vibration into horizontal transport. We demonstrate that this value is a constant independent of experimental conditions. The actual relaxation is successfully reproduced by the continuity equation with the proposed model. Finally, its specific applicability toward an astrophysical phenomenon is shown.
Effect of high pressure on the relaxation dynamics of glass-forming liquids
Energy Technology Data Exchange (ETDEWEB)
Paluch, M; Grzybowska, K; Grzybowski, A [Institute of Physics, Silesian University, ulica Uniwersytecka 4, 40-007 Katowice (Poland)
2007-05-23
A glass is usually formed by cooling a liquid at a rate sufficient to avoid crystallization. In the vicinity of the glass transition the structural relaxation time increases with lowering temperature in a non-Arrhenius fashion and the structural relaxation function reveals a non-Debye behaviour. However, liquid can be also vitrified by keeping it at a constant temperature and increasing the pressure. This pressure-induced transition to the glassy state is also accompanied by dramatic changes in the relaxation dynamics. Herein we discuss the behaviour of the structural relaxation times of glass-forming liquids and polymer melts under high pressure.
Effect of high pressure on the relaxation dynamics of glass-forming liquids
International Nuclear Information System (INIS)
Paluch, M; Grzybowska, K; Grzybowski, A
2007-01-01
A glass is usually formed by cooling a liquid at a rate sufficient to avoid crystallization. In the vicinity of the glass transition the structural relaxation time increases with lowering temperature in a non-Arrhenius fashion and the structural relaxation function reveals a non-Debye behaviour. However, liquid can be also vitrified by keeping it at a constant temperature and increasing the pressure. This pressure-induced transition to the glassy state is also accompanied by dramatic changes in the relaxation dynamics. Herein we discuss the behaviour of the structural relaxation times of glass-forming liquids and polymer melts under high pressure
Lunkenheimer, Peter
1997-01-01
Relaxations and fast dynamics of the plastic crystal cyclo-octanol investigated by broadband dielectric spectroscopy / R. Brand, P. Lunkenheimer, A. Loidl. - In: Physical review. B. 56. 1997. S. R5713-R5716
Molecular quenching and relaxation in a plasmonic tunable system
Baffou, Guillaume; Girard, Christian; Dujardin, Erik; Colas Des Francs, Gérard; Martin, Olivier J. F.
2008-03-01
Molecular fluorescence decay is significantly modified when the emitting molecule is located near a plasmonic structure. When the lateral sizes of such structures are reduced to nanometer-scale cross sections, they can be used to accurately control and amplify the emission rate. In this Rapid Communication, we extend Green’s dyadic method to quantitatively investigate both radiative and nonradiative decay channels experienced by a single fluorescent molecule confined in an adjustable dielectric-metal nanogap. The technique produces data in excellent agreement with current experimental work.
Modeling shockwave deformation via molecular dynamics
International Nuclear Information System (INIS)
Holian, B.L.
1987-01-01
Molecular dynamics (MD), where the equations of motion of up to thousands of interacting atoms are solved on the computer, has proven to be a powerful tool for investigating a wide variety of nonequilibrium processes from the atomistic viewpoint. Simulations of shock waves in three-dimensional (3D) solids and fluids have shown conclusively that shear-stress relaxation is achieved through atomic rearrangement. In the case of fluids, the transverse motion is viscous, and the constitutive model of Navier-Stokes hydrodynamics has been shown to be accurate - even on the time and distance scales of MD experiments. For strong shocks in solids, the plastic flow that leads to shear-stress relaxation in MD is highly localized near the shock front, involving a slippage along close-packed planes. For shocks of intermediate strength, MD calculations exhibit an elastic precursor running out in front of the steady plastic wave, where slippage similar in character to that in the very strong shocks leads to shear-stress relaxation. An interesting correlation between the maximum shear stress and the Hugoniot pressure jump is observed for both 3D and fluid shockwave calculations, which may have some utility in modeling applications. At low shock strengths, the MD simulations show only elastic compression, with no permanent transverse atomic strains. The result for perfect 3D crystals is also seen in calculations for 1D chains. It is speculated that, if it were practical, a very large MD system containing dislocations could be expected to exhibit more realistic plastic flow for weak shock waves, too
Space and time dynamical heterogeneity in glassy relaxation. The role of democratic clusters
International Nuclear Information System (INIS)
Appignanesi, G A; Rodriguez Fris, J A
2009-01-01
In this work we review recent computational advances in the understanding of the relaxation dynamics of supercooled glass-forming liquids. In such a supercooled regime these systems experience a striking dynamical slowing down which can be rationalized in terms of the picture of dynamical heterogeneities, wherein the dynamics can vary by orders of magnitude from one region of the sample to another and where the sizes and timescales of such slowly relaxing regions are expected to increase considerably as the temperature is decreased. We shall focus on the relaxation events at a microscopic level and describe the finding of the collective motions of particles responsible for the dynamical heterogeneities. In so doing, we shall demonstrate that the dynamics in different regions of the system is not only heterogeneous in space but also in time. In particular, we shall be interested in the events relevant to the long-time structural relaxation or α relaxation. In this regard, we shall focus on the discovery of cooperatively relaxing units involving the collective motion of relatively compact clusters of particles, called 'democratic clusters' or d-clusters. These events have been shown to trigger transitions between metabasins of the potential energy landscape (collections of similar configurations or structures) and to consist of the main steps in the α relaxation. Such events emerge in systems quite different in nature such as simple model glass formers and supercooled amorphous water. Additionally, another relevant issue in this context consists in the determination of a link between structure and dynamics. In this context, we describe the relationship between the d-cluster events and the constraints that the local structure poses on the relaxation dynamics, thus revealing their role in reformulating structural constraints. (topical review)
Ultrafast dissociation: An unexpected tool for probing molecular dynamics
International Nuclear Information System (INIS)
Morin, Paul; Miron, Catalin
2012-01-01
Highlights: ► Ultrafast dissociation has been investigated by means of XPS and mass spectrometry. ► The interplay between electron relaxation and molecular dynamics is evidenced. ► Extension toward polyatomics, clusters, adsorbed molecules is considered. ► Quantum effects (spectral hole, angular effects) evidence the molecular field anisotropy. -- Abstract: Ultrafast dissociation following core–shell excitation into an antibonding orbital led to the early observation in HBr of atomic Auger lines associated to the decay of dissociated excited atoms. The purpose of this article is to review the very large variety of systems where such a situation has been encountered, extending from simple diatomic molecules toward more complex systems like polyatomics, clusters, or adsorbed molecules. Interestingly, this phenomenon has revealed an extremely rich and powerful tool for probing nuclear dynamics and its subtle interplay with electron relaxation occurring on a comparable time scale. Consequently this review covers a surprisingly large period, starting in 1986 and still ongoing.
Revealing the Link between Structural Relaxation and Dynamic Heterogeneity in Glass-Forming Liquids.
Wang, Lijin; Xu, Ning; Wang, W H; Guan, Pengfei
2018-03-23
Despite the use of glasses for thousands of years, the nature of the glass transition is still mysterious. On approaching the glass transition, the growth of dynamic heterogeneity has long been thought to play a key role in explaining the abrupt slowdown of structural relaxation. However, it still remains elusive whether there is an underlying link between structural relaxation and dynamic heterogeneity. Here, we unravel the link by introducing a characteristic time scale hiding behind an identical dynamic heterogeneity for various model glass-forming liquids. We find that the time scale corresponds to the kinetic fragility of liquids. Moreover, it leads to scaling collapse of both the structural relaxation time and dynamic heterogeneity for all liquids studied, together with a characteristic temperature associated with the same dynamic heterogeneity. Our findings imply that studying the glass transition from the viewpoint of dynamic heterogeneity is more informative than expected.
Revealing the Link between Structural Relaxation and Dynamic Heterogeneity in Glass-Forming Liquids
Wang, Lijin; Xu, Ning; Wang, W. H.; Guan, Pengfei
2018-03-01
Despite the use of glasses for thousands of years, the nature of the glass transition is still mysterious. On approaching the glass transition, the growth of dynamic heterogeneity has long been thought to play a key role in explaining the abrupt slowdown of structural relaxation. However, it still remains elusive whether there is an underlying link between structural relaxation and dynamic heterogeneity. Here, we unravel the link by introducing a characteristic time scale hiding behind an identical dynamic heterogeneity for various model glass-forming liquids. We find that the time scale corresponds to the kinetic fragility of liquids. Moreover, it leads to scaling collapse of both the structural relaxation time and dynamic heterogeneity for all liquids studied, together with a characteristic temperature associated with the same dynamic heterogeneity. Our findings imply that studying the glass transition from the viewpoint of dynamic heterogeneity is more informative than expected.
Incorporation of quantum statistical features in molecular dynamics
International Nuclear Information System (INIS)
Ohnishi, Akira; Randrup, J.
1995-01-01
We formulate a method for incorporating quantum fluctuations into molecular-dynamics simulations of many-body systems, such as those employed for energetic nuclear collision processes. Based on Fermi's Golden Rule, we allow spontaneous transitions to occur between the wave packets which are not energy eigenstates. The ensuing diffusive evolution in the space of the wave packet parameters exhibits appealing physical properties, including relaxation towards quantum-statistical equilibrium. (author)
Energy Technology Data Exchange (ETDEWEB)
Cao, Qi-Long; Huang, Duo-Hui; Yang, Jun-Sheng; Wan, Min-Jie; Wang, Fan-Hou, E-mail: eatonch@gmail.com
2014-10-01
Molecular dynamics simulations were applied to study the dynamic and structural properties of supercooled liquid and glassy iron in the rapid-cooling processes. The mean-square displacement and the non-Gaussian parameter were used to describe the dynamic properties. The evolution of structural properties was investigated using the pair distribution functions and bond-angle distribution functions. Results for dynamic and structural relaxations indicate that the dynamic features are consistently correlated with the structure evolution, and there are three temperature regions as the temperature decreases: (1) at higher temperatures (1500 K, 1300 K, and 1100 K), the system remains in the liquid characteristics during the overall relaxation process. (2) At medial temperatures (1050 K, 900 K, and 700 K), a fast β-relaxation is followed by a much slower α-relaxation. There is a little change in the structural properties in the β-relaxation region, while major configuration rearrangements occurred in the α-relaxation range and the crystallization process was completed at the end of α-relaxation region. (3) At lower temperature (500 K), the system shows glassy characteristics during the overall relaxation process. In addition, the melting temperature, glass transition temperature and diffusion coefficients of supercooled liquid iron are also computed.
International Nuclear Information System (INIS)
Bernado, Pau; Fernandes, Miguel X.; Jacobs, Doris M.; Fiebig, Klaus; Garcia de la Torre, Jose; Pons, Miquel
2004-01-01
Many important proteins contain multiple domains connected by flexible linkers. Inter-domain motion is suggested to play a key role in many processes involving molecular recognition. Heteronuclear NMR relaxation is sensitive to motions in the relevant time scales and could provide valuable information on the dynamics of multi-domain proteins. However, the standard analysis based on the separation of global tumbling and fast local motions is no longer valid for multi-domain proteins undergoing internal motions involving complete domains and that take place on the same time scale than the overall motion.The complexity of the motions experienced even for the simplest two-domain proteins are difficult to capture with simple extensions of the classical Lipari-Szabo approach. Hydrodynamic effects are expected to dominate the motion of the individual globular domains, as well as that of the complete protein. Using Pin1 as a test case, we have simulated its motion at the microsecond time scale, at a reasonable computational expense, using Brownian Dynamic simulations on simplified models. The resulting trajectories provide insight on the interplay between global and inter-domain motion and can be analyzed using the recently published method of isotropic Reorientational Mode Dynamics which offer a way of calculating their contribution to heteronuclear relaxation rates. The analysis of trajectories computed with Pin1 models of different flexibility provides a general framework to understand the dynamics of multi-domain proteins and explains some of the observed features in the relaxation rate profile of free Pin1
Energy Technology Data Exchange (ETDEWEB)
Bernado, Pau [Institut de Biologie Structurale, Jean Pierre Ebel (France); Fernandes, Miguel X. [Universidad de Murcia, Departamento de Quimica Fisica, Facultad de Quimica (Spain); Jacobs, Doris M. [Johann Wolfgang Goethe-Universitaet Frankfurt, Institut fuer Organische Chemie und Chemische Biologie (Germany); Fiebig, Klaus [Affinium Pharmaceuticals (Canada); Garcia de la Torre, Jose [Universidad de Murcia, Departamento de Quimica Fisica, Facultad de Quimica (Spain); Pons, Miquel [Laboratori de RMN de Biomolecules, Parc Cientific de Barcelona (Spain)], E-mail: mpons@ub.edu
2004-05-15
Many important proteins contain multiple domains connected by flexible linkers. Inter-domain motion is suggested to play a key role in many processes involving molecular recognition. Heteronuclear NMR relaxation is sensitive to motions in the relevant time scales and could provide valuable information on the dynamics of multi-domain proteins. However, the standard analysis based on the separation of global tumbling and fast local motions is no longer valid for multi-domain proteins undergoing internal motions involving complete domains and that take place on the same time scale than the overall motion.The complexity of the motions experienced even for the simplest two-domain proteins are difficult to capture with simple extensions of the classical Lipari-Szabo approach. Hydrodynamic effects are expected to dominate the motion of the individual globular domains, as well as that of the complete protein. Using Pin1 as a test case, we have simulated its motion at the microsecond time scale, at a reasonable computational expense, using Brownian Dynamic simulations on simplified models. The resulting trajectories provide insight on the interplay between global and inter-domain motion and can be analyzed using the recently published method of isotropic Reorientational Mode Dynamics which offer a way of calculating their contribution to heteronuclear relaxation rates. The analysis of trajectories computed with Pin1 models of different flexibility provides a general framework to understand the dynamics of multi-domain proteins and explains some of the observed features in the relaxation rate profile of free Pin1.
International Nuclear Information System (INIS)
Dellerue, Serge
2000-01-01
Understand the structure-dynamics-function relation in the case of proteins is essential. But few experimental techniques allow to have access to knowledge of fast internal movements of biological macromolecules. With the neutron scattering method, it has been possible to study the reorientation dynamics of side chains and of polypeptide skeleton for two proteins in terms of water or detergent and of temperature. With the use of the molecular dynamics method, essential for completing and interpreting the experimental data, it has been possible to assess the different contributions of the whole structure of proteins to the overall dynamics. It has been shown that the polypeptide skeleton presents an energy relaxation comparable to those of the side chains. Moreover, it has been explained that the protein dynamics can only be understood in terms of relaxation time distribution. (author) [fr
Thomas-Fermi molecular dynamics
International Nuclear Information System (INIS)
Clerouin, J.; Pollock, E.L.; Zerah, G.
1992-01-01
A three-dimensional density-functional molecular-dynamics code is developed for the Thomas-Fermi density functional as a prototype for density functionals using only the density. Following Car and Parrinello [Phys. Rev. Lett. 55, 2471 (1985)], the electronic density is treated as a dynamical variable. The electronic densities are verified against a multi-ion Thomas-Fermi algorithm due to Parker [Phys. Rev. A 38, 2205 (1988)]. As an initial application, the effect of electronic polarization in enhancing ionic diffusion in strongly coupled plasmas is demonstrated
Symmetry of quantum molecular dynamics
International Nuclear Information System (INIS)
Burenin, A.V.
2002-01-01
The paper reviews the current state-of-art in describing quantum molecular dynamics based on symmetry principles alone. This qualitative approach is of particular interest as the only method currently available for a broad and topical class of problems in the internal dynamics of molecules. Besides, a molecule is a physical system whose collective internal motions are geometrically structured, and its perturbation theory description requires a symmetry analysis of this structure. The nature of the geometrical symmetry groups crucial for the closed formulation of the qualitative approach is discussed [ru
Inhomogeneous Relaxation of a Molecular Layer on an Insulator due to Compressive Stress
Bocquet, F.; Nony, L.; Mannsfeld, S. C. B.; Oison, V.; Pawlak, R.; Porte, L.; Loppacher, Ch.
2012-05-01
We discuss the inhomogeneous stress relaxation of a monolayer of hexahydroxytriphenylene (HHTP) which adopts the rare line-on-line (LOL) coincidence on KCl(001) and forms moiré patterns. The fact that the hexagonal HHTP layer is uniaxially compressed along the LOL makes this system an ideal candidate to discuss the influence of inhomogeneous stress relaxation. Our work is a combination of noncontact atomic force microscopy experiments, density functional theory and potential energy calculations, and a thorough interpretation by means of the Frenkel-Kontorova model. We show that the assumption of a homogeneous molecular layer is not valid for this organic-inorganic heteroepitaxial system since the best calculated energy configuration correlates with the experimental data only if inhomogeneous relaxations of the layer are taken into account.
Conformation analysis of trehalose. Molecular dynamics simulation and molecular mechanics
International Nuclear Information System (INIS)
Donnamaira, M.C.; Howard, E.I.; Grigera, J.R.
1992-09-01
Conformational analysis of the disaccharide trehalose is done by molecular dynamics and molecular mechanics. In spite of the different force fields used in each case, comparison between the molecular dynamics trajectories of the torsional angles of glycosidic linkage and energy conformational map shows a good agreement between both methods. By molecular dynamics it is observed a moderate mobility of the glycosidic linkage. The demands of computer time is comparable in both cases. (author). 6 refs, 4 figs
Effect of holographic grating period on its relaxation in a molecular glassy film
International Nuclear Information System (INIS)
Ozols, A; Augustovs, P; Kokars, V; Traskovskis, K; Saharov, D
2013-01-01
Holographic grating (HG) relaxation has been experimentally studied in 5,5,5-triphenylpentyl 4-((4-(bis(5,5,5-triphenylpentyl)amino) phenyl) diazenyl) benzoate molecular glassy film for HG periods (Λ) of 0.50, 2.0 and 8.6 μm. A strong effect of HG period on its relaxation is found manifesting itself differently in the volume and on the surface. The volume part of HG is fairly stable during 40 days if Λ > 0.50μm whereas the surface part of HG (most probably, surface relief grating) exhibits relaxational self-enhancement which is maximal at Λ = 8.6μm. It is proposed that thermostimulated directional mass transfer in the process of relaxation can be responsible for this relaxational self-enhancement. Weak HG recording and relatively fast HG decay takes place at Λ=0.50 μm. Therefore, effective chromophore photoorientation domain of about 0.2 μm is supposed
Molecular dynamics for dense matter
International Nuclear Information System (INIS)
Maruyama, Toshiki; Chiba, Satoshi; Watanabe, Gentaro
2012-01-01
We review a molecular dynamics method for nucleon many-body systems called quantum molecular dynamics (QMD), and our studies using this method. These studies address the structure and the dynamics of nuclear matter relevant to neutron star crusts, supernova cores, and heavy-ion collisions. A key advantage of QMD is that we can study dynamical processes of nucleon many-body systems without any assumptions about the nuclear structure. First, we focus on the inhomogeneous structures of low-density nuclear matter consisting not only of spherical nuclei but also of nuclear “pasta”, i.e., rod-like and slab-like nuclei. We show that pasta phases can appear in the ground and equilibrium states of nuclear matter without assuming nuclear shape. Next, we show our simulation of compression of nuclear matter which corresponds to the collapsing stage of supernovae. With the increase in density, a crystalline solid of spherical nuclei changes to a triangular lattice of rods by connecting neighboring nuclei. Finally, we discuss fragment formation in expanding nuclear matter. Our results suggest that a generally accepted scenario based on the liquid–gas phase transition is not plausible at lower temperatures. (author)
Molecular dynamics for dense matter
Maruyama, Toshiki; Watanabe, Gentaro; Chiba, Satoshi
2012-08-01
We review a molecular dynamics method for nucleon many-body systems called quantum molecular dynamics (QMD), and our studies using this method. These studies address the structure and the dynamics of nuclear matter relevant to neutron star crusts, supernova cores, and heavy-ion collisions. A key advantage of QMD is that we can study dynamical processes of nucleon many-body systems without any assumptions about the nuclear structure. First, we focus on the inhomogeneous structures of low-density nuclear matter consisting not only of spherical nuclei but also of nuclear "pasta", i.e., rod-like and slab-like nuclei. We show that pasta phases can appear in the ground and equilibrium states of nuclear matter without assuming nuclear shape. Next, we show our simulation of compression of nuclear matter which corresponds to the collapsing stage of supernovae. With the increase in density, a crystalline solid of spherical nuclei changes to a triangular lattice of rods by connecting neighboring nuclei. Finally, we discuss fragment formation in expanding nuclear matter. Our results suggest that a generally accepted scenario based on the liquid-gas phase transition is not plausible at lower temperatures.
Segmental dynamics in polymer melts by relaxation techniques and quasielastic neutron scattering
Colmenero, J.
1993-01-01
The dynamics of the segmental α-relaxation in three different polymeric systems, poly(vinyl methy ether) (PVME), poly(vinyl chloride) (PVC) and poly(bisphenol A, 2-hydroxypropylether) (PH) has been studied by means of relaxation techniques and quasielastic neutron scattering (backscattering spectrometers IN10 and IN13 at the ILL-Grenoble). By using these techniques we have covered a wide timescale ranging from mesoscopic to macroscopic times (10-10-101s). For analyzing the experimental data we have developed a phenomenological procedure in the frequency domain based on the Havriliak-Negami relaxation function which in fact implies a Kohlrausch-Williams-Watts relaxation function in the time domain. The results obtained indicate that the dynamics of the α-relaxation in a wide timescale shows a clear non-Debye behaviour. The shape of the relaxation function is found to be similar for the different techniques used and independent of temperature and momentum transfer (Q). Moreover the characteristic relaxation times deduced from the fitting of the experimental data can also be described using only one Vogel-Fulcher functional form. Besides we found that the Q-dependence of the relaxation times obtained by QENS is given by a power law, τ(Q) propto Q-n (n > 2) n being dependent on the system, and that the Q-behaviour and the non-Debye behaviour are directly correlated. We discuss this correlation taking into account several data of the dynamics of the α-relaxation previously reported in the literature. We also outline a possible scenario for explaining this empirical correlation.
Yi, Zheng; Lindner, Benjamin; Prinz, Jan-Hendrik; Noé, Frank; Smith, Jeremy C
2013-11-07
Neutron scattering experiments directly probe the dynamics of complex molecules on the sub pico- to microsecond time scales. However, the assignment of the relaxations seen experimentally to specific structural rearrangements is difficult, since many of the underlying dynamical processes may exist on similar timescales. In an accompanying article, we present a theoretical approach to the analysis of molecular dynamics simulations with a Markov State Model (MSM) that permits the direct identification of structural transitions leading to each contributing relaxation process. Here, we demonstrate the use of the method by applying it to the configurational dynamics of the well-characterized alanine dipeptide. A practical procedure for deriving the MSM from an MD is introduced. The result is a 9-state MSM in the space of the backbone dihedral angles and the side-chain methyl group. The agreement between the quasielastic spectrum calculated directly from the atomic trajectories and that derived from the Markov state model is excellent. The dependence on the wavevector of the individual Markov processes is described. The procedure means that it is now practicable to interpret quasielastic scattering spectra in terms of well-defined intramolecular transitions with minimal a priori assumptions as to the nature of the dynamics taking place.
Shete, Ganesh; Khomane, Kailas S; Bansal, Arvind Kumar
2014-01-01
The purpose of this paper was to investigate the relaxation behavior of amorphous hesperetin (HRN), using dielectric spectroscopy, and assessment of its crystallization kinetics above glass transition temperature (Tg ). Amorphous HRN exhibited both local (β-) and global (α-) relaxations. β-Relaxation was observed below Tg , whereas α-relaxation prominently emerged above Tg . β-Relaxation was found to be of Johari-Goldstein type and was correlated with α-process by coupling model. Secondly, isothermal crystallization experiments were performed at 363 K (Tg + 16.5 K), 373 K (Tg + 26.5 K), and 383 K (Tg + 36.5 K). The kinetics of crystallization, obtained from the normalized dielectric strength, was modeled using the Avrami model. Havriliak-Negami (HN) shape parameters, αHN and αHN .βHN , were analyzed during the course of crystallization to understand the dynamics of amorphous phase during the emergence of crystallites. HN shape parameters indicated that long range (α-like) were motions affected to a greater extent than short range (β-like) motions during isothermal crystallization studies at all temperature conditions. The variable behavior of α-like motions at different isothermal crystallization temperatures was attributed to evolving crystallites with time and increase in electrical conductivity with temperature. © 2013 Wiley Periodicals, Inc. and the American Pharmacists Association.
Introduction to Molecular Dynamics and Accelerated Molecular Dynamics
International Nuclear Information System (INIS)
Perez, Danny
2012-01-01
We first introduce classical molecular dynamics (MD) simulations. We discuss their main constituents - the interatomic potentials, the boundary conditions, and the integrators - and the discuss the various ensembles that can be sampled. We discuss the strengths and weaknesses of MD, specifically in terms of time and length-scales. We then move on to discuss accelerated MD (AMD) methods, techniques that were designed to circumvent the timescale limitations of MD for rare event systems. The different methods are introduced and examples of use given.
A nuclear magnetic relaxation study of hydrogen exchange and water dynamics in aqueous systems
International Nuclear Information System (INIS)
Lankhorst, D.
1983-01-01
In this thesis exchange of water protons in solutions of some weak electrolytes and polyelectrolytes is studied. Also the dynamical behaviour of water molecules in pure water is investigated. For these purposes nuclear magnetic resonance relaxation measurements, in solutions of oxygen-17 enriched water, are interpreted. The exchange rate of the water protons is derived from the contribution of 1 H- 17 O scalar coupling to the proton transverse relaxation rate. This rate is measured by the Carr-Purcell technique. (Auth.)
Flux ropes and 3D dynamics in the relaxation scaling experiment
International Nuclear Information System (INIS)
Intrator, T P; Feng, Y; Weber, T E; Swan, H O; Sun, X; Dorf, L; Sears, J A
2013-01-01
Flux ropes form basic building blocks for magnetic dynamics in many plasmas, are macroscopic analogues of magnetic field lines, and are irreducibly three dimensional (3D). We have used the relaxation scaling experiment (RSX) to study flux ropes, and have found many new features involving 3D dynamics, kink instability driven reconnection, nonlinearly stable but kinking flux ropes, and large flows. (paper)
A molecular dynamics simulation study of chloroform
Tironi, Ilario G.; van Gunsteren, Wilfred F.
Three different chloroform models have been investigated using molecular dynamics computer simulation. The thermodynamic, structural and dynamic properties of the various models were investigated in detail. In particular, the potential energies, diffusion coefficients and rotational correlation times obtained for each model are compared with experiment. It is found that the theory of rotational Brownian motion fails in describing the rotational diffusion of chloroform. The force field of Dietz and Heinzinger was found to give good overall agreement with experiment. An extended investigation of this chloroform model has been performed. Values are reported for the isothermal compressibility, the thermal expansion coefficient and the constant volume heat capacity. The values agree well with experiment. The static and frequency dependent dielectric permittivity were computed from a 1·2 ns simulation conducted under reaction field boundary conditions. Considering the fact that the model is rigid with fixed partial charges, the static dielectric constant and Debye relaxation time compare well with experiment. From the same simulation the shear viscosity was computed using the off-diagonal elements of the pressure tensor, both via an Einstein type relation and via a Green-Kubo equation. The calculated viscosities show good agreement with experimental values. The excess Helmholtz energy is calculated using the thermodynamic integration technique and simulations of 50 and 80 ps. The value obtained for the excess Helmholtz energy matches the theoretical value within a few per cent.
International Nuclear Information System (INIS)
Poveda, Ana; Asensio, Juan Luis; Martin-Pastor, Manuel; Jimenez-Barbero, Jesus
1997-01-01
1 H-NMR cross-relaxation rates and nonselective longitudinal relaxation times have been obtained at two magnetic fields (7.0 and 11.8 T) and at a variety of temperatures for the branched tetrasaccharide methyl 3-O-α-N-acetyl-galactosaminyl-β-galactopyranosyl-(1 → 4)[3-O-α-fucosyl] -glucopyranoside (1), an inhibitor of astrocyte growth. In addition, 13 C-NMR relaxation data have also been recorded at both fields. The 1 H-NMR relaxation data have been interpreted using different motional models to obtain proton-proton correlation times. The results indicate that the GalNAc and Fuc rings display more extensive local motion than the two inner Glc and Gal moieties, since those present significantly shorter local correlation times. The 13 C-NMR relaxation parameters have been interpreted in terms of the Lipari-Szabo model-free approach. Thus, order parameters and internal motion correlation times have been deduced. As obtained for the 1 H-NMR relaxation data, the two outer residues possess smaller order parameters than the two inner rings. Internal correlation times are in the order of 100 ps. The hydroxymethyl groups have also different behaviour,with the exocyclic carbon on the glucopyranoside unit showing the highestS 2 . Molecular dynamics simulations using a solvated system have also been performed and internal motion correlation functions have been deduced from these calculations. Order parameters and interproton distances have been compared to those inferred from the NMR measurements. The obtained results are in fair agreement with the experimental data
Energy Technology Data Exchange (ETDEWEB)
Poveda, Ana [Universidad Autonoma de Madrid, Servicio Interdepartamental de Investigacion (Spain); Asensio, Juan Luis; Martin-Pastor, Manuel; Jimenez-Barbero, Jesus [Instituto de Quimica Organica, CSIC, Grupo de Carbohidratos (Spain)
1997-07-15
{sup 1}H-NMR cross-relaxation rates and nonselective longitudinal relaxation times have been obtained at two magnetic fields (7.0 and 11.8 T) and at a variety of temperatures for the branched tetrasaccharide methyl 3-O-{alpha}-N-acetyl-galactosaminyl-{beta}-galactopyranosyl-(1{sup {yields}}4)[3-O-{alpha}-fucosyl] -glucopyranoside (1), an inhibitor of astrocyte growth. In addition, {sup 13}C-NMR relaxation data have also been recorded at both fields. The {sup 1}H-NMR relaxation data have been interpreted using different motional models to obtain proton-proton correlation times. The results indicate that the GalNAc and Fuc rings display more extensive local motion than the two inner Glc and Gal moieties, since those present significantly shorter local correlation times. The{sup 13}C-NMR relaxation parameters have been interpreted in terms of the Lipari-Szabo model-free approach. Thus, order parameters and internal motion correlation times have been deduced. As obtained for the{sup 1}H-NMR relaxation data, the two outer residues possess smaller order parameters than the two inner rings. Internal correlation times are in the order of 100 ps. The hydroxymethyl groups have also different behaviour,with the exocyclic carbon on the glucopyranoside unit showing the highestS{sup 2}. Molecular dynamics simulations using a solvated system have also been performed and internal motion correlation functions have been deduced from these calculations. Order parameters and interproton distances have been compared to those inferred from the NMR measurements. The obtained results are in fair agreement with the experimental data.
Konuma, Tsuyoshi; Harada, Erisa; Sugase, Kenji
2015-12-01
Protein dynamics plays important roles in many biological events, such as ligand binding and enzyme reactions. NMR is mostly used for investigating such protein dynamics in a site-specific manner. Recently, NMR has been actively applied to large proteins and intrinsically disordered proteins, which are attractive research targets. However, signal overlap, which is often observed for such proteins, hampers accurate analysis of NMR data. In this study, we have developed a new methodology called relaxation dispersion difference that can extract conformational exchange parameters from overlapped NMR signals measured using relaxation dispersion spectroscopy. In relaxation dispersion measurements, the signal intensities of fluctuating residues vary according to the Carr-Purcell-Meiboon-Gill pulsing interval, whereas those of non-fluctuating residues are constant. Therefore, subtraction of each relaxation dispersion spectrum from that with the highest signal intensities, measured at the shortest pulsing interval, leaves only the signals of the fluctuating residues. This is the principle of the relaxation dispersion difference method. This new method enabled us to extract exchange parameters from overlapped signals of heme oxygenase-1, which is a relatively large protein. The results indicate that the structural flexibility of a kink in the heme-binding site is important for efficient heme binding. Relaxation dispersion difference requires neither selectively labeled samples nor modification of pulse programs; thus it will have wide applications in protein dynamics analysis.
Energy Technology Data Exchange (ETDEWEB)
Konuma, Tsuyoshi [Icahn School of Medicine at Mount Sinai, Department of Structural and Chemical Biology (United States); Harada, Erisa [Suntory Foundation for Life Sciences, Bioorganic Research Institute (Japan); Sugase, Kenji, E-mail: sugase@sunbor.or.jp, E-mail: sugase@moleng.kyoto-u.ac.jp [Kyoto University, Department of Molecular Engineering, Graduate School of Engineering (Japan)
2015-12-15
Protein dynamics plays important roles in many biological events, such as ligand binding and enzyme reactions. NMR is mostly used for investigating such protein dynamics in a site-specific manner. Recently, NMR has been actively applied to large proteins and intrinsically disordered proteins, which are attractive research targets. However, signal overlap, which is often observed for such proteins, hampers accurate analysis of NMR data. In this study, we have developed a new methodology called relaxation dispersion difference that can extract conformational exchange parameters from overlapped NMR signals measured using relaxation dispersion spectroscopy. In relaxation dispersion measurements, the signal intensities of fluctuating residues vary according to the Carr-Purcell-Meiboon-Gill pulsing interval, whereas those of non-fluctuating residues are constant. Therefore, subtraction of each relaxation dispersion spectrum from that with the highest signal intensities, measured at the shortest pulsing interval, leaves only the signals of the fluctuating residues. This is the principle of the relaxation dispersion difference method. This new method enabled us to extract exchange parameters from overlapped signals of heme oxygenase-1, which is a relatively large protein. The results indicate that the structural flexibility of a kink in the heme-binding site is important for efficient heme binding. Relaxation dispersion difference requires neither selectively labeled samples nor modification of pulse programs; thus it will have wide applications in protein dynamics analysis.
Rheology via nonequilibrium molecular dynamics
International Nuclear Information System (INIS)
Hoover, W.G.
1982-10-01
The equilibrium molecular dynamics formulated by Newton, Lagrange, and Hamilton has been modified in order to simulate rheologial molecular flows with fast computers. This modified Nonequilibrium Molecular Dynamics (NEMD) has been applied to fluid and solid deformations, under both homogeneous and shock conditions, as well as to the transport of heat. The irreversible heating associated with dissipation could be controlled by carrying out isothermal NEMD calculations. The new isothermal NEMD equations of motion are consistent with Gauss' 1829 Least-Constraint principle as well as certain microscopic equilibrium and nonequilibrium statistical formulations due to Gibbs and Boltzmann. Application of isothermal NEMD revealed high-frequency and high-strain-rate behavior for simple fluids which resembled the behavior of polymer solutions and melts at lower frequencies and strain rates. For solids NEMD produces plastic flows consistent with experimental observations at much lower strain rates. The new nonequilibrium methods also suggest novel formulations of thermodynamics in nonequilibrium systems and shed light on the failure of the Principle of Material Frame Indifference
Temperature Dependence of Logarithmic-like Relaxational Dynamics of Hydrated tRNA.
Chu, Xiang-Qiang; Mamontov, Eugene; O'Neill, Hugh; Zhang, Qiu
2013-03-21
The dynamics of RNA within the β-relaxation region of 10 ps to 1 ns is crucial to its biological function. Because of its simpler chemical building blocks and the lack of the side methyl groups, faster relaxational dynamics of RNA compared to proteins can be expected. However, the situation is actually opposite. In this work, the relaxational dynamics of tRNA is measured by quasielastic neutron scattering and analyzed using the mode coupling theory, originally developed for glass-forming liquids. Our results reveal that the dynamics of tRNA follows a log-decay within the β-relaxation region, which is an important trait demonstrated by the dynamics of proteins. The dynamics of hydrated tRNA and lysozyme compared in the time domain further demonstrate that the slower dynamics of tRNA relative to proteins originates from the difference in the folded states of tRNA and proteins, as well as the influence of their hydration water.
Laser Controlled Molecular Orientation Dynamics
International Nuclear Information System (INIS)
Atabek, O.
2004-01-01
Molecular orientation is a challenging control issue covering a wide range of applications from reactive collisions, high order harmonic generation, surface processing and catalysis, to nanotechnologies. The laser control scenario rests on the following three steps: (i) depict some basic mechanisms producing dynamical orientation; (ii) use them both as computational and interpretative tools in optimal control schemes involving genetic algorithms; (iii) apply what is learnt from optimal control to improve the basic mechanisms. The existence of a target molecular rotational state combining the advantages of efficient and post-pulse long duration orientation is shown. A strategy is developed for reaching such a target in terms of a train of successive short laser pulses applied at predicted time intervals. Each individual pulse imparts a kick to the molecule which orients. Transposition of such strategies to generic systems is now under investigation
Parametrizing linear generalized Langevin dynamics from explicit molecular dynamics simulations
Energy Technology Data Exchange (ETDEWEB)
Gottwald, Fabian; Karsten, Sven; Ivanov, Sergei D., E-mail: sergei.ivanov@uni-rostock.de; Kühn, Oliver [Institute of Physics, Rostock University, Universitätsplatz 3, 18055 Rostock (Germany)
2015-06-28
Fundamental understanding of complex dynamics in many-particle systems on the atomistic level is of utmost importance. Often the systems of interest are of macroscopic size but can be partitioned into a few important degrees of freedom which are treated most accurately and others which constitute a thermal bath. Particular attention in this respect attracts the linear generalized Langevin equation, which can be rigorously derived by means of a linear projection technique. Within this framework, a complicated interaction with the bath can be reduced to a single memory kernel. This memory kernel in turn is parametrized for a particular system studied, usually by means of time-domain methods based on explicit molecular dynamics data. Here, we discuss that this task is more naturally achieved in frequency domain and develop a Fourier-based parametrization method that outperforms its time-domain analogues. Very surprisingly, the widely used rigid bond method turns out to be inappropriate in general. Importantly, we show that the rigid bond approach leads to a systematic overestimation of relaxation times, unless the system under study consists of a harmonic bath bi-linearly coupled to the relevant degrees of freedom.
Parametrizing linear generalized Langevin dynamics from explicit molecular dynamics simulations
International Nuclear Information System (INIS)
Gottwald, Fabian; Karsten, Sven; Ivanov, Sergei D.; Kühn, Oliver
2015-01-01
Fundamental understanding of complex dynamics in many-particle systems on the atomistic level is of utmost importance. Often the systems of interest are of macroscopic size but can be partitioned into a few important degrees of freedom which are treated most accurately and others which constitute a thermal bath. Particular attention in this respect attracts the linear generalized Langevin equation, which can be rigorously derived by means of a linear projection technique. Within this framework, a complicated interaction with the bath can be reduced to a single memory kernel. This memory kernel in turn is parametrized for a particular system studied, usually by means of time-domain methods based on explicit molecular dynamics data. Here, we discuss that this task is more naturally achieved in frequency domain and develop a Fourier-based parametrization method that outperforms its time-domain analogues. Very surprisingly, the widely used rigid bond method turns out to be inappropriate in general. Importantly, we show that the rigid bond approach leads to a systematic overestimation of relaxation times, unless the system under study consists of a harmonic bath bi-linearly coupled to the relevant degrees of freedom
International Nuclear Information System (INIS)
Vasudevan, Rama K.; Zhang, Shujun; Okatan, Mahmut Baris; Jesse, Stephen; Kalinin, Sergei V.; Bassiri-Gharb, Nazanin
2015-01-01
Compositional and charge disorder in ferroelectric relaxors lies at the heart of the unusual properties of these systems, such as aging and non-ergodicity, polarization rotations, and a host of temperature and field-driven phase transitions. However, much information about the field-dynamics of the polarization in the prototypical ferroelectric relaxor (1-x)Pb(Mg 1/3 Nb 2/3 )O 3-x PbTiO 3 (PMN-xPT) remains unprobed at the mesoscopic level. We use a piezoresponse force microscopy-based dynamic multimodal relaxation spectroscopy technique, enabling the study of ferroelectric switching and polarization relaxation at mesoscopic length scales, and carry out measurements on a PMN-0.28PT sample with minimal polishing. Results indicate that beyond a threshold DC bias the average relaxation increases as the system attempts to relax to the previous state. Phenomenological fitting reveals the presence of mesoscale heterogeneity in relaxation amplitudes and clearly suggests the presence of two distinct amplitudes. Independent component analysis reveals the presence of a disorder component of the relaxation, which is found to be strongly anti-correlated with the maximum piezoresponse at that location, suggesting smaller disorder effects where the polarization reversal is large and vice versa. The disorder in the relaxation amplitudes is postulated to arise from rhombohedral and field-induced tetragonal phase in the crystal, with each phase associated with its own relaxation amplitude. As a result, these studies highlight the crucial importance of the mixture of ferroelectric phases in the compositions in proximity of the morphotropic phase boundary in governing the local response and further highlight the ability of PFM voltage and time spectroscopies, in conjunction with big-data multivariate analyses, to locally map disorder and correlate it with parameters governing the dynamic behavior
Molecular dynamics study of silver
International Nuclear Information System (INIS)
Akhter, J.I.; Yaldram, K.; Ahmad, W.; Khan, M.K.; Rehman, T.S.
1995-03-01
We present results of molecular dynamics study using the embedded atom potential to examine the equilibrium bulk properties of Ag. We calculate the total energy and the lattice parameters as a function of temperature. From these we determine the specific heat and linear coefficient of thermal expansion. The comparison with experimental results of these two quantities is found to be excellent. We have also calculated the mean square displacement of the atoms in the three directions. As expected because of symmetry the displacements in the three directions are comparable and increase with increasing temperature. (author) 5 figs
Directory of Open Access Journals (Sweden)
Wu Xuebang
2015-09-01
Full Text Available The general trend in soft matter is to study systems of increasing complexity covering a wide range in time and frequency. Mechanical spectroscopy is a powerful tool for understanding the structure and relaxation dynamics of these materials over a large temperature range and frequency scale. In this work, we collect a few recent applications using low-frequency mechanical spectroscopy for elucidating the structural changes and relaxation dynamics in soft matter, largely based on the author’s group. We illustrate the potential of mechanical spectroscopy with three kinds of soft materials: colloids, polymers and granular systems. Examples include structural changes in colloids, segmental relaxations in amorphous polymers, and resonant dissipation of grain chains in three-dimensional media. The present work shows that mechanical spectroscopy has been applied as a necessary and complementary tool to study the dynamics of such complex systems.
Elkins, Madeline H.; Williams, Holly L.; Neumark, Daniel M.
2016-05-01
The excited state relaxation dynamics of the solvated electron in H2O and D2O are investigated using time-resolved photoelectron spectroscopy in a liquid microjet. The data show that the initial excited state decays on a time scale of 75 ± 12 fs in H2O and 102 ± 8 fs in D2O, followed by slower relaxation on time scales of 400 ± 70 fs and 390 ± 70 fs that are isotopically invariant within the precision of our measurements. Based on the time evolution of the transient signals, the faster and slower time constants are assigned to p → s internal conversion (IC) of the hydrated electron and relaxation on the ground electronic state, respectively. This assignment is consistent with the non-adiabatic mechanism for relaxation of the hydrated electron and yields an isotope effect of 1.4 ± 0.2 for IC of the hydrated electron.
Relaxation dynamics of a quantum emitter resonantly coupled to a metal nanoparticle
DEFF Research Database (Denmark)
Nerkararyan, K. V.; Bozhevolnyi, S. I.
2014-01-01
consequence of this relaxation process is that the emission, being largely determined by the MNP, comes out with a substantial delay. A large number of system parameters in our analytical description opens new possibilities for controlling quantum emitter dynamics. (C) 2014 Optical Society of America......The presence of a metal nanoparticle (MNP) near a quantum dipole emitter, when a localized surface plasmon mode is excited via the resonant coupling with an excited quantum dipole, dramatically changes the relaxation dynamics: an exponential decay changes to step-like behavior. The main physical...
Pohjola, Teemu; Schoeller, Herbert
2000-12-01
In this work, we study the spin dynamics of Mn12-acetate molecules in the regime of thermally assisted tunneling. In particular, we describe the system in the presence of a strong transverse magnetic field. Similar to recent experiments, the relaxation time/rate is found to display a series of resonances; their Lorentzian shape is found to stem from the tunneling. The dynamic susceptibility χ(ω) is calculated starting from the microscopic Hamiltonian and the resonant structure manifests itself also in χ(ω). Similar to recent results reported on another molecular magnet, Fe8, we find oscillations of the relaxation rate as a function of the transverse magnetic field when the field is directed along a hard axis of the molecules. This phenomenon is attributed to the interference of the geometrical or Berry phase. We propose susceptibility experiments to be carried out for strong transverse magnetic fields to study these oscillations and for a better resolution of the sharp satellite peaks in the relaxation rates.
A concurrent multiscale micromorphic molecular dynamics
International Nuclear Information System (INIS)
Li, Shaofan; Tong, Qi
2015-01-01
In this work, we have derived a multiscale micromorphic molecular dynamics (MMMD) from first principle to extend the (Andersen)-Parrinello-Rahman molecular dynamics to mesoscale and continuum scale. The multiscale micromorphic molecular dynamics is a con-current three-scale dynamics that couples a fine scale molecular dynamics, a mesoscale micromorphic dynamics, and a macroscale nonlocal particle dynamics together. By choosing proper statistical closure conditions, we have shown that the original Andersen-Parrinello-Rahman molecular dynamics is the homogeneous and equilibrium case of the proposed multiscale micromorphic molecular dynamics. In specific, we have shown that the Andersen-Parrinello-Rahman molecular dynamics can be rigorously formulated and justified from first principle, and its general inhomogeneous case, i.e., the three scale con-current multiscale micromorphic molecular dynamics can take into account of macroscale continuum mechanics boundary condition without the limitation of atomistic boundary condition or periodic boundary conditions. The discovered multiscale scale structure and the corresponding multiscale dynamics reveal a seamless transition from atomistic scale to continuum scale and the intrinsic coupling mechanism among them based on first principle formulation
Color molecular dynamics for dense matter
International Nuclear Information System (INIS)
Maruyama, Toshiki; Hatsuda, Tetsuo
2000-01-01
We propose a microscopic approach for quark many-body system based on molecular dynamics. Using color confinement and one-gluon exchange potentials together with meson exchange potentials between quarks, we construct nucleons and nuclear/quark matter. Dynamical transition between confinement and deconfinement phases are studied at high baryon density with this molecular dynamics simulation. (author)
Molecular motion of micellar solutes: a 13C NMR relaxation study
International Nuclear Information System (INIS)
Stark, R.E.; Kasakevich, M.L.; Granger, J.W.
