Operator evolution for ab initio nuclear theory
Schuster, Micah D; Johnson, Calvin W; Jurgenson, Eric D; Navratil, Petr
2014-01-01
The past two decades have seen a revolution in ab initio calculations of nuclear properties. One key element has been the development of a rigorous effective interaction theory, applying unitary transformations to soften the nuclear Hamiltonian and hence accelerate the convergence as a function of the model space size. For consistency, however, one ought to apply the same transformation to other operators when calculating transitions and mean values from the eigenstates of the renormalized Hamiltonian. Working in a translationally-invariant harmonic oscillator basis for the two- and three-nucleon systems, we evolve the Hamiltonian, square-radius and total dipole strength operators by the similarity renormalization group (SRG). The inclusion of up to three-body matrix elements in the 4He nucleus all but completely restores the invariance of the expectation values under the transformation. We also consider a Gaussian operator with adjustable range and find at short ranges an increased contribution from such ind...
Toward ab initio density functional theory for nuclei
Drut, J. E.; Furnstahl, R. J.; Platter, L.
2009-01-01
We survey approaches to nonrelativistic density functional theory (DFT) for nuclei using progress toward ab initio DFT for Coulomb systems as a guide. Ab initio DFT starts with a microscopic Hamiltonian and is naturally formulated using orbital-based functionals, which generalize the conventional local-density-plus-gradients form. The orbitals satisfy single-particle equations with multiplicative (local) potentials. The DFT functionals can be developed starting from internucleon forces using ...
Towards new horizons in ab initio nuclear structure theory
International Nuclear Information System (INIS)
We review recent advances in ab initio nuclear structure theory, which have changed the horizons of this field. Starting from chiral effective field theory to construct the nuclear Hamiltonian and the similarity renormalization group to further soften it, we address several many-body approaches that have seen major developments over the past few years. We show that the domain of ab initio nuclear structure theory has been pushed well beyond the p-shell and that quantitative QCD-based predictions are becoming possible all the way from the proton to the neutron drip line up into the medium-mass regime. (authors)
Ab Initio Nuclear Structure Theory: From Few to Many
International Nuclear Information System (INIS)
We summarize recent advances in ab initio nuclear structure theory, aiming to connect few- and many-body systems in a coherent theoretical framework. Starting from chiral effective field theory to construct the nuclear Hamiltonian and the similarity renormalization group to soften it, we address several many-body approaches that have seen major developments over the past few years. We show that the domain of ab initio nuclear structure theory has been pushed well beyond the p-shell and that quantitative predictions connected to QCD via chiral effective field theory are becoming possible all the way from the proton to the neutron drip line up into the medium-mass regime. (author)
Experimental, ab initio and density functional theory studies on sulfadiazine
Ogruc-Ildiz, Gulce; Akyuz, Sevim; Ozel, Aysen E.
2009-04-01
In the present study, combined experimental and computational study on molecular vibrations of free sulfadiazine has been reported. The theoretically possible stable conformers of free sulfadiazine molecule in electronically ground state were searched by means of torsion potential energy surfaces scan studies through C1 sbnd C7 sbnd S8 sbnd N8, C7 sbnd S8 sbnd N9 sbnd C10 and S8 sbnd N9 sbnd C10 sbnd N11 dihedral angles, at both semi-empirical PM3 and B3LYP/3-21G levels of theory. The final geometrical parameters for the obtained stable conformers were determined by means of geometry optimization carried out at ab initio HF/6-31G++(d,p) and DFT/B3LYP/6-31G++(d,p) theory levels. The harmonic and anharmonic vibrational wavenumbers and IR intensities were calculated at the same theory levels used in geometry optimization. The modes of the fundamental vibrations were characterized depending on their the total energy distribution (TED%). In order to fit the calculated harmonic wavenumbers to experimental ones, dual scale factors were used. The experimental infrared and Raman spectra of sulfadiazine in solid phase have been measured and compared with the calculated vibrational spectra of each conformer.
Song, Lingchun; Han, Jaebeom; Lin, Yen-lin; Xie, Wangshen; Gao, Jiali
2009-01-01
The explicit polarization (X-Pol) method has been examined using ab initio molecular orbital theory and density functional theory. The X-Pol potential was designed to provide a novel theoretical framework for developing next-generation force fields for biomolecular simulations. Importantly, the X-Pol potential is a general method, which can be employed with any level of electronic structure theory. The present study illustrates the implementation of the X-Pol method using ab initio Hartree—Fo...
DEFF Research Database (Denmark)
Abild-Pedersen, Frank; Nørskov, Jens Kehlet; Rostrup-Nielsen, Jens;
2006-01-01
Mechanisms and energetics of graphene growth catalyzed by nickel nanoclusters were studied using ab initio density functional theory calculations. It is demonstrated that nickel step-edge sites act as the preferential growth centers for graphene layers on the nickel surface. Carbon is transported...