1982-01-01
A series of simple NMR relaxation experiments have been performed on nitrobenzene and aniline dissolved in the ionic detergents sodium dodecyl sulfate (SDS) and hexadecyltrimethylammonium bromide (CTAB). Using 13 C relaxation rates at various molecular sites, and comparing data obtained in organic media with those for micellar solutions, the viscosity at the solubilization site was estimated and a detailed picture of motional restrictions imposed by the micellar enviroment was derived. Viscosities of 8 to 17 cp indicate a rather fluid environment for solubilized nitrobenzene; both additives exhibit altered motional preferences in CTAB solutions only. As an aid in interpretation of the NMR data, quasi-elastic light scattering and other physical techniques have been used to evaluate the influence of organic solutes on micellar size and shape. The NMR methods are examined critically in terms of their general usefulness for studies of solubilization in detergent micelles. 48 references
Atomic and Molecular Dynamics on and in Superfluid Helium Nanodroplets
Lehmann, Kevin K.
2003-03-01
Studies of intramolecular and intermolecular dynamics is at the core of Molecular Spectroscopic research several decades. Gas phase, particularly molecular beam, studies have greatly illuminated these processes in isolated molecules, bimolecular collisions, or small covalent and van der Waals complexes. Parallel to this effort have been studies in condensed phases, but there has unfortunately been little intellectual contact between these. The recent development of Helium Nanodropet Isolation Spectroscopy is providing an intellectual bridge between gas phase and condensed phase spectroscopy. While droplets of 10,000 He atoms are effectively a condensed phase, their low temperature ( 0.4 K) and ultralow heat capacities combined with their superfluid state make them an almost ideal matrix in which to study both molecular dynamics, including solute induced relaxations. The nsec times scales for many of the relaxation events, orders of magnitude slower than in classical liquids, results in spectra with unprecedented resolution for the liquid state. In this talk, studies of the Princeton group will be highlighted, with particular emphasis on those for which a combination of theory and experiment have combined to reveal dynamics in this unique Quantum Fluid.
Ricci, Matteo; Berardi, Roberto; Zannoni, Claudio
2015-08-01
We investigate the switching of a biaxial nematic filling a flat cell with planar homogeneous anchoring using a coarse-grained molecular dynamics simulation. We have found that an aligning field applied across the film, and acting on specific molecular axes, can drive the reorientation of the secondary biaxial director up to one order of magnitude faster than that for the principal director. While the π/2 switching of the secondary director does not affect the alignment of the long molecular axes, the field-driven reorientation of the principal director proceeds via a concerted rotation of the long and transversal molecular axes. More importantly, while upon switching off a (relatively) weak or intermediate field, the biaxial nematic liquid crystal is always able to relax to the initial surface aligned director state; this is not the case when using fields above a certain threshold. In that case, while the secondary director always recovers the initial state, the principal one remains, occasionally, trapped in a nonuniform director state due to the formation of domain walls.
Fluorescence relaxation spectroscopy : light on dynamical structures of flavoproteins
Burten - Bastiaens, P.I.H.
1992-01-01
Refinements in technique and data analysis have opened new avenues for a detailed interpretation of protein fluorescence. What is more, by combining new insights in protein structure and dynamics with improved knowledge of photophysics of biological chromophores, the coupling between
Traaseth, Nathaniel J; Chao, Fa-An; Masterson, Larry R; Mangia, Silvia; Garwood, Michael; Michaeli, Shalom; Seelig, Burckhard; Veglia, Gianluigi
2012-06-01
NMR relaxation methods probe biomolecular motions over a wide range of timescales. In particular, the rotating frame spin-lock R(1ρ) and Carr-Purcell-Meiboom-Gill (CPMG) R(2) experiments are commonly used to characterize μs to ms dynamics, which play a critical role in enzyme folding and catalysis. In an effort to complement these approaches, we introduced the Heteronuclear Adiabatic Relaxation Dispersion (HARD) method, where dispersion in rotating frame relaxation rate constants (longitudinal R(1ρ) and transverse R(2ρ)) is created by modulating the shape and duration of adiabatic full passage (AFP) pulses. Previously, we showed the ability of the HARD method to detect chemical exchange dynamics in the fast exchange regime (k(ex)∼10(4)-10(5) s(-1)). In this article, we show the sensitivity of the HARD method to slower exchange processes by measuring R(1ρ) and R(2ρ) relaxation rates for two soluble proteins (ubiquitin and 10C RNA ligase). One advantage of the HARD method is its nominal dependence on the applied radio frequency field, which can be leveraged to modulate the dispersion in the relaxation rate constants. In addition, we also include product operator simulations to define the dynamic range of adiabatic R(1ρ) and R(2ρ) that is valid under all exchange regimes. We conclude from both experimental observations and simulations that this method is complementary to CPMG-based and rotating frame spin-lock R(1ρ) experiments to probe conformational exchange dynamics for biomolecules. Finally, this approach is germane to several NMR-active nuclei, where relaxation rates are frequency-offset independent. Copyright © 2012 Elsevier Inc. All rights reserved.
Molecular dynamics of liquid crystals
Sarman, Sten
1997-02-01
We derive Green-Kubo relations for the viscosities of a nematic liquid crystal. The derivation is based on the application of a Gaussian constraint algorithm that makes the director angular velocity of a liquid crystal a constant of motion. Setting this velocity equal to zero means that a director-based coordinate system becomes an inertial frame and that the constraint torques do not do any work on the system. The system consequently remains in equilibrium. However, one generates a different equilibrium ensemble. The great advantage of this ensemble is that the Green-Kubo relations for the viscosities become linear combinations of time correlation function integrals, whereas they are complicated rational functions in the conventional canonical ensemble. This facilitates the numerical evaluation of the viscosities by molecular dynamics simulations.
The dynamic relaxation method in the structural analysis of concrete pressure vessels
International Nuclear Information System (INIS)
Davidson, I.; Assis Bastos, M.R. de; Camargo, P.B. de.
1977-01-01
The dynamic relaxation method, applied to 3 dimensional concrete structures, especially pressure vessels, is demonstrated. It utilizes the finite difference method and allows the growth of cracks to be followed up to the point of vessel rupture. A FORTRAN IV program is developed, which can also be utilized, with the necessary modifications, for other structure calculations [pt
Carrier relaxation dynamics in InAs/InGaAlAs quantum dashes
Ryasnyanskiy, A.I.; Biaggio, I.; Djie, Hery Susanto; Ooi, Boon S.; Tan, C.L.
2011-01-01
We characterize size-dependent carrier relaxation dynamics of partial laser structures containing quantum dashes by time-resolved degenerate four wave mixing between 1.2 and 1.6 ?m. © 2010 Elsevier B.V. All rights reserved.
Carrier relaxation dynamics in InAs/InGaAlAs quantum dashes
Ryasnyanskiy, A.I.
2011-03-01
We characterize size-dependent carrier relaxation dynamics of partial laser structures containing quantum dashes by time-resolved degenerate four wave mixing between 1.2 and 1.6 ?m. © 2010 Elsevier B.V. All rights reserved.
Dynamics of Exciton Relaxation in LH2 Antenna Probed by Multipulse Nonlinear Spectroscopy
Novoderezhkin, V.I.; Cohen Stuart, T.A.; van Grondelle, R.
2011-01-01
We explain the relaxation dynamics in the LH2-B850 antenna as revealed by multipulse pump - dump - probe spectroscopy (Th. A. Cohen StuartM. VengrisV. I. NovoderezhkinR. J. CogdellC. N. HunterR. van Grondelle, submitted). The theory of pump - dump - probe response is evaluated using the doorway -
Dynamics of merging: post-merger mixing and relaxation of an Illustris galaxy
Young, Anthony M.; Williams, Liliya L. R.; Hjorth, Jens
2018-02-01
During the merger of two galaxies, the resulting system undergoes violent relaxation and seeks stable equilibrium. However, the details of this evolution are not fully understood. Using Illustris simulation, we probe two physically related processes, mixing and relaxation. Though the two are driven by the same dynamics—global time-varying potential for the energy, and torques caused by asymmetries for angular momentum—we measure them differently. We define mixing as the redistribution of energy and angular momentum between particles of the two merging galaxies. We assess the degree of mixing as the difference between the shapes of their energy distributions, N(E)s, and their angular momentum distributions, N(L2)s. We find that the difference is decreasing with time, indicating mixing. To measure relaxation, we compare N(E) of the newly merged system to N(E) of a theoretical prediction for relaxed collisionless systems, DARKexp, and witness the system becoming more relaxed, in the sense that N(E) approaches DARKexp N(E). Because the dynamics driving mixing and relaxation are the same, the timescale is similar for both. We measure two sequential timescales: a rapid, 1 Gyr phase after the initial merger, during which the difference in N(E) of the two merging halos decreases by ~ 80%, followed by a slow phase, when the difference decreases by ~ 50% over ~ 8.5 Gyrs. This is a direct measurement of the relaxation timescale. Our work also draws attention to the fact that when a galaxy has reached Jeans equilibrium it may not yet have reached a fully relaxed state given by DARKexp, in that it retains information about its past history. This manifests itself most strongly in stars being centrally concentrated. We argue that it is particularly difficult for stars, and other tightly bound particles, to mix because they have less time to be influenced by the fluctuating potential, even across multiple merger events.
Hierarchical relaxation dynamics in a tilted two-band Bose-Hubbard model
Cosme, Jayson G.
2018-04-01
We numerically examine slow and hierarchical relaxation dynamics of interacting bosons described by a tilted two-band Bose-Hubbard model. The system is found to exhibit signatures of quantum chaos within the spectrum and the validity of the eigenstate thermalization hypothesis for relevant physical observables is demonstrated for certain parameter regimes. Using the truncated Wigner representation in the semiclassical limit of the system, dynamics of relevant observables reveal hierarchical relaxation and the appearance of prethermalized states is studied from the perspective of statistics of the underlying mean-field trajectories. The observed prethermalization scenario can be attributed to different stages of glassy dynamics in the mode-time configuration space due to dynamical phase transition between ergodic and nonergodic trajectories.
Theoretical Concepts in Molecular Photodissociation Dynamics
DEFF Research Database (Denmark)
Henriksen, Niels Engholm
1995-01-01
This chapter contains sections titled: Introduction Quantum Dynamics of Molecular Photofragmentation The Total Reaction Probability Final Product Distributions Time-Independent Approach, Stationary Scattering States Gaussian Wave Packet Dynamics Wigner Phase Space Representation The Diatomic...
Crossover from Rouse dynamics to the α-relaxation in poly(vinyl ...
Indian Academy of Sciences (India)
directly at the structure factor peak [1–3], is subject to the same universalities as ... Recently, molecular dynamics simulations [11] as well as mode coupling theory ..... appears to relate to the internal dynamics within the Gaussian blobs.
Water interactions with varying molecular states of bovine casein: 2H NMR relaxation studies
International Nuclear Information System (INIS)
Kumosinski, T.F.; Pessen, H.; Prestrelski, S.J.; Farrell, H.M. Jr.
1987-01-01
The caseins occur in milk as spherical colloidal complexes of protein and salts with an average diameter of 1200 A, the casein micelles. Removal of Ca2+ is thought to result in their dissociation into smaller protein complexes stabilized by hydrophobic interactions and called submicelles. Whether these submicelles actually occur within the micelles as discrete particles interconnected by calcium phosphate salt bridges has been the subject of much controversy. A variety of physical measurements have shown that casein micelles contain an inordinately high amount of trapped water (2 to 7 g H 2 O/g protein). With this in mind it was of interest to determine if NMR relaxation measurements could detect the presence of this trapped water within the micelles, and to evaluate whether it is a continuum with picosecond correlation times or is associated in part with discrete submicellar structures with nanosecond motions. For this purpose the variations in 2 H NMR longitudinal and transverse relaxation rates of water with protein concentration were determined for bovine casein at various temperatures, under both submicellar and micellar conditions. D 2 O was used instead of H 2 O to eliminate cross-relaxation effects. From the protein concentration dependence of the relaxation rates, the second virial coefficient of the protein was obtained by nonlinear regression analysis. Using either an isotropic tumbling or an intermediate asymmetry model, degrees of hydration, v, and correlation times, tau c, were calculated for the caseins; from the latter parameter the Stokes radius, r, was obtained. Next, estimates of molecular weights were obtained from r and the partial specific volume. Values were in the range of those published from other methodologies for the submicelles
Simple molecular mechanism of heat transfer: Debye relaxation versus power-law
International Nuclear Information System (INIS)
Gall, M.; Kutner, R.
2005-01-01
We study a simple molecular model (at coarse-grain level) as a basis of irreversible heat transfer through a diathermic partition. The partition separates into two adjacent parts a box containing ideal point particles that communicate only though this partition. We provide the basic mechanism of energy transfer between the left- and right-hand side gas samples by assuming equipartition of kinetic energy of all outgoing particles colliding with the partition at a given time. We analyse and compare two essentially different cases (A) the reference one, where we assume that the border walls of the box and the diathermic partitions can randomize the direction of motion of rebounding particles, and (B) the case where we assume the mirror collisions of particles with the border walls and the partition. In both cases the rebounding of the particles from border walls is elastic. The above introduced assumptions allow us to numerically simulate and analytically consider, for example, the relaxation of temperatures of both gas samples and the entropy of the system. However, in both cases the long-time relaxation is essentially different since in case (A) it is an exponential one, while in case (B) it seems to be a power-law relaxation. The obtained results well agree in case (A) with the predictions of the phenomenological, linear theory of irreversible theory had to be developed which assumes time-dependence of heat conductivity; it describes the relaxation of the system far from equilibrium. The explanation of the results obtained in this case is, nevertheless, an intriguing problem. (author)
Full quantum treatment of charge dynamics in amorphous molecular semiconductors
de Vries, Xander; Friederich, Pascal; Wenzel, Wolfgang; Coehoorn, Reinder; Bobbert, Peter A.
2018-02-01
We present a treatment of charge dynamics in amorphous molecular semiconductors that accounts for the coupling of charges to all intramolecular phonon modes in a fully quantum mechanical way. Based on ab initio calculations, we derive charge transfer rates that improve on the widely used semiclassical Marcus rate and obtain benchmark results for the mobility and energetic relaxation of electrons and holes in three semiconductors commonly applied in organic light-emitting diodes. Surprisingly, we find very similar results when using the simple Miller-Abrahams rate. We conclude that extracting the disorder strength from temperature-dependent charge transport studies is very possible but extracting the reorganization energy is not.
Directory of Open Access Journals (Sweden)
Vizcaíno Sergio F
2004-04-01
Full Text Available Abstract Background Comparative genomic data among organisms allow the reconstruction of their phylogenies and evolutionary time scales. Molecular timings have been recently used to suggest that environmental global change have shaped the evolutionary history of diverse terrestrial organisms. Living xenarthrans (armadillos, anteaters and sloths constitute an ideal model for studying the influence of past environmental changes on species diversification. Indeed, extant xenarthran species are relicts from an evolutionary radiation enhanced by their isolation in South America during the Tertiary era, a period for which major climate variations and tectonic events are relatively well documented. Results We applied a Bayesian approach to three nuclear genes in order to relax the molecular clock assumption while accounting for differences in evolutionary dynamics among genes and incorporating paleontological uncertainties. We obtained a molecular time scale for the evolution of extant xenarthrans and other placental mammals. Divergence time estimates provide substantial evidence for contemporaneous diversification events among independent xenarthran lineages. This correlated pattern of diversification might possibly relate to major environmental changes that occurred in South America during the Cenozoic. Conclusions The observed synchronicity between planetary and biological events suggests that global change played a crucial role in shaping the evolutionary history of extant xenarthrans. Our findings open ways to test this hypothesis further in other South American mammalian endemics like hystricognath rodents, platyrrhine primates, and didelphid marsupials.
DEFF Research Database (Denmark)
Roed, Lisa Anita; Niss, Kristine; Jakobsen, Bo
2015-01-01
The frequency dependent specific heat has been measured under pressure for the molecular glass forming liquid 5-polyphenyl-4-ether in the viscous regime close to the glass transition. The temperature and pressure dependences of the characteristic time scale associated with the specific heat...... is compared to the equivalent time scale from dielectric spectroscopy performed under identical conditions. It is shown that the ratio between the two time scales is independent of both temperature and pressure. This observation is non-trivial and demonstrates the existence of specially simple molecular...... liquids in which different physical relaxation processes are both as function of temperature and pressure/density governed by the same underlying “inner clock.” Furthermore, the results are discussed in terms of the recent conjecture that van der Waals liquids, like the measuredliquid, comply...
The Influence of the Relaxation Time on the Dynamic Hysteresis in Perovskite Solar Cells
Directory of Open Access Journals (Sweden)
Palici Alexandra
2018-01-01
Full Text Available We investigate the dynamic behavior of perovskite solar cells by focusing on the relaxation time τ, which corresponds to the slow de-polarization process from an initial bias pre-poled state. The dynamic electrical model (DEM is employed for simulating the J-V characteristics for different bias scan rates and pre-poling conditions. Depending on the sign of the initial polarization normal or inverted hysteresis may be induced. For fixed pre-poling conditions, the relaxation time, in relation to the bias scan rate, governs the magnitude of the dynamic hysteresis. In the limit of large τ the hysteretic effects are vanishing for the typical range of bias scan rates considered, while for very small τ the hysteresis is significant only when it is comparable with the measurement time interval.
Molecular dynamics for irradiation driven chemistry
DEFF Research Database (Denmark)
Sushko, Gennady B.; Solov'yov, Ilia A.; Solov'yov, Andrey V.
2016-01-01
A new molecular dynamics (MD) approach for computer simulations of irradiation driven chemical transformations of complex molecular systems is suggested. The approach is based on the fact that irradiation induced quantum transformations can often be treated as random, fast and local processes...... that describe the classical MD of complex molecular systems under irradiation. The proposed irradiation driven molecular dynamics (IDMD) methodology is designed for the molecular level description of the irradiation driven chemistry. The IDMD approach is implemented into the MBN Explorer software package...... involving small molecules or molecular fragments. We advocate that the quantum transformations, such as molecular bond breaks, creation and annihilation of dangling bonds, electronic charge redistributions, changes in molecular topologies, etc., could be incorporated locally into the molecular force fields...
Stress relaxation in vanadium under shock and shockless dynamic compression
International Nuclear Information System (INIS)
Kanel, G. I.; Razorenov, S. V.; Garkushin, G. V.; Savinykh, A. S.; Zaretsky, E. B.
2015-01-01
Evolutions of elastic-plastic waves have been recorded in three series of plate impact experiments with annealed vanadium samples under conditions of shockless and combined ramp and shock dynamic compression. The shaping of incident wave profiles was realized using intermediate base plates made of different silicate glasses through which the compression waves were entered into the samples. Measurements of the free surface velocity histories revealed an apparent growth of the Hugoniot elastic limit with decreasing average rate of compression. The growth was explained by “freezing” of the elastic precursor decay in the area of interaction of the incident and reflected waves. A set of obtained data show that the current value of the Hugoniot elastic limit and plastic strain rate is rather associated with the rate of the elastic precursor decay than with the local rate of compression. The study has revealed the contributions of dislocation multiplications in elastic waves. It has been shown that independently of the compression history the material arrives at the minimum point between the elastic and plastic waves with the same density of mobile dislocations
Non-equilibrium relaxation and near-arrest dynamics in colloidal suspensions
International Nuclear Information System (INIS)
Medina-Noyola, M; RamIrez-Gonzalez, Pedro
2009-01-01
In this work we propose a theory to describe the irreversible diffusive relaxation of the local concentration of a colloidal dispersion that proceeds toward its stable thermodynamic equilibrium state, but which may in the process be trapped in metastable or dynamically arrested states. The central assumption of this theory is that the irreversible relaxation of the macroscopically observed mean value n-bar(r,t) of the local concentration of colloidal particles is described by a diffusion equation involving a local mobility b*(r,t) that depends not only on the mean value n-bar(r,t) but also on the covariance σ(r,r';t)≡δn(r,t)δn(r',t)-bar of the fluctuations δn(r,t)≡n(r,t)-n-bar(r,t). This diffusion equation must hence be solved simultaneously with the relaxation equation for the covariance σ(r,r';t), and here we also derive the corresponding relaxation equation. The dependence of the local mobility b*(r,t) on the mean value and the covariance is determined by a self-consistent set of equations involving now the spatially and temporally non-local time-dependent correlation functions, which in a uniform system in equilibrium reduces to the self-consistent generalized Langevin equation (SCGLE) theory of colloid dynamics. The resulting general theory considers the possibility that these relaxation processes occur under the influence of external fields, such as gravitational forces acting in the process of sedimentation. In this paper, however, we describe a simpler application, in which the system remains spatially uniform during the irreversible relaxation process, and discuss the general features of the glass transition scenario predicted by this non-equilibrium theory.
Molecular dynamics simulations of lysozyme in water/sugar solutions
Energy Technology Data Exchange (ETDEWEB)
Lerbret, A. [Department of Food Science, Cornell University, 101 Stocking Hall, Ithaca, NY 14853 (United States); Affouard, F. [Laboratoire de Dynamique et Structure des Materiaux Moleculaires, UMR CNRS 8024, Universite Lille I, 59655 Villeneuve d' Ascq Cedex (France)], E-mail: frederic.affouard@univ-lille1.fr; Bordat, P. [Laboratoire de Chimie Theorique et de Physico-Chimie Moleculaire, UMR 5624, Universite de Pau et des Pays de l' Adour, 64000 Pau (France); Hedoux, A.; Guinet, Y.; Descamps, M. [Laboratoire de Dynamique et Structure des Materiaux Moleculaires, UMR CNRS 8024, Universite Lille I, 59655 Villeneuve d' Ascq Cedex (France)
2008-04-18
Structural and dynamical properties of the solvent at the protein/solvent interface have been investigated by molecular dynamics simulations of lysozyme in trehalose, maltose and sucrose solutions. Results are discussed in the framework of the bioprotection phenomena. The analysis of the relative concentration of water oxygen atoms around lysozyme suggests that lysozyme is preferentially hydrated. When comparing the three sugars, trehalose is seen more excluded than maltose and sucrose. The preferential exclusion of sugars from the protein surface induces some differences in the behavior of trehalose and maltose, particularly at 50 and 60 wt% concentrations, that are not observed experimentally in binary sugar/mixtures. The dynamical slowing down of the solvent is suggested to mainly arise from the homogeneity of the water/sugar matrices controlled by the percolation of the sugar hydrogen bonds networks. Furthermore, lysozyme strongly increases relaxation times of solvent molecules at the protein/solvent interface.
The classical and quantum dynamics of molecular spins on graphene
Cervetti, Christian; Rettori, Angelo; Pini, Maria Gloria; Cornia, Andrea; Repollés, Ana; Luis, Fernando; Dressel, Martin; Rauschenbach, Stephan; Kern, Klaus; Burghard, Marko; Bogani, Lapo
2016-02-01
Controlling the dynamics of spins on surfaces is pivotal to the design of spintronic and quantum computing devices. Proposed schemes involve the interaction of spins with graphene to enable surface-state spintronics and electrical spin manipulation. However, the influence of the graphene environment on the spin systems has yet to be unravelled. Here we explore the spin-graphene interaction by studying the classical and quantum dynamics of molecular magnets on graphene. Whereas the static spin response remains unaltered, the quantum spin dynamics and associated selection rules are profoundly modulated. The couplings to graphene phonons, to other spins, and to Dirac fermions are quantified using a newly developed model. Coupling to Dirac electrons introduces a dominant quantum relaxation channel that, by driving the spins over Villain’s threshold, gives rise to fully coherent, resonant spin tunnelling. Our findings provide fundamental insight into the interaction between spins and graphene, establishing the basis for electrical spin manipulation in graphene nanodevices.
Vinoth, K.; Ganesh, T.; Senthilkumar, P.; Sylvester, M. Maria; Karunakaran, D. J. S. Anand; Hudge, Praveen; Kumbharkhane, A. C.
2017-09-01
The aqueous solution of beta-alanine characterised and studied by their dispersive dielectric properties and relaxation process in the frequency domain of 10×106 Hz to 30×109 Hz with varying concentration in mole fractions and temperatures. The molecular interaction and dielectric parameters are discussed in terms of counter-ion concentration theory. The static permittivity (ε0), high frequency dielectric permittivity (ε∞) and excess dielectric parameters are accomplished by frequency depended physical properties and relaxation time (τ). Molecular orientation, ordering and correlation factors are reported as confirmation of intermolecular interactions. Ionic conductivity and thermo dynamical properties are concluded with the behaviour of the mixture constituents. Solute-solvent, solute-solute interaction, structure making and breaking abilities of the solute in aqueous medium are interpreted. Fourier Transform Infrared (FTIR) spectra of beta- alanine single crystal and liquid state have been studied. The 13C Nuclear Magnetic Resonance (NMR) spectral studies give the signature for resonating frequencies and chemical shifts of beta-alanine.
Complex dynamics of memristive circuits: Analytical results and universal slow relaxation
Caravelli, F.; Traversa, F. L.; Di Ventra, M.
2017-02-01
Networks with memristive elements (resistors with memory) are being explored for a variety of applications ranging from unconventional computing to models of the brain. However, analytical results that highlight the role of the graph connectivity on the memory dynamics are still few, thus limiting our understanding of these important dynamical systems. In this paper, we derive an exact matrix equation of motion that takes into account all the network constraints of a purely memristive circuit, and we employ it to derive analytical results regarding its relaxation properties. We are able to describe the memory evolution in terms of orthogonal projection operators onto the subspace of fundamental loop space of the underlying circuit. This orthogonal projection explicitly reveals the coupling between the spatial and temporal sectors of the memristive circuits and compactly describes the circuit topology. For the case of disordered graphs, we are able to explain the emergence of a power-law relaxation as a superposition of exponential relaxation times with a broad range of scales using random matrices. This power law is also universal, namely independent of the topology of the underlying graph but dependent only on the density of loops. In the case of circuits subject to alternating voltage instead, we are able to obtain an approximate solution of the dynamics, which is tested against a specific network topology. These results suggest a much richer dynamics of memristive networks than previously considered.
Thermally driven molecular linear motors - A molecular dynamics study
DEFF Research Database (Denmark)
Zambrano, Harvey A; Walther, Jens Honore; Jaffe, Richard Lawrence
2009-01-01
We conduct molecular dynamics simulations of a molecular linear motor consisting of coaxial carbon nanotubes with a long outer carbon nanotube confining and guiding the motion of an inner short, capsule-like nanotube. The simulations indicate that the motion of the capsule can be controlled by th...
Dynamical relaxation of the CP phases in next-to-minimal supersymmetry
International Nuclear Information System (INIS)
Demir, D.A.
1999-11-01
After promoting the phases of the soft masses to dynamical fields corresponding to Goldstone bosons of spontaneously broken global symmetries in the supersymmetry breaking sector, the next-to-minimal supersymmetric model is found to solve the μ problem and the strong CP problem simultaneously with an invisible axion. The domain wall problem persists in the form of axionic domain formation. Relaxation dynamics of the physical CP-violating phases is determined only by the short-distance physics and their relaxation values are not necessarily close to the CP-conserving points. Consequently, the solution of tile supersymmetric CP problem may require heavy enough superpartners and nonminimal flavor structures, where the latter may be also relevant for avoiding the formation of axionic domain walls. (author)
Low Parametric Sensitivity Realizations with relaxed L2-dynamic-range-scaling constraints
Hilaire , Thibault
2009-01-01
This paper presents a new dynamic-range scaling for the implementation of filters/controllers in state-space form. Relaxing the classical L2-scaling constraints by specific fixed-point considerations allows for a higher degree of freedom for the optimal L2-parametric sensitivity problem. However, overflows in the implementation are still prevented. The underlying constrained problem is converted into an unconstrained problem for which a solution can be provided. This leads to realizations whi...
Improved dynamical scaling analysis using the kernel method for nonequilibrium relaxation.
Echinaka, Yuki; Ozeki, Yukiyasu
2016-10-01
The dynamical scaling analysis for the Kosterlitz-Thouless transition in the nonequilibrium relaxation method is improved by the use of Bayesian statistics and the kernel method. This allows data to be fitted to a scaling function without using any parametric model function, which makes the results more reliable and reproducible and enables automatic and faster parameter estimation. Applying this method, the bootstrap method is introduced and a numerical discrimination for the transition type is proposed.
Reschke, S.; Wang, Zhe; Mayr, F.; Ruff, E.; Lunkenheimer, P.; Tsurkan, V.; Loidl, A.
2017-10-01
We report on THz time-domain spectroscopy on multiferroic GeV4S8 , which undergoes orbital ordering at a Jahn-Teller transition at 30.5 K and exhibits antiferromagnetic order below 14.6 K. The THz experiments are complemented by dielectric experiments at audio and radio frequencies. We identify a low-lying excitation close to 0.5 THz, which is only weakly temperature dependent and probably corresponds to a molecular excitation within the electronic level scheme of the V4 clusters. In addition, we detect complex temperature-dependent behavior of a low-lying phononic excitation, closely linked to the onset of orbitally driven ferroelectricity. In the high-temperature cubic phase, which is paramagnetic and orbitally disordered, this excitation is of relaxational character becomes an overdamped Lorentzian mode in the orbitally ordered phase below the Jahn-Teller transition, and finally appears as well-defined phonon excitation in the antiferromagnetic state. Abrupt changes in the real and imaginary parts of the complex dielectric permittivity show that orbital ordering appears via a structural phase transition with strong first-order character and that the onset of antiferromagnetic order is accompanied by significant structural changes, which are of first-order character, too. Dielectric spectroscopy documents that at low frequencies, significant dipolar relaxations are present in the orbitally ordered, paramagnetic phase only. In contrast to the closely related GaV4S8 , this relaxation dynamics that most likely mirrors coupled orbital and polar fluctuations does not seem to be related to the dynamic processes detected in the THz regime.
Sills, Scott; Gray, Tomoko; Overney, René M
2005-10-01
Nanoscale sliding friction involving a polystyrene melt near its glass transition temperature Tg (373 K) exhibited dissipation phenomena that provide insight into the underlying molecular relaxation processes. A dissipative length scale that shows significant parallelism with the size of cooperatively rearranging regions (CRRs) could be experimentally deduced from friction-velocity isotherms, combined with dielectric loss analysis. Upon cooling to approximately 10 K above Tg, the dissipation length Xd grew from a segmental scale of approximately 3 A to 2.1 nm, following a power-law relationship with the reduced temperature Xd approximately TR-phi. The resulting phi=1.89+/-0.08 is consistent with growth predictions for the length scale of CRRs in the heterogeneous regime of fragile glass formers. Deviations from the power-law behavior closer to Tg suggest that long-range processes, e.g., the normal mode or ultraslow Fischer modes, may couple with the alpha relaxation, leading to energy dissipation in domains of tens of nanometers.
Rational extended thermodynamics of a rarefied polyatomic gas with molecular relaxation processes
Arima, Takashi; Ruggeri, Tommaso; Sugiyama, Masaru
2017-10-01
We present a more refined version of rational extended thermodynamics of rarefied polyatomic gases in which molecular rotational and vibrational relaxation processes are treated individually. In this case, we need a triple hierarchy of the moment system and the system of balance equations is closed via the maximum entropy principle. Three different types of the production terms in the system, which are suggested by a generalized BGK-type collision term in the Boltzmann equation, are adopted. In particular, the rational extended thermodynamic theory with seven independent fields (ET7) is analyzed in detail. Finally, the dispersion relation of ultrasonic wave derived from the ET7 theory is confirmed by the experimental data for CO2, Cl2, and Br2 gases.
Dielectric relaxation spectra of liquid crystals in relation to molecular structure
International Nuclear Information System (INIS)
Wrobel, S.
1986-07-01
The dielectric spectra obtained for some members of two homologous series, i.e. for di-alkoxyazoxybenzenes and penthyl-alkoxythiobenzoates, are discussed qualitatively on the basis of the Nordio-Rigatti-Segre diffusion model. It is additionally assumed that the molecular reorientations take place about the principal axes of the inertia tensor. The distribution of correlation times, which is strongly temperature dependent in the vicinity of the clearing point, is interpreted as being caused by fluctuations of the principal axes frame which are due to conformation changes inside the end chains. The Bauer equation is used to describe both principal molecular reorientations, i.e. the reorientations about the long and short axis, observed in liquid crystalline structure by means of dielectric relaxation methods. The energies and entropies of activation have been computed for both principal reorientations. The differences between the high frequency limit of the dielectric permittivity and the refractive index squared of liquid crystals are explained in terms of two librational motions of the molecules observed by other experimental techniques, viz. far infra-red, Raman and inelastic neutron scattering spectroscopies, and found in this work on the basis of dielectrically measured energy barriers. It has been shown qualitatively that intramolecular libratory motions greatly effect the high frequency dielectric spectrum. Finally, molecular motions in liquid crystals are divided into two types: coherent and incoherent. 127 refs., 56 figs., 17 tabs. (author)
A Molecular Dynamics Study of Lunasin | Singh | South African ...
African Journals Online (AJOL)
A Molecular Dynamics Study of Lunasin. ... profile of lunasin,using classical molecular dynamics (MD) simulations at the time scale of 300 ns. ... Keywords: Lunasin, molecular dynamics, amber, CLASICO, α-helix, β-turn, PTRAJ, RGD, RMSD ...
Spin dynamics of the itinerant helimagnet MnSi studied by positive muon spin relaxation
International Nuclear Information System (INIS)
Kadono, R.; Matsuzaki, T.; Yamazaki, T.; Kreitzman, S.R.; Brewer, J.H.
1990-03-01
The local magnetic fields and spin dynamics of the itinerant helimagnet MnSi(T c ≅ 29.5 K) have been studied experimentally using positive muon spin rotation/relaxation (μ + SR) methods. In the ordered phase (T c ), zero-field μSR was used to measure the hyperfine fields at the muon sites as well as the muon spin-lattice relaxation time T 1 μ . Two magnetically inequivalent interstitial μ + sites were found with hyperfine coupling constants A hf (1) = -3.94 kOe/μ B and A hf (2) = -6.94 kOe/μ B , respectively. In the paramagnetic phase (T > T c ), the muon-nuclear spin double relaxation technique was used to simultaneously but independently determine the spin-lattice relaxation time T 1 Mn of 55 Mn spins and that of positive muons (T 1 μ ) over a wide temperature range (T c 1 Mn and T 1 μ in both phases shows systematic deviations from the predictions of self-consistent renormalization (SCR) theory. (author)
Energy Technology Data Exchange (ETDEWEB)
Elkins, Madeline H.; Williams, Holly L. [Department of Chemistry, University of California, Berkeley, California 94720 (United States); Neumark, Daniel M., E-mail: dneumark@berkeley.edu [Department of Chemistry, University of California, Berkeley, California 94720 (United States); Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720 (United States)
2016-05-14
The excited state relaxation dynamics of the solvated electron in H{sub 2}O and D{sub 2}O are investigated using time-resolved photoelectron spectroscopy in a liquid microjet. The data show that the initial excited state decays on a time scale of 75 ± 12 fs in H{sub 2}O and 102 ± 8 fs in D{sub 2}O, followed by slower relaxation on time scales of 400 ± 70 fs and 390 ± 70 fs that are isotopically invariant within the precision of our measurements. Based on the time evolution of the transient signals, the faster and slower time constants are assigned to p → s internal conversion (IC) of the hydrated electron and relaxation on the ground electronic state, respectively. This assignment is consistent with the non-adiabatic mechanism for relaxation of the hydrated electron and yields an isotope effect of 1.4 ± 0.2 for IC of the hydrated electron.
Ponzoni, Stefano
2014-10-16
By properly tuning the photon energy of a femtosecond laser pump, we disentangle, in carbon nanotube-Si (CNT/Si) heterojunctions, the fast relaxation dynamics occurring in CNT from the slow repopulation dynamics due to hole charge transfer at the junction. In this way we are able to track the transfer of the photogenerated holes from the Si depletion layer to the CNT layer, under the action of the built-in heterojunction potential. This also clarifies that CNT play an active role in the junction and do not act only as channels for charge collection and transport.
Ponzoni, Stefano; Galimberti, Gianluca; Sangaletti, L.; Castrucci, Paola; Del Gobbo, Silvano; Morbidoni, Maurizio; Scarselli, Manuela A.; Pagliara, Stefania
2014-01-01
By properly tuning the photon energy of a femtosecond laser pump, we disentangle, in carbon nanotube-Si (CNT/Si) heterojunctions, the fast relaxation dynamics occurring in CNT from the slow repopulation dynamics due to hole charge transfer at the junction. In this way we are able to track the transfer of the photogenerated holes from the Si depletion layer to the CNT layer, under the action of the built-in heterojunction potential. This also clarifies that CNT play an active role in the junction and do not act only as channels for charge collection and transport.
Dynamics of a model of two delay-coupled relaxation oscillators
Ruelas, R. E.; Rand, R. H.
2010-08-01
This paper investigates the dynamics of a new model of two coupled relaxation oscillators. The model replaces the usual DDE (differential-delay equation) formulation with a discrete-time approach with jumps. Existence, bifurcation and stability of in-phase periodic motions is studied. Simple periodic motions, which involve exactly two jumps per period, are found to have large plateaus in parameter space. These plateaus are separated by regions of complicated dynamics, reminiscent of the Devil's Staircase. Stability of motions in the in-phase manifold are contrasted with stability of motions in the full phase space.
Chirality in molecular collision dynamics
Lombardi, Andrea; Palazzetti, Federico
2018-02-01
Chirality is a phenomenon that permeates the natural world, with implications for atomic and molecular physics, for fundamental forces and for the mechanisms at the origin of the early evolution of life and biomolecular homochirality. The manifestations of chirality in chemistry and biochemistry are numerous, the striking ones being chiral recognition and asymmetric synthesis with important applications in molecular sciences and in industrial and pharmaceutical chemistry. Chiral discrimination phenomena, due to the existence of two enantiomeric forms, very well known in the case of interaction with light, but still nearly disregarded in molecular collision studies. Here we review some ideas and recent advances about the role of chirality in molecular collisions, designing and illustrating molecular beam experiments for the demonstration of chiral effects and suggesting a scenario for a stereo-directional origin of chiral selection.
Molecular dynamics for reactions of heterogeneous catalysis
Jansen, A.P.J.; Brongersma, H.H.; Santen, van R.A.
1991-01-01
An overview is given of Molecular Dynamics, and numerical integration techniques, system initialization, boundary conditions, force representation, statistics, system size, and simulations duration are discussed. Examples from surface science are used to illustrate the pros and cons of the method.
molecular dynamics simulations and quantum chemical calculations
African Journals Online (AJOL)
ABSTRACT. The molecular dynamic (MD) simulation and quantum chemical calculations for the adsorption of [2-(2-Henicos-10- .... electronic properties of molecule clusters, surfaces and ... The local reactivity was analyzed by determining the.
Molecular dynamics simulation of ribosome jam
Matsumoto, Shigenori; Takagi, Fumiko; Shimada, Takashi; Ito, Nobuyasu
2011-01-01
We propose a coarse-grained molecular dynamics model of ribosome molecules to study the dependence of translation process on environmental parameters. We found the model exhibits traffic jam property, which is consistent with an ASEP model. We
Toyama, Yuki; Kano, Hanaho; Mase, Yoko; Yokogawa, Mariko; Osawa, Masanori; Shimada, Ichio
2017-02-22
Heterotrimeric guanine-nucleotide-binding proteins (G proteins) serve as molecular switches in signalling pathways, by coupling the activation of cell surface receptors to intracellular responses. Mutations in the G protein α-subunit (Gα) that accelerate guanosine diphosphate (GDP) dissociation cause hyperactivation of the downstream effector proteins, leading to oncogenesis. However, the structural mechanism of the accelerated GDP dissociation has remained unclear. Here, we use magnetic field-dependent nuclear magnetic resonance relaxation analyses to investigate the structural and dynamic properties of GDP bound Gα on a microsecond timescale. We show that Gα rapidly exchanges between a ground-state conformation, which tightly binds to GDP and an excited conformation with reduced GDP affinity. The oncogenic D150N mutation accelerates GDP dissociation by shifting the equilibrium towards the excited conformation.
Dynamical processes in atomic and molecular physics
Ogurtsov, Gennadi
2012-01-01
Atomic and molecular physics underlie a basis for our knowledge of fundamental processes in nature and technology and in such applications as solid state physics, chemistry and biology. In recent years, atomic and molecular physics has undergone a revolutionary change due to great achievements in computing and experimental techniques. As a result, it has become possible to obtain information both on atomic and molecular characteristics and on dynamics of atomic and molecular processes. This e-book highlights the present state of investigations in the field of atomic and molecular physics. Rece
Visualizing Energy on Target: Molecular Dynamics Simulations
2017-12-01
ARL-TR-8234 ● DEC 2017 US Army Research Laboratory Visualizing Energy on Target: Molecular Dynamics Simulations by DeCarlos E...return it to the originator. ARL-TR-8234● DEC 2017 US Army Research Laboratory Visualizing Energy on Target: Molecular Dynamics...REPORT TYPE Technical Report 3. DATES COVERED (From - To) 1 October 2015–30 September 2016 4. TITLE AND SUBTITLE Visualizing Energy on Target
International Nuclear Information System (INIS)
Danilov, Denis; Nestler, Britta; Guerdane, Mohammed; Teichler, Helmar
2009-01-01
Results are presented from phase-field modelling and molecular dynamics simulations concerning the relaxation dynamics in a finite-temperature two-phase crystal-liquid sample subjected to an abrupt temperature drop. Relaxation takes place by propagation of the solidification front under formation of a spatially varying concentration profile in the melt. The molecular dynamics simulations are carried out with an interatomic model appropriate for the NiZr alloy system and provide the thermophysical data required for setting up the phase-field simulations. Regarding the concentration profile and velocity of the solidification front, best agreement between the phase-field model and molecular dynamics simulation is obtained when increasing the apparent diffusion coefficients in the phase-field treatment by a factor of four against their molecular dynamics estimates.
International Nuclear Information System (INIS)
Greene, Patrick T.; Schofield, Samuel P.; Nourgaliev, Robert
2017-01-01
A new mesh smoothing method designed to cluster cells near a dynamically evolving interface is presented. The method is based on weighted condition number mesh relaxation with the weight function computed from a level set representation of the interface. The weight function is expressed as a Taylor series based discontinuous Galerkin projection, which makes the computation of the derivatives of the weight function needed during the condition number optimization process a trivial matter. For cases when a level set is not available, a fast method for generating a low-order level set from discrete cell-centered fields, such as a volume fraction or index function, is provided. Results show that the low-order level set works equally well as the actual level set for mesh smoothing. Meshes generated for a number of interface geometries are presented, including cases with multiple level sets. Lastly, dynamic cases with moving interfaces show the new method is capable of maintaining a desired resolution near the interface with an acceptable number of relaxation iterations per time step, which demonstrates the method's potential to be used as a mesh relaxer for arbitrary Lagrangian Eulerian (ALE) methods.
Molecular ions, Rydberg spectroscopy and dynamics
International Nuclear Information System (INIS)
Jungen, Ch.
2015-01-01
Ion spectroscopy, Rydberg spectroscopy and molecular dynamics are closely related subjects. Multichannel quantum defect theory is a theoretical approach which draws on this close relationship and thereby becomes a powerful tool for the study of systems consisting of a positively charged molecular ion core interacting with an electron which may be loosely bound or freely scattering
Pattern recognition in molecular dynamics. [FORTRAN
Energy Technology Data Exchange (ETDEWEB)
Zurek, W H; Schieve, W C [Texas Univ., Austin (USA)
1977-07-01
An algorithm for the recognition of the formation of bound molecular states in the computer simulation of a dilute gas is presented. Applications to various related problems in physics and chemistry are pointed out. Data structure and decision processes are described. Performance of the FORTRAN program based on the algorithm in cooperation with the molecular dynamics program is described and the results are presented.
Molecular ions, Rydberg spectroscopy and dynamics
Energy Technology Data Exchange (ETDEWEB)
Jungen, Ch. [Laboratoire Aimé Cotton, Université de Paris-Sud, 91405 Orsay (France)
2015-01-22
Ion spectroscopy, Rydberg spectroscopy and molecular dynamics are closely related subjects. Multichannel quantum defect theory is a theoretical approach which draws on this close relationship and thereby becomes a powerful tool for the study of systems consisting of a positively charged molecular ion core interacting with an electron which may be loosely bound or freely scattering.
Lattice dynamics and molecular dynamics simulation of complex materials
International Nuclear Information System (INIS)
Chaplot, S.L.
1997-01-01
In this article we briefly review the lattice dynamics and molecular dynamics simulation techniques, as used for complex ionic and molecular solids, and demonstrate a number of applications through examples of our work. These computational studies, along with experiments, have provided microscopic insight into the structure and dynamics, phase transitions and thermodynamical properties of a variety of materials including fullerene, high temperature superconducting oxides and geological minerals as a function of pressure and temperature. The computational techniques also allow the study of the structures and dynamics associated with disorder, defects, surfaces, interfaces etc. (author)
International Nuclear Information System (INIS)
Krueger, P.; Stucky, T.; Boewe, M.; Neuhaeuser, H.
1993-01-01
Quasi-static stress relaxation and dynamic internal friction measurements of stress induced reversible structural relaxation were performed on the amorphous alloy Fe 40 Ni 40 B 20 . The kinetics can be well described by a stretched exponential Kohlrausch-Williams-Watts quasi-static relaxation. The thermally activated part of the internal friction shows an Arrhenius temperature behaviour for a fixed vibration frequency and an inverse power frequency behaviour for a fixed temperature. The activation energies calculated from the Arrhenius equation and from the frequency shift method are significantly different. In order to explain this discrepancy the relation between the quasi-static and the dynamic descriptions of the reversible relaxation is reexamined. In particular it is shown that these two activation energies are connected by the Kohlrausch exponent of the quasi-static relaxation. (orig.)
Fast evaluation of protein dynamics from deficient 15N relaxation data
Jaremko, Łukasz
2018-03-28
Simple and convenient method of protein dynamics evaluation from the insufficient experimental N relaxation data is presented basing on the ratios, products, and differences of longitudinal and transverse N relaxation rates obtained at a single magnetic field. Firstly, the proposed approach allows evaluating overall tumbling correlation time (nanosecond time scale). Next, local parameters of the model-free approach characterizing local mobility of backbone amide N–H vectors on two different time scales, S and R, can be elucidated. The generalized order parameter, S, describes motions on the time scale faster than the overall tumbling correlation time (pico- to nanoseconds), while the chemical exchange term, R, identifies processes slower than the overall tumbling correlation time (micro- to milliseconds). Advantages and disadvantages of different methods of data handling are thoroughly discussed.
Fast evaluation of protein dynamics from deficient 15N relaxation data
Jaremko, Łukasz; Jaremko, Mariusz; Ejchart, Andrzej; Nowakowski, Michał
2018-01-01
Simple and convenient method of protein dynamics evaluation from the insufficient experimental N relaxation data is presented basing on the ratios, products, and differences of longitudinal and transverse N relaxation rates obtained at a single magnetic field. Firstly, the proposed approach allows evaluating overall tumbling correlation time (nanosecond time scale). Next, local parameters of the model-free approach characterizing local mobility of backbone amide N–H vectors on two different time scales, S and R, can be elucidated. The generalized order parameter, S, describes motions on the time scale faster than the overall tumbling correlation time (pico- to nanoseconds), while the chemical exchange term, R, identifies processes slower than the overall tumbling correlation time (micro- to milliseconds). Advantages and disadvantages of different methods of data handling are thoroughly discussed.
Dielectric relaxation study of the dynamics of monosaccharides: D-ribose and 2-deoxy-D-ribose
Energy Technology Data Exchange (ETDEWEB)
Kaminski, K; Kaminska, E; Wlodarczyk, P; Paluch, M; Ziolo, J [Institute of Physics, Silesian University, ulica Uniwersytecka 4, 40-007 Katowice (Poland); Ngai, K L [Naval Research Laboratory, Washington, DC 20375-5320 (United States)
2008-08-20
The dielectric loss spectra of two closely related monosaccharides, D-ribose and 2-deoxy-D-ribose, measured at ambient and elevated pressures are presented. 2-deoxy-D-ribose and D-ribose are respectively the building blocks of the backbone chains in the nucleic acids DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). Small differences in the structure between D-ribose and 2-deoxy-D-ribose result in changes of the glass transition temperature T{sub g}, as well as the dielectric strength and activation enthalpy of the secondary relaxations. However, the frequency dispersion of the structural {alpha}-relaxation for the same relaxation time remains practically the same. Two secondary relaxations are present in both sugars. The slower secondary relaxation shifts to lower frequencies with increasing applied pressure, but not the faster one. This pressure dependence indicates that the slower secondary relaxation is the important and 'universal' Johari-Goldstein {beta}-relaxation of both sugars according to one of the criteria set up to classify secondary relaxations. Additional confirmation of this conclusion comes from good agreement of the observed relaxation time of the slower secondary relaxation with the primitive relaxation time calculated from the coupling model. All the dynamic properties of D-ribose and 2-deoxy-D-ribose are similar to the other monosaccharides, glucose, fructose, galactose and sorbose, except for the much larger relaxation strength of the {alpha}-relaxation of the former compared to the latter. The difference may distinguish the chemical and biological functions of D-ribose and 2-deoxy-D-ribose from the other monosaccharides.
Energy Technology Data Exchange (ETDEWEB)
Das, Suman [Chemical, Biological and Macromolecular Sciences, S. N. Bose National Centre for Basic Sciences, Block-JD, Sector-III, Salt Lake, Kolkata 700098 (India); Biswas, Ranjit, E-mail: ranjit@bose.res.in, E-mail: biswaroop.mukherjee@gmail.com [Chemical, Biological and Macromolecular Sciences, S. N. Bose National Centre for Basic Sciences, Block-JD, Sector-III, Salt Lake, Kolkata 700098 (India); Thematic Unit for Excellence – Computational Materials Science, S. N. Bose National Centre for Basic Sciences, Block-JD, Sector-III, Salt Lake, Kolkata 700098 (India); Mukherjee, Biswaroop, E-mail: ranjit@bose.res.in, E-mail: biswaroop.mukherjee@gmail.com [Thematic Unit for Excellence – Computational Materials Science, S. N. Bose National Centre for Basic Sciences, Block-JD, Sector-III, Salt Lake, Kolkata 700098 (India)
2016-08-28
The paper reports a detailed simulation study on collective reorientational relaxation, cooperative hydrogen bond (H-bond) fluctuations, and their connections to dielectric relaxation (DR) in deep eutectic solvents made of acetamide and three uni-univalent electrolytes, lithium nitrate (LiNO{sub 3}), lithium bromide (LiBr), and lithium perchlorate (LiClO{sub 4}). Because cooperative H-bond fluctuations and ion migration complicate the straightforward interpretation of measured DR timescales in terms of molecular dipolar rotations for these conducting media which support extensive intra- and inter-species H-bonding, one needs to separate out the individual components from the overall relaxation for examining the microscopic origin of various timescales. The present study does so and finds that reorientation of ion-complexed acetamide molecules generates relaxation timescales that are in sub-nanosecond to nanosecond range. This explains in molecular terms the nanosecond timescales reported by recent giga-Hertz DR measurements. Interestingly, the simulated survival timescale for the acetamide-Li{sup +} complex has been found to be a few tens of nanosecond, suggesting such a cation-complexed species may be responsible for a similar timescale reported by mega-Hertz DR measurements of acetamide/potassium thiocyanate deep eutectics near room temperature. The issue of collective versus single particle relaxation is discussed, and jump waiting time distributions are determined. Dependence on anion-identity in each of the cases has been examined. In short, the present study demonstrates that assumption of nano-sized domain formation is not required for explaining the DR detected nanosecond and longer timescales in these media.
Energy Technology Data Exchange (ETDEWEB)
Demontis, Pierfranco; Suffritti, Giuseppe B. [Dipartimento di Chimica e Farmacia, Università degli studi di Sassari, Sassari (Italy); Consorzio Interuniversitario Nazionale per la Scienza e Tecnologia dei Materiali (INSTM), Unità di ricerca di Sassari, Via Vienna, 2, I-07100 Sassari (Italy); Gulín-González, Jorge [Grupo de Matemática y Física Computacionales, Universidad de las Ciencias Informáticas (UCI), Carretera a San Antonio de los Baños, Km 21/2, La Lisa, La Habana (Cuba); Masia, Marco [Dipartimento di Chimica e Farmacia, Università degli studi di Sassari, Sassari (Italy); Consorzio Interuniversitario Nazionale per la Scienza e Tecnologia dei Materiali (INSTM), Unità di ricerca di Sassari, Via Vienna, 2, I-07100 Sassari (Italy); Istituto Officina dei Materiali del CNR, UOS SLACS, Via Vienna 2, 07100 Sassari (Italy); Sant, Marco [Dipartimento di Chimica e Farmacia, Università degli studi di Sassari, Sassari (Italy)
2015-06-28
In order to study the interplay between dynamical heterogeneities and structural properties of bulk liquid water in the temperature range 130–350 K, thus including the supercooled regime, we use the explicit trend of the distribution functions of some molecular properties, namely, the rotational relaxation constants, the atomic mean-square displacements, the relaxation of the cross correlation functions between the linear and squared displacements of H and O atoms of each molecule, the tetrahedral order parameter q and, finally, the number of nearest neighbors (NNs) and of hydrogen bonds (HBs) per molecule. Two different potentials are considered: TIP4P-Ew and a model developed in this laboratory for the study of nanoconfined water. The results are similar for the dynamical properties, but are markedly different for the structural characteristics. In particular, for temperatures higher than that of the dynamic crossover between “fragile” (at higher temperatures) and “strong” (at lower temperatures) liquid behaviors detected around 207 K, the rotational relaxation of supercooled water appears to be remarkably homogeneous. However, the structural parameters (number of NNs and of HBs, as well as q) do not show homogeneous distributions, and these distributions are different for the two water models. Another dynamic crossover between “fragile” (at lower temperatures) and “strong” (at higher temperatures) liquid behaviors, corresponding to the one found experimentally at T{sup ∗} ∼ 315 ± 5 K, was spotted at T{sup ∗} ∼ 283 K and T{sup ∗} ∼ 276 K for the TIP4P-Ew and the model developed in this laboratory, respectively. It was detected from the trend of Arrhenius plots of dynamic quantities and from the onset of a further heterogeneity in the rotational relaxation. To our best knowledge, it is the first time that this dynamical crossover is detected in computer simulations of bulk water. On the basis of the simulation results, the possible
Dynamics of exciton relaxation in LH2 antenna probed by multipulse nonlinear spectroscopy.
Novoderezhkin, Vladimir I; Cohen Stuart, Thomas A; van Grondelle, Rienk
2011-04-28
We explain the relaxation dynamics in the LH2-B850 antenna as revealed by multipulse pump-dump-probe spectroscopy (Th. A. Cohen Stuart, M. Vengris, V. I. Novoderezhkin, R. J. Cogdell, C. N. Hunter, R. van Grondelle, submitted). The theory of pump-dump-probe response is evaluated using the doorway-window approach in combination with the modified Redfield theory. We demonstrate that a simultaneous fit of linear spectra, pump-probe, and pump-dump-probe kinetics can be obtained at a quantitative level using the disordered exciton model, which is essentially the same as used to model the spectral fluctuations in single LH2 complexes (Novoderezhkin, V.; Rutkauskas, D.; van Grondelle, R. Biophys. J. 2006, 90, 2890). The present studies suggest that the observed relaxation rates are strongly dependent on the realization of the disorder. A big spread of the rates (exceeding 3 orders of magnitude) is correlated with the disorder-induced changes in delocalization length and overlap of the exciton wave functions. We conclude that the bulk kinetics reflect a superposition of many pathways corresponding to different physical limits of energy transfer, varying from sub-20 fs relaxation between delocalized and highly spatially overlapping exciton states to >20 ps jumps between states localized at the opposite sides of the ring.
Quantum dynamics of nuclear spins and spin relaxation in organic semiconductors
Mkhitaryan, V. V.; Dobrovitski, V. V.
2017-06-01
We investigate the role of the nuclear-spin quantum dynamics in hyperfine-induced spin relaxation of hopping carriers in organic semiconductors. The fast-hopping regime, when the carrier spin does not rotate much between subsequent hops, is typical for organic semiconductors possessing long spin coherence times. We consider this regime and focus on a carrier random-walk diffusion in one dimension, where the effect of the nuclear-spin dynamics is expected to be the strongest. Exact numerical simulations of spin systems with up to 25 nuclear spins are performed using the Suzuki-Trotter decomposition of the evolution operator. Larger nuclear-spin systems are modeled utilizing the spin-coherent state P -representation approach developed earlier. We find that the nuclear-spin dynamics strongly influences the carrier spin relaxation at long times. If the random walk is restricted to a small area, it leads to the quenching of carrier spin polarization at a nonzero value at long times. If the random walk is unrestricted, the carrier spin polarization acquires a long-time tail, decaying as 1 /√{t } . Based on the numerical results, we devise a simple formula describing the effect quantitatively.
Wavelet Analysis for Molecular Dynamics
2015-06-01
Our method takes as input the topology and sparsity of the bonding structure of a molecular system, and returns a hierarchical set of system-specific...problems, such as modeling crack initiation and propagation, or interfacial phenomena. In the present work, we introduce a wavelet-based approach to extend...Several functional forms are common for angle poten- tials complicating not only implementation but also choice of approximation. In all cases, the
Histidine side-chain dynamics and protonation monitored by C-13 CPMG NMR relaxation dispersion
DEFF Research Database (Denmark)
Hass, M. A. S.; Yilmaz, A.; Christensen, Hans Erik Mølager
2009-01-01
the chemical shift titration experiments, and the CPMG derived exchange rates agree with those obtained previously from N-15 backbone relaxation measurements. Compared to measurements of backbone nuclei, C-13(epsilon 1) dispersion provides a more direct method to monitor interchanging protonation states...... or other kinds of conformational changes of histidine side chains or their environment. Advantages and shortcomings of using the C-13(epsilon 1) dispersion experiments in combination with chemical shift titration experiments to obtain information on exchange dynamics of the histidine side chains...
Role of Orbital Dynamics in Spin Relaxation and Weak Antilocalization in Quantum Dots
Zaitsev, Oleg; Frustaglia, Diego; Richter, Klaus
2005-01-01
We develop a semiclassical theory for spin-dependent quantum transport to describe weak (anti)localization in quantum dots with spin-orbit coupling. This allows us to distinguish different types of spin relaxation in systems with chaotic, regular, and diffusive orbital classical dynamics. We find, in particular, that for typical Rashba spin-orbit coupling strengths, integrable ballistic systems can exhibit weak localization, while corresponding chaotic systems show weak antilocalization. We further calculate the magnetoconductance and analyze how the weak antilocalization is suppressed with decreasing quantum dot size and increasing additional in-plane magnetic field.
The elastoplastic calculation of disks with the help of dynamic relaxation
International Nuclear Information System (INIS)
Zerna, W.; Schnellenbach, G.; Ick, U.
1973-12-01
The possibilities for the computation of elasticplastic properties via dynamic relaxation are shown. From the various theories of plasticity the laws of Prandtl-Reuzs for solidifying materials were chosen for in this investigation. The calculation is carried out directly without further linearizations in a single computer run. It is possible to obtain an approximate solution via a direct process involving a fictitious elastic material law. Two disks with - according to the theory of elasticity - single stress points are used as examples. (orig.) [de
NMR and molecular dynamics of small solutes in liquid crystals
International Nuclear Information System (INIS)
Luyten, P.R.
1984-01-01
NMR relaxation measurements, using a wide variety of modern pulse techniques, can yield valuable information about molecular motions. In this thesis the applicability of theories developed to describe spin relaxation phenomena in partially ordered media is studied for small solutes in liquid crystals. 1 H, 2 H, 13 C and 14 N relaxation measurements are interpreted by means of a model, in which fast anisotropic re-orientational motion in an orienting potential combined with contributions from cooperative fluctuations in the surrounding liquid crystal molecules, induce the observed frequency dependent relaxation behavior. (orig.)
Dynamics and Thermodynamics of Molecular Machines
DEFF Research Database (Denmark)
Golubeva, Natalia
2014-01-01
to their microscopic size, molecular motors are governed by principles fundamentally different from those describing the operation of man-made motors such as car engines. In this dissertation the dynamic and thermodynamic properties of molecular machines are studied using the tools of nonequilibrium statistical......Molecular machines, or molecular motors, are small biophysical devices that perform a variety of essential metabolic processes such as DNA replication, protein synthesis and intracellular transport. Typically, these machines operate by converting chemical energy into motion and mechanical work. Due...... mechanics. The first part focuses on noninteracting molecular machines described by a paradigmatic continuum model with the aim of comparing and contrasting such a description to the one offered by the widely used discrete models. Many molecular motors, for example, kinesin involved in cellular cargo...
Electron-nuclear corellations for photoinduced dynamics in molecular dimers
Kilin, Dmitri S.; Pereversev, Yuryi V.; Prezhdo, Oleg V.
2003-03-01
Ultrafast photoinduced dynamics of electronic excitation in molecular dimers is drastically affected by dynamic reorganization of of inter- and intra- molecular nuclear configuration modelled by quantized nuclear degree of freedom [1]. The dynamics of the electronic population and nuclear coherence is analyzed with help of both numerical solution of the chain of coupled differential equations for mean coordinate, population inversion, electronic-vibrational correlation etc.[2] and by propagating the Gaussian wavepackets in relevant adiabatic potentials. Intriguing results were obtained in the approximation of small energy difference and small change of nuclear equilibrium configuration for excited electronic states. In the limiting case of resonance between electronic states energy difference and frequency of the nuclear mode these results have been justified by comparison to exactly solvable Jaynes-Cummings model. It has been found that the photoinduced processes in dimer are arranged according to their time scales:(i) fast scale of nuclear motion,(ii) intermediate scale of dynamical redistribution of electronic population between excited states as well as growth and dynamics of electronic -nuclear correlation,(iii) slow scale of electronic population approaching to the quasiequilibrium distribution, decay of electronic-nuclear correlation, and diminishing the amplitude of mean coordinate oscillations, accompanied by essential growth of the nuclear coordinate dispersion associated with the overall nuclear wavepacket width. Demonstrated quantum-relaxational features of photoinduced vibronic dinamical processess in molecular dimers are obtained by simple method, applicable to large biological systems with many degrees of freedom. [1] J. A. Cina, D. S. Kilin, T. S. Humble, J. Chem. Phys. (2003) in press. [2] O. V. Prezhdo, J. Chem. Phys. 117, 2995 (2002).
Advances in molecular vibrations and collision dynamics molecular clusters
Bacic, Zatko
1998-01-01
This volume focuses on molecular clusters, bound by van der Waals interactions and hydrogen bonds. Twelve chapters review a wide range of recent theoretical and experimental advances in the areas of cluster vibrations, spectroscopy, and reaction dynamics. The authors are leading experts, who have made significant contributions to these topics.The first chapter describes exciting results and new insights in the solvent effects on the short-time photo fragmentation dynamics of small molecules, obtained by combining heteroclusters with femtosecond laser excitation. The second is on theoretical work on effects of single solvent (argon) atom on the photodissociation dynamics of the solute H2O molecule. The next two chapters cover experimental and theoretical aspects of the energetics and vibrations of small clusters. Chapter 5 describes diffusion quantum Monte Carlo calculations and non additive three-body potential terms in molecular clusters. The next six chapters deal with hydrogen-bonded clusters, refle...
Magnetic nanoparticles in fluid environment: combining molecular dynamics and Lattice-Boltzmann
Energy Technology Data Exchange (ETDEWEB)
Melenev, Petr, E-mail: melenev@icmm.ru [Ural Federal University, 4, Turgeneva str., 620000 Ekaterinburg (Russian Federation); Institute of Continuous Media Mechanics, 1, Koroleva str., 614013 Perm (Russian Federation)
2017-06-01
Hydrodynamic interactions between magnetic nanoparticles suspended in the Newtonian liquid are accounted for using a combination of the lattice Boltzmann method and molecular dynamics simulations. Nanoparticle is modelled by the system of molecular dynamics material points (which form structure resembles raspberry) coupled to the lattice Boltzmann fluid. The hydrodynamic coupling between the colloids is studied by simulations of the thermo-induced rotational diffusion of two raspberry objects. It was found that for the considered range of model parameters the approaching of the raspberries leads to slight retard of the relaxation process. The presence of the weak magnetic dipolar interaction between the objects leads to modest decrease of the relaxation time and the extent of the acceleration of the diffusion is intensified along with magnetic forces. - Highlights: • The combination of molecular dynamics and lattice Boltzmann method is utilized for the reveal of the role of hydrodynamic interaction in rotational dynamics of colloid particles. • The verification of the model parameters is done based on the comparison with the results of Langevin dynamics. • For the task of free rotational diffusion of the pair of colloid particles the influence of the hydrodynamic interactions on the relaxation time is examined in the case of nonmagnetic particles and at the presence of weak dipolar interaction.
Molecular dynamics and diffusion a compilation
Fisher, David
2013-01-01
The molecular dynamics technique was developed in the 1960s as the outgrowth of attempts to model complicated systems by using either a) direct physical simulation or (following the great success of Monte Carlo methods) by b) using computer techniques. Computer simulation soon won out over clumsy physical simulation, and the ever-increasing speed and sophistication of computers has naturally made molecular dynamics simulation into a more and more successful technique. One of its most popular applications is the study of diffusion, and some experts now even claim that molecular dynamics simulation is, in the case of situations involving well-characterised elements and structures, more accurate than experimental measurement. The present double volume includes a compilation (over 600 items) of predicted solid-state diffusion data, for all of the major materials groups, dating back nearly four decades. The double volume also includes some original papers: "Determination of the Activation Energy for Formation and ...
Molecular Dynamics Studies of Nanofluidic Devices
DEFF Research Database (Denmark)
Zambrano Rodriguez, Harvey Alexander
of such devices. Computational nanofluidics complements experimental studies by providing detailed spatial and temporal information of the nanosystem. In this thesis, we conduct molecular dynamics simulations to study basic nanoscale devices. We focus our studies on the understanding of transport mechanism...... to drive fluids and solids at the nanoscale. Specifically, we present the results of three different research projects. Throughout the first part of this thesis, we include a comprenhensive introduction to computational nanofluidics and to molecular simulations, and describe the molecular dynamics...... in opposite direction to the imposed thermal gradient also we measure higher velocities as higher thermal gradients are imposed. Secondly, we present an atomistic analysis of a molecular linear motor fabricated of coaxial carbon nanotubes and powered by thermal gradients. The MD simulation results indicate...
Dynamics in a one-dimensional ferrogel model: relaxation, pairing, shock-wave propagation.
Goh, Segun; Menzel, Andreas M; Löwen, Hartmut
2018-05-23
Ferrogels are smart soft materials, consisting of a polymeric network and embedded magnetic particles. Novel phenomena, such as the variation of the overall mechanical properties by external magnetic fields, emerge consequently. However, the dynamic behavior of ferrogels remains largely unveiled. In this paper, we consider a one-dimensional chain consisting of magnetic dipoles and elastic springs between them as a simple model for ferrogels. The model is evaluated by corresponding simulations. To probe the dynamics theoretically, we investigate a continuum limit of the energy governing the system and the corresponding equation of motion. We provide general classification scenarios for the dynamics, elucidating the touching/detachment dynamics of the magnetic particles along the chain. In particular, it is verified in certain cases that the long-time relaxation corresponds to solutions of shock-wave propagation, while formations of particle pairs underlie the initial stage of the dynamics. We expect that these results will provide insight into the understanding of the dynamics of more realistic models with randomness in parameters and time-dependent magnetic fields.
Critical thickness and strain relaxation in molecular beam epitaxy-grown SrTiO3 films
International Nuclear Information System (INIS)
Wang, Tianqi; Ganguly, Koustav; Marshall, Patrick; Xu, Peng; Jalan, Bharat
2013-01-01
We report on the study of the critical thickness and the strain relaxation in epitaxial SrTiO 3 film grown on (La 0.3 Sr 0.7 )(Al 0.65 Ta 0.35 )O 3 (001) (LSAT) substrate using the hybrid molecular beam epitaxy approach. No change in the film's lattice parameter (both the in-plane and the out-of-plane) was observed up to a film thickness of 180 nm, which is in sharp contrast to the theoretical critical thickness of ∼12 nm calculated using the equilibrium theory of strain relaxation. For film thicknesses greater than 180 nm, the out-of-plane lattice parameter was found to decrease hyperbolically in an excellent agreement with the relaxation via forming misfit dislocations. Possible mechanisms are discussed by which the elastic strain energy can be accommodated prior to forming misfit dislocations leading to such anomalously large critical thickness
Molecular dynamics modeling of polymer flammability
International Nuclear Information System (INIS)
Nyden, M.R.; Brown, J.E.; Lomakin, S.M.
1992-01-01
Molecular dynamic simulations were used to identify factors which promote char formation during the thermal degradation of polymers. Computer movies based on these simulations, indicate that cross-linked model polymers tend to undergo further cross-linking when burned, eventually forming a high molecular weight, thermally stable char. This paper reports that the prediction was confirmed by char yield measurements made on γ and e - -irradiated polyethylene and chemically cross-linked poly(methyl methacrylate)
Simpkins, Blake S.; Fears, Kenan P.; Dressick, Walter J.; Dunkelberger, Adam D.; Spann, Bryan T.; Owrutsky, Jeffrey C.
2016-09-01
Coherent coupling between an optical transition and confined optical mode have been investigated for electronic-state transitions, however, only very recently have vibrational transitions been considered. Here, we demonstrate both static and dynamic results for vibrational bands strongly coupled to optical cavities. We experimentally and numerically describe strong coupling between a Fabry-Pérot cavity and carbonyl stretch ( 1730 cm 1) in poly-methylmethacrylate and provide evidence that the mixed-states are immune to inhomogeneous broadening. We investigate strong and weak coupling regimes through examination of cavities loaded with varying concentrations of a urethane monomer. Rabi splittings are in excellent agreement with an analytical description using no fitting parameters. Ultrafast pump-probe measurements reveal transient absorption signals over a frequency range well-separated from the vibrational band, as well as drastically modified relaxation rates. We speculate these modified kinetics are a consequence of the energy proximity between the vibration-cavity polariton modes and excited state transitions and that polaritons offer an alternative relaxation path for vibrational excitations. Varying the polariton energies by angle-tuning yields transient results consistent with this hypothesis. Furthermore, Rabi oscillations, or quantum beats, are observed at early times and we see evidence that these coherent vibration-cavity polariton excitations impact excited state population through cavity losses. Together, these results indicate that cavity coupling may be used to influence both excitation and relaxation rates of vibrations. Opening the field of polaritonic coupling to vibrational species promises to be a rich arena amenable to a wide variety of infrared-active bonds that can be studied in steady state and dynamically.
First-principles molecular dynamics for metals
International Nuclear Information System (INIS)
Fernando, G.W.; Qian, G.; Weinert, M.; Davenport, J.W.
1989-01-01
A Car-Parrinello-type first-principles molecular-dynamics approach capable of treating the partial occupancy of electronic states that occurs at the Fermi level in a metal is presented. The algorithms used to study metals are both simple and computationally efficient. We also discuss the connection between ordinary electronic-structure calculations and molecular-dynamics simulations as well as the role of Brillouin-zone sampling. This extension should be useful not only for metallic solids but also for solids that become metals in their liquid and/or amorphous phases
Scalable Molecular Dynamics for Large Biomolecular Systems
Directory of Open Access Journals (Sweden)
Robert K. Brunner
2000-01-01
Full Text Available We present an optimized parallelization scheme for molecular dynamics simulations of large biomolecular systems, implemented in the production-quality molecular dynamics program NAMD. With an object-based hybrid force and spatial decomposition scheme, and an aggressive measurement-based predictive load balancing framework, we have attained speeds and speedups that are much higher than any reported in literature so far. The paper first summarizes the broad methodology we are pursuing, and the basic parallelization scheme we used. It then describes the optimizations that were instrumental in increasing performance, and presents performance results on benchmark simulations.
Theory and application of quantum molecular dynamics
Zeng Hui Zhang, John
1999-01-01
This book provides a detailed presentation of modern quantum theories for treating the reaction dynamics of small molecular systems. Its main focus is on the recent development of successful quantum dynamics theories and computational methods for studying the molecular reactive scattering process, with specific applications given in detail for a number of benchmark chemical reaction systems in the gas phase and the gas surface. In contrast to traditional books on collision in physics focusing on abstract theory for nonreactive scattering, this book deals with both the development and the appli
Nonadiabatic electron wavepacket dynamics behind molecular autoionization
Matsuoka, Takahide; Takatsuka, Kazuo
2018-01-01
A theoretical method for real-time dynamics of nonadiabatic reorganization of electronic configurations in molecules is developed, with dual aim that the intramolecular electron dynamics can be probed by means of direct and/or indirect photoionizations and that the physical origins behind photoionization signals attained in the time domain can be identified in terms of the language of time-dependent quantum chemistry. In doing so, we first formulate and implement a new computational scheme for nonadiabatic electron dynamics associated with molecular ionization, which well fits in the general theory of nonadiabatic electron dynamics. In this method, the total nonadiabatic electron wavepackets are propagated in time directly with complex natural orbitals without referring to Hartree-Fock molecular orbitals, and the amount of electron flux from a molecular region leading to ionization is evaluated in terms of the relevant complex natural orbitals. In the second half of this paper, we apply the method to electron dynamics in the elementary processes consisting of the Auger decay to demonstrate the methodological significance. An illustrative example is taken from an Auger decay starting from the 2a1 orbital hole-state of H2O+. The roles of nuclear momentum (kinetic) couplings in electronic-state mixing during the decay process are analyzed in terms of complex natural orbitals, which are schematically represented in the conventional language of molecular symmetry of the Hartree-Fock orbitals.
Ultrafast molecular dynamics illuminated with synchrotron radiation
International Nuclear Information System (INIS)
Bozek, John D.; Miron, Catalin
2015-01-01
Highlights: • Ultrafast molecular dynamics probed with synchrotron radiation. • Core-excitation as probe of ultrafast dynamics through core-hole lifetime. • Review of experimental and theoretical methods in ultrafast dynamics using core-level excitation. - Abstract: Synchrotron radiation is a powerful tool for studying molecular dynamics in small molecules in spite of the absence of natural matching between the X-ray pulse duration and the time scale of nuclear motion. Promoting core level electrons to unoccupied molecular orbitals simultaneously initiates two ultrafast processes, nuclear dynamics on the potential energy surfaces of the highly excited neutral intermediate state of the molecule on the one hand and an ultrafast electronic decay of the intermediate excited state to a cationic final state, characterized by a core hole lifetime. The similar time scales of these processes enable core excited pump-probe-type experiments to be performed with long duration X-ray pulses from a synchrotron source. Recent results obtained at the PLIEADES beamline concerning ultrafast dissociation of core excited states and molecular potential energy curve mapping facilitated by changes in the geometry of the short-lived intermediate core excited state are reviewed. High brightness X-ray beams combined with state-of-the art electron and ion-electron coincidence spectrometers and highly sophisticated theoretical methods are required to conduct these experiments and to achieve a full understanding of the experimental results.
International Nuclear Information System (INIS)
Elkins, Madeline H.; Williams, Holly L.; Neumark, Daniel M.
2015-01-01
The charge-transfer-to-solvent dynamics (CTTS) and excited state relaxation mechanism of the solvated electron in methanol are studied by time-resolved photoelectron spectroscopy on a liquid methanol microjet by means of two-pulse and three-pulse experiments. In the two-pulse experiment, CTTS excitation is followed by a probe photoejection pulse. The resulting time-evolving photoelectron spectrum reveals multiple time scales characteristic of relaxation and geminate recombination of the initially generated electron which are consistent with prior results from transient absorption. In the three-pulse experiment, the relaxation dynamics of the solvated electron following electronic excitation are measured. The internal conversion lifetime of the excited electron is found to be 130 ± 40 fs, in agreement with extrapolated results from clusters and the non-adiabatic relaxation mechanism
Elkins, Madeline H.; Williams, Holly L.; Neumark, Daniel M.
2015-06-01
The charge-transfer-to-solvent dynamics (CTTS) and excited state relaxation mechanism of the solvated electron in methanol are studied by time-resolved photoelectron spectroscopy on a liquid methanol microjet by means of two-pulse and three-pulse experiments. In the two-pulse experiment, CTTS excitation is followed by a probe photoejection pulse. The resulting time-evolving photoelectron spectrum reveals multiple time scales characteristic of relaxation and geminate recombination of the initially generated electron which are consistent with prior results from transient absorption. In the three-pulse experiment, the relaxation dynamics of the solvated electron following electronic excitation are measured. The internal conversion lifetime of the excited electron is found to be 130 ± 40 fs, in agreement with extrapolated results from clusters and the non-adiabatic relaxation mechanism.
Energy Technology Data Exchange (ETDEWEB)
Elkins, Madeline H.; Williams, Holly L. [Department of Chemistry, University of California, Berkeley, California 94720 (United States); Neumark, Daniel M. [Department of Chemistry, University of California, Berkeley, California 94720 (United States); Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720 (United States)
2015-06-21
The charge-transfer-to-solvent dynamics (CTTS) and excited state relaxation mechanism of the solvated electron in methanol are studied by time-resolved photoelectron spectroscopy on a liquid methanol microjet by means of two-pulse and three-pulse experiments. In the two-pulse experiment, CTTS excitation is followed by a probe photoejection pulse. The resulting time-evolving photoelectron spectrum reveals multiple time scales characteristic of relaxation and geminate recombination of the initially generated electron which are consistent with prior results from transient absorption. In the three-pulse experiment, the relaxation dynamics of the solvated electron following electronic excitation are measured. The internal conversion lifetime of the excited electron is found to be 130 ± 40 fs, in agreement with extrapolated results from clusters and the non-adiabatic relaxation mechanism.
Salloum, Ahmed
Constraint relaxation by definition means that certain security, operational, or financial constraints are allowed to be violated in the energy market model for a predetermined penalty price. System operators utilize this mechanism in an effort to impose a price-cap on shadow prices throughout the market. In addition, constraint relaxations can serve as corrective approximations that help in reducing the occurrence of infeasible or extreme solutions in the day-ahead markets. This work aims to capture the impact constraint relaxations have on system operational security. Moreover, this analysis also provides a better understanding of the correlation between DC market models and AC real-time systems and analyzes how relaxations in market models propagate to real-time systems. This information can be used not only to assess the criticality of constraint relaxations, but also as a basis for determining penalty prices more accurately. Constraint relaxations practice was replicated in this work using a test case and a real-life large-scale system, while capturing both energy market aspects and AC real-time system performance. System performance investigation included static and dynamic security analysis for base-case and post-contingency operating conditions. PJM peak hour loads were dynamically modeled in order to capture delayed voltage recovery and sustained depressed voltage profiles as a result of reactive power deficiency caused by constraint relaxations. Moreover, impacts of constraint relaxations on operational system security were investigated when risk based penalty prices are used. Transmission lines in the PJM system were categorized according to their risk index and each category was as-signed a different penalty price accordingly in order to avoid real-time overloads on high risk lines. This work also extends the investigation of constraint relaxations to post-contingency relaxations, where emergency limits are allowed to be relaxed in energy market models
Numerical method of lines for the relaxational dynamics of nematic liquid crystals.
Bhattacharjee, A K; Menon, Gautam I; Adhikari, R
2008-08-01
We propose an efficient numerical scheme, based on the method of lines, for solving the Landau-de Gennes equations describing the relaxational dynamics of nematic liquid crystals. Our method is computationally easy to implement, balancing requirements of efficiency and accuracy. We benchmark our method through the study of the following problems: the isotropic-nematic interface, growth of nematic droplets in the isotropic phase, and the kinetics of coarsening following a quench into the nematic phase. Our results, obtained through solutions of the full coarse-grained equations of motion with no approximations, provide a stringent test of the de Gennes ansatz for the isotropic-nematic interface, illustrate the anisotropic character of droplets in the nucleation regime, and validate dynamical scaling in the coarsening regime.
Vibronic relaxation in molecular mixed crystals : Pentacene in naphthalene and p-terphenyl
Hesselink, Wim H.; Wiersma, Douwe A.
1981-01-01
Picosecond photon echo techniques are used to measure directly vibronic relaxation times in the first excited singlet state of pentacene in naphthalene and p-terphenyl. In regions of low (< 300 cm–1) and high (> 1000 cm–1) vibrational energy, relaxation is fast (τ <2 ps) due to direct phonon
Directory of Open Access Journals (Sweden)
Xinyu Li
2016-01-01
Full Text Available Water plays a very important role in wood and wood products. The molecular motion of water in wood is susceptible to thermal activation. Thermal energy makes water molecules more active and weakens the force between water and wood; therefore, the water molecules dynamic properties are greatly influenced. Molecular dynamics study is important for wood drying; this paper therefore focuses on water molecular dynamics in wood through fast field cycling nuclear magnetic resonance relaxometry techniques. The results show that the spin-lattice relaxation rate decreases with the Larmor frequency. Nuclear magnetic resonance dispersion profiles at different temperatures could separate the relaxation contribution of water in bigger pores and smaller pores. The T1 distribution from wide to narrow at 10 MHz Larmor frequency reflects the shrinkage of pore size with the higher temperature. The dependence of spin-lattice relaxation rate on correlation time for water molecular motion based on BPP (proposed by Bloembergen, Purcell, and Pound theory shows that water correlation time increases with higher temperature, and its activation energy, calculated using the Arrhenius transformation equation, is 9.06±0.53 kJ/mol.
International Nuclear Information System (INIS)
Guseva, D. V.; Komarov, P. V.; Lyulin, Alexey V.
2014-01-01
Constant temperature–constant pressure (NpT) molecular-dynamics computer simulations have been carried out for the united-atom model of a non-crosslinked (1,4) cis-polyisoprene (PI) melt confined between two amorphous, fully coordinated silica surfaces. The Lennard-Jones 12-6 potential was implemented to describe the polymer–silica interactions. The thickness H of the produced PI–silica film has been varied in a wide range, 1 g g is the individual PI chain radius of gyration measured under the imposed confinement. After a thorough equilibration, the PI film stratified structure and polymer segmental dynamics have been studied. The chain structure in the middle of the films resembles that in a corresponding bulk, but the polymer-density profile shows a pronounced ordering of the polymer segments in the vicinity of silica surfaces; this ordering disappears toward the film middles. Tremendous slowing down of the polymer segmental dynamics has been observed in the film surface layers, with the segmental relaxation more than 150 times slower as compared to that in a PI bulk. This effect increases with decreasing the polymer-film thickness. The segmental relaxation in the PI film middles shows additional relaxation process which is absent in a PI bulk. Even though there are fast relaxation processes in the film middle, its overall relaxation is slower as compared to that in a bulk sample. The interpretation of the results in terms of polymer glassy bridges has been discussed
Electronic relaxation dynamics of a metal atom deposited on argon cluster
International Nuclear Information System (INIS)
Awali, Slim
2014-01-01
This thesis is a study on the interaction between electronically excited atomic states and a non-reactive environment. We have theoretically and experimentally studied situations where a metal atom (Ba or K) is placed in a finite size environment (argon cluster). The presence of the medium affects the electronic levels of the atom. On the other side, the excitation of the atom induces a relaxation dynamics of the electronic energy through the deformation of the cluster. The experimental part of this work focuses on two aspects: the spectroscopy and the dynamics. In both cases a first laser electronically excites the metal atom and the second ionizes the excited system. The observable is the photoelectron spectrum recorded after photoionization and possibly information on the photoion which are also produced. This pump/probe technique, with also two lasers, provide the ultrafast dynamic when the lasers pulses used are of ultrashort (60 fs). The use of nanosecond lasers leads to resonance spectroscopic measurement, unresolved temporally, which give information on the position of the energy levels of the studied system. From a theoretical point-of-view, the excited states of M-Ar n were calculated at the ab initio level, using large core pseudo-potential to limit the active electrons of the metal to valence electrons. The study of alkali metals (potassium) is especially well adapted to this method since only one electron is active. The ab-initio calculation and a Monte-Carlo simulation where coupled to optimize the geometry of the KAr n (n = 1-10) cluster when K is in the ground state of the neutral and the ion, or excited in the 4p or 5s state. Calculations were also conducted in collaboration with B. Gervais (CIMAP, Caen) on KAr n clusters having several tens of argon atoms. Absorption spectra were also calculated. From an experimental point-of-view, we were able to characterize the excited states of potassium and barium perturbed by the clusters. In both cases a
Molecular dynamics simulation of impact test
International Nuclear Information System (INIS)
Akahoshi, Y.; Schmauder, S.; Ludwig, M.
1998-01-01
This paper describes an impact test by molecular dynamics (MD) simulation to evaluate embrittlement of bcc Fe at different temperatures. A new impact test model is developed for MD simulation. The typical fracture behaviors show transition from brittle to ductile fracture, and a history of the impact loads also demonstrates its transition. We conclude that the impact test by MD could be feasible. (orig.)
Ab Initio molecular dynamics with excited electrons
Alavi, A.; Kohanoff, J.; Parrinello, M.; Frenkel, D.
1994-01-01
A method to do ab initio molecular dynamics suitable for metallic and electronically hot systems is described. It is based on a density functional which is costationary with the finite-temperature functional of Mermin, with state being included with possibly fractional occupation numbers.
Molecular dynamics simulations of RNA motifs
Czech Academy of Sciences Publication Activity Database
Csaszar, K.; Špačková, Naďa; Šponer, Jiří; Leontis, N. B.
2002-01-01
Roč. 223, - (2002), s. 154 ISSN 0065-7727. [Annual Meeting of the American Chemistry Society /223./. 07.04.2002-11.04.2002, Orlando ] Institutional research plan: CEZ:AV0Z5004920 Keywords : molecular dynamics * RNA * hydration Subject RIV: BO - Biophysics
Molecular dynamics simulation of impact test
Energy Technology Data Exchange (ETDEWEB)
Akahoshi, Y. [Kyushu Inst. of Tech., Kitakyushu, Fukuoka (Japan); Schmauder, S.; Ludwig, M. [Stuttgart Univ. (Germany). Staatliche Materialpruefungsanstalt
1998-11-01
This paper describes an impact test by molecular dynamics (MD) simulation to evaluate embrittlement of bcc Fe at different temperatures. A new impact test model is developed for MD simulation. The typical fracture behaviors show transition from brittle to ductile fracture, and a history of the impact loads also demonstrates its transition. We conclude that the impact test by MD could be feasible. (orig.)
Molecular dynamics simulation of a phospholipid membrane
Egberts, Egbert; Marrink, Siewert-Jan; Berendsen, Herman J.C.
We present the results of molecular dynamics (MD) simulations of a phospholipid membrane in water, including full atomic detail. The goal of the simulations was twofold: first we wanted to set up a simulation system which is able to reproduce experimental results and can serve as a model membrane in
Molecular dynamics simulations and quantum chemical calculations ...
African Journals Online (AJOL)
Molecular dynamic simulation results indicate that the imidazoline derivative molecules uses the imidazoline ring to effectively adsorb on the surface of iron, with the alkyl hydrophobic tail forming an n shape (canopy like covering) at geometry optimization and at 353 K. The n shape canopy like covering to a large extent may ...
Nanotribology investigations with classical molecular dynamics
Solhjoo, Soheil
2017-01-01
This thesis presents a number of nanotribological problems investigated by means of classical molecular dynamics (MD) simulations, within the context of the applicability of continuum mechanics contact theories at the atomic scale. Along these lines, three different themes can be recognized herein:
Catalysis and communication in dynamic molecular networks
Fanlo Virgos, Hugo
2015-01-01
The interactions of a Dynamic Combinatorial Library (DCL) of molecules with specific targets leads to composition changes of the library which can reveal potential guests and / or catalysts. In this thesis some chemical systems have been proposed to achieve a certain level of molecular complexity
Reaction dynamics in polyatomic molecular systems
Energy Technology Data Exchange (ETDEWEB)
Miller, W.H. [Lawrence Berkeley Laboratory, CA (United States)
1993-12-01
The goal of this program is the development of theoretical methods and models for describing the dynamics of chemical reactions, with specific interest for application to polyatomic molecular systems of special interest and relevance. There is interest in developing the most rigorous possible theoretical approaches and also in more approximate treatments that are more readily applicable to complex systems.
Kinetics from Replica Exchange Molecular Dynamics Simulations.
Stelzl, Lukas S; Hummer, Gerhard
2017-08-08
Transitions between metastable states govern many fundamental processes in physics, chemistry and biology, from nucleation events in phase transitions to the folding of proteins. The free energy surfaces underlying these processes can be obtained from simulations using enhanced sampling methods. However, their altered dynamics makes kinetic and mechanistic information difficult or impossible to extract. Here, we show that, with replica exchange molecular dynamics (REMD), one can not only sample equilibrium properties but also extract kinetic information. For systems that strictly obey first-order kinetics, the procedure to extract rates is rigorous. For actual molecular systems whose long-time dynamics are captured by kinetic rate models, accurate rate coefficients can be determined from the statistics of the transitions between the metastable states at each replica temperature. We demonstrate the practical applicability of the procedure by constructing master equation (Markov state) models of peptide and RNA folding from REMD simulations.
Directory of Open Access Journals (Sweden)
M.M. Sevryukova
2017-12-01
Full Text Available The optical properties and dynamics of transport of electron excitation and the ways of its relaxation in the supramolecular D–π–A complex on the basis of merocyanines have been investigated. There have been found two components in the transfer of charge: fast and slow, which correspond to different conformational states of the carbon chain in merocyanines. It was found that the main photoluminescence of the studied molecular solutions of merocyanines by its nature is similar to the exciplex luminescence, as a manifestation of resonant and charge transfer interaction in an excited state. The lifetime in this state is about 2000 ps.
Dynamical quenching of tunneling in molecular magnets
Energy Technology Data Exchange (ETDEWEB)
José Santander, María, E-mail: maria.jose.noemi@gmail.com [Recursos Educativos Quántica, Santiago (Chile); Departamento de Física, Universidad de Santiago de Chile and CEDENNA, Avda. Ecuador 3493, Santiago (Chile); Nunez, Alvaro S., E-mail: alnunez@dfi.uchile.cl [Departamento de Física, Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Casilla 487-3, Santiago (Chile); Roldán-Molina, A. [Instituto de Física, Pontificia Universidad Católica de Valparaíso, Avenida Universidad 330, Curauma, Valparaíso (Chile); Troncoso, Roberto E., E-mail: r.troncoso.c@gmail.com [Centro para el Desarrollo de la Nanociencia y la Nanotecnología, CEDENNA, Avda. Ecuador 3493, Santiago 9170124 (Chile); Departamento de Física, Universidad Técnica Federico Santa María, Avenida España 1680, Valparaíso (Chile)
2015-12-15
It is shown that a single molecular magnet placed in a rapidly oscillating magnetic field displays the phenomenon of quenching of tunneling processes. The results open a way to manipulate the quantum states of molecular magnets by means of radiation in the terahertz range. Our analysis separates the time evolution into slow and fast components thereby obtaining an effective theory for the slow dynamics. This effective theory presents quenching of the tunnel effect, in particular, stands out its difference with the so-called coherent destruction of tunneling. We support our prediction with numerical evidence based on an exact solution of Schrödinger's equation. - Highlights: • Single molecular magnets under rapidly oscillating magnetic fields is studied. • It is shown that this system displays the quenching of tunneling processes. • Our findings provide a control of quantum molecular magnets via terahertz radiation.
Dynamical quenching of tunneling in molecular magnets
International Nuclear Information System (INIS)
José Santander, María; Nunez, Alvaro S.; Roldán-Molina, A.; Troncoso, Roberto E.
2015-01-01
It is shown that a single molecular magnet placed in a rapidly oscillating magnetic field displays the phenomenon of quenching of tunneling processes. The results open a way to manipulate the quantum states of molecular magnets by means of radiation in the terahertz range. Our analysis separates the time evolution into slow and fast components thereby obtaining an effective theory for the slow dynamics. This effective theory presents quenching of the tunnel effect, in particular, stands out its difference with the so-called coherent destruction of tunneling. We support our prediction with numerical evidence based on an exact solution of Schrödinger's equation. - Highlights: • Single molecular magnets under rapidly oscillating magnetic fields is studied. • It is shown that this system displays the quenching of tunneling processes. • Our findings provide a control of quantum molecular magnets via terahertz radiation
Molecular Dynamics Simulation of Salt Diffusion in Polyelectrolyte Assemblies.
Zhang, Ran; Duan, Xiaozheng; Ding, Mingming; Shi, Tongfei
2018-06-05
The diffusion of salt ions and charged probe molecules in polyelectrolyte assemblies is often assumed to follow a theoretical hopping model, in which the diffusing ion is hopping between charged sites of chains based on electroneutrality. However, experimental verification of diffusing pathway at such microscales is difficult, and the corresponding molecular mechanisms remain elusive. In this study, we perform all-atom molecular dynamics (MD) simulations of salt diffusion in polyelectrolyte (PE) assembly of poly (sodium 4-styrenesulfonate) (PSS) and poly (diallyldimethylammonium chloride) (PDAC). Besides the ion hopping mode, the diffusing trajectories are found presenting common features of a jump process, i.e., subjecting to PE relaxation, water pockets in the structure open and close, thus the ion can move from one pocket to another. Anomalous subdiffusion of ions and water is observed due to the trapping scenarios in these water pockets. The jump events are much rarer compared with ion hopping but significantly increases salt diffusion with increasing temperature. Our result strongly indicates that salt diffusion in hydrated PDAC/PSS is a combined process of ion hopping and jump motion. This provides new molecular explanation for the coupling of salt motion with chain motion and the nonlinear increase of salt diffusion at glass transition temperature.
Spin dynamics at level crossing in molecular AF rings probed by NMR
Energy Technology Data Exchange (ETDEWEB)
Lascialfari, A. E-mail: lascialfari@fisicavolta.unipv.it; Borsa, F.; Julien, M.-H.; Micotti, E.; Furukawa, Y.; Jang, Z.H.; Cornia, A.; Gatteschi, D.; Horvatic, M.; Van Slageren, J
2004-05-01
The low-temperature spin dynamics in molecular rings with a finite number (N{<=}10) of magnetic ions was studied by means of {sup 1}H NMR. When an external magnetic field (B) induces a crossing between energy levels, peaks are observed in the spin-lattice relaxation rate of protons, 1/T{sub 1}(B), at constant temperature. We discuss similarities and differences in the data from three different rings: Fe10, Fe6:Li and Cr8.
Spin dynamics at level crossing in molecular AF rings probed by NMR
International Nuclear Information System (INIS)
Lascialfari, A.; Borsa, F.; Julien, M.-H.; Micotti, E.; Furukawa, Y.; Jang, Z.H.; Cornia, A.; Gatteschi, D.; Horvatic, M.; Van Slageren, J.
2004-01-01
The low-temperature spin dynamics in molecular rings with a finite number (N≤10) of magnetic ions was studied by means of 1 H NMR. When an external magnetic field (B) induces a crossing between energy levels, peaks are observed in the spin-lattice relaxation rate of protons, 1/T 1 (B), at constant temperature. We discuss similarities and differences in the data from three different rings: Fe10, Fe6:Li and Cr8
Analysis of Twisting of Cellulose Nanofibrils in Atomistic Molecular Dynamics Simulations
DEFF Research Database (Denmark)
Paavilainen, S.; Rog, T.; Vattulainen, I.
2011-01-01
We use atomistic molecular dynamics simulations to study the crystal structure of cellulose nanofibrils, whose sizes are comparable with the crystalline parts in commercial nanocellulose. The simulations show twisting, whose rate of relaxation is strongly temperature dependent. Meanwhile......, no significant bending or stretching of nanocellulose is discovered. Considerations of atomic-scale interaction patterns bring about that the twisting arises from hydrogen bonding within and between the chains in a fibril....
Kargovsky, A. V.; Chichigina, O. A.; Anashkina, E. I.; Valenti, D.; Spagnolo, B.
2015-10-01
The relaxation dynamics of a system described by a Langevin equation with pulse multiplicative noise sources with different correlation properties is considered. The solution of the corresponding Fokker-Planck equation is derived for Gaussian white noise. Moreover, two pulse processes with regulated periodicity are considered as a noise source: the dead-time-distorted Poisson process and the process with fixed time intervals, which is characterized by an infinite correlation time. We find that the steady state of the system is dependent on the correlation properties of the pulse noise. An increase of the noise correlation causes the decrease of the mean value of the solution at the steady state. The analytical results are in good agreement with the numerical ones.
Relaxation dynamics of glass transition in PMMA + SWCNT composites by temperature-modulated DSC
Pradhan, N. R.; Iannacchione, G. S.
2010-03-01
The experimental technique offered by temperature-modulated differential scanning calorimeter (TMDSC) used to investigate the thermal relaxation dynamics through the glass transition as a function of frequency was studied for pure PMMA and PMMA-single wall carbon nanotubes (SWCNTs) composites. A strong dependence of the temperature dependence peak in the imaginary part of complex heat capacity (Tmax) is found during the transition from the glass-like to the liquid-like region. The frequency dependence of Tmax of the imaginary part of heat capacity (Cp) is described by Arrhenius law. The activation energy obtained from the fitting shows increases while the characteristic relaxation time decreases with increasing mass fraction (phim) of SWCNTs. The dynamics of the composites during glass transition, at slow and high scan rates, are also the main focus of this experimental study. The change in enthalpy during heating and cooling is also reported as a function of scan rate and frequency of temperature modulation. The glass transition temperature (Tg) shows increases with increasing frequency of temperature modulation and phim of SWCNTs inside the polymer host. Experimental results show that Tg is higher at higher scan rates but as the frequency of temperature modulation increases, the Tg values of different scan rates coincide with each other and alter the scan rate dependence. From the imaginary part of heat capacity, it is obvious that Tmax is not the actual glass transition temperature of pure polymer but Tmax and Tg values can be superimposed when phim increases in the polymer host or when the sample undergoes a transition with a certain frequency of temperature modulation.
Energy Technology Data Exchange (ETDEWEB)
Unger, Isaak
2017-07-01
About a decade ago new types of electronic non-radiative relaxation processes, involving the environment of an electronically excited or ionized monomer, have been predicted for van der Waals clusters and these were also the first systems where such processes have been detected experimentally. These new autoionization channels encompass the recombination of an electron and a hole, and the energy transfer to a neighboring atom or molecule. Two processes can be distinguished here. In the intermolecular Coulombic decay (ICD) the hole created upon ionization of a monomer is filled by a valence electron of the same species, and the energy released in this electron-hole recombination is used to ionize a neighboring species. In the electron transfer mediated decay (ETMD) the initial hole is filled by an electron from a neighboring species, and the energy released by this recombination is either used to ionize the same neighbor species, or to ionize a third monomer. In more recent experiments on liquid water it has been discovered that these non- local autoionization processes are strongly coupled with ultrafast nuclear dynamics. The core ionization initiates proton motion along a hydrogen donor-bond of the electronically excited water cation. This nuclear dynamics leads to the formation of transient cationic species where a proton is shared by two neighboring water molecules. Subsequent autoionization, either via Auger decay, ICD or ETMD, then occurs from any of such structure transients. This relaxation process is termed proton transfer mediated charge separation, PTM-CS. It has been found in a number of experiments that the probability of PTM-CS to occur depends on the hydrogen-bond strength between the core-ionized molecule and solvent molecules.
Orthonormal Wavelet Bases for Quantum Molecular Dynamics
International Nuclear Information System (INIS)
Tymczak, C.; Wang, X.
1997-01-01
We report on the use of compactly supported, orthonormal wavelet bases for quantum molecular-dynamics (Car-Parrinello) algorithms. A wavelet selection scheme is developed and tested for prototypical problems, such as the three-dimensional harmonic oscillator, the hydrogen atom, and the local density approximation to atomic and molecular systems. Our method shows systematic convergence with increased grid size, along with improvement on compression rates, thereby yielding an optimal grid for self-consistent electronic structure calculations. copyright 1997 The American Physical Society
Superdomain dynamics in ferroelectric-ferroelastic films: Switching, jamming, and relaxation
Scott, J. F.; Hershkovitz, A.; Ivry, Y.; Lu, H.; Gruverman, A.; Gregg, J. M.
2017-12-01
Recent experimental work shows that ferroelectric switching can occur in large jumps in which ferroelastic superdomains switch together, rather than having the numerous smaller ferroelectric domains switch within them. In this sense, the superdomains play a role analogous to that of Abrikosov vortices in thin superconducting films under the Kosterlitz-Thouless framework, which control the dynamics more than individual Cooper pairs within them do. Here, we examine the dynamics of ferroelastic superdomains in ferroelastic ferroelectrics and their role in switching devices such as memories. Jamming of ferroelectric domains in thin films has revealed an unexpected time dependence of t-1/4 at long times (hours), but it is difficult to discriminate between power-law and exponential relaxation. Other aspects of this work, including spatial period doubling of domains, led to a description of ferroelastic domains as nonlinear processes in a viscoelastic medium, which produce folding and metastable kinetically limited states. This ¼ exponent is a surprising agreement with the well-known value of ¼ for coarsening dynamics in viscoelastic media. We try to establish a link between these two processes, hitherto considered unrelated, and with superdomains and domain bundles. We note also that high-Tc superconductors share many of the ferroelastic domain properties discussed here and that several new solar cell materials and metal-insulator transition systems are ferroelastic.
Redox-controlled backbone dynamics of human cytochrome c revealed by 15N NMR relaxation measurements
International Nuclear Information System (INIS)
Sakamoto, Koichi; Kamiya, Masakatsu; Uchida, Takeshi; Kawano, Keiichi; Ishimori, Koichiro
2010-01-01
Research highlights: → The dynamic parameters for the backbone dynamics in Cyt c were determined. → The backbone mobility of Cyt c is highly restricted due to the covalently bound heme. → The backbone mobility of Cyt c is more restricted upon the oxidation of the heme. → The redox-dependent dynamics are shown in the backbone of Cyt c. → The backbone dynamics of Cyt c would regulate the electron transfer from Cyt c. -- Abstract: Redox-controlled backbone dynamics in cytochrome c (Cyt c) were revealed by 2D 15 N NMR relaxation experiments. 15 N T 1 and T 2 values and 1 H- 15 N NOEs of uniformly 15 N-labeled reduced and oxidized Cyt c were measured, and the generalized order parameters (S 2 ), the effective correlation time for internal motion (τ e ), the 15 N exchange broadening contributions (R ex ) for each residue, and the overall correlation time (τ m ) were estimated by model-free dynamics formalism. These dynamic parameters clearly showed that the backbone dynamics of Cyt c are highly restricted due to the covalently bound heme that functions as the stable hydrophobic core. Upon oxidation of the heme iron in Cyt c, the average S 2 value was increased from 0.88 ± 0.01 to 0.92 ± 0.01, demonstrating that the mobility of the backbone is further restricted in the oxidized form. Such increases in the S 2 values were more prominent in the loop regions, including amino acid residues near the thioether bonds to the heme moiety and positively charged region around Lys87. Both of the regions are supposed to form the interaction site for cytochrome c oxidase (CcO) and the electron pathway from Cyt c to CcO. The redox-dependent mobility of the backbone in the interaction site for the electron transfer to CcO suggests an electron transfer mechanism regulated by the backbone dynamics in the Cyt c-CcO system.
Excited-state molecular photoionization dynamics
International Nuclear Information System (INIS)
Pratt, S.T.
1995-01-01
This review presents a survey of work using resonance-enhanced multiphoton ionization and double-resonance techniques to study excited-state photoionization dynamics in molecules. These techniques routinely provide detail and precision that are difficult to achieve in single-photon ionization from the ground state. The review not only emphasizes new aspects of photoionization revealed in the excited-state experiments but also shows how the excited-state techniques can provide textbook illustrations of some fundamental mechanisms in molecular photoionization dynamics. Most of the examples are confined to diatomic molecules. (author)
Towards the molecular bases of polymerase dynamics
International Nuclear Information System (INIS)
Chela Flores, J.
1991-03-01
One aspect of the strong relationship that is known to exist between the processes of DNA replication and transcription is manifest in the coupling of the rates of movement of the replication fork (r f ) and RNA polymerase (r t ). We address two issues concerning the largely unexplored area of polymerase dynamics: (i) The validity of an approximate kinematic formula linking r f and r t suggested by experiments in which transcription is initiated in some prokaryotes with the antibiotic streptolydigin, and (ii) What are the molecular bases of the kinematic formula? An analysis of the available data suggests possible molecular bases for polymerase dynamics. In particular, we are led to a hypothesis: In active chromatin r t may depend on the length (λ t ) of the transcript of the primary messenger RNA (pre-mRNA). This new effect is subject to experimental verification. We discuss possible experiments that may be performed in order to test this prediction. (author). Refs, 6 tabs
Electron-nuclear dynamics of molecular systems
International Nuclear Information System (INIS)
Diz, A.; Oehrn, Y.
1994-01-01
The content of an ab initio time-dependent theory of quantum molecular dynamics of electrons and atomic nuclei is presented. Employing the time-dependent variational principle and a family of approximate state vectors yields a set of dynamical equations approximating the time-dependent Schroedinger equation. These equations govern the time evolution of the relevant state vector parameters as molecular orbital coefficients, nuclear positions, and momenta. This approach does not impose the Born-Oppenheimer approximation, does not use potential energy surfaces, and takes into account electron-nuclear coupling. Basic conservation laws are fully obeyed. The simplest model of the theory employs a single determinantal state for the electrons and classical nuclei and is implemented in the computer code ENDyne. Results from this ab-initio theory are reported for ion-atom and ion-molecule collisions
Molecular quantum dynamics. From theory to applications
International Nuclear Information System (INIS)
Gatti, Fabien
2014-01-01
An educational and accessible introduction to the field of molecular quantum dynamics. Illustrates the importance of the topic for broad areas of science: from astrophysics and the physics of the atmosphere, over elementary processes in chemistry, to biological processes. Presents chosen examples of striking applications, highlighting success stories, summarized by the internationally renowned experts. Including a foreword by Lorenz Cederbaum (University Heidelberg, Germany). This book focuses on current applications of molecular quantum dynamics. Examples from all main subjects in the field, presented by the internationally renowned experts, illustrate the importance of the domain. Recent success in helping to understand experimental observations in fields like heterogeneous catalysis, photochemistry, reactive scattering, optical spectroscopy, or femto- and attosecond chemistry and spectroscopy underline that nuclear quantum mechanical effects affect many areas of chemical and physical research. In contrast to standard quantum chemistry calculations, where the nuclei are treated classically, molecular quantum dynamics can cover quantum mechanical effects in their motion. Many examples, ranging from fundamental to applied problems, are known today that are impacted by nuclear quantum mechanical effects, including phenomena like tunneling, zero point energy effects, or non-adiabatic transitions. Being important to correctly understand many observations in chemical, organic and biological systems, or for the understanding of molecular spectroscopy, the range of applications covered in this book comprises broad areas of science: from astrophysics and the physics and chemistry of the atmosphere, over elementary processes in chemistry, to biological processes (such as the first steps of photosynthesis or vision). Nevertheless, many researchers refrain from entering this domain. The book ''Molecular Quantum Dynamics'' offers them an accessible introduction. Although the
Molecular quantum dynamics. From theory to applications
Energy Technology Data Exchange (ETDEWEB)
Gatti, Fabien (ed.) [Montpellier 2 Univ. (France). Inst. Charles Gerhardt - CNRS 5253
2014-09-01
An educational and accessible introduction to the field of molecular quantum dynamics. Illustrates the importance of the topic for broad areas of science: from astrophysics and the physics of the atmosphere, over elementary processes in chemistry, to biological processes. Presents chosen examples of striking applications, highlighting success stories, summarized by the internationally renowned experts. Including a foreword by Lorenz Cederbaum (University Heidelberg, Germany). This book focuses on current applications of molecular quantum dynamics. Examples from all main subjects in the field, presented by the internationally renowned experts, illustrate the importance of the domain. Recent success in helping to understand experimental observations in fields like heterogeneous catalysis, photochemistry, reactive scattering, optical spectroscopy, or femto- and attosecond chemistry and spectroscopy underline that nuclear quantum mechanical effects affect many areas of chemical and physical research. In contrast to standard quantum chemistry calculations, where the nuclei are treated classically, molecular quantum dynamics can cover quantum mechanical effects in their motion. Many examples, ranging from fundamental to applied problems, are known today that are impacted by nuclear quantum mechanical effects, including phenomena like tunneling, zero point energy effects, or non-adiabatic transitions. Being important to correctly understand many observations in chemical, organic and biological systems, or for the understanding of molecular spectroscopy, the range of applications covered in this book comprises broad areas of science: from astrophysics and the physics and chemistry of the atmosphere, over elementary processes in chemistry, to biological processes (such as the first steps of photosynthesis or vision). Nevertheless, many researchers refrain from entering this domain. The book ''Molecular Quantum Dynamics'' offers them an accessible
Molecular Dynamics with Helical Periodic Boundary Conditions
Czech Academy of Sciences Publication Activity Database
Kessler, Jiří; Bouř, Petr
2014-01-01
Roč. 35, č. 21 (2014), s. 1552-1559 ISSN 0192-8651 R&D Projects: GA ČR GAP208/11/0105; GA MŠk(CZ) LH11033 Grant - others:GA AV ČR(CZ) M200551205; GA MŠk(CZ) LM2010005 Institutional support: RVO:61388963 Keywords : periodic boundary conditions * helical symmetry * molecular dynamics * protein structure * amyloid fibrils Subject RIV: CF - Physical ; Theoretical Chemistry Impact factor: 3.589, year: 2014
Molecular dynamics simulation of a chemical reaction
International Nuclear Information System (INIS)
Gorecki, J.; Gryko, J.
1988-06-01
Molecular dynamics is used to study the chemical reaction A+A→B+B. It is shown that the reaction rate constant follows the Arrhenius law both for Lennard-Jones and hard sphere interaction potentials between substrate particles. A. For the denser systems the reaction rate is proportional to the value of the radial distribution function at the contact point of two hard spheres. 10 refs, 4 figs
Nonequilibrium molecular dynamics theory, algorithms and applications
Todd, Billy D
2017-01-01
Written by two specialists with over twenty-five years of experience in the field, this valuable text presents a wide range of topics within the growing field of nonequilibrium molecular dynamics (NEMD). It introduces theories which are fundamental to the field - namely, nonequilibrium statistical mechanics and nonequilibrium thermodynamics - and provides state-of-the-art algorithms and advice for designing reliable NEMD code, as well as examining applications for both atomic and molecular fluids. It discusses homogenous and inhomogenous flows and pays considerable attention to highly confined fluids, such as nanofluidics. In addition to statistical mechanics and thermodynamics, the book covers the themes of temperature and thermodynamic fluxes and their computation, the theory and algorithms for homogenous shear and elongational flows, response theory and its applications, heat and mass transport algorithms, applications in molecular rheology, highly confined fluids (nanofluidics), the phenomenon of slip and...
In situ structure and dynamics of DNA origami determined through molecular dynamics simulations.
Yoo, Jejoong; Aksimentiev, Aleksei
2013-12-10
The DNA origami method permits folding of long single-stranded DNA into complex 3D structures with subnanometer precision. Transmission electron microscopy, atomic force microscopy, and recently cryo-EM tomography have been used to characterize the properties of such DNA origami objects, however their microscopic structures and dynamics have remained unknown. Here, we report the results of all-atom molecular dynamics simulations that characterized the structural and mechanical properties of DNA origami objects in unprecedented microscopic detail. When simulated in an aqueous environment, the structures of DNA origami objects depart from their idealized targets as a result of steric, electrostatic, and solvent-mediated forces. Whereas the global structural features of such relaxed conformations conform to the target designs, local deformations are abundant and vary in magnitude along the structures. In contrast to their free-solution conformation, the Holliday junctions in the DNA origami structures adopt a left-handed antiparallel conformation. We find the DNA origami structures undergo considerable temporal fluctuations on both local and global scales. Analysis of such structural fluctuations reveals the local mechanical properties of the DNA origami objects. The lattice type of the structures considerably affects global mechanical properties such as bending rigidity. Our study demonstrates the potential of all-atom molecular dynamics simulations to play a considerable role in future development of the DNA origami field by providing accurate, quantitative assessment of local and global structural and mechanical properties of DNA origami objects.
Lattice Distortion Mediated Paramagnetic Relaxation in High-Spin High-Symmetry Molecular Magnets
Garg, Anupam
1998-08-01
Field-dependent maxima in the relaxation rate of the magnetic molecules Mn12-Ac and Fe8-tacn have commonly been ascribed to some resonant tunneling phenomena. We argue instead that the relaxation is purely due to phonons. The rate maxima arise because of a Jahn-Teller-like distortion caused by the coupling of phonons to degenerate Zeeman levels of the molecule at the top of the barrier. The binding energy of the distorted intermediate states lowers the barrier height and increases the relaxation rate. A nonperturbative calculation of this effect is carried out for a model system. An approximate result for the field variation near a maximum is found to agree reasonably with experiment.
Molecular theory for nuclear magnetic relaxation in protein solutions and tissue
International Nuclear Information System (INIS)
Kimmich, R.; Nusser, W.; Gneiting, T.
1990-01-01
A model theory is presented explaining a series of striking phenomena observed with nuclear magnetic relaxation in protein systems such as solutions or tissue. The frequency, concentration and temperature dependences of proton or deuteron relaxation times of protein solutions and tissue are explained. It is concluded that the translational diffusion of water molecules along the rugged surfaces of proteins and, to a minor degree, protein backbone fluctuations are crucial processes. The rate limiting factor of macromolecular tumbling is assumed to be given by the free water content in a certain analogy to the free-volume model of Cohen ad Turnbull. There are two characteristic water mass fractions indicating the saturation of the hydration shells and the onset of protein tumbling. A closed and relatively simple set of relaxation formulas is presented. The potentially fractal nature of the diffusion of water molecules on the protein surface is discussed. (author). 43 refs.; 4 figs
Ottou Abe, M. T.; Correia, N. T.; Ndjaka, J. M. B.; Affouard, F.
2015-10-01
In this paper, structural and dynamical properties of ibuprofen and ketoprofen glass-forming liquids have been investigated by means of molecular dynamics simulations. Molecular mobility of both materials is analyzed with respect to the different inter-molecular linear/cyclic hydrogen bonding associations. For ibuprofen, the dominant organization is found to be composed of small hydrogen bonding aggregates corresponding to cyclic dimers through the carboxyl group. For ketoprofen, the propensity of cyclic dimers is significantly reduced by the formation of hydrogen bonds with the ketone oxygen of the molecule altering the hydrogen bond (HB) associating structures that can be formed and thus molecular dynamics. The issue of the presence/absence of the peculiar low frequency Debye-type process in dielectric relaxation spectroscopy (DRS) data in these materials is addressed. Results obtained from simulations confirm that the Debye process originates from the internal cis-trans conversion of the —COOH carboxyl group. It is shown that the specific intermolecular HB structures associated to a given profen control the main dynamical features of this conversion, in particular its separation from the α-process, which make it detectable or not from DRS. For ibuprofen, the possible role of the —CCCO torsion motion, more "local" than the —COOH motion since it is less influenced by the intermolecular HBs, is suggested in the microscopic origin of the quite intense secondary γ-relaxation process detected from DRS.
Gnanasekaran, Ramachandran; Xu, Yao; Leitner, David M
2010-12-23
Water confined in proteins exhibits dynamics distinct from the dynamics of water in the bulk or near the surface of a biomolecule. We examine the water dynamics at the interface of the two globules of the homodimeric hemoglobin from Scapharca inaequivalvis (HbI) by molecular dynamics (MD) simulations, with focus on water-protein hydrogen bond lifetimes and rotational anisotropy of the interfacial waters. We find that relaxation of the waters at the interface of both deoxy- and oxy-HbI, which contain a cluster of 17 and 11 interfacial waters, respectively, is well described by stretched exponentials with exponents from 0.1 to 0.6 and relaxation times of tens to thousands of picoseconds. The interfacial water molecules of oxy-HbI exhibit slower rotational relaxation and hydrogen bond rearrangement than those of deoxy-HbI, consistent with an allosteric transition from unliganded to liganded conformers involving the expulsion of several water molecules from the interface. Though the interfacial waters are translationally and rotationally static on the picosecond time scale, they contribute to fast communication between the globules via vibrations. We find that the interfacial waters enhance vibrational energy transport across the interface by ≈10%.
Coulomb interactions via local dynamics: a molecular-dynamics algorithm
International Nuclear Information System (INIS)
Pasichnyk, Igor; Duenweg, Burkhard
2004-01-01
We derive and describe in detail a recently proposed method for obtaining Coulomb interactions as the potential of mean force between charges which are dynamically coupled to a local electromagnetic field. We focus on the molecular dynamics version of the method and show that it is intimately related to the Car-Parrinello approach, while being equivalent to solving Maxwell's equations with a freely adjustable speed of light. Unphysical self-energies arise as a result of the lattice interpolation of charges, and are corrected by a subtraction scheme based on the exact lattice Green function. The method can be straightforwardly parallelized using standard domain decomposition. Some preliminary benchmark results are presented
Prompers, J.J.; Brüschweiler, R.
2002-01-01
A general framework is presented for the interpretation of NMR relaxation data of proteins. The method, termed isotropic reorientational eigenmode dynamics (iRED), relies on a principal component analysis of the isotropically averaged covariance matrix of the lattice functions of the spin
International Nuclear Information System (INIS)
Salmon, L.; Licinio, A.; Jensen, M.R.; Blackledge, M.; Ortega Roldan, J.L.; Van Nuland, N.; Lescop, E.
2011-01-01
We have recently presented a titration approach for the determination of residual dipolar couplings (RDCs) from experimentally inaccessible complexes. Here, we extend this approach to the measurement of 15 N spin relaxation rates and demonstrate that this can provide long-range structural, dynamic, and kinetic information about these elusive systems. (authors)
Molecular dynamics studies of superionic conductors
International Nuclear Information System (INIS)
Rahman, A.; Vashishta, P.
1983-01-01
Structural and dynamical properties of superionic conductors AgI and CuI are studied using molecular dynamics (MD) techniques. The model of these superionic conductors is based on the use of effective pair potentials. To determine the constants in these potentials, cohesive energy and bulk modulus are used as input: in addition one uses notions of ionic size based on the known crystal structure. Salient features of the MD technique are outlined. Methods of treating long range Coulomb forces are discussed in detail. This includes the manner of doing Ewald sum for MD cells of arbitrary shape. Features that can be incorporated to expedite the MD calculations are also discussed. A novel MD technique which allows for a dynamically controlled variation of the shape and size of the MD cell is described briefly. The development of this novel technique has made it possible to study structural phase transitions in superionic conductors. 68 references, 17 figures, 2 tables
Energy Technology Data Exchange (ETDEWEB)
Greene, Patrick T. [Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States); Schofield, Samuel P. [Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States); Nourgaliev, Robert [Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
2016-06-21
A new mesh smoothing method designed to cluster mesh cells near a dynamically evolving interface is presented. The method is based on weighted condition number mesh relaxation with the weight function being computed from a level set representation of the interface. The weight function is expressed as a Taylor series based discontinuous Galerkin projection, which makes the computation of the derivatives of the weight function needed during the condition number optimization process a trivial matter. For cases when a level set is not available, a fast method for generating a low-order level set from discrete cell-centered elds, such as a volume fraction or index function, is provided. Results show that the low-order level set works equally well for the weight function as the actual level set. Meshes generated for a number of interface geometries are presented, including cases with multiple level sets. Dynamic cases for moving interfaces are presented to demonstrate the method's potential usefulness to arbitrary Lagrangian Eulerian (ALE) methods.
Sappey, R; Ocio, M; Hammann, J
2000-01-01
We discuss here the nature of the low-temperature magnetic relaxation in samples of magnetic nanoparticles. In addition to usual magnetic viscosity measurement, we have used the residual memory ratio (RMR) method. This procedure enables us to overcome the uncertainties usually associated with the energy barrier distribution, thus giving a more detailed insight on the nature of the observed dynamics. A custom-made apparatus coupling dilution refrigeration and SQUID magnetometry allowed measurements of very diluted samples at temperatures ranging between 60 mK and 7 K. Two types of particles have been studied: gamma-Fe sub 2 O sub 3 of moderate anisotropy, and CoFe sub 2 O sub 4 of higher anisotropy where quantum effects are more likely to occur. In both cases, the data cannot simply be interpreted in terms of mere thermally activated dynamics of independent particles. The deviation from thermal activation seems to go opposite of what is expected from the possible effect of particle interactions. We therefore b...
International Nuclear Information System (INIS)
Mahieu, Nathalie
1992-01-01
The work reported in this research thesis aimed at characterizing micellar phases formed by some surfactants (sodium carboxylates) in aqueous solution. After some recalls on nuclear magnetic resonance dealing with spin relaxation (longitudinal relaxation, transverse relaxation, relaxation in the rotating coordinate system, and crossed relaxation), and comments on the dipolar mechanism responsible of relaxation phenomena, the author presents the methods used for relaxation parameter measurement and the data processing software issued from experiments. He presents experiments which allowed the self-diffusion coefficient to be measured, reports data processing, and addresses problems of special diffusion and of coherence transfers during diffusion measurements. Results of proton relaxation measurements are then presented and discussed. They are used to determine the micellar state of the studied carboxylates. The case of the oleate is also addressed. Measurements of carbon-13 relaxation times are reported, and exploited in terms of structural parameters by using the Relaxator software. An original method of the hetero-nuclear Overhauser method is presented, and used to assess the average distance between water molecules and micelle surface [fr
Three-stage classical molecular dynamics model for simulation of heavy-ion fusion
Directory of Open Access Journals (Sweden)
Godre Subodh S.
2015-01-01
Full Text Available A three-stage Classical Molecular Dynamics (3S-CMD approach for heavy-ion fusion is developed. In this approach the Classical Rigid-Body Dynamics simulation for heavy-ion collision involving light deformed nucleus is initiated on their Rutherford trajectories at very large initial separation. Collision simulation is then followed by relaxation of the rigid-body constrains for one or both the colliding nuclei at distances close to the barrier when the trajectories of all the nucleons are obtained in a Classical Molecular Dynamics approach. This 3S-CMD approach explicitly takes into account not only the long range Coulomb reorientation of the deformed collision partner but also the internal vibrational excitations of one or both the nuclei at distances close to the barrier. The results of the dynamical simulation for 24Mg+208Pb collision show significant modification of the fusion barrier and calculated fusion cross sections due to internal excitations.
Ultraviolet relaxation dynamics of aniline, N, N-dimethylaniline and 3,5-dimethylaniline at 250 nm
Energy Technology Data Exchange (ETDEWEB)
Thompson, James O. F.; Saalbach, Lisa; Crane, Stuart W. [Institute of Photonics and Quantum Sciences, Heriot-Watt University, Edinburgh EH14 4AS (United Kingdom); Paterson, Martin J. [Institute of Chemical Sciences, Heriot-Watt University, Edinburgh EH14 4AS (United Kingdom); Townsend, Dave, E-mail: D.Townsend@hw.ac.uk [Institute of Photonics and Quantum Sciences, Heriot-Watt University, Edinburgh EH14 4AS (United Kingdom); Institute of Chemical Sciences, Heriot-Watt University, Edinburgh EH14 4AS (United Kingdom)
2015-03-21
Time-resolved photoelectron imaging was used to investigate the electronic relaxation dynamics of gas-phase aniline, N, N-dimethylaniline, and 3,5-dimethylaniline following ultraviolet excitation at 250 nm. Our analysis was supported by ab initio coupled-cluster calculations evaluating excited state energies and (in aniline) the evolution of a range of excited state physical properties as a function of N–H bond extension. Due to a lack of consistency between several earlier studies undertaken in aniline, the specific aim of this present work was to gain new insight into the previously proposed non-adiabatic coupling interaction between the two lowest lying singlet excited states S{sub 1}(ππ{sup ∗}) and S{sub 2}(3s/πσ{sup ∗}). The methyl-substituted systems N, N-dimethylaniline and 3,5-dimethylaniline were included in order to obtain more detailed dynamical information about the key internal molecular coordinates that drive the S{sub 1}(ππ{sup ∗})/S{sub 2}(3s/πσ{sup ∗}) coupling mechanism. Our findings suggest that in all three systems, both electronic states are directly populated during the initial excitation, with the S{sub 2}(3s/πσ{sup ∗}) state then potentially decaying via either direct dissociation along the N–X stretching coordinate (X = H or CH{sub 3}) or internal conversion to the S{sub 1}(ππ{sup ∗}) state. In aniline and N, N-dimethylaniline, both pathways most likely compete in the depletion of S{sub 2}(3s/πσ{sup ∗}) state population. However, in 3,5-dimethylaniline, only the direct dissociation mechanism appears to be active. This is rationalized in terms of changes in the relative rates of the two decay pathways upon methylation of the aromatic ring system.
DEFF Research Database (Denmark)
da Graça Thrige, D; Buur, J R; Jørgensen, Flemming Steen
1997-01-01
cereus including a docked substrate molecule was subjected to a stepwise molecular mechanics energy minimization. Second, the location of the nucleophilic water molecule in the active site of the fully relaxed enzyme-substrate complex was determined by evaluation of nonbonded interaction energies between...... water molecule was verified during a 100 ps molecular dynamics simulation. During the simulation the substrate undergoes a conformational change, but retains its localization in the active site. The contacts between the enzyme, the substrate, and the nucleophilic water molecule display some fluctuations...... the strong electrostatic interactions in the active site realistically during energy minimization, delocalization of the charges from the three zinc ions was considered. Therefore, quantum mechanics calculations on the zinc ions and the zinc-coordinating residues were carried out prior to the molecular...
Cooperativity in Molecular Dynamics Structural Models and the Dielectric Spectra of 1,2-Ethanediol
Usacheva, T. M.
2018-05-01
Linear relationships are established between the experimental equilibrium correlation factor and the molecular dynamics (MD) mean value of the O-H···O bond angle and the longitudinal component of the unit vector of the mean statistical dipole moment of the cluster in liquid 1,2-ethanediol (12ED). The achievements of modern MD models in describing the experimental dispersion of the permittivity of 12ED by both continuous and discrete relaxation time spectra are analyzed. The advantage computer MD experiments have over dielectric spectroscopy for calculating relaxation time and determining the molecular diffusion mechanisms of the rearrangement of the network 12ED structure, which is more complex than water, is demonstrated.
Molecular theory of mass transfer kinetics and dynamics at gas-water interface
International Nuclear Information System (INIS)
Morita, Akihiro; Garrett, Bruce C
2008-01-01
The mass transfer mechanism across gas-water interface is studied with molecular dynamics (MD) simulation. The MD results provide a robust and qualitatively consistent picture to previous studies about microscopic aspects of mass transfer, including interface structure, free energy profiles for the uptake, scattering dynamics and energy relaxation of impinging molecules. These MD results are quantitatively compared with experimental uptake measurements, and we find that the apparent inconsistency between MD and experiment could be partly resolved by precise decomposition of the observed kinetics into elemental steps. Remaining issues and future perspectives toward constructing a comprehensive multi-scale description of interfacial mass transfer are summarized.
Molecular Dynamics: New Frontier in Personalized Medicine.
Sneha, P; Doss, C George Priya
2016-01-01
The field of drug discovery has witnessed infinite development over the last decade with the demand for discovery of novel efficient lead compounds. Although the development of novel compounds in this field has seen large failure, a breakthrough in this area might be the establishment of personalized medicine. The trend of personalized medicine has shown stupendous growth being a hot topic after the successful completion of Human Genome Project and 1000 genomes pilot project. Genomic variant such as SNPs play a vital role with respect to inter individual's disease susceptibility and drug response. Hence, identification of such genetic variants has to be performed before administration of a drug. This process requires high-end techniques to understand the complexity of the molecules which might bring an insight to understand the compounds at their molecular level. To sustenance this, field of bioinformatics plays a crucial role in revealing the molecular mechanism of the mutation and thereby designing a drug for an individual in fast and affordable manner. High-end computational methods, such as molecular dynamics (MD) simulation has proved to be a constitutive approach to detecting the minor changes associated with an SNP for better understanding of the structural and functional relationship. The parameters used in molecular dynamic simulation elucidate different properties of a macromolecule, such as protein stability and flexibility. MD along with docking analysis can reveal the synergetic effect of an SNP in protein-ligand interaction and provides a foundation for designing a particular drug molecule for an individual. This compelling application of computational power and the advent of other technologies have paved a promising way toward personalized medicine. In this in-depth review, we tried to highlight the different wings of MD toward personalized medicine. © 2016 Elsevier Inc. All rights reserved.
Weysser, F; Puertas, A M; Fuchs, M; Voigtmann, Th
2010-07-01
We analyze the slow glassy structural relaxation as measured through collective and tagged-particle density correlation functions obtained from Brownian dynamics simulations for a polydisperse system of quasi-hard spheres in the framework of the mode-coupling theory (MCT) of the glass transition. Asymptotic analyses show good agreement for the collective dynamics when polydispersity effects are taken into account in a multicomponent calculation, but qualitative disagreement at small q when the system is treated as effectively monodisperse. The origin of the different small-q behavior is attributed to the interplay between interdiffusion processes and structural relaxation. Numerical solutions of the MCT equations are obtained taking properly binned partial static structure factors from the simulations as input. Accounting for a shift in the critical density, the collective density correlation functions are well described by the theory at all densities investigated in the simulations, with quantitative agreement best around the maxima of the static structure factor and worst around its minima. A parameter-free comparison of the tagged-particle dynamics however reveals large quantitative errors for small wave numbers that are connected to the well-known decoupling of self-diffusion from structural relaxation and to dynamical heterogeneities. While deviations from MCT behavior are clearly seen in the tagged-particle quantities for densities close to and on the liquid side of the MCT glass transition, no such deviations are seen in the collective dynamics.
Parallelization of quantum molecular dynamics simulation code
International Nuclear Information System (INIS)
Kato, Kaori; Kunugi, Tomoaki; Shibahara, Masahiko; Kotake, Susumu
1998-02-01
A quantum molecular dynamics simulation code has been developed for the analysis of the thermalization of photon energies in the molecule or materials in Kansai Research Establishment. The simulation code is parallelized for both Scalar massively parallel computer (Intel Paragon XP/S75) and Vector parallel computer (Fujitsu VPP300/12). Scalable speed-up has been obtained with a distribution to processor units by division of particle group in both parallel computers. As a result of distribution to processor units not only by particle group but also by the particles calculation that is constructed with fine calculations, highly parallelization performance is achieved in Intel Paragon XP/S75. (author)
Lipid Configurations from Molecular Dynamics Simulations
DEFF Research Database (Denmark)
Pezeshkian, Weria; Khandelia, Himanshu; Marsh, Derek
2018-01-01
of dihedral angles in palmitoyl-oleoyl phosphatidylcholine from molecular dynamics simulations of hydrated fluid bilayer membranes. We compare results from the widely used lipid force field of Berger et al. with those from the most recent C36 release of the CHARMM force field for lipids. Only the CHARMM force......The extent to which current force fields faithfully reproduce conformational properties of lipids in bilayer membranes, and whether these reflect the structural principles established for phospholipids in bilayer crystals, are central to biomembrane simulations. We determine the distribution...
Molecular dynamics studies of actinide nitrides
International Nuclear Information System (INIS)
Kurosaki, Ken; Uno, Masayoshi; Yamanaka, Shinsuke; Minato, Kazuo
2004-01-01
The molecular dynamics (MD) calculation was performed for actinide nitrides (UN, NpN, and PuN) in the temperature range from 300 to 2800 K to evaluate the physical properties viz., the lattice parameter, thermal expansion coefficient, compressibility, and heat capacity. The Morse-type potential function added to the Busing-Ida type potential was employed for the ionic interactions. The interatomic potential parameters were determined by fitting to the experimental data of the lattice parameter. The usefulness and applicability of the MD method to evaluate the physical properties of actinide nitrides were studied. (author)
Viscosity calculations at molecular dynamics simulations
International Nuclear Information System (INIS)
Kirova, E M; Norman, G E
2015-01-01
Viscosity and diffusion are chosen as an example to demonstrate the universality of diagnostics methods in the molecular dynamics method. To emphasize the universality, three diverse systems are investigated, which differ from each other drastically: liquids with embedded atom method and pairwise interatomic interaction potentials and dusty plasma with a unique multiparametric interparticle interaction potential. Both the Einstein-Helfand and Green-Kubo relations are used. Such a particular process as glass transition is analysed at the simulation of the aluminium melt. The effect of the dust particle charge fluctuation is considered. The results are compared with the experimental data. (paper)
Molecular dynamics simulation of ribosome jam
Matsumoto, Shigenori
2011-09-01
We propose a coarse-grained molecular dynamics model of ribosome molecules to study the dependence of translation process on environmental parameters. We found the model exhibits traffic jam property, which is consistent with an ASEP model. We estimated the influence of the temperature and concentration of molecules on the hopping probability used in the ASEP model. Our model can also treat environmental effects on the translation process that cannot be explained by such cellular automaton models. © 2010 Elsevier B.V. All rights reserved.
Molecular dynamics of surfactant protein C
DEFF Research Database (Denmark)
Ramírez, Eunice; Santana, Alberto; Cruz, Anthony
2006-01-01
Surfactant protein C (SP-C) is a membrane-associated protein essential for normal respiration. It has been found that the alpha-helix form of SP-C can undergo, under certain conditions, a transformation from an alpha-helix to a beta-strand conformation that closely resembles amyloid fibrils, which...... are possible contributors to the pathogenesis of pulmonary alveolar proteinosis. Molecular dynamics simulations using the NAMD2 package were performed for systems containing from one to seven SP-C molecules to study their behavior in water. The results of our simulations show that unfolding of the protein...
Lerbret, Adrien; Affouard, Frédéric
2017-01-01
Water and glycerol are well-known to facilitate the structural relaxation of amorphous protein matrices. However, several studies evidenced that they may also limit fast ($\\sim$ pico-nanosecond, ps-ns) and small-amplitude ($\\sim$ \\AA ) motions of proteins, which govern their stability in freeze-dried sugar mixtures. To determine how they interact with proteins and sugars in glassy matrices and, thereby, modulate their fast dynamics, we performed molecular dynamics (MD) simulations of lysozyme...
Energy Technology Data Exchange (ETDEWEB)
Bousige, Colin; Boţan, Alexandru; Coasne, Benoît, E-mail: coasne@mit.edu [Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139 (United States); UMI 3466 CNRS-MIT, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139 (United States); Ulm, Franz-Josef [Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139 (United States); Pellenq, Roland J.-M. [Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139 (United States); UMI 3466 CNRS-MIT, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139 (United States); CINaM, CNRS/Aix Marseille Université, Campus de Luminy, 13288 Marseille Cedex 09 (France)
2015-03-21
We report an efficient atom-scale reconstruction method that consists of combining the Hybrid Reverse Monte Carlo algorithm (HRMC) with Molecular Dynamics (MD) in the framework of a simulated annealing technique. In the spirit of the experimentally constrained molecular relaxation technique [Biswas et al., Phys. Rev. B 69, 195207 (2004)], this modified procedure offers a refined strategy in the field of reconstruction techniques, with special interest for heterogeneous and disordered solids such as amorphous porous materials. While the HRMC method generates physical structures, thanks to the use of energy penalties, the combination with MD makes the method at least one order of magnitude faster than HRMC simulations to obtain structures of similar quality. Furthermore, in order to ensure the transferability of this technique, we provide rational arguments to select the various input parameters such as the relative weight ω of the energy penalty with respect to the structure optimization. By applying the method to disordered porous carbons, we show that adsorption properties provide data to test the global texture of the reconstructed sample but are only weakly sensitive to the presence of defects. In contrast, the vibrational properties such as the phonon density of states are found to be very sensitive to the local structure of the sample.
McCune, Matthew; Kosztin, Ioan
2013-03-01
Cellular Particle Dynamics (CPD) is a theoretical-computational-experimental framework for describing and predicting the time evolution of biomechanical relaxation processes of multi-cellular systems, such as fusion, sorting and compression. In CPD, cells are modeled as an ensemble of cellular particles (CPs) that interact via short range contact interactions, characterized by an attractive (adhesive interaction) and a repulsive (excluded volume interaction) component. The time evolution of the spatial conformation of the multicellular system is determined by following the trajectories of all CPs through numerical integration of their equations of motion. Here we present CPD simulation results for the fusion of both spherical and cylindrical multi-cellular aggregates. First, we calibrate the relevant CPD model parameters for a given cell type by comparing the CPD simulation results for the fusion of two spherical aggregates to the corresponding experimental results. Next, CPD simulations are used to predict the time evolution of the fusion of cylindrical aggregates. The latter is relevant for the formation of tubular multi-cellular structures (i.e., primitive blood vessels) created by the novel bioprinting technology. Work supported by NSF [PHY-0957914]. Computer time provided by the University of Missouri Bioinformatics Consortium.
Relaxation Dynamics of the Glass Transition in PMMA+SWCNT Composites by Temperature-Modulated DSC
Pradhan, Nihar; Iannacchione, Germano
2010-03-01
Temperature Modulated Differential Scanning Calorimeter (TMDSC) used to investigate the thermal relaxation dynamics of PMMA-Single wall carbon nanotubes (SWCNTs) through the glass transition as a function of frequency. A strong dependence of the temperature dependence peak in imaginary part of complex heat capacity (Tmax) was found during the transition from glass like to liquid like region and can be described by Arhenius law. The activation energy shows increases while the charactersistic time decreases with increasing mass fraction (φm) of SWCNTs. Decreasing of enthalpy, while heating and slowly increasing while cooling at 2.0 K/min scan rate was observed and as frequency of temperature modulation increases. There is no relative change of enthalpy in heating and cooling observed at sufficiently slow scan rate. The glass transition temperature (Tg) shows increases as a function of frequency of temperature modulation, φm of SWCNTs and with increasing scan rate. From imaginary part of heat capacity, it obvious that Tmax is not the actual glass transition temperature of pure polymer but Tmax and Tg values can be superimpose when φm of SWCNT increases in polymer.
Little evidence for dynamic divergences in ultraviscous molecular liquids
DEFF Research Database (Denmark)
Hecksher, Tina; Nielsen, Albena; Olsen, Niels Boye
2008-01-01
The physics of the ultraviscous liquid phase preceding glass formation continues to pose major problems that remain unsolved. It is actively debated, for instance, whether the marked increase of the relaxation time reflects an underlying phase transition to a state of infinite relaxation time....... To elucidate the empirical evidence for this intriguing scenario, some of the most accurate relaxationtime data available for any class of ultraviscous liquids-those obtained by dielectric relaxation experiments on organic liquids just above the glass transition-were compiled. Analysis of data for 42 liquids...... shows that there is no compelling evidence for the Vogel-Fulcher-Tammann (VFT) prediction that the relaxation time diverges at a finite temperature.We conclude that theories with a dynamic divergence of the VFT formlack a direct experimental basis....
Molecular dynamics in high electric fields
International Nuclear Information System (INIS)
Apostol, M.; Cune, L.C.
2016-01-01
Highlights: • New method for rotation molecular spectra in high electric fields. • Parametric resonances – new features in spectra. • New elementary excitations in polar solids from dipolar interaction (“dipolons”). • Discussion about a possible origin of the ferroelectricity from dipolar interactions. - Abstract: Molecular rotation spectra, generated by the coupling of the molecular electric-dipole moments to an external time-dependent electric field, are discussed in a few particular conditions which can be of some experimental interest. First, the spherical-pendulum molecular model is reviewed, with the aim of introducing an approximate method which consists in the separation of the azimuthal and zenithal motions. Second, rotation spectra are considered in the presence of a static electric field. Two particular cases are analyzed, corresponding to strong and weak fields. In both cases the classical motion of the dipoles consists of rotations and vibrations about equilibrium positions; this motion may exhibit parametric resonances. For strong fields a large macroscopic electric polarization may appear. This situation may be relevant for polar matter (like pyroelectrics, ferroelectrics), or for heavy impurities embedded in a polar solid. The dipolar interaction is analyzed in polar condensed matter, where it is shown that new polarization modes appear for a spontaneous macroscopic electric polarization (these modes are tentatively called “dipolons”); one of the polarization modes is related to parametric resonances. The extension of these considerations to magnetic dipoles is briefly discussed. The treatment is extended to strong electric fields which oscillate with a high frequency, as those provided by high-power lasers. It is shown that the effect of such fields on molecular dynamics is governed by a much weaker, effective, renormalized, static electric field.
Quantum dynamics of crystals of molecular magnets inside microwave resonators
Energy Technology Data Exchange (ETDEWEB)
Amigo, R.; Tejada, J.; Chudnovsky, E.M.; Hernandez, J.M.; Garcia-Santiago, A. E-mail: antonio@ubxlab.comtoni@ubxlab.com
2004-05-01
It is shown that crystals of molecular nanomagnets exhibit enhanced magnetic relaxation when placed inside a resonant cavity. Strong dependence of the magnetization curve on the geometry of the cavity has been observed, providing evidence of the coherent microwave radiation by the crystals. These observations open the possibility of building a nanomagnetic microwave laser pumped by the magnetic field.
Quantum dynamics of crystals of molecular magnets inside microwave resonators
International Nuclear Information System (INIS)
Amigo, R.; Tejada, J.; Chudnovsky, E.M.; Hernandez, J.M.; Garcia-Santiago, A.
2004-01-01
It is shown that crystals of molecular nanomagnets exhibit enhanced magnetic relaxation when placed inside a resonant cavity. Strong dependence of the magnetization curve on the geometry of the cavity has been observed, providing evidence of the coherent microwave radiation by the crystals. These observations open the possibility of building a nanomagnetic microwave laser pumped by the magnetic field
Directory of Open Access Journals (Sweden)
Takashi Arima
2018-04-01
Full Text Available After summarizing the present status of Rational Extended Thermodynamics (RET of gases, which is an endeavor to generalize the Navier–Stokes and Fourier (NSF theory of viscous heat-conducting fluids, we develop the molecular RET theory of rarefied polyatomic gases with 15 independent fields. The theory is justified, at mesoscopic level, by a generalized Boltzmann equation in which the distribution function depends on two internal variables that take into account the energy exchange among the different molecular modes of a gas, that is, translational, rotational, and vibrational modes. By adopting the generalized Bhatnagar, Gross and Krook (BGK-type collision term, we derive explicitly the closed system of field equations with the use of the Maximum Entropy Principle (MEP. The NSF theory is derived from the RET theory as a limiting case of small relaxation times via the Maxwellian iteration. The relaxation times introduced in the theory are shown to be related to the shear and bulk viscosities and heat conductivity.
Classical molecular dynamics simulation of nuclear fuels
International Nuclear Information System (INIS)
Devanathan, R.; Krack, M.; Bertolus, M.
2015-01-01
Molecular dynamics simulation using forces calculated from empirical potentials, commonly called classical molecular dynamics, is well suited to study primary damage production by irradiation, defect interactions with fission gas atoms, gas bubble nucleation, grain boundary effects on defect and gas bubble evolution in nuclear fuel, and the resulting changes in thermomechanical properties. This enables one to obtain insights into fundamental mechanisms governing the behaviour of nuclear fuel, as well as parameters that can be used as inputs for mesoscale models. The interaction potentials used for the force calculations are generated by fitting properties of interest to experimental data and electronic structure calculations (see Chapter 12). We present here the different types of potentials currently available for UO 2 and illustrations of applications to the description of the behaviour of this material under irradiation. The results obtained from the present generation of potentials for UO 2 are qualitatively similar, but quantitatively different. There is a need to refine these existing potentials to provide a better representation of the performance of polycrystalline fuel under a variety of operating conditions, develop models that are equipped to handle deviations from stoichiometry, and validate the models and assumptions used. (authors)
Molecular dynamics simulation study of nonconcatenated ring polymers in a melt. II. Dynamics
Halverson, Jonathan D.; Lee, Won Bo; Grest, Gary S.; Grosberg, Alexander Y.; Kremer, Kurt
2011-05-01
Molecular dynamics simulations were conducted to investigate the dynamic properties of melts of nonconcatenated ring polymers and compared to melts of linear polymers. The longest rings were composed of N = 1600 monomers per chain which corresponds to roughly 57 entanglement lengths for comparable linear polymers. The ring melts were found to diffuse faster than their linear counterparts, with both architectures approximately obeying a D ˜ N-2.4 scaling law for large N. The mean-square displacement of the center-of-mass of the rings follows a sub-diffusive behavior for times and distances beyond the ring extension , neither compatible with the Rouse nor the reptation model. The rings relax stress much faster than linear polymers, and the zero-shear viscosity was found to vary as η0 ˜ N1.4 ± 0.2 which is much weaker than the N3.4 behavior of linear chains, not matching any commonly known model for polymer dynamics when compared to the observed mean-square displacements. These findings are discussed in view of the conformational properties of the rings presented in the preceding paper [J. D. Halverson, W. Lee, G. S. Grest, A. Y. Grosberg, and K. Kremer, J. Chem. Phys. 134, 204904 (2011)], 10.1063/1.3587137.
Dynamic relaxation processes in compressible multiphase flows. Application to evaporation phenomena
Directory of Open Access Journals (Sweden)
Le Métayer O.
2013-07-01
Full Text Available Phase changes and heat exchanges are examples of physical processes appearing in many industrial applications involving multiphase compressible flows. Their knowledge is of fundamental importance to reproduce correctly the resulting effects in simulation tools. A fine description of the flow topology is thus required to obtain the interfacial area between phases. This one is responsible for the dynamics and the kinetics of heat and mass transfer when evaporation or condensation occurs. Unfortunately this exchange area cannot be obtained easily and accurately especially when complex mixtures (drops, bubbles, pockets of very different sizes appear inside the transient medium. The natural way to solve this specific trouble consists in using a thin grid to capture interfaces at all spatial scales. But this possibility needs huge computing resources and can be hardly used when considering physical systems of large dimensions. A realistic method is to consider instantaneous exchanges between phases by the way of additional source terms in a full non-equilibrium multiphase flow model [2,15,17]. In this one each phase obeys its own equation of state and has its own set of equations and variables (pressure, temperature, velocity, energy, entropy,.... When enabling the relaxation source terms the multiphase mixture instantaneously tends towards a mechanical or thermodynamic equilibrium state at each point of the flow. This strategy allows to mark the boundaries of the real flow behavior and to magnify the dominant physical effects (heat exchanges, evaporation, drag,... inside the medium. A description of the various relaxation processes is given in the paper. Les changements de phase et les transferts de chaleur sont des exemples de phénomènes physiques présents dans de nombreuses applications industrielles faisant intervenir des écoulements compressibles multiphasiques. La connaissance des mécanismes associés est primordiale afin de reproduire
The 2011 Dynamics of Molecular Collisions Conference
Energy Technology Data Exchange (ETDEWEB)
Nesbitt, David J. [JILA, NIST
2011-07-11
The Dynamics of Molecular Collisions Conference focuses on all aspects of molecular collisions--experimental & theoretical studies of elastic, inelastic, & reactive encounters involving atoms, molecules, ions, clusters, & surfaces--as well as half collisions--photodissociation, photo-induced reaction, & photodesorption. The scientific program for the meeting in 2011 included exciting advances in both the core & multidisciplinary forefronts of the study of molecular collision processes. Following the format of the 2009 meeting, we also invited sessions in special topics that involve interfacial dynamics, novel emerging spectroscopies, chemical dynamics in atmospheric, combustion & interstellar environments, as well as a session devoted to theoretical & experimental advances in ultracold molecular samples. Researchers working inside & outside the traditional core topics of the meeting are encouraged to join the conference. We invite contributions of work that seeks understanding of how inter & intra-molecular forces determine the dynamics of the phenomena under study. In addition to invited oral sessions & contributed poster sessions, the scientific program included a formal session consisting of five contributed talks selected from the submitted poster abstracts. The DMC has distinguished itself by having the Herschbach Medal Symposium as part of the meeting format. This tradition of the Herschbach Medal was first started in the 2007 meeting chaired by David Chandler, based on a generous donation of funds & artwork design by Professor Dudley Herschbach himself. There are two such awards made, one for experimental & one for theoretical contributions to the field of Molecular Collision Dynamics, broadly defined. The symposium is always held on the last night of the meeting & has the awardees are asked to deliver an invited lecture on their work. The 2011 Herschbach Medal was dedicated to the contributions of two long standing leaders in Chemical Physics, Professor
Molecular structures and intramolecular dynamics of pentahalides
Ischenko, A. A.
2017-03-01
This paper reviews advances of modern gas electron diffraction (GED) method combined with high-resolution spectroscopy and quantum chemical calculations in studies of the impact of intramolecular dynamics in free molecules of pentahalides. Some recently developed approaches to the electron diffraction data interpretation, based on direct incorporation of the adiabatic potential energy surface parameters to the diffraction intensity are described. In this way, complementary data of different experimental and computational methods can be directly combined for solving problems of the molecular structure and its dynamics. The possibility to evaluate some important parameters of the adiabatic potential energy surface - barriers to pseudorotation and saddle point of intermediate configuration from diffraction intensities in solving the inverse GED problem is demonstrated on several examples. With increasing accuracy of the electron diffraction intensities and the development of the theoretical background of electron scattering and data interpretation, it has become possible to investigate complex nuclear dynamics in fluxional systems by the GED method. Results of other research groups are also included in the discussion.
Nuclear relaxation study of the spin dynamics in a one-dimensional Heisenberg system, TMMC
International Nuclear Information System (INIS)
Bakheit, M.A.
1974-01-01
Changes in the nuclear relaxation time as a function of the magnetic field intensity in TMMC are very different wether the field direction is parallel or perpendicular to the direction of the exchange chains (vector c). In parallel field, the relaxation probability increases as the field decreases. The process of spin diffusion in a one-dimensional system is well illustrated by the changes experimentally observed. In perpendicular field, the relaxation probability is constant as far as H 0 >2kG, it clearly decreases for H 0 [fr
Dynamic stress relaxation due to cyclic variation of strain at elevated temperature
International Nuclear Information System (INIS)
Suzuki, F.
1975-01-01
The relaxation of stress which occurs when low amplitude alternating strains are superimposed on constant mean total strains is studied in this paper. Experiments were carried out on a 0.16 per cent carbon steel and an AISI 347 stainless steel at 450 0 C and 650 0 C respectively in which the decrease of axial mean stress was measured as a function of time. When even a low amplitude alternating strain was applied, the rate of stress relaxation was observed to increase. Analytical predictions based on creep and static relaxation data show fairly good agreement with experiments in the period corresponding to transient creep. (author)
International Nuclear Information System (INIS)
Giugni, A; Cunsolo, A
2006-01-01
We describe an experimental study of the dynamic structure factor of liquid glycerol performed by complementary inelastic techniques such as Brillouin visible, ultraviolet and x-ray scattering. The spectra have been collected as a function of both temperature and momentum transfer. The relevant hypersonic parameters are evaluated from the spectral lineshape analysis modelling the data with a simple hydrodynamic profile. The study of their frequency dependence allows us to observe the occurrence of an active structural relaxation and to measure the related timescale. We also find signatures of further relaxation processes occurring below the accessible frequency window. As a result, the dynamic window traditionally probed in spectroscopic experiments is greatly extended and partially bridges the gap between MHz and THz techniques
A molecular dynamics study of energetic particle bombardment on diamond
International Nuclear Information System (INIS)
Li Rongbin; Dai Yongbing; Hu Xiaojun; Shen Hesheng; He Xianchang
2003-01-01
Molecular dynamic simulations, utilizing the Tersoff many-body potential, are used to investigate the microscopic processes of a single boron atom with an energy of 500 eV implanted into the diamond (001) 2 x 1 reconstructed surface. By calculating the variation of the mean coordination number with time, the lifetime of a thermal spike created by B bombardment is about 0.18 ps. Formation of the split-interstitial composed of projectile and lattice atom (B-C) is observed. The total potential energy of the system decreases about 0.56 eV with a stable B split-interstitial existing in diamond. Lattice relaxations in the diamond (001) 2 x 1 reconstructed surface or near surface of the simulated have been discussed, and the results show that the outermost layer atoms tend to move inward and other atoms move outward, while the interplanar distance between the outermost layer and the second layer has been shortened by 15%, compared with its starting interplanar distance. Stress distribution in the calculated diamond configuration is inhomogeneous. After boron implanted into diamond with an energy of 500 eV, there is an excess of compressively stressed atoms in the lattice, which induces the total stress being compressive
Stresses and elastic constants of crystalline sodium, from molecular dynamics
International Nuclear Information System (INIS)
Schiferl, S.K.
1985-02-01
The stresses and the elastic constants of bcc sodium are calculated by molecular dynamics (MD) for temperatures to T = 340K. The total adiabatic potential of a system of sodium atoms is represented by pseudopotential model. The resulting expression has two terms: a large, strictly volume-dependent potential, plus a sum over ion pairs of a small, volume-dependent two-body potential. The stresses and the elastic constants are given as strain derivatives of the Helmholtz free energy. The resulting expressions involve canonical ensemble averages (and fluctuation averages) of the position and volume derivatives of the potential. An ensemble correction relates the results to MD equilibrium averages. Evaluation of the potential and its derivatives requires the calculation of integrals with infinite upper limits of integration, and integrand singularities. Methods for calculating these integrals and estimating the effects of integration errors are developed. A method is given for choosing initial conditions that relax quickly to a desired equilibrium state. Statistical methods developed earlier for MD data are extended to evaluate uncertainties in fluctuation averages, and to test for symmetry. 45 refs., 10 figs., 4 tabs
Energy Technology Data Exchange (ETDEWEB)
Glowacki, David R., E-mail: drglowacki@gmail.com [School of Chemistry, University of Bristol, Bristol BS8 1TS (United Kingdom); Department of Computer Science, University of Bristol, Bristol BS8 1UB (United Kingdom); PULSE Institute and Department of Chemistry, Stanford University, Stanford, California 94305 (United States); SLAC National Accelerator Laboratory, Menlo Park, California 94025 (United States); Orr-Ewing, Andrew J. [School of Chemistry, University of Bristol, Bristol BS8 1TS (United Kingdom); Harvey, Jeremy N. [Department of Chemistry, KU Leuven, Celestijnenlaan 200F, B-3001 Heverlee (Belgium)
2015-07-28
We describe a parallelized linear-scaling computational framework developed to implement arbitrarily large multi-state empirical valence bond (MS-EVB) calculations within CHARMM and TINKER. Forces are obtained using the Hellmann-Feynman relationship, giving continuous gradients, and good energy conservation. Utilizing multi-dimensional Gaussian coupling elements fit to explicitly correlated coupled cluster theory, we built a 64-state MS-EVB model designed to study the F + CD{sub 3}CN → DF + CD{sub 2}CN reaction in CD{sub 3}CN solvent (recently reported in Dunning et al. [Science 347(6221), 530 (2015)]). This approach allows us to build a reactive potential energy surface whose balanced accuracy and efficiency considerably surpass what we could achieve otherwise. We ran molecular dynamics simulations to examine a range of observables which follow in the wake of the reactive event: energy deposition in the nascent reaction products, vibrational relaxation rates of excited DF in CD{sub 3}CN solvent, equilibrium power spectra of DF in CD{sub 3}CN, and time dependent spectral shifts associated with relaxation of the nascent DF. Many of our results are in good agreement with time-resolved experimental observations, providing evidence for the accuracy of our MS-EVB framework in treating both the solute and solute/solvent interactions. The simulations provide additional insight into the dynamics at sub-picosecond time scales that are difficult to resolve experimentally. In particular, the simulations show that (immediately following deuterium abstraction) the nascent DF finds itself in a non-equilibrium regime in two different respects: (1) it is highly vibrationally excited, with ∼23 kcal mol{sup −1} localized in the stretch and (2) its post-reaction solvation environment, in which it is not yet hydrogen-bonded to CD{sub 3}CN solvent molecules, is intermediate between the non-interacting gas-phase limit and the solution-phase equilibrium limit. Vibrational
Glowacki, David R; Orr-Ewing, Andrew J; Harvey, Jeremy N
2015-07-28
We describe a parallelized linear-scaling computational framework developed to implement arbitrarily large multi-state empirical valence bond (MS-EVB) calculations within CHARMM and TINKER. Forces are obtained using the Hellmann-Feynman relationship, giving continuous gradients, and good energy conservation. Utilizing multi-dimensional Gaussian coupling elements fit to explicitly correlated coupled cluster theory, we built a 64-state MS-EVB model designed to study the F + CD3CN → DF + CD2CN reaction in CD3CN solvent (recently reported in Dunning et al. [Science 347(6221), 530 (2015)]). This approach allows us to build a reactive potential energy surface whose balanced accuracy and efficiency considerably surpass what we could achieve otherwise. We ran molecular dynamics simulations to examine a range of observables which follow in the wake of the reactive event: energy deposition in the nascent reaction products, vibrational relaxation rates of excited DF in CD3CN solvent, equilibrium power spectra of DF in CD3CN, and time dependent spectral shifts associated with relaxation of the nascent DF. Many of our results are in good agreement with time-resolved experimental observations, providing evidence for the accuracy of our MS-EVB framework in treating both the solute and solute/solvent interactions. The simulations provide additional insight into the dynamics at sub-picosecond time scales that are difficult to resolve experimentally. In particular, the simulations show that (immediately following deuterium abstraction) the nascent DF finds itself in a non-equilibrium regime in two different respects: (1) it is highly vibrationally excited, with ∼23 kcal mol(-1) localized in the stretch and (2) its post-reaction solvation environment, in which it is not yet hydrogen-bonded to CD3CN solvent molecules, is intermediate between the non-interacting gas-phase limit and the solution-phase equilibrium limit. Vibrational relaxation of the nascent DF results in a spectral
Comparison of single-ion molecular dynamics in common solvents
Muralidharan, A.; Pratt, L. R.; Chaudhari, M. I.; Rempe, S. B.
2018-06-01
Laying a basis for molecularly specific theory for the mobilities of ions in solutions of practical interest, we report a broad survey of velocity autocorrelation functions (VACFs) of Li+ and PF6- ions in water, ethylene carbonate, propylene carbonate, and acetonitrile solutions. We extract the memory function, γ(t), which characterizes the random forces governing the mobilities of ions. We provide comparisons controlling for the effects of electrolyte concentration and ion-pairing, van der Waals attractive interactions, and solvent molecular characteristics. For the heavier ion (PF6-), velocity relaxations are all similar: negative tail relaxations for the VACF and a clear second relaxation for γ (t ), observed previously also for other molecular ions and with n-pentanol as the solvent. For the light Li+ ion, short time-scale oscillatory behavior masks simple, longer time-scale relaxation of γ (t ). But the corresponding analysis of the solventberg Li+(H2O)4 does conform to the standard picture set by all the PF6- results.
A Coupling Tool for Parallel Molecular Dynamics-Continuum Simulations
Neumann, Philipp; Tchipev, Nikola
2012-01-01
We present a tool for coupling Molecular Dynamics and continuum solvers. It is written in C++ and is meant to support the developers of hybrid molecular - continuum simulations in terms of both realisation of the respective coupling algorithm
Backbone dynamics of the EIAV-Tat protein from 15N relaxation studies
International Nuclear Information System (INIS)
Ejchart, A.; Herrmann, F.; Roesch, P.; Sticht, H.; Willbold, D.
1994-01-01
The work investigates the mobility of EIAV-Tat protein backbone by measuring the relaxation parameters of the 15 N nitrogens. High degree of the flexibility, non-typical of rigid, well structured proteins was shown
Ngai, K. L.; Paluch, M.
2017-12-01
Successful thermodynamic scaling of the structural alpha-relaxation time or transport coefficients of glass-forming liquids determined at various temperatures T and pressures P means the data conform to a single function of the product variable TVgamma, where V is the specific volume and gamma is a material specific constant. In the past two decades we have witnessed successful TVgamma-scaling in many molecular, polymeric, and even metallic glass-formers, and gamma is related to the slope of the repulsive part of the intermolecular potential. The advances made indicate TVgamma-scaling is an important aspect of the dynamic and thermodynamic properties of glass-formers. In this paper we show the origin of TVgamma-scaling is not from the structural alpha-relaxation time. Instead it comes from its precursor, the Johari-Goldstein beta-relaxation or the primitive relaxation of the Coupling Model and their relaxation times or tau_0 respectively. It is remarkable that all relaxation times are functions of TVgamma with the same gama, as well as the fractional exponent of the Kohlrausch correlation function of the structural alpha-relaxation. We arrive at this conclusion convincingly based on corroborative evidences from a number of experiments and molecular dynamics simulations performed on a wide variety of glass-formers and in conjunction with consistency with the predictions of the Coupling Model.
Huynh, Tan Vu; Messinger, Robert J.; Sarou-Kanian, Vincent; Fayon, Franck; Bouchet, Renaud; Deschamps, Michaël
2017-10-01
The intrinsic ionic conductivity of polyethylene oxide (PEO)-based block copolymer electrolytes is often assumed to be identical to the conductivity of the PEO homopolymer. Here, we use high-field 7Li nuclear magnetic resonance (NMR) relaxation and pulsed-field-gradient (PFG) NMR diffusion measurements to probe lithium ion dynamics over nanosecond and millisecond time scales in PEO and polystyrene (PS)-b-PEO-b-PS electrolytes containing the lithium salt LiTFSI. Variable-temperature longitudinal (T1) and transverse (T2) 7Li NMR relaxation rates were acquired at three magnetic field strengths and quantitatively analyzed for the first time at such fields, enabling us to distinguish two characteristic time scales that describe fluctuations of the 7Li nuclear electric quadrupolar interaction. Fast lithium motions [up to O (ns)] are essentially identical between the two polymer electrolytes, including sub-nanosecond vibrations and local fluctuations of the coordination polyhedra between lithium and nearby oxygen atoms. However, lithium dynamics over longer time scales [O (10 ns) and greater] are slower in the block copolymer compared to the homopolymer, as manifested experimentally by their different transverse 7Li NMR relaxation rates. Restricted dynamics and altered thermodynamic behavior of PEO chains anchored near PS domains likely explain these results.
Molecular dynamics simulation of laser shock phenomena
Energy Technology Data Exchange (ETDEWEB)
Fukumoto, Ichirou [Japan Atomic Energy Research Inst., Kansai Research Establishment, Advanced Photon Research Center, Neyagawa, Osaka (Japan).
2001-10-01
Recently, ultrashort-pulse lasers with high peak power have been developed, and their application to materials processing is expected as a tool of precision microfabrication. When a high power laser irradiates, a shock wave propagates into the material and dislocations are generated. In this paper, laser shock phenomena of the metal were analyzed using the modified molecular dynamics method, which has been developed by Ohmura and Fukumoto. The main results obtained are summarized as follows: (1) The shock wave induced by the Gaussian beam irradiation propagates radially from the surface to the interior. (2) A lot of dislocations are generated at the solid-liquid interface by the propagation of a shock wave. (3) Some dislocations are moved instantaneously with the velocity of the longitudinal wave when the shock wave passes, and their velocity is not larger than the transverse velocity after the shock wave has passed. (author)
Molecular dynamic simulation study of molten cesium
Directory of Open Access Journals (Sweden)
Yeganegi Saeid
2017-01-01
Full Text Available Molecular dynamics simulations were performed to study thermodynamics and structural properties of expanded caesium fluid. Internal pressure, radial distribution functions (RDFs, coordination numbers and diffusion coefficients have been calculated at temperature range 700–1600 K and pressure range 100–800 bar. We used the internal pressure to predict the metal–non-metal transition occurrence region. RDFs were calculated at wide ranges of temperature and pressure. The coordination numbers decrease and positions of the first peak of RDFs slightly increase as the temperature increases and pressure decreases. The calculated self-diffusion coefficients at various temperatures and pressures show no distinct boundary between Cs metallic fluid and its expanded fluid where it continuously increases with temperature.
Nonequilibrium molecular dynamics: The first 25 years
International Nuclear Information System (INIS)
Hoover, W.G.
1992-08-01
Equilibrium Molecular Dynamics has been generalized to simulate Nonequilibrium systems by adding sources of thermodynamic heat and work. This generalization incorporates microscopic mechanical definitions of macroscopic thermodynamic and hydrodynamic variables, such as temperature and stress, and augments atomistic forces with special boundary, constraint, and driving forces capable of doing work on, and exchanging heat with, an otherwise Newtonian system. The underlying Lyapunov instability of these nonequilibrium equations of motion links microscopic time-reversible deterministic trajectories to macroscopic time-irreversible hydrodynamic behavior as described by the Second Law of Thermodynamics. Green-Kubo linear-response theory has been checked. Nonlinear plastic deformation, intense heat conduction, shockwave propagation, and nonequilibrium phase transformation have all been simulated. The nonequilibrium techniques, coupled with qualitative improvements in parallel computer hardware, are enabling simulations to approximate real-world microscale and nanoscale experiments
Dynamics and Thermodynamics of Transthyretin Association from Molecular Dynamics Simulations
Directory of Open Access Journals (Sweden)
Cedrix J. Dongmo Foumthuim
2018-01-01
Full Text Available Molecular dynamics simulations are used in this work to probe the structural stability and the dynamics of engineered mutants of transthyretin (TTR, i.e., the double mutant F87M/L110M (MT-TTR and the triple mutant F87M/L110M/S117E (3M-TTR, in relation to wild-type. Free energy analysis from end-point simulations and statistical effective energy functions are used to analyze trajectories, revealing that mutations do not have major impact on protein structure but rather on protein association, shifting the equilibria towards dissociated species. The result is confirmed by the analysis of 3M-TTR which shows dissociation within the first 10 ns of the simulation, indicating that contacts are lost at the dimer-dimer interface, whereas dimers (formed by monomers which pair to form two extended β-sheets appear fairly stable. Overall the simulations provide a detailed view of the dynamics and thermodynamics of wild-type and mutant transthyretins and a rationale of the observed effects.
International Nuclear Information System (INIS)
Beretta, Gian Paolo
2006-01-01
We discuss a nonlinear model for relaxation by energy redistribution within an isolated, closed system composed of noninteracting identical particles with energy levels e i with i=1,2,...,N. The time-dependent occupation probabilities p i (t) are assumed to obey the nonlinear rate equations τ dp i /dt=-p i ln p i -α(t)p i -β(t)e i p i where α(t) and β(t) are functionals of the p i (t)'s that maintain invariant the mean energy E=Σ i=1 N e i p i (t) and the normalization condition 1=Σ i=1 N p i (t). The entropy S(t)=-k B Σ i=1 N p i (t)ln p i (t) is a nondecreasing function of time until the initially nonzero occupation probabilities reach a Boltzmann-like canonical distribution over the occupied energy eigenstates. Initially zero occupation probabilities, instead, remain zero at all times. The solutions p i (t) of the rate equations are unique and well defined for arbitrary initial conditions p i (0) and for all times. The existence and uniqueness both forward and backward in time allows the reconstruction of the ancestral or primordial lowest entropy state. By casting the rate equations in terms not of the p i 's but of their positive square roots √(p i ), they unfold from the assumption that time evolution is at all times along the local direction of steepest entropy ascent or, equivalently, of maximal entropy generation. These rate equations have the same mathematical structure and basic features as the nonlinear dynamical equation proposed in a series of papers ending with G. P. Beretta, Found. Phys. 17, 365 (1987) and recently rediscovered by S. Gheorghiu-Svirschevski [Phys. Rev. A 63, 022105 (2001);63, 054102 (2001)]. Numerical results illustrate the features of the dynamics and the differences from the rate equations recently considered for the same problem by M. Lemanska and Z. Jaeger [Physica D 170, 72 (2002)]. We also interpret the functionals k B α(t) and k B β(t) as nonequilibrium generalizations of the thermodynamic-equilibrium Massieu
Tsukushi, I.; Yamamuro, O.; Matsuo, T.
The organic crystals of tri-O-methyl-β-cyclodextrin (TMCD) and its three clathrate compounds containing benzoic acid (BA), p-nitrobenzoic acid (NBA) and p-hydroxybenzoic acid (HBA), sucrose (SUC), salicin (SAL), phenolphthalein (PP), 1,3,5-tri-α-naphthylbenzene (TNB) were amorphized by milling with a vibrating mill for 2 ˜ 16 hours at room temperature. The amorphization was checked by differential scanning calorimetry (DSC) and X-ray powder diffraction. The heat capacities of crystals, liquid quenched glasses (LQG), and mechanically-milled amorphous solid (MMAS) of TMCD and TNB were measured with an adiabatic calorimeter in the temperature range between 12 and 375 K. For both compounds, the enthalpy relaxation of MMAS appeared in the wide temperature range below Tg and the released configurational enthalpy was much larger than that of LQG, indicating that MMAS is more disordered and strained than LQG.
Vibrational and Rotational Energy Relaxation in Liquids
DEFF Research Database (Denmark)
Petersen, Jakob
Vibrational and rotational energy relaxation in liquids are studied by means of computer simulations. As a precursor for studying vibrational energy relaxation of a solute molecule subsequent to the formation of a chemical bond, the validity of the classical Bersohn-Zewail model for describing......, the vibrational energy relaxation of I2 subsequent to photodissociation and recombination in CCl4 is studied using classical Molecular Dynamics simulations. The vibrational relaxation times and the time-dependent I-I pair distribution function are compared to new experimental results, and a qualitative agreement...... is found in both cases. Furthermore, the rotational energy relaxation of H2O in liquid water is studied via simulations and a power-and-work analysis. The mechanism of the energy transfer from the rotationally excited H2O molecule to its water neighbors is elucidated, i.e. the energy-accepting degrees...
Armen, Roger S; Chen, Jianhan; Brooks, Charles L
2009-10-13
Incorporating receptor flexibility into molecular docking should improve results for flexible proteins. However, the incorporation of explicit all-atom flexibility with molecular dynamics for the entire protein chain may also introduce significant error and "noise" that could decrease docking accuracy and deteriorate the ability of a scoring function to rank native-like poses. We address this apparent paradox by comparing the success of several flexible receptor models in cross-docking and multiple receptor ensemble docking for p38α mitogen-activated protein (MAP) kinase. Explicit all-atom receptor flexibility has been incorporated into a CHARMM-based molecular docking method (CDOCKER) using both molecular dynamics (MD) and torsion angle molecular dynamics (TAMD) for the refinement of predicted protein-ligand binding geometries. These flexible receptor models have been evaluated, and the accuracy and efficiency of TAMD sampling is directly compared to MD sampling. Several flexible receptor models are compared, encompassing flexible side chains, flexible loops, multiple flexible backbone segments, and treatment of the entire chain as flexible. We find that although including side chain and some backbone flexibility is required for improved docking accuracy as expected, docking accuracy also diminishes as additional and unnecessary receptor flexibility is included into the conformational search space. Ensemble docking results demonstrate that including protein flexibility leads to to improved agreement with binding data for 227 active compounds. This comparison also demonstrates that a flexible receptor model enriches high affinity compound identification without significantly increasing the number of false positives from low affinity compounds.
Molecular beam studies of reaction dynamics
International Nuclear Information System (INIS)
Lee, Yuan T.
1991-03-01
The major thrust of this research project is to elucidate detailed dynamics of simple elementary reactions that are theoretically important and to unravel the mechanism of complex chemical reactions or photochemical processes that play important roles in many macroscopic processes. Molecular beams of reactants are used to study individual reactive encounters between molecules or to monitor photodissociation events in a collision-free environment. Most of the information is derived from measurement of the product fragment energy, angular, and state distributions. Recent activities are centered on the mechanisms of elementary chemical reactions involving oxygen atoms with unsaturated hydrocarbons, the dynamics of endothermic substitution reactions, the dependence of the chemical reactivity of electronically excited atoms on the alignment of excited orbitals, the primary photochemical processes of polyatomic molecules, intramolecular energy transfer of chemically activated and locally excited molecules, the energetics of free radicals that are important to combustion processes, the infrared-absorption spectra of carbonium ions and hydrated hydronium ions, and bond-selective photodissociation through electric excitation
A molecular dynamics approach to barrodiffusion
Cooley, James; Marciante, Mathieu; Murillo, Michael
2016-10-01
Unexpected phenomena in the reaction rates for Inertial Confinement Fusion (ICF) capsules have led to a renewed interest in the thermo-dynamically driven diffusion process for the past 10 years, often described collectively as barodiffusion. In the current context, barodiffusion would manifest as a process that separates ions of differing mass and charge ratios due to pressure and temperature gradients set-up through shock structures in the capsule core. Barrodiffusion includes additional mass transfer terms that account for the irreversible transport of species due to gradients in the system, both thermodynamic and electric e.g, i = - ρD [ ∇c +kp ∇ln(pi) +kT(i) ∇ln(Ti) +kt(e) ∇ln(Te) +eke/Ti ∇ϕ ] . Several groups have attacked this phenomena using continuum scale models and supplemented with kinetic theory to derive coefficients for the different diffusion terms based on assumptions about the collisional processes. In contrast, we have applied a molecular dynamics (MD) simulation to this system to gain a first-principle understanding of the rate kinetics and to assess the accuracy of the differin
Molecular beam studies of reaction dynamics
Energy Technology Data Exchange (ETDEWEB)
Lee, Y.T. [Lawrence Berkeley Laboratory, CA (United States)
1993-12-01
The major thrust of this research project is to elucidate detailed dynamics of simple elementary reactions that are theoretically important and to unravel the mechanism of complex chemical reactions or photochemical processes that play important roles in many macroscopic processes. Molecular beams of reactants are used to study individual reactive encounters between molecules or to monitor photodissociation events in a collision-free environment. Most of the information is derived from measurement of the product fragment energy, angular, and state distributions. Recent activities are centered on the mechanisms of elementary chemical reactions involving oxygen atoms with unsaturated hydrocarbons, the dynamics of endothermic substitution reactions, the dependence of the chemical reactivity of electronically excited atoms on the alignment of excited orbitals, the primary photochemical processes of polyatomic molecules, intramolecular energy transfer of chemically activated and locally excited molecules, the energetics of free radicals that are important to combustion processes, the infrared-absorption spectra of carbonium ions and hydrated hydronium ions, and bond-selective photodissociation through electric excitation.
Isotopic effects in vibrational relaxation dynamics of H on a Si(100) surface
Bouakline, F.; Lorenz, U.; Melani, G.; Paramonov, G. K.; Saalfrank, P.
2017-10-01
In a recent paper [U. Lorenz and P. Saalfrank, Chem. Phys. 482, 69 (2017)], we proposed a robust scheme to set up a system-bath model Hamiltonian, describing the coupling of adsorbate vibrations (system) to surface phonons (bath), from first principles. The method is based on an embedded cluster approach, using orthogonal coordinates for system and bath modes, and an anharmonic phononic expansion of the system-bath interaction up to second order. In this contribution, we use this model Hamiltonian to calculate vibrational relaxation rates of H-Si and D-Si bending modes, coupled to a fully H(D)-covered Si(100)-( 2 × 1 ) surface, at zero temperature. The D-Si bending mode has an anharmonic frequency lying inside the bath frequency spectrum, whereas the H-Si bending mode frequency is outside the bath Debye band. Therefore, in the present calculations, we only take into account one-phonon system-bath couplings for the D-Si system and both one- and two-phonon interaction terms in the case of H-Si. The computation of vibrational lifetimes is performed with two different approaches, namely, Fermi's golden rule, and a generalized Bixon-Jortner model built in a restricted vibrational space of the adsorbate-surface zeroth-order Hamiltonian. For D-Si, the Bixon-Jortner Hamiltonian can be solved by exact diagonalization, serving as a benchmark, whereas for H-Si, an iterative scheme based on the recursive residue generation method is applied, with excellent convergence properties. We found that the lifetimes obtained with perturbation theory, albeit having almost the same order of magnitude—a few hundred fs for D-Si and a couple of ps for H-Si—, are strongly dependent on the discretized numerical representation of the bath spectral density. On the other hand, the Bixon-Jortner model is free of such numerical deficiencies, therefore providing better estimates of vibrational relaxation rates, at a very low computational cost. The results obtained with this model clearly show
International Nuclear Information System (INIS)
Brot, C.; Virlet, J.
1979-01-01
14 N and 1 H NMR relaxation times have been measured in quinuclidine in its plastic phase. These measurements rule out isotropic motion. Correlation times for several anisotropic reorientational models are calculated from these NMR data. The best agreement with the values calculated from neutron scattering experiments (preceding paper) is obtained for a model where the molecules reorient by +-90 0 jumps about the crystallographic C 4 axes with a residence time of (22.2+-2).10 -12 s, and by +-120 0 jumps about the molecular C 3 axes with a residence of (5.25+-2.8).10 -12 s, at room temperature. The activation enthalpy is 15.3 kJ.mol. -1 for the +-90 0 jumps, and higher for the +-120 0 jumps. Translational correlation times have also been measured at high temperature, below the melting point
Low-relaxation spin waves in laser-molecular-beam epitaxy grown nanosized yttrium iron garnet films
Energy Technology Data Exchange (ETDEWEB)
Lutsev, L. V., E-mail: l-lutsev@mail.ru; Korovin, A. M.; Bursian, V. E.; Gastev, S. V.; Fedorov, V. V.; Suturin, S. M.; Sokolov, N. S. [Ioffe Physical-Technical Institute, Russian Academy of Sciences, 194021 St. Petersburg (Russian Federation)
2016-05-02
Synthesis of nanosized yttrium iron garnet (Y{sub 3}Fe{sub 5}O{sub 12}, YIG) films followed by the study of ferromagnetic resonance (FMR) and spin wave propagation in these films is reported. The YIG films were grown on gadolinium gallium garnet substrates by laser molecular beam epitaxy. It has been shown that spin waves propagating in YIG deposited at 700 °C have low damping. At the frequency of 3.29 GHz, the spin-wave damping parameter is less than 3.6 × 10{sup −5}. Magnetic inhomogeneities of the YIG films give the main contribution to the FMR linewidth. The contribution of the relaxation processes to the FMR linewidth is as low as 1.2%.
Approximation of quantum observables by molecular dynamics simulations
Sandberg, Mattias
2016-01-01
In this talk I will discuss how to estimate the uncertainty in molecular dynamics simulations. Molecular dynamics is a computational method to study molecular systems in materials science, chemistry, and molecular biology. The wide popularity of molecular dynamics simulations relies on the fact that in many cases it agrees very well with experiments. If we however want the simulation to predict something that has no comparing experiment, we need a mathematical estimate of the accuracy of the computation. In the case of molecular systems with few particles, such studies are made by directly solving the Schrodinger equation. In this talk I will discuss theoretical results on the accuracy between quantum mechanics and molecular dynamics, to be used for systems that are too large to be handled computationally by the Schrodinger equation.
Approximation of quantum observables by molecular dynamics simulations
Sandberg, Mattias
2016-01-06
In this talk I will discuss how to estimate the uncertainty in molecular dynamics simulations. Molecular dynamics is a computational method to study molecular systems in materials science, chemistry, and molecular biology. The wide popularity of molecular dynamics simulations relies on the fact that in many cases it agrees very well with experiments. If we however want the simulation to predict something that has no comparing experiment, we need a mathematical estimate of the accuracy of the computation. In the case of molecular systems with few particles, such studies are made by directly solving the Schrodinger equation. In this talk I will discuss theoretical results on the accuracy between quantum mechanics and molecular dynamics, to be used for systems that are too large to be handled computationally by the Schrodinger equation.
Investigation of impurity defects in α-iron by molecular dynamics method
International Nuclear Information System (INIS)
Kevorkyan, Yu.R.
1986-01-01
Investigation of the configuration of impurity defects in α-iron by the molecular dynamics method is presented. The Jhonson model potential has been used to calculate the interaction of matrix atoms. The impurity-matrix atom interaction is described by the same form of the potential shifted along the axis of interatomic distances for a definite value. The correspondence between the shift value and change in the radius of the impurity defect is established on the basis of calculation of the relaxation volume. Possible configurations of the impurity - interstitial matrix atom complexes are obtained for the given model of the impurity defect, dimensional boundaries of possible transitions between different configurations are determined. Formation and bound energies, relaxation volumes of impurity defects are calculated
Oesterling, Sven; Schalk, Oliver; Geng, Ting; Thomas, Richard D; Hansson, Tony; de Vivie-Riedle, Regina
2017-01-18
For the series furan, furfural and β-furfural we investigated the effect of substituents and their positioning on the photoinduced relaxation dynamics in a combined theoretical and experimental approach. Using time resolved photoelectron spectroscopy with a high intensity probe pulse, we can, for the first time, follow the whole deactivation process of furan through a two photon probe signal. Using the extended 2-electron 2-orbital model [Nenov et al., J. Chem. Phys., 2011, 135, 034304] we explain the formation of one central conical intersection and predict the influence of the aldehyde group of the derivatives on its geometry. This, as well as the relaxation mechanisms from photoexcitation to the final outcome was investigated using a variety of theoretical methods. Complete active space self consistent field was used for on-the-fly calculations while complete active space perturbation theory and coupled cluster theory were used to accurately describe critical configurations. Experiment and theory show the relaxation dynamics of furfural and β-furfural to be slowed down, and together they disclose an additional deactivation pathway, which is attributed to the n O lonepair state introduced with the aldehyde group.
Ultrafast relaxation dynamics of a biologically relevant probe dansyl at the micellar surface.
Sarkar, Rupa; Ghosh, Manoranjan; Pal, Samir Kumar
2005-02-01
We report picosecond-resolved measurement of the fluorescence of a well-known biologically relevant probe, dansyl chromophore at the surface of a cationic micelle (cetyltrimethylammonium bromide, CTAB). The dansyl chromophore has environmentally sensitive fluorescence quantum yields and emission maxima, along with large Stokes shift. In order to study the solvation dynamics of the micellar environment, we measured the fluorescence of dansyl chromophore attached to the micellar surface. The fluorescence transients were observed to decay (with time constant approximately 350 ps) in the blue end and rise with similar timescale in the red end, indicative of solvation dynamics of the environment. The solvation correlation function is measured to decay with time constant 338 ps, which is much slower than that of ordinary bulk water. Time-resolved anisotropy of the dansyl chromophore shows a bi-exponential decay with time constants 413 ps (23%) and 1.3 ns (77%), which is considerably slower than that in free solvents revealing the rigidity of the dansyl-micelle complex. Time-resolved area-normalized emission spectroscopic (TRANES) analysis of the time dependent emission spectra of the dansyl chromophore in the micellar environment shows an isoemissive point at 21066 cm-1. This indicates the fluorescence of the chromophore contains emission from two kinds of excited states namely locally excited state (prior to charge transfer) and charge transfer state. The nature of the solvation dynamics in the micellar environments is therefore explored from the time-resolved anisotropy measurement coupled with the TRANES analysis of the fluorescence transients. The time scale of the solvation is important for the mechanism of molecular recognition.
Zheng, Yexin; Wang, Shi-Qing; Tsige, Mesfin
The Kremer-Grest bead-spring model has been the standard model in molecular dynamics simulation of polymer glasses. However, due to current computational limitations in accessing relevant time scales in polymer glasses in a reasonable amount of CPU time, simulation of mechanical response of polymer glasses in molecular dynamic simulations requires a much higher quenching rate and deformation rate than used in experiments. Despite several orders of magnitude difference in time scale between simulation and experiment, previous studies have shown that simulations can produce meaningful results that can be directly compared with experimental results. In this work we show that by tuning the quenching rate and deformation rate relative to the segmental relaxation times, a reasonable mechanical response shows up in the glassy state. Specifically, we show a younger glass prepared with a faster quenching rate shows glassy responses only when the imposed deformation rate is proportionally higher. the National Science Foundation (DMR-1444859 and DMR-1609977).
Incipient plasticity and indentation response of MgO surfaces using molecular dynamics
Tran, Anh-Son; Hong, Zheng-Han; Chen, Ming-Yuan; Fang, Te-Hua
2018-05-01
The mechanical characteristics of magnesium oxide (MgO) based on nanoindentation are studied using molecular dynamics (MD) simulation. The effects of indenting speed and temperature on the structural deformation and loading-unloading curve are investigated. Results show that the strained surface of the MgO expands to produce a greater relaxation of atoms in the surroundings of the indent. The dislocation propagation and pile-up for MgO occur more significantly with the increasing temperature from 300 K to 973 K. In addition, with increasing temperature, the high strained atoms with a great perturbation appearing at the groove location.
Thermal transpiration: A molecular dynamics study
Energy Technology Data Exchange (ETDEWEB)
T, Joe Francis [Computational Nanotechnology Laboratory, School of Nano Science and Technology, National Institute of Technology Calicut, Kozhikode (India); Sathian, Sarith P. [Department of Applied Mechanics, Indian Institute of Technology Madras, Chennai (India)
2014-12-09
Thermal transpiration is a phenomenon where fluid molecules move from the cold end towards the hot end of a channel under the influence of longitudinal temperature gradient alone. Although the phenomenon of thermal transpiration is observed at rarefied gas conditions in macro systems, the phenomenon can occur at atmospheric pressure if the characteristic dimensions of the channel is less than 100 nm. The flow through these nanosized channels is characterized by the free molecular flow regimes and continuum theory is inadequate to describe the flow. Thus a non-continuum method like molecular dynamics (MD) is necessary to study such phenomenon. In the present work, MD simulations were carried out to investigate the occurance of thermal transpiration in copper and platinum nanochannels at atmospheric pressure conditions. The mean pressure of argon gas confined inside the nano channels was maintained around 1 bar. The channel height is maintained at 2nm. The argon atoms interact with each other and with the wall atoms through the Lennard-Jones potential. The wall atoms are modelled using an EAM potential. Further, separate simulations were carried out where a Harmonic potential is used for the atom-atom interaction in the platinum channel. A thermally insulating wall was introduced between the low and high temperature regions and those wall atoms interact with fluid atoms through a repulsive potential. A reduced cut off radius were used to achieve this. Thermal creep is induced by applying a temperature gradient along the channel wall. It was found that flow developed in the direction of the increasing temperature gradient of the wall. An increase in the volumetric flux was observed as the length of the cold and the hot regions of the wall were increased. The effect of temperature gradient and the wall-fluid interaction strength on the flow parameters have been studied to understand the phenomenon better.
Investigating Ebola virus pathogenicity using molecular dynamics.
Pappalardo, Morena; Collu, Francesca; Macpherson, James; Michaelis, Martin; Fraternali, Franca; Wass, Mark N
2017-08-11
Ebolaviruses have been known to cause deadly disease in humans for 40 years and have recently been demonstrated in West Africa to be able to cause large outbreaks. Four Ebolavirus species cause severe disease associated with high mortality in humans. Reston viruses are the only Ebolaviruses that do not cause disease in humans. Conserved amino acid changes in the Reston virus protein VP24 compared to VP24 of other Ebolaviruses have been suggested to alter VP24 binding to host cell karyopherins resulting in impaired inhibition of interferon signalling, which may explain the difference in human pathogenicity. Here we used protein structural analysis and molecular dynamics to further elucidate the interaction between VP24 and KPNA5. As a control experiment, we compared the interaction of wild-type and R137A-mutant (known to affect KPNA5 binding) Ebola virus VP24 with KPNA5. Results confirmed that the R137A mutation weakens direct VP24-KPNA5 binding and enables water molecules to penetrate at the interface. Similarly, Reston virus VP24 displayed a weaker interaction with KPNA5 than Ebola virus VP24, which is likely to reduce the ability of Reston virus VP24 to prevent host cell interferon signalling. Our results provide novel molecular detail on the interaction of Reston virus VP24 and Ebola virus VP24 with human KPNA5. The results indicate a weaker interaction of Reston virus VP24 with KPNA5 than Ebola virus VP24, which is probably associated with a decreased ability to interfere with the host cell interferon response. Hence, our study provides further evidence that VP24 is a key player in determining Ebolavirus pathogenicity.
Yasaka, Yoshiro; Klein, Michael L; Nakahara, Masaru; Matubayasi, Nobuyuki
2012-02-21
The rotational dynamics of benzene and water in the ionic liquid (IL) 1-butyl-3-methylimidazolium chloride are studied using molecular dynamics (MD) simulation and NMR T(1) measurements. MD trajectories based on an effective potential are used to calculate the (2)H NMR relaxation time, T(1) via Fourier transform of the relevant rotational time correlation function, C(2R)(t). To compensate for the lack of polarization in the standard fixed-charge modeling of the IL, an effective ionic charge, which is smaller than the elementary charge is employed. The simulation results are in closest agreement with NMR experiments with respect to the temperature and Larmor frequency dependencies of T(1) when an effective charge of ±0.5e is used for the anion and the cation, respectively. The computed C(2R)(t) of both solutes shows a bi-modal nature, comprised of an initial non-diffusive ps relaxation plus a long-time ns tail extending to the diffusive regime. Due to the latter component, the solute dynamics is not under the motional narrowing condition with respect to the prevalent Larmor frequency. It is shown that the diffusive tail of the C(2R)(t) is most important to understand frequency and temperature dependencies of T(1) in ILs. On the other hand, the effect of the initial ps relaxation is an increase of T(1) by a constant factor. This is equivalent to an "effective" reduction of the quadrupolar coupling constant (QCC). Thus, in the NMR T(1) analysis, the rotational time correlation function can be modeled analytically in the form of aexp (-t/τ) (Lipari-Szabo model), where the constant a, the Lipari-Szabo factor, contains the integrated contribution of the short-time relaxation and τ represents the relaxation time of the exponential (diffusive) tail. The Debye model is a special case of the Lipari-Szabo model with a = 1, and turns out to be inappropriate to represent benzene and water dynamics in ILs since a is as small as 0.1. The use of the Debye model would result in
Nonadiabatic excited-state molecular dynamics: On-the-fly limiting of essential excited states
Energy Technology Data Exchange (ETDEWEB)
Nelson, Tammie [Los Alamos National Laboratory, Los Alamos, NM 87545 (United States); Naumov, Artem [Skolkovo Institute of Science and Technology, Moscow 143026 (Russian Federation); Fernandez-Alberti, Sebastian [Universidad Nacional de Quilmes, Roque Saenz Pea 352, B1876BXD Bernal (Argentina); Tretiak, Sergei, E-mail: serg@lanl.gov [Los Alamos National Laboratory, Los Alamos, NM 87545 (United States)
2016-12-20
The simulation of nonadiabatic dynamics in extended molecular systems involving hundreds of atoms and large densities of states is particularly challenging. Nonadiabatic coupling terms (NACTs) represent a significant numerical bottleneck in surface hopping approaches. Rather than using unreliable NACT cutting schemes, here we develop “on-the-fly” state limiting methods to eliminate states that are no longer essential for the non-radiative relaxation dynamics as a trajectory proceeds. We propose a state number criteria and an energy-based state limit. The latter is more physically relevant by requiring a user-imposed energy threshold. For this purpose, we introduce a local kinetic energy gauge by summing contributions from atoms within the spatial localization of the electronic wavefunction to define the energy available for upward hops. The proposed state limiting schemes are implemented within the nonadiabatic excited-state molecular dynamics framework to simulate photoinduced relaxation in poly-phenylene vinylene (PPV) and branched poly-phenylene ethynylene (PPE) oligomers for benchmark evaluation.
How Dynamic Visualization Technology Can Support Molecular Reasoning
Levy, Dalit
2013-01-01
This paper reports the results of a study aimed at exploring the advantages of dynamic visualization for the development of better understanding of molecular processes. We designed a technology-enhanced curriculum module in which high school chemistry students conduct virtual experiments with dynamic molecular visualizations of solid, liquid, and…
Molecular beam studies of adsorption dynamics
International Nuclear Information System (INIS)
Arumainayagam, C.R.; McMaster, M.C.; Madix, R.J.
1991-01-01
We have investigated the trapping dynamics of C 1 -C 3 alkanes and Xe on Pt(111) using supersonic molecular beams and a direct technique to measure trapping probabilities. We have extended a one-dimensional model based on classical mechanics to include trapping and have found semiquantitative agreement with experimental results for the dependence of the initial trapping probability on incident translational energy at normal incidence. Our measurements of the initial trapping probability as a function of incident translational energy at normal incidence are in agreement with previous mean translational energy measurements for Xe and CH 4 desorbing near the surface normal, in accordance with detailed balance. However, the angular dependence of the initial trapping probability shows deviations from normal energy scaling, demonstrating the importance of parallel momentum in the trapping process and the inadequacy of one-dimensional models. The dependence of the initial trapping probability of Xe on incident translational energy and angle is quite well fit by three-dimensional stochastic classical trajectory calculations utilizing a Morse potential. Angular distributions of the scattered molecules indicate that the trapping probability is not a sensitive function of surface temperature. The trapping probability increases with surface coverage in quantitative agreement with a modified Kisliuk model which incorporates enhanced trapping onto the monolayer. We have also used the direct technique to study trapping onto a saturated monolayer state to investigate the dynamics of extrinsic precursor adsorption and find that the initial trapping probability onto the monolayer is higher than on the clean surface. The initial trapping probability onto the monolayer scales with total energy, indicating a highly corrugated interaction potential
Directory of Open Access Journals (Sweden)
Debashis Mukherjee
2002-06-01
Full Text Available Abstract: We present in this paper two new versions of Rayleigh-SchrÃ‚Â¨odinger (RS and the Brillouin-Wigner (BW state-specific multi-reference perturbative theories (SSMRPT which stem from our state-specific multi-reference coupled-cluster formalism (SS-MRCC, developed with a complete active space (CAS. They are manifestly sizeextensive and are designed to avoid intruders. The combining coefficients cÃŽÂ¼ for the model functions ÃÂ†ÃŽÂ¼ are completely relaxed and are obtained by diagonalizing an effective operator in the model space, one root of which is the target eigenvalue of interest. By invoking suitable partitioning of the hamiltonian, very convenient perturbative versions of the formalism in both the RS and the BW forms are developed for the second order energy. The unperturbed hamiltonians for these theories can be chosen to be of both MÃÂ†ller-Plesset (MP and Epstein-Nesbet (EN type. However, we choose the corresponding Fock operator fÃŽÂ¼ for each model function ÃÂ†ÃŽÂ¼, whose diagonal elements are used to define the unperturbed hamiltonian in the MP partition. In the EN partition, we additionally include all the diagonal direct and exchange ladders. Our SS-MRPT thus utilizes a multi-partitioning strategy. Illustrative numerical applications are presented for potential energy surfaces (PES of the ground (1ÃŽÂ£+ and the first delta (1ÃŽÂ” states of CH+ which possess pronounced multi-reference character. Comparison of the results with the corresponding full CI values indicates the efficacy of our formalisms.
International Nuclear Information System (INIS)
Bertotti, G.; Basso, V.; Beatrice, C.; LoBue, M.; Magni, A.; Tiberto, P.
2001-01-01
An overview is given of the present understanding of hysteresis phenomena in magnetic materials. The problem is addressed from three approximate viewpoints: the connection between rate-independent hysteresis and micromagnetics; the modifications brought into this picture by thermal relaxation effects; the role of rate-dependent magnetization mechanisms, like eddy-current-damped domain wall motion
Folding very short peptides using molecular dynamics.
Directory of Open Access Journals (Sweden)
Bosco K Ho
2006-04-01
Full Text Available Peptides often have conformational preferences. We simulated 133 peptide 8-mer fragments from six different proteins, sampled by replica-exchange molecular dynamics using Amber7 with a GB/SA (generalized-Born/solvent-accessible electrostatic approximation to water implicit solvent. We found that 85 of the peptides have no preferred structure, while 48 of them converge to a preferred structure. In 85% of the converged cases (41 peptides, the structures found by the simulations bear some resemblance to their native structures, based on a coarse-grained backbone description. In particular, all seven of the beta hairpins in the native structures contain a fragment in the turn that is highly structured. In the eight cases where the bioinformatics-based I-sites library picks out native-like structures, the present simulations are largely in agreement. Such physics-based modeling may be useful for identifying early nuclei in folding kinetics and for assisting in protein-structure prediction methods that utilize the assembly of peptide fragments.
Molecular dynamics studies of displacement cascades
International Nuclear Information System (INIS)
Averback, R.S.; Hsieh, Horngming; Diaz de la Rubia, T.
1990-02-01
Molecular-dynamics simulations of cascades in Cu and Ni with primary-knock-on energies up to 5 keV and lattice temperatures in the range 0 K--700 K are described. Interatomic forces were represented by either the Gibson II (Cu) or Johnson-Erginsoy (Ni) potentials in most of this work, although some simulations using ''Embedded Atom Method'' potentials, e.g., for threshold events in Ni 3 Al, are also presented. The results indicate that the primary state of damage produced by displacement cascades is controlled by two phenomena, replacement collision sequences during the collisional phase of the cascade and local melting during the thermal spike. As expected, the collisional phase is rather similar in Cu and Ni, however, the thermal spike is of longer duration and has a more pronounced influence in Cu than Ni. When the ambient temperature of the lattice is increased, the melt zones are observed to both increase in size and cool more slowly. This has the effect of reducing defect production and enhancing atomic mixing and disordering. The implications of these results for defect production, cascade collapse, atomic disordering will be discussed. 34 refs., 7 figs., 2 tabs
A molecular dynamics simulation code ISIS
International Nuclear Information System (INIS)
Kambayashi, Shaw
1992-06-01
Computer simulation based on the molecular dynamics (MD) method has become an important tool complementary to experiments and theoretical calculations in a wide range of scientific fields such as physics, chemistry, biology, and so on. In the MD method, the Newtonian equations-of-motion of classical particles are integrated numerically to reproduce a phase-space trajectory of the system. In the 1980's, several new techniques have been developed for simulation at constant-temperature and/or constant-pressure in convenient to compare result of computer simulation with experimental results. We first summarize the MD method for both microcanonical and canonical simulations. Then, we present and overview of a newly developed ISIS (Isokinetic Simulation of Soft-spheres) code and its performance on various computers including vector processors. The ISIS code has a capability to make a MD simulation under constant-temperature condition by using the isokinetic constraint method. The equations-of-motion is integrated by a very accurate fifth-order finite differential algorithm. The bookkeeping method is also utilized to reduce the computational time. Furthermore, the ISIS code is well adopted for vector processing: Speedup ratio ranged from 16 to 24 times is obtained on a VP2600/10 vector processor. (author)
Spin-diffusions and diffusive molecular dynamics
Farmer, Brittan; Luskin, Mitchell; Plecháč, Petr; Simpson, Gideon
2017-12-01
Metastable configurations in condensed matter typically fluctuate about local energy minima at the femtosecond time scale before transitioning between local minima after nanoseconds or microseconds. This vast scale separation limits the applicability of classical molecular dynamics (MD) methods and has spurned the development of a host of approximate algorithms. One recently proposed method is diffusive MD which aims at integrating a system of ordinary differential equations describing the likelihood of occupancy by one of two species, in the case of a binary alloy, while quasistatically evolving the locations of the atoms. While diffusive MD has shown itself to be efficient and provide agreement with observations, it is fundamentally a model, with unclear connections to classical MD. In this work, we formulate a spin-diffusion stochastic process and show how it can be connected to diffusive MD. The spin-diffusion model couples a classical overdamped Langevin equation to a kinetic Monte Carlo model for exchange amongst the species of a binary alloy. Under suitable assumptions and approximations, spin-diffusion can be shown to lead to diffusive MD type models. The key assumptions and approximations include a well-defined time scale separation, a choice of spin-exchange rates, a low temperature approximation, and a mean field type approximation. We derive several models from different assumptions and show their relationship to diffusive MD. Differences and similarities amongst the models are explored in a simple test problem.
International Nuclear Information System (INIS)
Choudhury, Niharendu
2013-01-01
Highlights: • We present atomistic MD simulation of water confined between two paraffin-like plates. • Effect of plate hydrophobicity on the confined water dynamics is investigated. • Diffusivity of confined water is calculated from mean squared displacements. • Rotational dynamics of the confined water has bimodal nature of relaxation. • Monotonic dependence of translational and rotational dynamics on hydrophobicity. - Abstract: We present detailed molecular dynamics simulations of water in and around a pair of plates immersed in water to investigate the effect of degree of hydrophobicity or hydrophilicity of the plates on dynamics of water confined between the two plates. The nature of the plate has been tuned from hydrophobic to hydrophilic and vice versa by varying plate-water dispersion interaction. Analyses of the translational dynamics as performed by calculating mean squared displacements of the confined water reveal a monotonically decreasing trend of the diffusivity with increasing hydrophilicity of the plates. Orientational dynamics of the confined water also follows the same monotonic trend. Although orientational time constant almost does not change with the increase of plate-water dispersion interaction in the hydrophobic regime corresponding to the smaller plate-water attraction, it changes considerably in the hydrophilic regime corresponding to larger plate-water dispersion interactions
Chirped-pulse manipulated carrier dynamics in low-temperature molecular-beam-epitaxy grown GaAs
International Nuclear Information System (INIS)
Lee, Chao-Kuei; Lin, Yuan-Yao; Lin, Sung-Hui; Lin, Gong-Ru; Pan, Ci-Ling
2014-01-01
Chirped pulse controlled carrier dynamics in low-temperature molecular-beam-epitaxy grown GaAs are investigated by degenerate pump-probe technique. Varying the chirped condition of excited pulse from negative to positive increases the carrier relaxation time so as to modify the dispersion and reshape current pulse in time domain. The spectral dependence of carrier dynamics is analytically derived and explained by Shockley-Read Hall model. This observation enables the new feasibility of controlling carrier dynamics in ultrafast optical devices via the chirped pulse excitations
Wu, Bin
hydrogen bonding. It is found that the population of this bonding would increase and the corresponding relaxation would slow down as molecular charge increases. We perceive that through more and longer interaction between penetrating water molecules and polymeric part of dendrimer, the dynamics of latter could be enhanced.
Interfacial effects revealed by ultrafast relaxation dynamics in BiFeO 3 / YBa 2 Cu 3 O 7 bilayers
Springer, D.
2016-02-12
The temperature dependence of the relaxation dynamics in the bilayer thin film heterostructure composed of multiferroic BiFeO3 (BFO) and superconducting YBa2Cu3O7 (YBCO) grown on a (001) SrTiO3 substrate is studied by a time-resolved pump-probe technique, and compared with that of pure YBCO thin film grown under the same growth conditions. The superconductivity of YBCO is found to be retained in the heterostructure. We observe a speeding up of the YBCO recombination dynamics in the superconducting state of the heterostructure, and attribute it to the presence of weak ferromagnetism at the BFO/YBCO interface as observed in magnetization data. An extension of the Rothwarf-Taylor model is used to fit the ultrafast dynamics of BFO/YBCO, that models an increased quasiparticle occupation of the ferromagnetic interfacial layer in the superconducting state of YBCO.
Interfacial effects revealed by ultrafast relaxation dynamics in BiFeO 3 / YBa 2 Cu 3 O 7 bilayers
Springer, D.; Nair, Saritha K.; He, Mi; Lu, C. L.; Cheong, S. A.; Wu, Tao; Panagopoulos, C.; Chia, Elbert E. M.; Zhu, Jian-Xin
2016-01-01
The temperature dependence of the relaxation dynamics in the bilayer thin film heterostructure composed of multiferroic BiFeO3 (BFO) and superconducting YBa2Cu3O7 (YBCO) grown on a (001) SrTiO3 substrate is studied by a time-resolved pump-probe technique, and compared with that of pure YBCO thin film grown under the same growth conditions. The superconductivity of YBCO is found to be retained in the heterostructure. We observe a speeding up of the YBCO recombination dynamics in the superconducting state of the heterostructure, and attribute it to the presence of weak ferromagnetism at the BFO/YBCO interface as observed in magnetization data. An extension of the Rothwarf-Taylor model is used to fit the ultrafast dynamics of BFO/YBCO, that models an increased quasiparticle occupation of the ferromagnetic interfacial layer in the superconducting state of YBCO.
International Nuclear Information System (INIS)
Wang, J.; Gutierre, M.S.
2010-01-01
This paper presents results of a molecular dynamics simulation study of dehydrated 2:1 clay minerals using the Parrinello-Rahman constant-pressure molecular dynamics method. The method is capable of simulating a system under the most general applied stress conditions by considering the changes of MD cell size and shape. Given the advantage of the method, it is the major goal of the paper to investigate the influence of imposed cell boundary conditions on the molecular structural transformation of 2:1 clay minerals under different normal pressures. Simulation results show that the degrees of freedom of the simulation cell (i.e., whether the cell size or shape change is allowed) determines the final equilibrated crystal structure of clay minerals. Both the MD method and the static method have successfully revealed unforeseen structural transformations of clay minerals upon relaxation under different normal pressures. It is found that large shear distortions of clay minerals occur when full allowance is given to the cell size and shape change. A complete elimination of the interlayer spacing is observed in a static simulation. However, when only the cell size change is allowed, interlayer spacing is retained, but large internal shear stresses also exist.
Polymer deformation in Brownian ratchets: theory and molecular dynamics simulations.
Kenward, Martin; Slater, Gary W
2008-11-01
We examine polymers in the presence of an applied asymmetric sawtooth (ratchet) potential which is periodically switched on and off, using molecular dynamics (MD) simulations with an explicit Lennard-Jones solvent. We show that the distribution of the center of mass for a polymer in a ratchet is relatively wide for potential well depths U0 on the order of several kBT. The application of the ratchet potential also deforms the polymer chains. With increasing U0 the Flory exponent varies from that for a free three-dimensional (3D) chain, nu=35 (U0=0), to that corresponding to a 2D compressed (pancake-shaped) polymer with a value of nu=34 for moderate U0. This has the added effect of decreasing a polymer's diffusion coefficient from its 3D value D3D to that of a pancaked-shaped polymer moving parallel to its minor axis D2D. The result is that a polymer then has a time-dependent diffusion coefficient D(t) during the ratchet off time. We further show that this suggests a different method to operate a ratchet, where the off time of the ratchet, toff, is defined in terms of the relaxation time of the polymer, tauR. We also derive a modified version of the Bader ratchet model [Bader, Proc. Natl. Acad. Sci. U.S.A. 96, 13165 (1999)] which accounts for this deformation and we present a simple expression to describe the time dependent diffusion coefficient D(t). Using this model we then illustrate that polymer deformation can be used to modulate polymer migration in a ratchet potential.
International Nuclear Information System (INIS)
Huo, L.S.; Zeng, J.F.; Wang, W.H.; Liu, C.T.; Yang, Y.
2013-01-01
Starting from the nanoscale structural heterogeneities intrinsic to metallic glasses (MGs), here we show that there are two concurrent contributions to their microscale quasi-static shear modulus G I : one (μ) is related to the atomic bonding strength of solid-like regions and the other (G II ) to the change in the possible configurations of liquid-like regions (dynamic relaxation). Through carefully designed high-rate nanoscale indentation tests, a simple constitutive relation (μ = G I + G II ) is experimentally verified. On a fundamental level, our current work provides a structure–property correlation that may be applicable to a wide range of glassy materials
Laus, Sabrina; Sour, Angélique; Ruloff, Robert; Tóth, Eva; Merbach, André E
2005-05-06
The EPTPA5) chelate, which ensures fast water exchange in GdIII complexes, has been coupled to three different generations (5, 7, and 9) of polyamidoamine (PAMAM) dendrimers through benzylthiourea linkages (H5EPTPA = ethylenepropylenetriamine-N,N,N',N'',N''-pentaacetic acid). The proton relaxivities measured at pH 7.4 for the dendrimer complexes G5-(GdEPTPA)111, G7-(GdEPTPA)253 and G9-(GdEPTPA)1157 decrease with increasing temperature, indicating that, for the first time for dendrimers, slow water exchange does not limit relaxivity. At a given field and temperature, the relaxivity increases from G5 to G7, and then slightly decreases for G9 (r1 = 20.5, 28.3 and 27.9 mM(-1) s(-1), respectively, at 37 degrees C, 30 MHz). The relaxivities show a strong and reversible pH dependency for all three dendrimer complexes. This originates from the pH-dependent rotational dynamics of the dendrimer skeleton, which was evidenced by a combined variable-temperature and multiple-field 17O NMR and 1H relaxivity study performed at pH 6.0 and 9.9 on G5-(GdEPTPA)111. The longitudinal 17O and 1H relaxation rates of the dendrimeric complex are strongly pH-dependent, whereas they are not for the [Gd(EPTPA)(H2O)]2- monomer chelate. The longitudinal 17O and 1H relaxation rates have been analysed by the Lipari-Szabo spectral density functions and correlation times have been calculated for the global motion of the entire macromolecule (tau(gO)) and the local motion of the GdIII chelates on the surface (tau(lO)), correlated by means of an order parameter S2. The dendrimer complex G5-(GdEPTPA)111 has a considerably higher tau(gO) under acidic than under basic conditions (tau(298)gO = 4040 ps and 2950 ps, respectively), while local motions are less influenced by pH (tau(298)lO = 150 and 125 ps). The order parameter, characterizing the rigidity of the macromolecule, is also higher at pH 6.0 than at pH 9.9 (S2 = 0.43 vs 0.36, respectively). The pH dependence of the global correlation time can be
Energy Technology Data Exchange (ETDEWEB)
Rennella, Enrico; Schuetz, Anne K.; Kay, Lewis E., E-mail: kay@pound.med.utoronto.ca [University of Toronto, Departments of Molecular Genetics, Biochemistry and Chemistry (Canada)
2016-06-15
Methyl groups have emerged as powerful probes of protein dynamics with timescales from picoseconds to seconds. Typically, studies involving high molecular weight complexes exploit {sup 13}CH{sub 3}- or {sup 13}CHD{sub 2}-labeling in otherwise highly deuterated proteins. The {sup 13}CHD{sub 2} label offers the unique advantage of providing {sup 13}C, {sup 1}H and {sup 2}H spin probes, however a disadvantage has been the lack of an experiment to record {sup 13}C Carr–Purcell–Meiboom–Gill relaxation dispersion that monitors millisecond time-scale dynamics, implicated in a wide range of biological processes. Herein we develop an experiment that eliminates artifacts that would normally result from the scalar coupling between {sup 13}C and {sup 2}H spins that has limited applications in the past. The utility of the approach is established with a number of applications, including measurement of ms dynamics of a disease mutant of a 320 kDa p97 complex.
Structure and Dynamics of Polymers in Cylindrical Nanoconfinement: A Molecular Dynamics Study
Pressly, James; Riggleman, Robert; Winey, Karen
The structure and dynamics of polymers under nanoconfinement is critical for understanding how polymers behave in applications from hydraulic fracking to fabricating integrated circuits. We previously used simulations to explore the effect of the diameter of cylindrical pores (d = 10-40 σ, where σ is the unit length in reduced units) on polymer end-to-end distance (Ree,perp, Ree,par) , entanglement density, melt diffusion coefficient (D), and local relaxation time (τperp, τpar) at fixed polymer chain length (N = 350). These studies found D, Ree,par, and τperp increased with increasing confinement while entanglement density, Ree,perp, and τpar decreased. Experiments also found that D increased but to a lesser extent. Here, we examine the molecular weight dependence of these properties using N = 25, 50, 100, 200, 350, and 500 confined to pores of diameter 14 σ to examine a range of confinements. Our preliminary results show that as N increases D and Ree,par, increase as well, relative to the unconfined state, while entanglement density and Ree,perp decrease, consistent with our previous work. Interestingly, τ is shown to be independent of chain length indicating the impact of confinement imposed by reducing pore diameter is distinct from that imposed by increasing chain length.
Ultrafast Relaxation Dynamics of the Optical Nonlinearity in Nanometric Gold Particles
International Nuclear Information System (INIS)
Puech, K.; Blau, W.J.
2001-01-01
Measurements of the resonantly enhanced, third-order nonlinear optical properties of gold nanostructures exhibiting reduced charge-carrier mobility in three dimensions were performed with a number of ultrafast nonlinear optical techniques. The size of the particles investigated was varied between 5 and 40 nm. The magnitude of the nonlinear susceptibility is of the order of 5.10 -16 m 2 V -2 at resonance and an order of magnitude lower off-resonance. The response time of the nonlinearity is found to be extremely fast and could not be resolved in the experiments undertaken here. The only statement that can be made in this regard is that the phase relaxation time is of the order of or less than 20 fs while the energy relaxation time is of the order of or less than 75 fs
International Nuclear Information System (INIS)
Hawlitzky, M; Horbach, J; Binder, K; Ispas, S; Krack, M
2008-01-01
A molecular dynamics (MD) study of the static and dynamic properties of molten and glassy germanium dioxide is presented. The interactions between the atoms are modeled by the classical pair potential proposed by Oeffner and Elliott (OE) (1998 Phys. Rev. B 58 14791). We compare our results to experiments and previous simulations. In addition, an 'ab initio' method, the so-called Car-Parrinello molecular dynamics (CPMD), is applied to check the accuracy of the structural properties, as obtained by the classical MD simulations with the OE potential. As in a similar study for SiO 2 , the structure predicted by CPMD is only slightly softer than that resulting from the classical MD. In contrast to earlier simulations, both the static structure and dynamic properties are in very good agreement with pertinent experimental data. MD simulations with the OE potential are also used to study the relaxation dynamics. As previously found for SiO 2 , for high temperatures the dynamics of molten GeO 2 is compatible with a description in terms of mode coupling theory
Energy Technology Data Exchange (ETDEWEB)
Zhang, Pei; He, Li [Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, WI 53706 (United States); Besser, Matthew F. [Materials Science and Engineering, Ames Laboratory, Iowa State University, Ames, IA 50011 (United States); Liu, Ze; Schroers, Jan [Department of Mechanical Engineering and Materials Science, Yale University, New Haven, CT 06511 (United States); Kramer, Matthew J. [Materials Science and Engineering, Ames Laboratory, Iowa State University, Ames, IA 50011 (United States); Voyles, Paul M., E-mail: paul.voyles@wisc.edu [Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, WI 53706 (United States)
2017-07-15
Electron correlation microscopy (ECM) is a way to measure structural relaxation times, τ, of liquids with nanometer-scale spatial resolution using coherent electron scattering equivalent of photon correlation spectroscopy. We have applied ECM with a 3.5 nm diameter probe to Pt{sub 57.5}Cu{sub 14.7}Ni{sub 5.3}P{sub 22.5} amorphous nanorods and Pd{sub 40}Ni{sub 40}P{sub 20} bulk metallic glass (BMG) heated inside the STEM into the supercooled liquid region. These data demonstrate that the ECM technique is limited by the characteristics of the time series, which must be at least 40τ to obtain a well-converged correlation function g{sub 2}(t), and the time per frame, which must be less than 0.1τ to obtain sufficient sampling. A high-speed direct electron camera enables fast acquisition and affords reliable g{sub 2}(t) data even with low signal per frame. - Highlights: • Electron Correlation Microscopy (ECM) technique was applied to measure structural relaxation times of supercooled liquids in metallic glass. • In Pt{sub 57.5}Cu{sub 14.7}Ni{sub 5.3}P{sub 22.5} nanowire, τ and β decreases over the measured supercooled liquid regime. • In Pd{sub 40}Ni{sub 40}P{sub 20} bulk alloy, τ decreases from T{sub g}+28 °C to T{sub g}+48 °C, then increases as the temperature approaches T{sub x}. • ECM experiment requires a length of time series at least 40 times the characteristic relaxation time and a time per diffraction pattern at most 0.1 times the relaxation time.
Quantum efficiency and excited-state relaxation dynamics in neodymium-doped phosphate laser glasses
International Nuclear Information System (INIS)
Caird, J.A.; Ramponi, A.J.; Staver, P.R.
1991-01-01
Radiometrically calibrated spectroscopic techniques employing an integrating-sphere detection system have been used to determine the fluorescence quantum efficiencies for two commercially available Nd 3+ -doped phosphate laser glasses, LG-750 and LG-760. Quantum efficiencies and fluorescence lifetimes were measured for samples with various neodymium concentrations. It is shown that the effects of concentration quenching are accurately described when both resonant nonradiative excitation hopping (the Burshtein model) and annihilation by cross relaxation are accounted for by Foerster--Dexter dipole--dipole energy-transfer theory. The Foerster--Dexter critical range for nonradiative excitation hopping was found to be R DD =11 A, while the critical range for cross relaxation was close to R DA =4 A in these glasses. The quantum efficiency at low Nd 3+ concentrations was (92±5)%, implying a nonradiative relaxation rate of 210±150 s -1 for isolated ions. Improved values for the radiative lifetimes and the stimulated emission cross sections for these glasses were also deduced from the measurements
Analysis of Time Reversible Born-Oppenheimer Molecular Dynamics
Directory of Open Access Journals (Sweden)
Lin Lin
2013-12-01
Full Text Available We analyze the time reversible Born-Oppenheimer molecular dynamics (TRBOMD scheme, which preserves the time reversibility of the Born-Oppenheimer molecular dynamics even with non-convergent self-consistent field iteration. In the linear response regime, we derive the stability condition, as well as the accuracy of TRBOMD for computing physical properties, such as the phonon frequency obtained from the molecular dynamics simulation. We connect and compare TRBOMD with Car-Parrinello molecular dynamics in terms of accuracy and stability. We further discuss the accuracy of TRBOMD beyond the linear response regime for non-equilibrium dynamics of nuclei. Our results are demonstrated through numerical experiments using a simplified one-dimensional model for Kohn-Sham density functional theory.
Non-adiabatic molecular dynamic simulations of opening reaction of molecular junctions
Czech Academy of Sciences Publication Activity Database
Zobač, Vladimír; Lewis, J.P.; Jelínek, Pavel
2016-01-01
Roč. 27, č. 28 (2016), 1-8, č. článku 285202. ISSN 0957-4484 R&D Projects: GA ČR(CZ) GA14-02079S Institutional support: RVO:68378271 Keywords : non-adiabatic molecular dynamics * molecular junctions * molecular switches * DFT Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 3.440, year: 2016
Kasyanenko, Valeriy Mitrofanovich
Measuring the three-dimensional structure of molecules, dynamics of structural changes, and energy transport on a molecular scale is important for many areas of natural science. Supplementing the widely used methods of x-ray diffraction, NMR, and optical spectroscopies, a two-dimensional infrared spectroscopy (2DIR) method was introduced about a decade ago. The 2DIR method measures pair-wise interactions between vibrational modes in molecules, thus acquiring molecular structural constraints such as distances between vibrating groups and the angles between their transition dipoles. The 2DIR method has been applied to a variety of molecular systems but in studying larger molecules such as proteins and peptides the method is facing challenges associated with the congestion of their vibrational spectra and delocalized character of their vibrational modes. To help extract structural information from such spectra and make efficient use of vibrational modes separated by large distances, a novel relaxation-assisted 2DIR method (RA 2DIR) has recently been proposed, which exploits the transport of excess vibrational energy from the initially excited mode. With the goal of further development of RA 2DIR, we applied it to a variety of molecular systems, including model compounds, transition-metal complexes, and isomers. The experiments revealed several novel effects which both enhance the power of RA 2DIR and bring a deeper understanding to the fundamental process of energy transport on a molecular level. We demonstrated that RA 2DIR can enhance greatly (27-fold) the cross-peak amplitude among spatially remote modes, which leads to an increase of the range of distances accessible for structural measurements by several fold. We demonstrated that the energy transport time correlates with the intermode distance. This correlation offers a new way for identifying connectivity patterns in molecules. We developed two models of energy transport in molecules. In one, a spatial overlap
Molecular dynamics using quasielastic neutron scattering
Mitra, S
2003-01-01
Quasielastic neutron scattering (QENS) technique is well suited to study the molecular motions (rotations and translations) in solids or liquids. It offers a unique possibility of analysing spatial dimensions of atomic or molecular processes in their development over time. We describe here some of the systems studied using the QENS spectrometer, designed, developed and commissioned at Dhruva reactor in Trombay. We have studied a variety of systems to investigate the molecular motion, for example, simple molecular solids, molecules adsorbed in confined medium like porous systems or zeolites, monolayer-protected nano-sized metal clusters, water in Portland cement as it cures with time, etc. (author)
Uncovering molecular relaxation processes with nonlinear spectroscopies in the deep UV
International Nuclear Information System (INIS)
West, Brantley A.; Molesky, Brian P.; Giokas, Paul G.; Moran, Andrew M.
2013-01-01
Highlights: • We discuss the outlook for multidimensional spectroscopies in the deep UV. • Photophysics are examined in small DNA components at cryogenic temperatures. • Wavepacket motions are detected in ring-opening systems with 2DUV spectroscopy. • Measurements of electronic wavepacket motions in molecules are proposed. - Abstract: Nonlinear laser spectroscopies in the deep UV spectral range are motivated by studies of biological systems and elementary processes in small molecules. This perspective article discusses recent technical advances in this area with a particular emphasis on diffractive optic based approaches to four-wave mixing spectroscopies. Applications to two classes of systems illustrate present experimental capabilities. First, experiments on DNA components at cryogenic temperatures are used to uncover features of excited state potential energy surfaces and vibrational cooling mechanisms. Second, sub-200 fs internal conversion processes and coherent wavepacket motions are investigated in cyclohexadiene and α-terpinene. Finally, we propose new experimental directions that combine methods for producing few-cycle UV laser pulses in noble gases with incoherent detection methods (e.g., photoionization) in experiments with time resolution near a singlefemtosecond. These measurements are motivated by knowledge of extremely fast non-adiabatic dynamics and the resolution of electronic wavepacket motions in molecules
Pal, P.; Ghosh, A.
2016-07-01
In this paper, we have studied the dynamics and relaxation of charge carriers in poly(methylmethacrylate)-lithium salt based polymer electrolytes plasticized with ethylene carbonate. Structural and thermal properties have been examined using X-ray diffraction and differential scanning calorimetry, respectively. We have analyzed the complex conductivity spectra by using power law model coupled with the contribution of electrode polarization at low frequencies and high temperatures. The temperature dependence of the ionic conductivity and crossover frequency exhibits Vogel-Tammann-Fulcher type behavior indicating a strong coupling between the ionic and the polymer chain segmental motions. The scaling of the ac conductivity indicates that relaxation dynamics of charge carriers follows a common mechanism for all temperatures and ethylene carbonate concentrations. The analysis of the ac conductivity also shows the existence of a nearly constant loss in these polymer electrolytes at low temperatures and high frequencies. The fraction of free anions and ion pairs in polymer electrolyte have been obtained from the analysis of Fourier transform infrared spectra. It is observed that these quantities influence the behavior of the composition dependence of the ionic conductivity.
Energy Technology Data Exchange (ETDEWEB)
Pal, P.; Ghosh, A., E-mail: sspag@iacs.res.in [Department of Solid State Physics, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032 (India)
2016-07-28
In this paper, we have studied the dynamics and relaxation of charge carriers in poly(methylmethacrylate)-lithium salt based polymer electrolytes plasticized with ethylene carbonate. Structural and thermal properties have been examined using X-ray diffraction and differential scanning calorimetry, respectively. We have analyzed the complex conductivity spectra by using power law model coupled with the contribution of electrode polarization at low frequencies and high temperatures. The temperature dependence of the ionic conductivity and crossover frequency exhibits Vogel-Tammann-Fulcher type behavior indicating a strong coupling between the ionic and the polymer chain segmental motions. The scaling of the ac conductivity indicates that relaxation dynamics of charge carriers follows a common mechanism for all temperatures and ethylene carbonate concentrations. The analysis of the ac conductivity also shows the existence of a nearly constant loss in these polymer electrolytes at low temperatures and high frequencies. The fraction of free anions and ion pairs in polymer electrolyte have been obtained from the analysis of Fourier transform infrared spectra. It is observed that these quantities influence the behavior of the composition dependence of the ionic conductivity.
Xu, Jianhua; Chen, Binbin; Callis, Patrik Robert; Muiño, Pedro L; Rozeboom, Henriette J; Broos, Jaap; Toptygin, Dmitri; Brand, Ludwig; Knutson, Jay R
2015-01-01
Time Dependent Fluorescence Stokes (emission wavelength) Shifts (TDFSS) from tryptophan (Trp) following sub-picosecond excitation are increasingly used to investigate protein dynamics, most recently enabling active research interest into water dynamics near the surface of proteins. Unlike many
Czech Academy of Sciences Publication Activity Database
Levy, Yoann; Derrien, Thibault; Bulgakova, Nadezhda M.; Gurevich, E.L.; Mocek, Tomáš
2016-01-01
Roč. 374, Jun (2016), s. 157-164 ISSN 0169-4332 R&D Projects: GA MŠk ED2.1.00/01.0027 Grant - others:HILASE(XE) CZ.1.05/2.1.00/01.0027; OP VK 6(XE) CZ.1.07/2.3.00/20.0143; OP VK 6(XE) CZ.1.07/2.3.00/20.0143 Institutional support: RVO:68378271 Keywords : LIPSS * modulated temperature relaxation * two-temperature model * nano-melting Subject RIV: BH - Optics, Masers, Lasers Impact factor: 3.387, year: 2016
Structural and dynamical characterization of piroxicam by 1H- and 13C-NMR relaxation studies
International Nuclear Information System (INIS)
Rossi, C.; Casini, A.; Picchi, M.P.; Laschi, F.; Calabria, A.; Marcolongo, R.
1987-01-01
Carbon spin-lattice relaxation rates of anti-inflammatory drug, piroxicam, have been measured. These results have been used in determining the reorientational rates of the proton carbon vectors. An analysis of internal motions within the pyridinyl moiety of piroxicam was carried out. Selective proton-carbon nuclear Overhauser effect (NOE) measurements were made in order to determine the solution structure of piroxicam. The effect of indirect NOE arising from exchangeable protons has been analyzed and considered. 20 refs.; 4 figs.; 3 tabs
Analysis of the backbone dynamics of capsicein using 15N NMR relaxation rate measurements
International Nuclear Information System (INIS)
Van Heijenoort, C.; Bouaziz, S.; Guittet, E.
1994-01-01
15 N relaxation times T 1 and T 1ρ , and heteronuclear steady state nOes, were measured on capsicein, a 98 residue protein. The classical analysis of these data using directly the Lipari and Szabo formalism was shown to give incoherent results, probably due to the presence of a slow exchange along the whole protein. This global exchange broadening made the usual preliminary evaluation of the overall correlation time of capsicein using the Lipari and Szabo expression for the spectral densities impossible. (authors). 2 figs., 23 refs
Semiclassical Monte Carlo: A first principles approach to non-adiabatic molecular dynamics
International Nuclear Information System (INIS)
White, Alexander J.; Gorshkov, Vyacheslav N.; Wang, Ruixi; Tretiak, Sergei; Mozyrsky, Dmitry
2014-01-01
Modeling the dynamics of photophysical and (photo)chemical reactions in extended molecular systems is a new frontier for quantum chemistry. Many dynamical phenomena, such as intersystem crossing, non-radiative relaxation, and charge and energy transfer, require a non-adiabatic description which incorporate transitions between electronic states. Additionally, these dynamics are often highly sensitive to quantum coherences and interference effects. Several methods exist to simulate non-adiabatic dynamics; however, they are typically either too expensive to be applied to large molecular systems (10's-100's of atoms), or they are based on ad hoc schemes which may include severe approximations due to inconsistencies in classical and quantum mechanics. We present, in detail, an algorithm based on Monte Carlo sampling of the semiclassical time-dependent wavefunction that involves running simple surface hopping dynamics, followed by a post-processing step which adds little cost. The method requires only a few quantities from quantum chemistry calculations, can systematically be improved, and provides excellent agreement with exact quantum mechanical results. Here we show excellent agreement with exact solutions for scattering results of standard test problems. Additionally, we find that convergence of the wavefunction is controlled by complex valued phase factors, the size of the non-adiabatic coupling region, and the choice of sampling function. These results help in determining the range of applicability of the method, and provide a starting point for further improvement
International Nuclear Information System (INIS)
Lascialfari, A.; Borsa, F.; Carretta, P.; Jang, Z.H.; Borsa, F.; Gatteschi, D.
1998-01-01
Measurements of the spin-lattice relaxation rate are reported for muons and protons as a function of temperature for different values of the applied magnetic field in the Mn 12 O 12 molecular cluster. Strongly field dependent maxima in the relaxation rate versus temperature are observed below 50thinspthinspK. The results are explained in terms of thermal fluctuations of the total magnetization of the cluster among the different orientations with respect to the anisotropy axis. The lifetimes of the different m components of the total spin, S T =10 , of the molecule are obtained from the experiment and shown to be consistent with the ones expected from a spin-phonon coupling mechanism. No clear evidence for macroscopic quantum tunneling was observed in the field dependence of the proton relaxation rate at low T . copyright 1998 The American Physical Society
Probing spin dynamics and quantum relaxation in Li Y0.998 Ho0.002 F4 via 19F NMR
Graf, M. J.; Lascialfari, A.; Borsa, F.; Tkachuk, A. M.; Barbara, B.
2006-01-01
We report measurements of F19 nuclear spin-lattice relaxation 1/T1 as a function of temperature and external magnetic field in a LiY0.998Ho0.002F4 single crystal, a single-ion magnet exhibiting interesting quantum effects. The F19 1/T1 is found to depend on the coupling with the diluted rare-earth (RE) moments, making it an effective probe of the rare-earth spin dynamics. The results for 1/T1 show a behavior similar to that observed in molecular nanomagnets, a result which we attribute to the discreteness of the energy levels in both cases. At intermediate temperatures the lifetime broadening of the crystal field split RE magnetic levels follows a T3 power law. At low temperature the field dependence of 1/T1 shows peaks in correspondence to the critical magnetic fields for energy level crossings (LC). A key result of this study is that the broadening of the levels at LC is found to become extremely small at low temperatures, about 1.7mT , a value which is comparable to the weak dipolar fields at the RE lattice positions. Thus, unlike the molecular magnets, decoherence effects are strongly suppressed, and it may be possible to measure directly the level repulsions at avoided level crossings.
Crystal structure and pair potentials: A molecular-dynamics study
Energy Technology Data Exchange (ETDEWEB)
Parrinello, M.; Rahman, A.
1980-10-06
With use of a Lagrangian which allows for the variation of the shape and size of the periodically repeating molecular-dynamics cell, it is shown that different pair potentials lead to different crystal structures.
Molecular Interactions and Reaction Dynamics in Supercritical Water Oxidation
National Research Council Canada - National Science Library
Johnston, K
1998-01-01
.... From UV-vis spectroscopic measurements and molecular dynamics simulation of chemical equilibria, we have shown that density effects on broad classes of reactions may be explained in terms of changes...
Molecular Dynamics and Bioactivity of a Novel Mutated Human ...
African Journals Online (AJOL)
Keywords: Parathyroid hormone, Mutation prediction, Molecular dynamics, RANKL/OPG, UAMS-32P cell. Tropical .... PTH1R were used as MD simulation starting points. A full-atom ... Values of RMSD, Rg, and potential energy evaluation ...
Olefin Metathesis in Peptidomimetics, Dynamic Combinatorial Chemistry, and Molecular Imprinting
National Research Council Canada - National Science Library
Low, Tammy K
2006-01-01
.... Our research goals consisted of employing olefin metathesis in the synthesis of peptidomimetics, and studying the feasibility of this method in dynamic combinatorial chemistry and molecular imprinting of nerve agents...
A molecular dynamics calculation of solid phase of malonic acid ...
Indian Academy of Sciences (India)
Sathya S R R Perumal
Keywords. Hydrogen bond chain; elastic constants; molecular dynamics. 1. Introduction ... theory - a probabilistic model to determine the hydro- gen bonds within the .... compares poorly with the experimental value of 108.5. Similarly β and γ ...
Energy Technology Data Exchange (ETDEWEB)
Borg, Matthew K., E-mail: matthew.borg@strath.ac.uk [Department of Mechanical and Aerospace Engineering, University of Strathclyde, Glasgow G1 1XJ (United Kingdom); Lockerby, Duncan A., E-mail: duncan.lockerby@warwick.ac.uk [School of Engineering, University of Warwick, Coventry CV4 7AL (United Kingdom); Reese, Jason M., E-mail: jason.reese@ed.ac.uk [School of Engineering, University of Edinburgh, Edinburgh EH9 3JL (United Kingdom)
2014-02-21
The emergence of new applications of molecular dynamics (MD) simulation calls for the development of mass-statting procedures that insert or delete particles on-the-fly. In this paper we present a new mass-stat which we term FADE, because it gradually “fades-in” (inserts) or “fades-out” (deletes) molecules over a short relaxation period within a MD simulation. FADE applies a time-weighted relaxation to the intermolecular pair forces between the inserting/deleting molecule and any neighbouring molecules. The weighting function we propose in this paper is a piece-wise polynomial that can be described entirely by two parameters: the relaxation time scale and the order of the polynomial. FADE inherently conserves overall system momentum independent of the form of the weighting function. We demonstrate various simulations of insertions of atomic argon, polyatomic TIP4P water, polymer strands, and C{sub 60} Buckminsterfullerene molecules. We propose FADE parameters and a maximum density variation per insertion-instance that restricts spurious potential energy changes entering the system within desired tolerances. We also demonstrate in this paper that FADE compares very well to an existing insertion algorithm called USHER, in terms of accuracy, insertion rate (in dense fluids), and computational efficiency. The USHER algorithm is applicable to monatomic and water molecules only, but we demonstrate that FADE can be generally applied to various forms and sizes of molecules, such as polymeric molecules of long aspect ratio, and spherical carbon fullerenes with hollow interiors.
International Nuclear Information System (INIS)
Borg, Matthew K.; Lockerby, Duncan A.; Reese, Jason M.
2014-01-01
The emergence of new applications of molecular dynamics (MD) simulation calls for the development of mass-statting procedures that insert or delete particles on-the-fly. In this paper we present a new mass-stat which we term FADE, because it gradually “fades-in” (inserts) or “fades-out” (deletes) molecules over a short relaxation period within a MD simulation. FADE applies a time-weighted relaxation to the intermolecular pair forces between the inserting/deleting molecule and any neighbouring molecules. The weighting function we propose in this paper is a piece-wise polynomial that can be described entirely by two parameters: the relaxation time scale and the order of the polynomial. FADE inherently conserves overall system momentum independent of the form of the weighting function. We demonstrate various simulations of insertions of atomic argon, polyatomic TIP4P water, polymer strands, and C 60 Buckminsterfullerene molecules. We propose FADE parameters and a maximum density variation per insertion-instance that restricts spurious potential energy changes entering the system within desired tolerances. We also demonstrate in this paper that FADE compares very well to an existing insertion algorithm called USHER, in terms of accuracy, insertion rate (in dense fluids), and computational efficiency. The USHER algorithm is applicable to monatomic and water molecules only, but we demonstrate that FADE can be generally applied to various forms and sizes of molecules, such as polymeric molecules of long aspect ratio, and spherical carbon fullerenes with hollow interiors
Tarnacka, M; Madejczyk, O; Adrjanowicz, K; Pionteck, J; Kaminska, E; Kamiński, K; Paluch, M
2015-06-14
Pressure-Volume-Temperature (PVT) measurements and broadband dielectric spectroscopy were carried out to investigate molecular dynamics and to test the validity of thermodynamic scaling of two homologous compounds of pharmaceutical activity: itraconazole and ketoconazole in the wide range of thermodynamic conditions. The pressure coefficients of the glass transition temperature (dT(g)/dp) for itraconazole and ketoconazole were determined to be equal to 183 and 228 K/GPa, respectively. However, for itraconazole, the additional transition to the nematic phase was observed and characterized by the pressure coefficient dT(n)/dp = 258 K/GPa. From PVT and dielectric data, we obtained that the liquid-nematic phase transition is governed by the relaxation time since it occurred at constant τ(α) = 10(-5) s. Furthermore, we plotted the obtained relaxation times as a function of T(-1)v(-γ), which has revealed that the validity of thermodynamic scaling with the γ exponent equals to 3.69 ± 0.04 and 3.64 ± 0.03 for itraconazole and ketoconazole, respectively. Further analysis of the scaling parameter in itraconazole revealed that it unexpectedly decreases with increasing relaxation time, which resulted in dramatic change of the shape of the thermodynamic scaling master curve. While in the case of ketoconazole, it remained the same within entire range of data (within experimental uncertainty). We suppose that in case of itraconazole, this peculiar behavior is related to the liquid crystals' properties of itraconazole molecule.
Heyes, D. M.; Nuevo, M. J.; Morales, J. J.
Following on from our previous study (Heyes, D. M., Nuevo, M. J, and Morales, J. J., 1996, Molec. Phys., 88, 1503), molecular dynamics simulations have been carried out of translational and rotational diffusion of atomistically rough near-spherical solid Lennard-Jones (LJ) clusters immersed in a Weeks-Chandler-Andersen liquid solvent. A single cluster consisting of up to about 100LJ particles as part of an 8000 atom fluid system was considered in each case. The translational and rotational diffusion coefficients decrease with increasing cluster size and solvent density (roughly in proportion to the molar volume of the solvent). The simulations reveal that for clusters in excess of about 30LJ atoms there is a clear separation of timescales between angular velocity and orientation relaxation which adhere well to the small-step diffusion model encapsulated in Hubbard's relationship. For 100 atom clusters both the StokesEinstein (translation) and Stokes-Einstein-Debye (rotation) equations apply approximately. The small departures from these reference solutions indicate that the translational relaxation experiences a local viscosity in excess of the bulk value (typically by ~ 30%), whereas rotational relaxation experiences a smaller viscosity than the bulk (typically by ~ 30%) reasonably in accord with the Gierer-Wirtz model. Both of these observations are consistent with an observed layering of the liquid molecules next to the cluster observed in our previous study.
Molecular dynamics and Monte Carlo calculations in statistical mechanics
International Nuclear Information System (INIS)
Wood, W.W.; Erpenbeck, J.J.
1976-01-01
Monte Carlo and molecular dynamics calculations on statistical mechanical systems is reviewed giving some of the more significant recent developments. It is noted that the term molecular dynamics refers to the time-averaging technique for hard-core and square-well interactions and for continuous force-law interactions. Ergodic questions, methodology, quantum mechanical, Lorentz, and one-dimensional, hard-core, and square and triangular-well systems, short-range soft potentials, and other systems are included. 268 references
Next generation extended Lagrangian first principles molecular dynamics.
Niklasson, Anders M N
2017-08-07
Extended Lagrangian Born-Oppenheimer molecular dynamics [A. M. N. Niklasson, Phys. Rev. Lett. 100, 123004 (2008)] is formulated for general Hohenberg-Kohn density-functional theory and compared with the extended Lagrangian framework of first principles molecular dynamics by Car and Parrinello [Phys. Rev. Lett. 55, 2471 (1985)]. It is shown how extended Lagrangian Born-Oppenheimer molecular dynamics overcomes several shortcomings of regular, direct Born-Oppenheimer molecular dynamics, while improving or maintaining important features of Car-Parrinello simulations. The accuracy of the electronic degrees of freedom in extended Lagrangian Born-Oppenheimer molecular dynamics, with respect to the exact Born-Oppenheimer solution, is of second-order in the size of the integration time step and of fourth order in the potential energy surface. Improved stability over recent formulations of extended Lagrangian Born-Oppenheimer molecular dynamics is achieved by generalizing the theory to finite temperature ensembles, using fractional occupation numbers in the calculation of the inner-product kernel of the extended harmonic oscillator that appears as a preconditioner in the electronic equations of motion. Material systems that normally exhibit slow self-consistent field convergence can be simulated using integration time steps of the same order as in direct Born-Oppenheimer molecular dynamics, but without the requirement of an iterative, non-linear electronic ground-state optimization prior to the force evaluations and without a systematic drift in the total energy. In combination with proposed low-rank and on the fly updates of the kernel, this formulation provides an efficient and general framework for quantum-based Born-Oppenheimer molecular dynamics simulations.
Dynamical analysis of highly excited molecular spectra
Energy Technology Data Exchange (ETDEWEB)
Kellman, M.E. [Univ. of Oregon, Eugene (United States)
1993-12-01
The goal of this program is new methods for analysis of spectra and dynamics of highly excited vibrational states of molecules. In these systems, strong mode coupling and anharmonicity give rise to complicated classical dynamics, and make the simple normal modes analysis unsatisfactory. New methods of spectral analysis, pattern recognition, and assignment are sought using techniques of nonlinear dynamics including bifurcation theory, phase space classification, and quantization of phase space structures. The emphasis is chaotic systems and systems with many degrees of freedom.
Molecular Dynamics Simulations of Kinetic Models for Chiral Dominance in Soft Condensed Matter
DEFF Research Database (Denmark)
Toxvaerd, Søren
2001-01-01
Molecular dynamics simulation, models for isomerization kinetics, origin of biomolecular chirality......Molecular dynamics simulation, models for isomerization kinetics, origin of biomolecular chirality...
A fermionic molecular dynamics technique to model nuclear matter
International Nuclear Information System (INIS)
Vantournhout, K.; Jachowicz, N.; Ryckebusch, J.
2009-01-01
Full text: At sub-nuclear densities of about 10 14 g/cm 3 , nuclear matter arranges itself in a variety of complex shapes. This can be the case in the crust of neutron stars and in core-collapse supernovae. These slab like and rod like structures, designated as nuclear pasta, have been modelled with classical molecular dynamics techniques. We present a technique, based on fermionic molecular dynamics, to model nuclear matter at sub-nuclear densities in a semi classical framework. The dynamical evolution of an antisymmetric ground state is described making the assumption of periodic boundary conditions. Adding the concepts of antisymmetry, spin and probability distributions to classical molecular dynamics, brings the dynamical description of nuclear matter to a quantum mechanical level. Applications of this model vary from investigation of macroscopic observables and the equation of state to the study of fundamental interactions on the microscopic structure of the matter. (author)
Current-driven dynamics in molecular-scale devices
International Nuclear Information System (INIS)
Seideman, Tamar
2003-01-01
We review recent theoretical work on current-triggered processes in molecular-scale devices - a field at the interface between solid state physics and chemical dynamics with potential applications in diverse areas, including artificial molecular machines, unimolecular transport, surface nanochemistry and nanolithography. The qualitative physics underlying current-triggered dynamics is first discussed and placed in context with several well-studied phenomena with which it shares aspects. A theory for modelling these dynamics is next formulated within a time-dependent scattering approach. Our end result provides useful insight into the system properties that determine the reaction outcome as well as a computationally convenient framework for numerical realization. The theory is applied to study single-molecule surface reactions induced by a scanning tunnelling microscope and current-triggered dynamics in single-molecule transistors. We close with a discussion of several potential applications of current-induced dynamics in molecular devices and several opportunities for future research. (topical review)
Molecular Dynamic Modeling and Simulation for Polymers
National Research Council Canada - National Science Library
Harrell, Anthony
2003-01-01
... the mechanical properties of polymers. In particular, the goal was to develop insights as to how a molecular level structure is connected to the bulk properties of materials assuming homogeneity...
Molecular dynamics of a proguanil derivative
African Journals Online (AJOL)
pc
Proguanil is a prophylactic antimalarial drug t .... presence of resistance to individual component. ... This is the mathematical ... predicting equilibrium structures of molecular systems ..... for the modeling and subsequent development of.
Bhattacharya, Sisir; Bhardwaj, Sunny P; Suryanarayanan, Raj
2014-10-01
To determine the effect of annealing on the two secondary relaxations in amorphous sucrose and in sucrose solid dispersions. Sucrose was co-lyophilized with either PVP or sorbitol, annealed for different time periods and analyzed by dielectric spectroscopy. In an earlier investigation, we had documented the effect of PVP and sorbitol on the primary and the two secondary relaxations in amorphous sucrose solid dispersions (1). Here we investigated the effect of annealing on local motions, both in amorphous sucrose and in the dispersions. The average relaxation time of the local motion (irrespective of origin) in sucrose, decreased upon annealing. However, the heterogeneity in relaxation time distribution as well as the dielectric strength decreased only for β1- (the slower relaxation) but not for β2-relaxations. The effect of annealing on β2-relaxation times was neutralized by sorbitol while PVP negated the effect of annealing on both β1- and β2-relaxations. An increase in local mobility of sucrose brought about by annealing could be negated with an additive.
Visualizing functional motions of membrane transporters with molecular dynamics simulations.
Shaikh, Saher A; Li, Jing; Enkavi, Giray; Wen, Po-Chao; Huang, Zhijian; Tajkhorshid, Emad
2013-01-29
Computational modeling and molecular simulation techniques have become an integral part of modern molecular research. Various areas of molecular sciences continue to benefit from, indeed rely on, the unparalleled spatial and temporal resolutions offered by these technologies, to provide a more complete picture of the molecular problems at hand. Because of the continuous development of more efficient algorithms harvesting ever-expanding computational resources, and the emergence of more advanced and novel theories and methodologies, the scope of computational studies has expanded significantly over the past decade, now including much larger molecular systems and far more complex molecular phenomena. Among the various computer modeling techniques, the application of molecular dynamics (MD) simulation and related techniques has particularly drawn attention in biomolecular research, because of the ability of the method to describe the dynamical nature of the molecular systems and thereby to provide a more realistic representation, which is often needed for understanding fundamental molecular properties. The method has proven to be remarkably successful in capturing molecular events and structural transitions highly relevant to the function and/or physicochemical properties of biomolecular systems. Herein, after a brief introduction to the method of MD, we use a number of membrane transport proteins studied in our laboratory as examples to showcase the scope and applicability of the method and its power in characterizing molecular motions of various magnitudes and time scales that are involved in the function of this important class of membrane proteins.
Energy Technology Data Exchange (ETDEWEB)
Holderna-Natkaniec, K.; Wasicki, J.W. [Univ. A. Mickiewicza, Poznan (Poland); Natkaniec, I. [Lab. of Neutron Physics, Joint Inst. for Nuclear Research, Dubna (Joint Institute for Nuclear Research (JINR))
1995-12-31
Inelastic incoherent neutron scattering as well as NMR methods have been used for the study of molecular dynamics of bornyl chloride. The temperature dependence of spin-lattice relaxation time has been determined. The different type of molecular reorientations have been postulated in three terpene phases existed in 10-186 K temperature range. 5 refs, 3 figs.
Pseudorotational dynamics of small molecular species
International Nuclear Information System (INIS)
Hagelberg, F.
2002-01-01
The electron nuclear dynamics (END) theory was designed to provide a full description of the dynamic development of the electronic system. It is independent of any potential energy surface constructions. The dynamic behavior of molecules close to the threshold of dissociation was the objective of this study. Thus, simulations based on END theory were performed with the aim to extend the current understanding of the dynamic features of pseudorotational into a non-adiabatic regime. Electron dynamics of triatomic species (H 3 + and Li 3 + ) in terms of electronic angular momentum expectation values were characterized. Finally, it is shown that the expansion coefficients which carry the information about the excitation content of the electronic system at any stage of the motional process can be calculated. (nevyjel)
Crossover from Rouse dynamics to the α- relaxation in a poly(vinyl ethylene)
International Nuclear Information System (INIS)
Arbe, A.; Colmenero, J.; Richter, D.; Monkenbusch, M.; Willner, L.; Farago, B.
2004-01-01
By means of neutron spin echo (NSE) we have explored the dynamics of poly(vinyl ethylene) on length scales covering Rouse dynamics and below. The results establish the simultaneous existence of a generic sublinear diffusion regime which underlies the α-process in addition to the Rouse process. Both regimes are separated by a well- defined dynamic crossover. From that the size of the Gaussian blobs making up the Rouse model is determined directly. The glassy dynamics may thus be identified with subdiffusive motions occurring within these Gaussian blobs. (author)
Migliardo, F; Angell, C A; Magazù, S
2017-01-01
Glycerol and sorbitol are glass-forming hydrogen-bonded systems characterized by intriguing properties which make these systems very interesting also from the applications point of view. The goal of this work is to relate the hydrogen-bonded features, relaxation dynamics, glass transition properties and fragility of these systems, in particular to seek insight into their very different liquid fragilities. The comparison between glycerol and sorbitol is carried out by collecting the elastic incoherent neutron scattering (EINS) intensity as a function of temperature and of the instrumental energy resolution. Intensity data vs temperature and resolution are analyzed in terms of thermal restraint and Resolution Elastic Neutron Scattering (RENS) approaches. The number of OH groups, which are related to the connecting sites, is a significant parameter both in the glass transition and in the dynamical transition. On the other hand, the disordered nature of sorbitol is confirmed by the existence of different relaxation processes. From the applications point of view, glycerol and sorbitol have remarkable bioprotectant properties which make these systems useful in different technological and industrial fields. Furthermore, polyols are rich in glassforming liquid phenomenology and highly deserving of study in their own right. The comparison of EINS and calorimetric data on glycerol and sorbitol helps provide a connection between structural relaxation, dynamical transition, glass transition, and fragility. The evaluation of the inflection point in the elastic intensity behavior as a function of temperature and instrumental energy resolution provides a confirmation of the validity of the RENS approach. This article is part of a Special Issue entitled "Science for Life" Guest Editor: Dr. Austen Angell, Dr. Salvatore Magazù and Dr. Federica Migliardo. Copyright © 2016. Published by Elsevier B.V.
Talantsev, A.; Lu, Y.; Fache, T.; Lavanant, M.; Hamadeh, A.; Aristov, A.; Koplak, O.; Morgunov, R.; Mangin, S.
2018-04-01
Two synthetic antiferromagnet bilayer systems with strong perpendicular anisotropy CoFeB/Ta/CoFeB and Pt/Co/Ir/Co/Pt have been grown using sputtering techniques. For both systems two types of magnetization transitions have been studied. The first one concerns transitions from a state where magnetizations of the two magnetic layers are parallel (P state) to a state where magnetizations of the two layers are aligned antiparallel (AP state). The second one concerns transitions between the two possible antiparallel alignments (AP+ to AP-). For both systems and both transitions after-effect measurements can be understood in the frame of nucleation—propagation model. Time derivative analysis of magnetic relaxation curves and mapping of the first order reversal curves at different temperature allowed us to demonstrate the presence of different pinning centers, which number can be controlled by magnetic field and temperature.
Attractors of relaxation discrete-time systems with chaotic dynamics on a fast time scale
International Nuclear Information System (INIS)
Maslennikov, Oleg V.; Nekorkin, Vladimir I.
2016-01-01
In this work, a new type of relaxation systems is considered. Their prominent feature is that they comprise two distinct epochs, one is slow regular motion and another is fast chaotic motion. Unlike traditionally studied slow-fast systems that have smooth manifolds of slow motions in the phase space and fast trajectories between them, in this new type one observes, apart the same geometric objects, areas of transient chaos. Alternating periods of slow regular motions and fast chaotic ones as well as transitions between them result in a specific chaotic attractor with chaos on a fast time scale. We formulate basic properties of such attractors in the framework of discrete-time systems and consider several examples. Finally, we provide an important application of such systems, the neuronal electrical activity in the form of chaotic spike-burst oscillations.
Colmenero, J.; Alegría, A.; Arbe, A.; Frick, B.
1992-12-01
The dynamics of the α-relaxation in three glass-forming polymeric systems, poly(vinyl methyl ether) (PVME), poly(vinyl chloride) (PVC), and poly(bisphenol A, 2-hydroxypropylether) (PH) has been studied by means of quasielastic neutron scattering and compared with the results obtained from relaxation techniques. The results indicate that the dynamics of the α-relaxation in a wide timescale shows a clear non-Debye behaviour and can be well described by means of the same spectral shape, which is found to be independent of temperature and momentum transfer ( Q). Moreover, the Havriliak-Negami characteristic times deduced from the fitting of the experimental data can also be described using only one Vogel-Fulcher functional form. This implies a self-consistent description of the dynamics of the α-relaxation obtained by very different probes. Besides, we found that the Q-dependence of the characteristic times obtained by QENS is given by a power law, τ(Q) ∝ Q - n ( n > 2), n being dependent on the system, and that the Q-behaviour and the non-Debye behaviour are directly correlated. These results have main implications about the physical mechanisms behind the dynamics of the α-relaxation.
Dynamic combinatorial libraries based on hydrogen-bonde molecular boxes
Kerckhoffs, J.M.C.A.; Mateos timoneda, Miguel; Reinhoudt, David; Crego Calama, Mercedes
2007-01-01
This article describes two different types of dynamic combinatorial libraries of host and guest molecules. The first part of this article describes the encapsulation of alizarin trimer 2 a3 by dynamic mixtures of up to twenty different self-assembled molecular receptors together with the
Energy conservation in molecular dynamics simulations of classical systems
DEFF Research Database (Denmark)
Toxværd, Søren; Heilmann, Ole; Dyre, J. C.
2012-01-01
Classical Newtonian dynamics is analytic and the energy of an isolated system is conserved. The energy of such a system, obtained by the discrete “Verlet” algorithm commonly used in molecular dynamics simulations, fluctuates but is conserved in the mean. This is explained by the existence...
International Nuclear Information System (INIS)
Gonzalez, R L; Leyet, Y; Guerrero, F; Guerra, J de Los S; Venet, M; Eiras, J A
2007-01-01
The relaxation dynamics of the conductive process present in PbNb 2 O 6 piezoelectric ceramics was investigated. A relaxation function in the time domain, Φ(t), was found from the frequency dependence of the dielectric modulus (imaginary component, M'') by using a relaxation function in the frequency domain, F*(ω). The best relaxation function, F*(ω), was found to be a Cole-Cole distribution function, in which relaxation characteristic parameters, such as α and τ CC , are involved. On the other hand, the relaxation function, Φ(t), obtained by the time domain method, was found to be a Kohlrausch-Williams-Watts (KWW) function type. The thermal evolution of the characteristics parameters of the KWW function (β and τ*) was analysed. The values of the activation energy (E a ), obtained in the whole investigated temperature interval, suggest the existence of a relaxation mechanism (a conductive process), which may be interpreted by an ion hopping between neighbouring sites within the crystalline lattice. The results are corroborated with the formalism of the AC conductivity
Classical molecular dynamics simulation of weakly-bound projectile heavy-ion reactions
Directory of Open Access Journals (Sweden)
Morker Mitul R.
2015-01-01
Full Text Available A 3-body classical molecular dynamics approach for heavy-ion reactions involving weakly bound projectiles is developed. In this approach a weakly bound projectile is constructed as a two-body cluster of the constituent tightly bound nuclei in a configuration corresponding to the observed breakup energy. This 3-body system with their individual nucleon configuration in their ground state is dynamically evolved for given initial conditions using the three-stage classical molecular dynamics approach (3S-CMD. Various levels of rigidbody constraints on the projectile constituents and the target are considered at appropriate stages. This 3-dimensional approach explicitly takes into account not only the long range Coulomb reorientation of the deformed collision partner but internal excitations and breakup probabilities at distances close to the barrier also. Dynamical simulations of 6Li+209Bi show all the possible reaction mechanism like complete fusion, incomplete fusion, scattering and breakup scattering. Complete fusion cross sections of 6Li+209Bi and 7Li+209Bi reactions are calculated in this approach with systematic relaxations of the rigid-body constraints on one or more constituent nuclei.
International Nuclear Information System (INIS)
Sahu, Sarata C.; Bhuyan, Abani K.; Udgaonkar, Jayant B.; Hosur, R.V.
2000-01-01
Backbone dynamics of uniformly 15 N-labeled free barnase and its complex with unlabelled barstar have been studied at 40 deg. C, pH 6.6, using 15 N relaxation data obtained from proton-detected 2D { 1 H}- 15 N NMR spectroscopy. 15 N spin-lattice relaxation rate constants (R 1 ), spin-spin relaxation rate constants (R 2 ), and steady-state heteronuclear { 1 H}- 15 N NOEs have been measured at a magnetic field strength of 14.1 Tesla for 91 residues of free barnase and for 90 residues out of a total of 106 in the complex (excluding three prolines and the N-terminal residue) backbone amide 15 N sites of barnase. The primary relaxation data for both the cases have been analyzed in the framework of the model-free formalism using both isotropic and axially symmetric models of the rotational diffusion tensor. As per the latter, the overall rotational correlation times (τ m ) are 5.0 and 9.5 ns for the free and complexed barnase, respectively. The average order parameter is found to be 0.80 for free barnase and 0.86 for the complex. However, the changes are not uniform along the backbone and for about 5 residues near the binding interface there is actually a significant decrease in the order parameters on complex formation. These residues are not involved in the actual binding. For the residues where the order parameter increases, the magnitudes vary significantly. It is observed that the complex has much less internal mobility, compared to free barnase. From the changes in the order parameters, the entropic contribution of NH bond vector motion to the free energy of complex formation has been calculated. It is apparent that these motions cause significant unfavorable contributions and therefore must be compensated by many other favorable contributions to effect tight complex formation. The observed variations in the motion and their different locations with regard to the binding interface may have important implications for remote effects and regulation of the enzyme
Invariant molecular-dynamics approach to structural phase transitions
International Nuclear Information System (INIS)
Wentzcovitch, R.M.
1991-01-01
Two fictitious Lagrangians to be used in molecular-dynamics simulations with variable cell shape and suitable to study problems like structural phase transitions are introduced. Because they are invariant with respect to the choice of the simulation cell edges and eliminate symmetry breaking associated with the fictitious part of the dynamics, they improve the physical content of numerical simulations that up to now have been done by using Parrinello-Rahman dynamics
Ab initio molecular dynamics in a finite homogeneous electric field.
Umari, P; Pasquarello, Alfredo
2002-10-07
We treat homogeneous electric fields within density functional calculations with periodic boundary conditions. A nonlocal energy functional depending on the applied field is used within an ab initio molecular dynamics scheme. The reliability of the method is demonstrated in the case of bulk MgO for the Born effective charges, and the high- and low-frequency dielectric constants. We evaluate the static dielectric constant by performing a damped molecular dynamics in an electric field and avoiding the calculation of the dynamical matrix. Application of this method to vitreous silica shows good agreement with experiment and illustrates its potential for systems of large size.
Ramos, Sergio Luis L M; Ogino, Michihiko; Oguni, Masaharu
2015-01-28
We investigated the thermal properties of liquid methylcyclohexane and racemic sec-butylcyclohexane, as representatives of a molecular system with only dispersion-force intermolecular interactions, confined in the pores (thickness/diameter d = 12, 6, 1.1 nm) of silica gels by adiabatic calorimetry. The results imply a heterogeneous picture for molecular aggregate under confinement consisting of an interfacial region and an inner pore one. In the vicinity of a glass-transition temperature T(g,bulk) of bulk liquid, two distinguishable relaxation phenomena were observed for the confined systems and their origins were attributed to the devitrification, namely glass transition, processes of (1) a layer of interfacial molecules adjacent to the pore walls and (2) the molecules located in the middle of the pore. A third glass-transition phenomenon was observed at lower temperatures and ascribed to a secondary relaxation process. The glass transition of the interfacial-layer molecules was found to proceed at temperatures rather above T(g,bulk), whereas that of the molecules located in the inner pore region occurred at temperatures below T(g,bulk). We discuss the reason why the molecules located in different places in the pores reveal the respectively different dynamical properties.
Classical and quantum molecular dynamics in NMR spectra
Szymański, Sławomir
2018-01-01
The book provides a detailed account of how condensed-phase molecular dynamics are reflected in the line shapes of NMR spectra. The theories establishing connections between random, time-dependent molecular processes and lineshape effects are exposed in depth. Special emphasis is placed on the theoretical aspects, involving in particular intermolecular processes in solution, and molecular symmetry issues. The Liouville super-operator formalism is briefly introduced and used wherever it is beneficial for the transparency of presentation. The proposed formal descriptions of the discussed problems are sufficiently detailed to be implemented on a computer. Practical applications of the theory in solid- and liquid-phase studies are illustrated with appropriate experimental examples, exposing the potential of the lineshape method in elucidating molecular dynamics NMR-observable molecular phenomena where quantization of the spatial nuclear degrees of freedom is crucial are addressed in the last part of the book. As ...
Dynamics of molecular superrotors in an external magnetic field
Korobenko, Aleksey; Milner, Valery
2015-08-01
We excite diatomic oxygen and nitrogen to high rotational states with an optical centrifuge and study their dynamics in an external magnetic field. Ion imaging is employed to directly visualize, and follow in time, the rotation plane of the molecular superrotors. The two different mechanisms of interaction between the magnetic field and the molecular angular momentum in paramagnetic oxygen and non-magnetic nitrogen lead to qualitatively different behaviour. In nitrogen, we observe the precession of the molecular angular momentum around the field vector. In oxygen, strong spin-rotation coupling results in faster and richer dynamics, encompassing the splitting of the rotation plane into three separate components. As the centrifuged molecules evolve with no significant dispersion of the molecular wave function, the observed magnetic interaction presents an efficient mechanism for controlling the plane of molecular rotation.
Dynamics of molecular superrotors in an external magnetic field
International Nuclear Information System (INIS)
Korobenko, Aleksey; Milner, Valery
2015-01-01
We excite diatomic oxygen and nitrogen to high rotational states with an optical centrifuge and study their dynamics in an external magnetic field. Ion imaging is employed to directly visualize, and follow in time, the rotation plane of the molecular superrotors. The two different mechanisms of interaction between the magnetic field and the molecular angular momentum in paramagnetic oxygen and non-magnetic nitrogen lead to qualitatively different behaviour. In nitrogen, we observe the precession of the molecular angular momentum around the field vector. In oxygen, strong spin–rotation coupling results in faster and richer dynamics, encompassing the splitting of the rotation plane into three separate components. As the centrifuged molecules evolve with no significant dispersion of the molecular wave function, the observed magnetic interaction presents an efficient mechanism for controlling the plane of molecular rotation. (paper)
Kenkre, V. M.; Chase, M.
2017-08-01
The approach to equilibrium of a quantum mechanical system in interaction with a bath is studied from a practical as well as a conceptual point of view. Explicit memory functions are derived for given models of bath couplings. If the system is a harmonic oscillator representing a molecule in interaction with a reservoir, the generalized master equation derived becomes an extension into the coherent domain of the well-known Montroll-Shuler equation for vibrational relaxation and unimolecular dissociation. A generalization of the Bethe-Teller result regarding energy relaxation is found for short times. The theory has obvious applications to relaxation dynamics at ultra-short times as in observations on the femtosecond time scale and to the investigation of quantum coherence at those short times. While vibrational relaxation in chemical physics is a primary target of the study, another system of interest in condensed matter physics, an electron or hole in a lattice subjected to a strong DC electric field that gives rise to well-known Wannier-Stark ladders, is naturally addressed with the theory. Specific system-bath interactions are explored to obtain explicit details of the dynamics. General phenomenological descriptions of the reservoir are considered rather than specific microscopic realizations.
Molecular dynamics with deterministic and stochastic numerical methods
Leimkuhler, Ben
2015-01-01
This book describes the mathematical underpinnings of algorithms used for molecular dynamics simulation, including both deterministic and stochastic numerical methods. Molecular dynamics is one of the most versatile and powerful methods of modern computational science and engineering and is used widely in chemistry, physics, materials science and biology. Understanding the foundations of numerical methods means knowing how to select the best one for a given problem (from the wide range of techniques on offer) and how to create new, efficient methods to address particular challenges as they arise in complex applications. Aimed at a broad audience, this book presents the basic theory of Hamiltonian mechanics and stochastic differential equations, as well as topics including symplectic numerical methods, the handling of constraints and rigid bodies, the efficient treatment of Langevin dynamics, thermostats to control the molecular ensemble, multiple time-stepping, and the dissipative particle dynamics method...
Molecular sieving through a graphene nanopore: non-equilibrium molecular dynamics simulation
Institute of Scientific and Technical Information of China (English)
Chengzhen Sun; Bofeng Bai
2017-01-01
Two-dimensional graphene nanopores have shown great promise as ultra-permeable molecular sieves based on their size-sieving effects.We design a nitrogen/hydrogen modified graphene nanopore and conduct a transient non-equilibrium molecular dynamics simulation on its molecular sieving effects.The distinct time-varying molecular crossing numbers show that this special nanopore can efficiently sieve CO2 and H2S molecules from CH4 molecules with high selectivity.By analyzing the molecular structure and pore functionalization-related molecular orientation and permeable zone in the nanopore,density distribution in the molecular adsorption layer on the graphene surface,as well as other features,the molecular sieving mechanisms of graphene nanopores are revealed.Finally,several implications on the design of highly-efficient graphene nanopores,especially for determining the porosity and chemical functionalization,as gas separation membranes are summarized based on the identified phenomena and mechanisms.
First principles molecular dynamics without self-consistent field optimization
International Nuclear Information System (INIS)
Souvatzis, Petros; Niklasson, Anders M. N.
2014-01-01
We present a first principles molecular dynamics approach that is based on time-reversible extended Lagrangian Born-Oppenheimer molecular dynamics [A. M. N. Niklasson, Phys. Rev. Lett. 100, 123004 (2008)] in the limit of vanishing self-consistent field optimization. The optimization-free dynamics keeps the computational cost to a minimum and typically provides molecular trajectories that closely follow the exact Born-Oppenheimer potential energy surface. Only one single diagonalization and Hamiltonian (or Fockian) construction are required in each integration time step. The proposed dynamics is derived for a general free-energy potential surface valid at finite electronic temperatures within hybrid density functional theory. Even in the event of irregular functional behavior that may cause a dynamical instability, the optimization-free limit represents a natural starting guess for force calculations that may require a more elaborate iterative electronic ground state optimization. Our optimization-free dynamics thus represents a flexible theoretical framework for a broad and general class of ab initio molecular dynamics simulations
Fang, Shaoyin; Zhu, Ruidan; Lai, Tianshu
2017-03-21
Spin relaxation dynamics of holes in intrinsic GaAs quantum wells is studied using time-resolved circular dichromatic absorption spectroscopy at room temperature. It is found that ultrafast dynamics is dominated by the cooperative contributions of band filling and many-body effects. The relative contribution of the two effects is opposite in strength for electrons and holes. As a result, transient circular dichromatic differential transmission (TCD-DT) with co- and cross-circularly polarized pump and probe presents different strength at several picosecond delay time. Ultrafast spin relaxation dynamics of excited holes is sensitively reflected in TCD-DT with cross-circularly polarized pump and probe. A model, including coherent artifact, thermalization of nonthermal carriers and the cooperative contribution of band filling and many-body effects, is developed, and used to fit TCD-DT with cross-circularly polarized pump and probe. Spin relaxation time of holes is achieved as a function of excited hole density for the first time at room temperature, and increases with hole density, which disagrees with a theoretical prediction based on EY spin relaxation mechanism, implying that EY mechanism may be not dominant hole spin relaxation mechanism at room temperature, but DP mechanism is dominant possibly.
Dynamics of photoexcited carrier relaxation and recombination in CdTe/CdS thin films
Energy Technology Data Exchange (ETDEWEB)
Levi, D.H.; Fluegel, B.D.; Ahrenkiel, R.K. [National Renewable Energy Lab., Golden, CO (United States)] [and others
1996-05-01
Efficiency-limiting defects in photovoltaic devices are readily probed by time-resolved spectroscopy. This paper presents the first direct optical measurements of the relaxation and recombination pathways of photoexcited carriers in the CdS window layer of CdTe/CdS polycrystalline thin films. Femtosecond time-resolved pump/probe measurements indicate the possible existence of a two-phase CdS/CdSTe layer, rather than a continuously graded alloy layer at the CdTe/CdS interface. Complementary time-resolved photoluminescence (PL) measurements show that the photoexcited carriers are rapidly captured by deep-level defects. The temporal and density-dependent properties of the photoluminescence prove that the large Stokes shift of the PL relative to the band edge is due to strong phonon coupling to deep-level defects in CdS. The authors suggest that modifications in the CdS processing may enhance carrier collection efficiency in the blue spectral region.
Next Generation Extended Lagrangian Quantum-based Molecular Dynamics
Negre, Christian
2017-06-01
A new framework for extended Lagrangian first-principles molecular dynamics simulations is presented, which overcomes shortcomings of regular, direct Born-Oppenheimer molecular dynamics, while maintaining important advantages of the unified extended Lagrangian formulation of density functional theory pioneered by Car and Parrinello three decades ago. The new framework allows, for the first time, energy conserving, linear-scaling Born-Oppenheimer molecular dynamics simulations, which is necessary to study larger and more realistic systems over longer simulation times than previously possible. Expensive, self-consinstent-field optimizations are avoided and normal integration time steps of regular, direct Born-Oppenheimer molecular dynamics can be used. Linear scaling electronic structure theory is presented using a graph-based approach that is ideal for parallel calculations on hybrid computer platforms. For the first time, quantum based Born-Oppenheimer molecular dynamics simulation is becoming a practically feasible approach in simulations of +100,000 atoms-representing a competitive alternative to classical polarizable force field methods. In collaboration with: Anders Niklasson, Los Alamos National Laboratory.
Energy Technology Data Exchange (ETDEWEB)
Kandel, Saugat; Salomon-Ferrer, Romelia; Larsen, Adrien B.; Vaidehi, Nagarajan, E-mail: nvaidehi@coh.org [Division of Immunology, Beckman Research Institute of the City of Hope, Duarte, California 91010 (United States); Jain, Abhinandan, E-mail: Abhi.Jain@jpl.nasa.gov [Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, California 91109 (United States)
2016-01-28
The Internal Coordinate Molecular Dynamics (ICMD) method is an attractive molecular dynamics (MD) method for studying the dynamics of bonded systems such as proteins and polymers. It offers a simple venue for coarsening the dynamics model of a system at multiple hierarchical levels. For example, large scale protein dynamics can be studied using torsional dynamics, where large domains or helical structures can be treated as rigid bodies and the loops connecting them as flexible torsions. ICMD with such a dynamic model of the protein, combined with enhanced conformational sampling method such as temperature replica exchange, allows the sampling of large scale domain motion involving high energy barrier transitions. Once these large scale conformational transitions are sampled, all-torsion, or even all-atom, MD simulations can be carried out for the low energy conformations sampled via coarse grained ICMD to calculate the energetics of distinct conformations. Such hierarchical MD simulations can be carried out with standard all-atom forcefields without the need for compromising on the accuracy of the forces. Using constraints to treat bond lengths and bond angles as rigid can, however, distort the potential energy landscape of the system and reduce the number of dihedral transitions as well as conformational sampling. We present here a two-part solution to overcome such distortions of the potential energy landscape with ICMD models. To alleviate the intrinsic distortion that stems from the reduced phase space in torsional MD, we use the Fixman compensating potential. To additionally alleviate the extrinsic distortion that arises from the coupling between the dihedral angles and bond angles within a force field, we propose a hybrid ICMD method that allows the selective relaxing of bond angles. This hybrid ICMD method bridges the gap between all-atom MD and torsional MD. We demonstrate with examples that these methods together offer a solution to eliminate the potential
DEFF Research Database (Denmark)
Hellgren, Lars; Sandelius, A.S.
2001-01-01
Plant sterols differ from cholesterol in having an alkyl group at Delta -24, and, in the case of stigmasterol, also a Delta -22 double bond. The effects of 10 mol% of three plant sterols (campesterol, fl-sitosterol, stigmasterol) and cholesterol on the molecular dynamics and phase behavior...... in multilamellar liposomes made from different phosphatidylcholine (PC) molecular species have been compared, utilizing the fluorescent probe Laurdan (2-dimethyl-amino-6-laurylnaphthalene). Laurdan reports the molecular mobility in the hydrophilic/hydrophobic interface of the membrane by determining the rate...... of dipolar relaxation of water molecules close to the glycerol backbone of PC. Our results showed that the Delta -24 alkyl group of plant sterols did not affect their ability to reduce molecular mobility in this region of the PC membranes. However, the plant sterols had a decreased capacity compared...
Molecular dynamics simulations of the dielectric properties of fructose aqueous solutions
International Nuclear Information System (INIS)
Sonoda, Milton T; Dolores Elola, M; Skaf, Munir S
2016-01-01
The static dielectric permittivity and dielectric relaxation properties of fructose aqueous solutions of different concentrations ranging from 1.0 to 4.0 mol l −1 are investigated by means of molecular dynamics simulations. The contributions from intra- and interspecies molecular correlations were computed individually for both the static and frequency-dependent dielectric properties, and the results were compared with the available experimental data. Simulation results in the time- and frequency-domains were analyzed and indicate that the presence of fructose has little effect on the position of the fast, high-frequency (>500 cm −1 ) components of the dielectric response spectrum. The low-frequency (<0.1 cm −1 ) components, however, are markedly influenced by sugar concentration. Our analysis indicates that fructose–fructose and fructose–water interactions strongly affect the rotational-diffusion regime of molecular motions in the solutions. Increasing fructose concentration not only enhances sugar–sugar and sugar-water low frequency contributions to the dielectric loss spectrum but also slows down the reorientational dynamics of water molecules. These results are consistent with previous computer simulations carried out for other disaccharide aqueous solutions. (paper)
Sridhar, S.; Touma, Jihad R.
2017-02-01
We study the resonant relaxation (RR) of an axisymmetric, low-mass (or Keplerian) stellar disc orbiting a more massive black hole (MBH). Our recent work on the general kinetic theory of RR is simplified in the standard manner by the neglect of 'gravitational polarization' and applied to a razor-thin axisymmetric disc. The wake of a stellar orbit is expressed in terms of the angular momenta exchanged with other orbits, and used to derive a kinetic equation for RR under the combined actions of self-gravity, 1 PN and 1.5 PN general relativistic effects of the MBH and an arbitrary external axisymmetric potential. This is a Fokker-Planck equation for the stellar distribution function (DF), wherein the diffusion coefficients are given self-consistently in terms of contributions from apsidal resonances between pairs of stellar orbits. The physical kinetics is studied for the two main cases of interest. (1) 'Lossless' discs in which the MBH is not a sink of stars, and disc mass, angular momentum and energy are conserved: we prove that general H-functions can increase or decrease during RR, but the Boltzmann entropy is (essentially) unique in being a non-decreasing function of time. Therefore, secular thermal equilibria are maximum entropy states, with DFs of the Boltzmann form; the two-ring correlation function at equilibrium is computed. (2) Discs that lose stars to the MBH through an 'empty loss cone': we derive expressions for the MBH feeding rates of mass, angular momentum and energy in terms of the diffusive fluxes at the loss-cone boundaries.
Unoccupied electronic structure and relaxation dynamics of Pb/Si(1 1 1)
International Nuclear Information System (INIS)
Sandhofer, M.; Sklyadneva, I.Yu.; Sharma, V.; Trontl, V. Mikšić; Zhou, P.; Ligges, M.; Heid, R.; Bohnen, K.-P.; Chulkov, E.V.; Bovensiepen, U.
2014-01-01
Highlights: • The electronic structure of unoccupied quantum well states in Pb/Si(1 1 1) is analyzed. • The Si substrate modifies the binding energy and dispersion of the Pb bands. • The direct Pb 6p z orbital overlap with Si orbitals is marginal. • We conclude a 6p x,y mediated interaction of the 6p z states with the Si bulk bands. - Abstract: The unoccupied electronic structure of epitaxial Pb films on Si(1 1 1) is analyzed by angle-resolved two-photon photoemission in the Γ ¯ →M ¯ direction close to the Brillouin zone center. The experimental results are compared to density functional theory calculations and we focus on the nature of the interaction of the 6p z states with the Si substrate. The experimentally obtained dispersion E(k || ) of the unoccupied quantum well states is weaker than expected for freestanding films, in good agreement with their occupied counterparts. Following E(k || ) of quantum well states as a function of momentum at different energies, which are degenerate and non-degenerate with the Si conduction band, we observe no influence of the Si bulk band and conclude a vanishing direct interaction of the Pb 6p z states with the Si band. However, the momentum range at which mixing of 6p z and 6p x,y derived subbands is found to occur in the presence of the Si substrate is closer to Γ ¯ than in the corresponding freestanding film, which indicates a substrate-mediated enhancement of the mixing of these states. Additional femtosecond time-resolved measurements show a constant relaxation time of hot electrons in unoccupied quantum well states as a function of parallel electron momentum which supports our conclusion of a p x,y mediated interaction of the p z states with the Si conduction band
Depasquale, Roberto; Lee, Sau L; Saluja, Bhawana; Shur, Jagdeep; Price, Robert
2015-06-01
This study investigated the structural relaxation of micronized fluticasone propionate (FP) under different lagering conditions and its influence on aerodynamic particle size distribution (APSD) of binary and tertiary carrier-based dry powder inhaler (DPI) formulations. Micronized FP was lagered under low humidity (LH 25 C, 33% RH [relative humidity]), high humidity (HH 25°C, 75% RH) for 30, 60, and 90 days, respectively, and high temperature (HT 60°C, 44% RH) for 14 days. Physicochemical, surface interfacial properties via cohesive-adhesive balance (CAB) measurements and amorphous disorder levels of the FP samples were characterized. Particle size, surface area, and rugosity suggested minimal morphological changes of the lagered FP samples, with the exception of the 90-day HH (HH90) sample. HH90 FP samples appeared to undergo surface reconstruction with a reduction in surface rugosity. LH and HH lagering reduced the levels of amorphous content over 90-day exposure, which influenced the CAB measurements with lactose monohydrate and salmeterol xinafoate (SX). CAB analysis suggested that LH and HH lagering led to different interfacial interactions with lactose monohydrate but an increasing adhesive affinity with SX. HT lagering led to no detectable levels of the amorphous disorder, resulting in an increase in the adhesive interaction with lactose monohydrate. APSD analysis suggested that the fine particle mass of FP and SX was affected by the lagering of the FP. In conclusion, environmental conditions during the lagering of FP may have a profound effect on physicochemical and interfacial properties as well as product performance of binary and tertiary carrier-based DPI formulations.
Anisotropic relaxation dynamics in a dipolar Fermi gas driven out of equilibrium
DEFF Research Database (Denmark)
Aikawa, K.; Frisch, A.; Mark, M.
2014-01-01
We report on the observation of a large anisotropy in the rethermalization dynamics of an ultracold dipolar Fermi gas driven out of equilibrium. Our system consists of an ultracold sample of strongly magnetic $^{167}$Er fermions, spin-polarized in the lowest Zeeman sublevel. In this system, elastic...
Molecular dynamics study of atomic displacements in disordered solid alloys
Puzyrev, Yevgeniy S.
The effects of atomic displacements on the energetics of alloys plays important role in the determining the properties of alloys. We studied the atomic displacements in disordered solid alloys using molecular dynamics and Monte-Carlo methods. The diffuse scattering of pure materials, copper, gold, nickel, and palladium was calculated. The experimental data for pure Cu was obtained from diffuse scattering intensity of synchrotron x-ray radiation. The comparison showed the advantages of molecular dynamics method for calculating the atomic displacements in solid alloys. The individual nearest neighbor separations were calculated for Cu 50Au50 alloy and compared to the result of XAFS experiment. The molecular dynamics method provided theoretical predictions of nearest neighbor pair separations in other binary alloys, Cu-Pd and Cu-Al for wide range of the concentrations. We also experimentally recovered the diffuse scattering maps for the Cu47.3Au52.7 and Cu85.2Al14.8 alloy.
Atomistic interactions of clusters on surfaces using molecular dynamics and hyper molecular dynamics
International Nuclear Information System (INIS)
Sanz-Navarro, Carlos F.
2002-01-01
The work presented in this thesis describes the results of Molecular Dynamics (MD) simulations applied to the interaction of silver clusters with graphite surfaces and some numerical and theoretical methods concerning the extension of MD simulations to longer time scales (hyper-MD). The first part of this thesis studies the implantation of clusters at normal incidence onto a graphite surface in order to determine the scaling of the penetration depth (PD) against the impact energy. A comparison with experimental results is made with good agreement. The main physical observations of the impact process are described and analysed. It is shown that there is a threshold impact velocity above which the linear dependence on PD on impact energy changes to a linear dependence on velocity. Implantation of silver clusters at oblique incidence is also considered. The second part of this work analyses the validity and feasibility of the three minimisation methods for the hyper-MD simulation method whereby time scales of an MD simulation can be extended. A correct mathematical basis for the iterative method is derived. It is found that one of the iterative methods, upon which hyper-lD is based, is very likely to fail in high-dimensional situations because it requires a too expensive convergence. Two new approximations to the hyper-MD approach are proposed, which reduce the computational effort considerably. Both approaches, although not exact, can help to search for some of the most likely transitions in the system. Some examples are given to illustrate this. (author)
Dynamic molecular oxygen production in cometary comae
Yao, Yunxi; Giapis, Konstantinos P.
2017-05-01
Abundant molecular oxygen was discovered in the coma of comet 67P/Churyumov-Gerasimenko. Its origin was ascribed to primordial gaseous O2 incorporated into the nucleus during the comet's formation. This thesis was put forward after discounting several O2 production mechanisms in comets, including photolysis and radiolysis of water, solar wind-surface interactions and gas-phase collisions. Here we report an original Eley-Rideal reaction mechanism, which permits direct O2 formation in single collisions of energetic water ions with oxidized cometary surface analogues. The reaction proceeds by H2O+ abstracting a surface O-atom, then forming an excited precursor state, which dissociates to produce O2-. Subsequent photo-detachment leads to molecular O2, whose presence in the coma may thus be linked directly to water molecules and their interaction with the solar wind. This abiotic O2 production mechanism is consistent with reported trends in the 67P coma and raises awareness of the role of energetic negative ions in comets.
Kovalev, Vadim M; Tse, Wang-Kong
2017-11-22
We develop a microscopic theory for the relaxation dynamics of an optically pumped two-level system (TLS) coupled to a bath of weakly interacting Bose gas. Using Keldysh formalism and diagrammatic perturbation theory, expressions for the relaxation times of the TLS Rabi oscillations are derived when the boson bath is in the normal state and the Bose-Einstein condensate (BEC) state. We apply our general theory to consider an irradiated quantum dot coupled with a boson bath consisting of a two-dimensional dipolar exciton gas. When the bath is in the BEC regime, relaxation of the Rabi oscillations is due to both condensate and non-condensate fractions of the bath bosons for weak TLS-light coupling and pre dominantly due to the non-condensate fraction for strong TLS-light coupling. Our theory also shows that a phase transition of the bath from the normal to the BEC state strongly influences the relaxation rate of the TLS Rabi oscillations. The TLS relaxation rate is approximately independent of the pump field frequency and monotonically dependent on the field strength when the bath is in the low-temperature regime of the normal phase. Phase transition of the dipolar exciton gas leads to a non-monotonic dependence of the TLS relaxation rate on both the pump field frequency and field strength, providing a characteristic signature for the detection of BEC phase transition of the coupled dipolar exciton gas.
Energy conserving, linear scaling Born-Oppenheimer molecular dynamics.
Cawkwell, M J; Niklasson, Anders M N
2012-10-07
Born-Oppenheimer molecular dynamics simulations with long-term conservation of the total energy and a computational cost that scales linearly with system size have been obtained simultaneously. Linear scaling with a low pre-factor is achieved using density matrix purification with sparse matrix algebra and a numerical threshold on matrix elements. The extended Lagrangian Born-Oppenheimer molecular dynamics formalism [A. M. N. Niklasson, Phys. Rev. Lett. 100, 123004 (2008)] yields microcanonical trajectories with the approximate forces obtained from the linear scaling method that exhibit no systematic drift over hundreds of picoseconds and which are indistinguishable from trajectories computed using exact forces.
Femtochemistry and femtobiology ultrafast dynamics in molecular science
Douhal, Abderrazzak
2002-01-01
This book contains important contributions from top international scientists on the-state-of-the-art of femtochemistry and femtobiology at the beginning of the new millennium. It consists of reviews and papers on ultrafast dynamics in molecular science.The coverage of topics highlights several important features of molecular science from the viewpoint of structure (space domain) and dynamics (time domain). First of all, the book presents the latest developments, such as experimental techniques for understanding ultrafast processes in gas, condensed and complex systems, including biological mol
AceCloud: Molecular Dynamics Simulations in the Cloud.
Harvey, M J; De Fabritiis, G
2015-05-26
We present AceCloud, an on-demand service for molecular dynamics simulations. AceCloud is designed to facilitate the secure execution of large ensembles of simulations on an external cloud computing service (currently Amazon Web Services). The AceCloud client, integrated into the ACEMD molecular dynamics package, provides an easy-to-use interface that abstracts all aspects of interaction with the cloud services. This gives the user the experience that all simulations are running on their local machine, minimizing the learning curve typically associated with the transition to using high performance computing services.
State-to-state dynamics of molecular energy transfer
Energy Technology Data Exchange (ETDEWEB)
Gentry, W.R.; Giese, C.F. [Univ. of Minnesota, Minneapolis (United States)
1993-12-01
The goal of this research program is to elucidate the elementary dynamical mechanisms of vibrational and rotational energy transfer between molecules, at a quantum-state resolved level of detail. Molecular beam techniques are used to isolate individual molecular collisions, and to control the kinetic energy of collision. Lasers are used both to prepare specific quantum states prior to collision by stimulated-emission pumping (SEP), and to measure the distribution of quantum states in the collision products by laser-induced fluorescence (LIF). The results are interpreted in terms of dynamical models, which may be cast in a classical, semiclassical or quantum mechanical framework, as appropriate.
Chin, A. W.; Mangaud, E.; Atabek, O.; Desouter-Lecomte, M.
2018-06-01
Engineering and harnessing coherent excitonic transport in organic nanostructures has recently been suggested as a promising way towards improving manmade light-harvesting materials. However, realizing and testing the dissipative system-environment models underlying these proposals is presently very challenging in supramolecular materials. A promising alternative is to use simpler and highly tunable "quantum simulators" built from programmable qubits, as recently achieved in a superconducting circuit by Potočnik et al. [A. Potočnik et al., Nat. Commun. 9, 904 (2018), 10.1038/s41467-018-03312-x]. We simulate the real-time dynamics of an exciton coupled to a quantum bath as it moves through a network based on the quantum circuit of Potočnik et al. Using the numerically exact hierarchical equations of motion to capture the open quantum system dynamics, we find that an ultrafast but completely incoherent relaxation from a high-lying "bright" exciton into a doublet of closely spaced "dark" excitons can spontaneously generate electronic coherences and oscillatory real-space motion across the network (quantum beats). Importantly, we show that this behavior also survives when the environmental noise is classically stochastic (effectively high temperature), as in present experiments. These predictions highlight the possibilities of designing matched electronic and spectral noise structures for robust coherence generation that do not require coherent excitation or cold environments.
Molecular dynamics simulation of nonlinear spectroscopies of intermolecular motions in liquid water.
Yagasaki, Takuma; Saito, Shinji
2009-09-15
elucidated by introducing the "translation-free" molecular dynamics simulation. The isotropic pump-probe signal and the polarization anisotropy decay show fast transfer of the librational energy to the surrounding water molecules, followed by relaxation to the hot ground state. These theoretical methods do not require frequently used assumptions and can thus be called ab initio methods; together with multidimensional nonlinear spectroscopies, they provide powerful methods for examining the inter- and intramolecular details of water dynamics.
Chain networking revealed by molecular dynamics simulation
Zheng, Yexin; Tsige, Mesfin; Wang, Shi-Qing
Based on Kremer-Grest model for entangled polymer melts, we demonstrate how the response of a polymer glass depends critically on the chain length. After quenching two melts of very different chain lengths (350 beads per chain and 30 beads per chain) into deeply glassy states, we subject them to uniaxial extension. Our MD simulations show that the glass of long chains undergoes stable necking after yielding whereas the system of short chains is unable to neck and breaks up after strain localization. During ductile extension of the polymer glass made of long chain significant chain tension builds up in the load-bearing strands (LBSs). Further analysis is expected to reveal evidence of activation of the primary structure during post-yield extension. These results lend support to the recent molecular model 1 and are the simulations to demonstrate the role of chain networking. This work is supported, in part, by a NSF Grant (DMR-EAGER-1444859)
Multiple time step integrators in ab initio molecular dynamics
International Nuclear Information System (INIS)
Luehr, Nathan; Martínez, Todd J.; Markland, Thomas E.
2014-01-01
Multiple time-scale algorithms exploit the natural separation of time-scales in chemical systems to greatly accelerate the efficiency of molecular dynamics simulations. Although the utility of these methods in systems where the interactions are described by empirical potentials is now well established, their application to ab initio molecular dynamics calculations has been limited by difficulties associated with splitting the ab initio potential into fast and slowly varying components. Here we present two schemes that enable efficient time-scale separation in ab initio calculations: one based on fragment decomposition and the other on range separation of the Coulomb operator in the electronic Hamiltonian. We demonstrate for both water clusters and a solvated hydroxide ion that multiple time-scale molecular dynamics allows for outer time steps of 2.5 fs, which are as large as those obtained when such schemes are applied to empirical potentials, while still allowing for bonds to be broken and reformed throughout the dynamics. This permits computational speedups of up to 4.4x, compared to standard Born-Oppenheimer ab initio molecular dynamics with a 0.5 fs time step, while maintaining the same energy conservation and accuracy
Energy Technology Data Exchange (ETDEWEB)
Kida, Shogo; Yamamoto, Masaya; Kawata, Hiroaki; Hirai, Yoshihiko; Yasuda, Masaaki, E-mail: yasuda@pe.osakafu-u.ac.jp [Department of Physics and Electronics, Osaka Prefecture University, Sakai, Osaka 599-8531 (Japan); Tada, Kazuhiro [Department of Electrical and Control Systems Engineering, National Institute of Technology, Toyama College, Toyama 939-8630 (Japan)
2015-09-15
Molecular dynamics (MD) simulations are performed to study the correlation between electron irradiation defects and applied stress in graphene. The electron irradiation effect is introduced by the binary collision model in the MD simulation. By applying a tensile stress to graphene, the number of adatom-vacancy (AV) and Stone–Wales (SW) defects increase under electron irradiation, while the number of single-vacancy defects is not noticeably affected by the applied stress. Both the activation and formation energies of an AV defect and the activation energy of an SW defect decrease when a tensile stress is applied to graphene. Applying tensile stress also relaxes the compression stress associated with SW defect formation. These effects induced by the applied stress cause the increase in AV and SW defect formation under electron irradiation.
Ab Initio Molecular-Dynamics Simulation of Neuromorphic Computing in Phase-Change Memory Materials.
Skelton, Jonathan M; Loke, Desmond; Lee, Taehoon; Elliott, Stephen R
2015-07-08
We present an in silico study of the neuromorphic-computing behavior of the prototypical phase-change material, Ge2Sb2Te5, using ab initio molecular-dynamics simulations. Stepwise changes in structural order in response to temperature pulses of varying length and duration are observed, and a good reproduction of the spike-timing-dependent plasticity observed in nanoelectronic synapses is demonstrated. Short above-melting pulses lead to instantaneous loss of structural and chemical order, followed by delayed partial recovery upon structural relaxation. We also investigate the link between structural order and electrical and optical properties. These results pave the way toward a first-principles understanding of phase-change physics beyond binary switching.
Molecular rotation-vibration dynamics of low-symmetric hydrate crystal in the terahertz region.
Fu, Xiaojian; Wu, Hongya; Xi, Xiaoqing; Zhou, Ji
2014-01-16
The rotational and vibrational dynamics of molecules in copper sulfate pentahydrate crystal are investigated with terahertz dielectric spectra. It is shown that the relaxation-like dielectric dispersion in the low frequency region is related to the reorientation of water molecules under the driving of terahertz electric field, whereas the resonant dispersion can be ascribed to lattice vibration. It is also found that, due to the hydrogen-bond effect, the vibrational mode at about 1.83 THz along [-111] direction softens with decreasing temperature, that is, the crystal expands in this direction when cooled. On the contrary, the mode hardens in the direction perpendicular to [-111] during the cooling process. This contributes to the further understanding of the molecular structure and bonding features of hydrate crystals.
Connection between slow and fast dynamics of molecular liquids around the glass transition
International Nuclear Information System (INIS)
Niss, Kristine; Dalle-Ferrier, Cecile; Frick, Bernhard; Russo, Daniela; Dyre, Jeppe; Alba-Simionesco, Christiane
2010-01-01
The mean-square displacement (MSD) was measured by neutron scattering at various temperatures and pressures for a number of molecular glass-forming liquids. The MSD is invariant along the glass-transition line at the pressure studied, thus establishing an 'intrinsic' Lindemann criterion for any given liquid. A one-to-one connection between the MSD's temperature dependence and the liquid's fragility is found when the MSD is evaluated on a time scale of ∼4 ns, but does not hold when the MSD is evaluated at shorter times. The findings are discussed in terms of the elastic model and the role of relaxations, and the correlations between slow and fast dynamics are addressed.
Energy Technology Data Exchange (ETDEWEB)
Yurasov, D. V., E-mail: Inquisitor@ipm.sci-nnov.ru [Russian Academy of Sciences, Institute for Physics of Microstructures (Russian Federation); Bobrov, A. I. [Lobachevsky State University of Nizhny Novgorod (Russian Federation); Daniltsev, V. M.; Novikov, A. V. [Russian Academy of Sciences, Institute for Physics of Microstructures (Russian Federation); Pavlov, D. A. [Lobachevsky State University of Nizhny Novgorod (Russian Federation); Skorokhodov, E. V.; Shaleev, M. V.; Yunin, P. A. [Russian Academy of Sciences, Institute for Physics of Microstructures (Russian Federation)
2015-11-15
Influence of the Ge layer thickness and annealing conditions on the parameters of relaxed Ge/Si(001) layers grown by molecular beam epitaxy via two-stage growth is investigated. The dependences of the threading dislocation density and surface roughness on the Ge layer thickness, annealing temperature and time, and the presence of a hydrogen atmosphere are obtained. As a result of optimization of the growth and annealing conditions, relaxed Ge/Si(001) layers which are thinner than 1 μm with a low threading dislocation density on the order of 10{sup 7} cm{sup –2} and a root mean square roughness of less than 1 nm are obtained.
A Coupling Tool for Parallel Molecular Dynamics-Continuum Simulations
Neumann, Philipp
2012-06-01
We present a tool for coupling Molecular Dynamics and continuum solvers. It is written in C++ and is meant to support the developers of hybrid molecular - continuum simulations in terms of both realisation of the respective coupling algorithm as well as parallel execution of the hybrid simulation. We describe the implementational concept of the tool and its parallel extensions. We particularly focus on the parallel execution of particle insertions into dense molecular systems and propose a respective parallel algorithm. Our implementations are validated for serial and parallel setups in two and three dimensions. © 2012 IEEE.
Energy Technology Data Exchange (ETDEWEB)
Kimmich, R; Nusser, W; Gneiting, T [Ulm Universitaet (Federal Republic of Germany). Sektion Kernresonanzspektroskopie
1990-04-01
A model theory is presented explaining a series of striking phenomena observed with nuclear magnetic relaxation in protein systems such as solutions or tissue. The frequency, concentration and temperature dependences of proton or deuteron relaxation times of protein solutions and tissue are explained. It is concluded that the translational diffusion of water molecules along the rugged surfaces of proteins and, to a minor degree, protein backbone fluctuations are crucial processes. The rate limiting factor of macromolecular tumbling is assumed to be given by the free water content in a certain analogy to the free-volume model of Cohen ad Turnbull. There are two characteristic water mass fractions indicating the saturation of the hydration shells and the onset of protein tumbling. A closed and relatively simple set of relaxation formulas is presented. The potentially fractal nature of the diffusion of water molecules on the protein surface is discussed. (author). 43 refs.; 4 figs.
Plastic dislocation motion via nonequilibrium molecular and continuum dynamics
International Nuclear Information System (INIS)
Hoover, W.G.; Ladd, A.J.C.; Hoover, N.E.
1980-01-01
The classical two-dimensional close-packed triangular lattice, with nearest-neighbor spring forces, is a convenient standard material for the investigation of dislocation motion and plastic flow. Two kinds of calculations, based on this standard material, are described here: (1) Molecular Dynamics simulations, incorporating adiabatic strains described with the help of Doll's Tensor, and (2) Continuum Dynamics simulations, incorporating periodic boundaries and dislocation interaction through stress-field superposition
Molecular Dynamics Simulations of Poly(dimethylsiloxane) Properties
Czech Academy of Sciences Publication Activity Database
Fojtíková, J.; Kalvoda, L.; Sedlák, Petr
2015-01-01
Roč. 128, č. 4 (2015), s. 637-639 ISSN 0587-4246 R&D Projects: GA ČR GB14-36566G Institutional support: RVO:61388998 Keywords : molecular dynamics * poly(dimethylsiloxane) * dissipative particle dynamics Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 0.525, year: 2015 http://przyrbwn.icm.edu.pl/APP/PDF/128/a128z4p40.pdf
Investigation of nuclear multifragmentation using molecular dynamics and restructured aggregation
International Nuclear Information System (INIS)
Paula, L. de; Nemeth, J.; Ben-Hao, Sa.; Leray, S.; Ngo, C.; Souza, S.R.; Yu-Ming, Zheng; Paula, L. de; Nemeth, J.; Ben-Hao, Sa.; Yu-Ming, Zheng; Ngo, H.
1991-01-01
We study the stability of excited 197 Au nuclei with respect to multifragmentation. For that we use a dynamical simulation based on molecular dynamics and restructured aggregation. A particular attention is paid to check the stability of the ground state nuclei generated by the simulation. Four kinds of excitations are considered: heat, compression, rotation and a geometrical instability created when a projectile drills a hole in a 197 Au nucleus
A kinetic model of trp-cage folding from multiple biased molecular dynamics simulations.
Directory of Open Access Journals (Sweden)
Fabrizio Marinelli
2009-08-01
Full Text Available Trp-cage is a designed 20-residue polypeptide that, in spite of its size, shares several features with larger globular proteins.Although the system has been intensively investigated experimentally and theoretically, its folding mechanism is not yet fully understood. Indeed, some experiments suggest a two-state behavior, while others point to the presence of intermediates. In this work we show that the results of a bias-exchange metadynamics simulation can be used for constructing a detailed thermodynamic and kinetic model of the system. The model, although constructed from a biased simulation, has a quality similar to those extracted from the analysis of long unbiased molecular dynamics trajectories. This is demonstrated by a careful benchmark of the approach on a smaller system, the solvated Ace-Ala3-Nme peptide. For theTrp-cage folding, the model predicts that the relaxation time of 3100 ns observed experimentally is due to the presence of a compact molten globule-like conformation. This state has an occupancy of only 3% at 300 K, but acts as a kinetic trap.Instead, non-compact structures relax to the folded state on the sub-microsecond timescale. The model also predicts the presence of a state at Calpha-RMSD of 4.4 A from the NMR structure in which the Trp strongly interacts with Pro12. This state can explain the abnormal temperature dependence of the Pro12-delta3 and Gly11-alpha3 chemical shifts. The structures of the two most stable misfolded intermediates are in agreement with NMR experiments on the unfolded protein. Our work shows that, using biased molecular dynamics trajectories, it is possible to construct a model describing in detail the Trp-cage folding kinetics and thermodynamics in agreement with experimental data.
Thermal parameter identification for non-Fourier heat transfer from molecular dynamics
Singh, Amit; Tadmor, Ellad B.
2015-10-01
Fourier's law leads to a diffusive model of heat transfer in which a thermal signal propagates infinitely fast and the only material parameter is the thermal conductivity. In micro- and nano-scale systems, non-Fourier effects involving coupled diffusion and wavelike propagation of heat can become important. An extension of Fourier's law to account for such effects leads to a Jeffreys-type model for heat transfer with two relaxation times. We propose a new Thermal Parameter Identification (TPI) method for obtaining the Jeffreys-type thermal parameters from molecular dynamics simulations. The TPI method makes use of a nonlinear regression-based approach for obtaining the coefficients in analytical expressions for cosine and sine-weighted averages of temperature and heat flux over the length of the system. The method is applied to argon nanobeams over a range of temperature and system sizes. The results for thermal conductivity are found to be in good agreement with standard Green-Kubo and direct method calculations. The TPI method is more efficient for systems with high diffusivity and has the advantage, that unlike the direct method, it is free from the influence of thermostats. In addition, the method provides the thermal relaxation times for argon. Using the determined parameters, the Jeffreys-type model is able to reproduce the molecular dynamics results for a short-duration heat pulse where wavelike propagation of heat is observed thereby confirming the existence of second sound in argon. An implementation of the TPI method in MATLAB is available as part of the online supplementary material.
Molecular dynamics in porous media studied by nuclear magnetic resonance techniques
International Nuclear Information System (INIS)
Mattea, C.
2006-01-01
Field cycling NMR relaxometry was used to study dynamics of fluids under confinement in different scenarios: fluids flowing through porous media, fluids partially filling porous media and polymer melts in nanoscopic pores. Diffusion in partially filled porous media was also studied with the aid of an NMR diffusometry technique. It is shown that hydrodynamic flow influences the spin-lattice relaxation rate of water confined in mesoscopic porous media under certain conditions. The effect is predicted by an analytical theory and Monte Carlo simulations, and confirmed experimentally by field-cycling NMR relaxometry. Field-cycling NMR relaxometry has been applied to polar and non polar adsorbates in partially filled silica porous glasses. The dependence of the spin-lattice relaxation rate on the filling degree shows that limits for slow and fast exchange between different phases can be distinguished and identified depending on the pore size and polarity of the solvents. Diffusion in the same unsaturated systems was studied with the aid of NMR diffusometry technique. The effective diffusion coefficient of solvents with different polarities displays opposite tendencies as a function of the liquid content. A two-phase fast exchange model including Knudsen and ordinary diffusion and different effective tortuosities is presented accounting for these phenomena. In the case of polymer melts confined in narrow artificial tubes of a porous solid matrix with variable diameter (9 to 57 nm), the characteristics of reptation were experimentally verified using proton field cycling NMR relaxometry technique. This observation is independent of the molecular mass and pore size. In bulk, the same polymer melts show either Rouse or renormalized Rouse dynamics, depending on the molecular mass. The polymers under confinement show features specific for reptation even with a pore diameter 15 times larger than the Flory radius while bulk melts of the same polymers do not. (orig.)
Stress relaxation in viscous soft spheres.
Boschan, Julia; Vasudevan, Siddarth A; Boukany, Pouyan E; Somfai, Ellák; Tighe, Brian P
2017-10-04
We report the results of molecular dynamics simulations of stress relaxation tests in athermal viscous soft sphere packings close to their unjamming transition. By systematically and simultaneously varying both the amplitude of the applied strain step and the pressure of the initial condition, we access both linear and nonlinear response regimes and control the distance to jamming. Stress relaxation in viscoelastic solids is characterized by a relaxation time τ* that separates short time scales, where viscous loss is substantial, from long time scales, where elastic storage dominates and the response is essentially quasistatic. We identify two distinct plateaus in the strain dependence of the relaxation time, one each in the linear and nonlinear regimes. The height of both plateaus scales as an inverse power law with the distance to jamming. By probing the time evolution of particle velocities during relaxation, we further identify a correlation between mechanical relaxation in the bulk and the degree of non-affinity in the particle velocities on the micro scale.
Energy Technology Data Exchange (ETDEWEB)
Schubert, Alexander, E-mail: schubert@irsamc.ups-tlse.fr; Meier, Christoph [Laboratoire Collisions Agrégats et Réactivité, IRSAMC, UMR CNRS 5589, Université Paul Sabatier, 31062 Toulouse (France); Falvo, Cyril [Institut des Sciences Moléculaires d’Orsay (ISMO), CNRS, Univ. Paris-Sud, Université Paris-Saclay, 91405 Orsay (France)
2016-08-07
We present mixed quantum-classical simulations on relaxation and dephasing of vibrationally excited carbon monoxide within a protein environment. The methodology is based on a vibrational surface hopping approach treating the vibrational states of CO quantum mechanically, while all remaining degrees of freedom are described by means of classical molecular dynamics. The CO vibrational states form the “surfaces” for the classical trajectories of protein and solvent atoms. In return, environmentally induced non-adiabatic couplings between these states cause transitions describing the vibrational relaxation from first principles. The molecular dynamics simulation yields a detailed atomistic picture of the energy relaxation pathways, taking the molecular structure and dynamics of the protein and its solvent fully into account. Using the ultrafast photolysis of CO in the hemoprotein FixL as an example, we study the relaxation of vibrationally excited CO and evaluate the role of each of the FixL residues forming the heme pocket.
International Nuclear Information System (INIS)
Khenfouch, M.; Wéry, J.; Baïtoul, M.; Maaza, M.
2014-01-01
Generally, porphyrin nanostructured materials are known by playing many roles such as photoconductors, photovoltaics and capable of light induced charging. Also their combination with acceptors like graphene, the rising two dimension material, added exciting physical and chemical properties. In this work, Morphology, optical absorption and photoluminescence properties were investigated in order to elucidate the interaction between the few layered graphene oxide (FGO) and pophyrin nanorods. Reporting on the photoluminescence (PL) of both porphyrin nanorods and FGO/porphyrin nanorods composite, synthesized via a self-assembly method, we have experimentally demonstrated the generation of a new photoluminescence band giving rise to a white light. This luminescence was studied by the analysis of its origins and dynamics which show a huge change of exciton life time found to be longer after the interaction with graphene oxide (GO) sheets. -- Highlights: • We prepared FGO-porphyrin nanorods composite via a simple chemical method. • Luminescence properties were studied presenting the absorption, photoluminescence and dynamics measurements. • These results show the emission of a white light which we studied its emissions origins. • TEM images show FGO sheets decorated with porphyrin nanorods. • FGO had like effect an increase of the exciton lifetime in porphyrin nanorods
Ultrafast intramolecular relaxation dynamics of Mg- and Zn-bacteriochlorophyll a
Energy Technology Data Exchange (ETDEWEB)
Kosumi, Daisuke [Osaka City University Advanced Research Institute for Natural Science and Technology, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585 (Japan); CREST/JST, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012 (Japan); Nakagawa, Katsunori; Sakai, Shunsuke [Graduate School of Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555 (Japan); Nagaoka, Yuya; Maruta, Satoshi; Sugisaki, Mitsuru [CREST/JST, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012 (Japan); Department of Physics, Graduate School of Science, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585 (Japan); Dewa, Takehisa [Graduate School of Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555 (Japan); PRESTO/JST, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012 (Japan); Nango, Mamoru [The Osaka City University Advanced Research Institute for Natural Science and Technology, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585 (Japan); CREST/JST, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012 (Japan); Graduate School of Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555 (Japan); Hashimoto, Hideki [The Osaka City University Advanced Research Institute for Natural Science and Technology, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585 (Japan); CREST/JST, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012 (Japan); Department of Physics, Graduate School of Science, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585 (Japan)
2013-07-21
Ultrafast excited-state dynamics of the photosynthetic pigment (Mg-)bacteriochlorophyll a and its Zn-substituted form were investigated by steady-state absorption/fluorescence and femtosecond pump-probe spectroscopic measurements. The obtained steady-state absorption and fluorescence spectra of bacteriochlorophyll a in solution showed that the central metal compound significantly affects the energy of the Q{sub x} state, but has almost no effect on the Q{sub y} state. Photo-induced absorption spectra were recorded upon excitation of Mg- and Zn-bacteriochlorophyll a into either their Q{sub x} or Q{sub y} state. By comparing the kinetic traces of transient absorption, ground-state beaching, and stimulated emission after excitation to the Q{sub x} or Q{sub y} state, we showed that the Q{sub x} state was substantially incorporated in the ultrafast excited-state dynamics of bacteriochlorophyll a. Based on these observations, the lifetime of the Q{sub x} state was determined to be 50 and 70 fs for Mg- and Zn-bacteriochlorophyll a, respectively, indicating that the lifetime was influenced by the central metal atom due to the change of the energy gap between the Q{sub x} and Q{sub y} states.
Energy Technology Data Exchange (ETDEWEB)
Khenfouch, M., E-mail: khenfouch@yahoo.fr [University Sidi Mohamed Ben Abdellah, Faculty of Sciences Dhar el Mahraz, Laboratory of Solid State Physics, Group of Polymers and Nanomaterials, BP 1796 Atlas, Fez 30 000 (Morocco); iThemba LABS-National Research Foundation of South Africa, Old Faure Road, PO Box 722, Somerset West 7129, Western Cape Province (South Africa); UNESCO-UNISA Africa Chair in Nanosciences-Nanotechnology, College of Graduate Studies, University of South Africa, Muckleneuk ridge, PO Box 392, Pretoria (South Africa); Wéry, J. [Institut des Matériaux Jean Rouxel, Nantes, 2 rue de la Houssinière, BP 32229, 44322 Nantes, Cedex 3 (France); Baïtoul, M., E-mail: baitoul@yahoo.fr [University Sidi Mohamed Ben Abdellah, Faculty of Sciences Dhar el Mahraz, Laboratory of Solid State Physics, Group of Polymers and Nanomaterials, BP 1796 Atlas, Fez 30 000 (Morocco); Maaza, M. [iThemba LABS-National Research Foundation of South Africa, Old Faure Road, PO Box 722, Somerset West 7129, Western Cape Province (South Africa); UNESCO-UNISA Africa Chair in Nanosciences-Nanotechnology, College of Graduate Studies, University of South Africa, Muckleneuk ridge, PO Box 392, Pretoria (South Africa)
2014-01-15
Generally, porphyrin nanostructured materials are known by playing many roles such as photoconductors, photovoltaics and capable of light induced charging. Also their combination with acceptors like graphene, the rising two dimension material, added exciting physical and chemical properties. In this work, Morphology, optical absorption and photoluminescence properties were investigated in order to elucidate the interaction between the few layered graphene oxide (FGO) and pophyrin nanorods. Reporting on the photoluminescence (PL) of both porphyrin nanorods and FGO/porphyrin nanorods composite, synthesized via a self-assembly method, we have experimentally demonstrated the generation of a new photoluminescence band giving rise to a white light. This luminescence was studied by the analysis of its origins and dynamics which show a huge change of exciton life time found to be longer after the interaction with graphene oxide (GO) sheets. -- Highlights: • We prepared FGO-porphyrin nanorods composite via a simple chemical method. • Luminescence properties were studied presenting the absorption, photoluminescence and dynamics measurements. • These results show the emission of a white light which we studied its emissions origins. • TEM images show FGO sheets decorated with porphyrin nanorods. • FGO had like effect an increase of the exciton lifetime in porphyrin nanorods.
Theory of multiexciton dynamics in molecular chains
Wang, Luxia; May, Volkhard
2016-11-01
Ultrafast and strong optical excitation of a molecular system is considered which is formed by a regular one-dimensional arrangement of identical molecules. As it is typical for zinc chlorine-type molecules the transition energy from the ground state to the first excited singlet state is assumed to be smaller than the energy difference between the first excited state and the following one. This enables the creation of many excitons without their immediate quenching due to exciton-exciton annihilation. As a first step into the field of dense Frenkel-exciton systems the present approach stays at a mean-field type of description and ignores vibrational contributions. The resulting nonlinear kinetic equations mix Rabi-type oscillations with those caused by energy transfer and suggest an excitation-dependent narrowing of the exciton band. The indication of this effect in the framework of a two-color pump-probe experiment and of the detection of photon emission is discussed.
International Nuclear Information System (INIS)
De Grazia, M.
2007-12-01
The new sources of radiation in the extreme-UV (X-UV: 10-100 nm), which deliver spatially coherent, ultra-short and intense pulses, allow studying high flux processes and ultra-fast dynamics in various domains. The thesis work presents two applications of the high-order laser harmonics (HH) to solid state physics. In Part I, we describe the optimization of the harmonic for studies of X-UV/solids interaction. In Part II, we investigate effects of high excitation density in the dynamics of electron relaxation in dielectric scintillator crystals - tungstates and fluorides, using time-resolved luminescence spectroscopy. Quenching of luminescence at short time gives evidence of the competition between radiative and non-radiative recombination of self-trapped excitons (STE). The non-radiative channel is identified to mutual interaction of STE at high excitation density. In Part III, we study the X-UV induced damage mechanism in various materials, either conductor (amorphous carbon) or insulators (organic polymers, e.g., PMMA). In PMMA-Plexiglas, in the desorption regime (0.2 mJ/cm 2 , i.e., below damage threshold), the surface modifications reflect X-UV induced photochemical processes that are tentatively identified, as a function of dose: at low dose, polymer chain scission followed by the blow-up of the volatile, low-molecular fragments leads to crater formation; at high dose, cross-linking in the near-surface layer of remaining material leads to surface hardening. These promising results have great perspectives considering the performances already attained and planned in the next future in the development of the harmonic sources. (author)
International Nuclear Information System (INIS)
Canales-Mayordomo, Angeles; Fayos, Rosa; Angulo, Jesus; Ojeda, Rafael; Martin-Pastor, Manuel; Nieto, Pedro M.; Martin-Lomas, Manuel; Lozano, Rosa; Gimenez-Gallego, Guillermo; Jimenez-Barbero, Jesus
2006-01-01
The binding site and backbone dynamics of a bioactive complex formed by the acidic fibroblast growth factor (FGF-1) and a specifically designed heparin hexasaccharide has been investigated by HSQC and relaxation NMR methods. The comparison of the relaxation data for the free and bound states has allowed showing that the complex is monomeric, and still induces mutagenesis, and that the protein backbone presents reduced motion in different timescale in its bound state, except in certain points that are involved in the interaction with the fibroblast growth factor receptor (FGFR)
Energy Technology Data Exchange (ETDEWEB)
Canales-Mayordomo, Angeles; Fayos, Rosa [Centro de Investigaciones Biologicas, CSIC, Departamento de Estructura y Funcion de Proteinas (Spain); Angulo, Jesus; Ojeda, Rafael [Instituto de Investigaciones Quimicas, CSIC, Grupo de Carbohidratos (Spain); Martin-Pastor, Manuel [Unidad de RM y Unidad de RMN de Biomoleculas Asociada al CSIC, Laboratorio de Estructura e Estructura de Biomoleculas Jose Carracido (Spain); Nieto, Pedro M.; Martin-Lomas, Manuel [Instituto de Investigaciones Quimicas, CSIC, Grupo de Carbohidratos (Spain); Lozano, Rosa; Gimenez-Gallego, Guillermo; Jimenez-Barbero, Jesus [Centro de Investigaciones Biologicas, CSIC, Departamento de Estructura y Funcion de Proteinas (Spain)], E-mail: jjbarbero@cib.csic.es
2006-08-15
The binding site and backbone dynamics of a bioactive complex formed by the acidic fibroblast growth factor (FGF-1) and a specifically designed heparin hexasaccharide has been investigated by HSQC and relaxation NMR methods. The comparison of the relaxation data for the free and bound states has allowed showing that the complex is monomeric, and still induces mutagenesis, and that the protein backbone presents reduced motion in different timescale in its bound state, except in certain points that are involved in the interaction with the fibroblast growth factor receptor (FGFR)
Mori, Mirko; Kateb, Fatiha; Bodenhausen, Geoffrey; Piccioli, Mario; Abergel, Daniel
2010-03-17
Multiple quantum relaxation in proteins reveals unexpected relationships between correlated or anti-correlated conformational backbone dynamics in alpha-helices or beta-sheets. The contributions of conformational exchange to the relaxation rates of C'N coherences (i.e., double- and zero-quantum coherences involving backbone carbonyl (13)C' and neighboring amide (15)N nuclei) depend on the kinetics of slow exchange processes, as well as on the populations of the conformations and chemical shift differences of (13)C' and (15)N nuclei. The relaxation rates of C'N coherences, which reflect concerted fluctuations due to slow chemical shift modulations (CSMs), were determined by direct (13)C detection in diamagnetic and paramagnetic proteins. In well-folded proteins such as lanthanide-substituted calbindin (CaLnCb), copper,zinc superoxide dismutase (Cu,Zn SOD), and matrix metalloproteinase (MMP12), slow conformational exchange occurs along the entire backbone. Our observations demonstrate that relaxation rates of C'N coherences arising from slow backbone dynamics have positive signs (characteristic of correlated fluctuations) in beta-sheets and negative signs (characteristic of anti-correlated fluctuations) in alpha-helices. This extends the prospects of structure-dynamics relationships to slow time scales that are relevant for protein function and enzymatic activity.
Molecular dynamics studies of superionic conductors
International Nuclear Information System (INIS)
Rahman, A.
1979-01-01
Over the last fifteen years computer modeling of liquids and solids has become a useful method of understanding the structural and dynamical correlations in these systems. Some characteristics of the method are presented with an example from work on homogeneous nucleation in monoatomic liquids; the interaction potential determines the structure: a Lennard--Jones system nucleates a close packed structure while an alkali metal potential nucleates a bcc packing. In the study of ionic systems like CaF 2 the Coulomb interaction together with the short range repulsion is enough to produce a satisfactory model for the motion of F - ions in CaF 2 at approx. 1600 0 K. Analysis of this motion shows that F - ions reside at their fluorite sites for about 6 x 10 -12 s and that the diffusion is mainly due to F - jumps in the 100 direction. The motion can be analyzed in terms of the generation and annihilation of anti-Frenkel pairs. The temperature dependence of the F - diffusion constant at two different densities has also been calculated. The computer model does not correspond with experiment in this regard
Photoionization dynamics of excited molecular states
International Nuclear Information System (INIS)
Dehmer, J.L.; O'Halloran, M.A.; Tomkins, F.S.; Dehmer, P.M.; Pratt, S.T.
1987-01-01
Resonance Enhanced Multiphoton Ionization (REMPI) utilizes tunable dye lasers to ionize an atom or molecule by first preparing an excited state by multiphoton absorption and then ionizing that state before it can decay. This process is highly selective with respect to both the initial and resonant intermediate states of the target, and it can be extremely sensitive. In addition, the products of the REMPI process can be detected as needed by analyzing the resulting electrons, ions, fluorescence, or by additional REMPI. This points to a number of opportunities for exploring excited state physics and chemistry at the quantum-state-specific level. Here we will first give a brief overview of the large variety of experimental approaches to excited state phenomena made possible by REMPI. Then we will examine in more detail, recent studies of the three photon resonant, four photon (3 + 1) ionization of H 2 via the C 'PI/sup u/ state. Strong non-Franck-Condon behavior in the photoelectron spectra of this nominally simple Rydberg state has led to the examination of a variety of dynamical mechanisms. Of these, the role of doubly excited autoionizing states now seems decisive. Progress on photoelectron studies of autoionizing states in H 2 , excited in a (2 + 1) REMPI process via the E, F 1 Σ/sub g/ + will also be briefly discussed. 26 refs., 7 figs
Dynamical photo-induced electronic properties of molecular junctions
Beltako, K.; Michelini, F.; Cavassilas, N.; Raymond, L.
2018-03-01
Nanoscale molecular-electronic devices and machines are emerging as promising functional elements, naturally flexible and efficient, for next-generation technologies. A deeper understanding of carrier dynamics in molecular junctions is expected to benefit many fields of nanoelectronics and power devices. We determine time-resolved charge current flowing at the donor-acceptor interface in molecular junctions connected to metallic electrodes by means of quantum transport simulations. The current is induced by the interaction of the donor with a Gaussian-shape femtosecond laser pulse. Effects of the molecular internal coupling, metal-molecule tunneling, and light-donor coupling on photocurrent are discussed. We then define the time-resolved local density of states which is proposed as an efficient tool to describe the absorbing molecule in contact with metallic electrodes. Non-equilibrium reorganization of hybridized molecular orbitals through the light-donor interaction gives rise to two phenomena: the dynamical Rabi shift and the appearance of Floquet-like states. Such insights into the dynamical photoelectronic structure of molecules are of strong interest for ultrafast spectroscopy and open avenues toward the possibility of analyzing and controlling the internal properties of quantum nanodevices with pump-push photocurrent spectroscopy.
Molecular dynamics of the structure and thermodynamics of dusty ...
African Journals Online (AJOL)
The static structure and thermodynamic properties of two-dimensional dusty plasma are analyzed for some typical values of coupling and screening parameters using classical molecular dynamics. Radial distribution function and static structure factor are computed. The radial distribution functions display the typical ...
A MOLECULAR-DYNAMICS STUDY OF LECITHIN MONOLAYERS
AHLSTROM, P; BERENDSEN, HJC
1993-01-01
Two monolayers of didecanoyllecithin at the air-water interface have been studied using molecular dynamics simulations. The model system consisted of two monolayers of 42 lecithin molecules each separated by a roughly 4 nm thick slab of SPC water. The area per lecithin molecule was 0.78 nm(2)
Structure of hydrogenated amorphous silicon from ab initio molecular dynamics
Energy Technology Data Exchange (ETDEWEB)
Buda, F. (Department of Physics, The Ohio State University, 174 West 18th Avenue, Columbus, Ohio (USA)); Chiarotti, G.L. (International School for Advanced Studies, Strada Costiera 11, I-34014 Trieste (Italy) Laboratorio Tecnologie Avanzate Superfici e Catalisi del Consorzio Interuniversitario Nazionale di Fisica della Materia, Padriciano 99, I-34012 Trieste (Italy)); Car, R. (International School for Advanced Studies, Strada Costiera 11, I-34014 Trieste (Italy) Institut Romard de Recherche Numerique en Physique des Materiaux, CH-1015 Lausanne, Switzerland Department of Condensed Matter Physics, University of Geneva, CH-1211 Geneva (Switzerland)); Parrinello, M. (IBM Research Division, Zurich Research Laboratory, CH-8803 Rueschlikon (Switzerland))
1991-09-15
We have generated a model of hydrogenated amorphous silicon by first-principles molecular dynamics. Our results are in good agreement with the available experimental data and provide new insight into the microscopic structure of this material. The calculation lends support to models in which monohydride complexes are prevalent, and indicates a strong tendency of hydrogen to form small clusters.
Ab initio molecular dynamics simulation of laser melting of silicon
Silvestrelli, P.-L.; Alavi, A.; Parrinello, M.; Frenkel, D.
1996-01-01
The method of ab initio molecular dynamics, based on finite temperature density functional theory, is used to simulate laser heating of crystal silicon. We have found that a high concentration of excited electrons dramatically weakens the covalent bond. As a result, the system undergoes a melting
Microsecond atomic-scale molecular dynamics simulations of polyimides
Lyulin, S.V.; Gurtovenko, A.A.; Larin, S.V.; Nazarychev, V.M.; Lyulin, A.V.
2013-01-01
We employ microsecond atomic-scale molecular dynamics simulations to get insight into the structural and thermal properties of heat-resistant bulk polyimides. As electrostatic interactions are essential for the polyimides considered, we propose a two-step equilibration protocol that includes long
Molecular dynamics study of the silica-water-SDA interactions
Szyja, B.M.; Jansen, A.P.J.; Verstraelen, T.; Santen, van R.A.
2009-01-01
In this paper we have applied the molecular dynamics simulations in order to analyse the role of the structure directing tetrapropylammonium ions in the aggregation process that leads to silicalite formation. We address the specific question of how the interactions between silica precursor species
Molecular dynamics simulations of ballistic He penetration into W fuzz
Klaver, T. P. C.; Nordlund, K.; Morgan, T. W.; Westerhof, E.; Thijsse, B. J.; van de Sanden, M. C. M.
2016-01-01
Results are presented of large-scale Molecular Dynamics simulations of low-energy He bombardment of W nanorods, or so-called ‘fuzz’ structures. The goal of these simulations is to see if ballistic He penetration through W fuzz offers a more realistic scenario for how He moves through fuzz layers
Clustering Molecular Dynamics Trajectories for Optimizing Docking Experiments
Directory of Open Access Journals (Sweden)
Renata De Paris
2015-01-01
Full Text Available Molecular dynamics simulations of protein receptors have become an attractive tool for rational drug discovery. However, the high computational cost of employing molecular dynamics trajectories in virtual screening of large repositories threats the feasibility of this task. Computational intelligence techniques have been applied in this context, with the ultimate goal of reducing the overall computational cost so the task can become feasible. Particularly, clustering algorithms have been widely used as a means to reduce the dimensionality of molecular dynamics trajectories. In this paper, we develop a novel methodology for clustering entire trajectories using structural features from the substrate-binding cavity of the receptor in order to optimize docking experiments on a cloud-based environment. The resulting partition was selected based on three clustering validity criteria, and it was further validated by analyzing the interactions between 20 ligands and a fully flexible receptor (FFR model containing a 20 ns molecular dynamics simulation trajectory. Our proposed methodology shows that taking into account features of the substrate-binding cavity as input for the k-means algorithm is a promising technique for accurately selecting ensembles of representative structures tailored to a specific ligand.
Toluene model for molecular dynamics simulations in the ranges 298
Fioroni, M.; Vogt, D.
2004-01-01
An all-atom model for toluene is presented in the framework of classical molecular dynamics (MD). The model has been parametrized under the GROMOS96 force field to reproduce the physicochemical properties of the neat liquid. Four new atom types have been introduced, distinguishing between carbons
Young Modulus of Crystalline Polyethylene from ab Initio Molecular Dynamics
Hageman, J.C.L.; Meier, Robert J.; Heinemann, M.; Groot, R.A. de
1997-01-01
The Young modulus for crystalline polyethylene is calculated using ab initio molecular dynamics based on density functional theory in the local density approximation (DFT-LDA). This modulus, which can be seen as the ultimate value for the Young modulus of polyethylene fibers, is found to be 334 GPa.
Molecular dynamics simulations and free energy profile of ...
Indian Academy of Sciences (India)
aDepartment of Chemical Engineering, bDepartment of Chemistry, Amirkabir University of Technology,. 15875-4413 ... Lipid bilayers; Paracetamol; free energy; molecular dynamics simulation; membrane. 1. ..... bilayer is less favourable due to the hydrophobic nature .... Orsi M and Essex J W 2010 Soft Matter 6 3797. 54.
Molecular dynamics simulations of lipid vesicle fusion in atomic detail
Knecht, Volker; Marrink, Siewert-Jan
The fusion of a membrane-bounded vesicle with a target membrane is a key step in intracellular trafficking, exocytosis, and drug delivery. Molecular dynamics simulations have been used to study the fusion of small unilamellar vesicles composed of a dipalmitoyl-phosphatidylcholine (DPPC)/palmitic
Molecular dynamic analysis of the structure of dendrimers
Energy Technology Data Exchange (ETDEWEB)
Canetta, E.; Maino, G. E-mail: maino@bologna.enea.it
2004-01-01
We present main results of molecular dynamics simulations that we have carried out in order to investigate structural properties of polyamidoamine (PAMAM) dendrimers. Obtained data confirm the PAMAM dendrimer structure proposed by experiments, performed by means of X-ray scattering (SAXS) and quasi-elastic light scattering (QELS) techniques.