Ab initio Nuclear structure Theory with chiral two- plus three-nucleon interactions
International Nuclear Information System (INIS)
Low-energy nuclear theory has entered an era of ab initio nuclear structure and reaction calculations based on input from QCD. One of the most promising paths from QCD to nuclear observables employs Hamiltonians constructed within chiral effective field theory as consistent starting point for precise ab initio nuclear structure and reaction studies. However, the full inclusion of chiral two- plus three-nucleon (NN+3N) interactions in exact and approximate many-body calculations still poses a formidable challenge. We discuss recent developments towards this goal, ranging from consistent Similarity Renormalization Group evolutions of NN+3N Hamiltonians to large-scale ab initio calculations for ground states and spectra in the Importance-Truncated No-Core Shell Model with full 3N interactions. We highlight recent achievements and discuss open issues and future perspectives for nuclear structure theory with QCD-based interactions. Moreover, we discuss successful steps towards merging ab initio structure and reaction theory and show applications to low-energy reactions in the p-shell relevant for astrophysics.
International Nuclear Information System (INIS)
The ability of simple levels of ab initio molecular orbital theory to describe with reasonable accuracy the energetics of isotopic exchange processes is demonstrated. Three levels of ab initio molecular orbital theory have been surveyed. The first two levels are single-determinant Hartree-Foch methods utilizing the 3-21G split-valence and 6-31G* polarization basis sets. The third level, which is computationally the most complex, uses the 6-31G* basis set but allows for partial account of electron correlation by way of Moller-Plesset perturbation theory terminated at second order. Theoretical and spectroscopic equilibrium constants for reactions XH + XD reversible XD + H2 where XD is a hydride of the first row of the periodic table are tabulated
Ab initio theory of galvanomagnetic phenomena in ferromagnetic metals and disordered alloys
Turek, Ilja; Kudrnovsky, Josef; Drchal, Vaclav
2011-01-01
We present an ab initio theory of transport quantities of metallic ferromagnets developed in the framework of the fully relativistic tight-binding linear muffin-tin orbital method. The approach is based on the Kubo-Streda formula for the conductivity tensor, on the coherent potential approximation for random alloys, and on the concept of interatomic electron transport. The developed formalism is applied to pure 3d transition metals (Fe, Co, Ni) and to random Ni-based ferromagnetic alloys (Ni-...
Blomqvist, Andreas
2010-01-01
In this thesis, density functional theory (DFT) calculations and DFT based ab initio molecular dynamics simulations have been employed in order to gain insights into materials properties like diffusion, adsorption, catalysis, and structure. In transition metals, absorbed hydrogen atoms self-trap due to localization of metal d-electrons. The self-trapping state is shown to highly influence hydrogen diffusion in the classical over-barrier jump temperature region. Li diffusion in Li-N-H systems ...
Ab initio theory of helix <-> coil phase transition
DEFF Research Database (Denmark)
Yakubovich, Alexander V.; Solov'yov, Ilia; Solov'yov, Andrey V.;
2008-01-01
In this paper, we suggest a theoretical method based on the statistical mechanics for treating the alpha-helix <-> random coil transition in alanine polypeptides. We consider this process as a first-order phase transition and develop a theory which is free of model parameters and is based solely on...... fundamental physical principles. It describes essential thermodynamical properties of the system such as heat capacity, the phase transition temperature and others from the analysis of the polypeptide potential energy surface calculated as a function of two dihedral angles, responsible for the polypeptide...... twisting. The suggested theory is general and with some modification can be applied for the description of phase transitions in other complex molecular systems (e.g. proteins, DNA, nanotubes, atomic clusters, fullerenes)....
Ab initio theory of thermal properties of germanane
Heine, Matthew; Lindsay, Lucas; Carrete, Jesús; Mingo, Natalio; Hellman, Olle; Broido, David
Germanane(GeH) is a germanium based hydrogen-terminated multi-layered graphane analogue semiconductor, which may be a promising thermoelectric due to its high electron mobility and the capability to tune its transport properties. We have performed first principles calculations of the thermal properties of germanane. Harmonic and anharmonic interatomic force constants are calculated within the framework of density functional theory, from which phonon dispersions, specific heat, thermal expansion are obtained. The phonon Boltzmann equation is solved to obtain the lattice thermal conductivity. The disparity in constituent masses in GeH gives phonon modes that are distinctly Ge or H in character and causes the specific heat not to saturate until much higher temperatures than in bulk Ge. Weak interlayer bonding and strong phonon-phonon scattering result in highly anisotropic and quite low intrinsic lattice thermal conductivity compared to Ge.
Marsalek, Ondrej
2015-01-01
Path integral molecular dynamics simulations, combined with an ab initio evaluation of interactions using electronic structure theory, incorporate the quantum mechanical nature of both the electrons and nuclei, which are essential to accurately describe systems containing light nuclei. However, path integral simulations have traditionally required a computational cost around two orders of magnitude greater than treating the nuclei classically, making them prohibitively costly for most applications. Here we show that the cost of path integral simulations can be dramatically reduced by extending our ring polymer contraction approach to ab initio molecular dynamics simulations. By using density functional tight binding as a reference system, we show that our ab initio ring polymer contraction (AI-RPC) scheme gives rapid and systematic convergence to the full path integral density functional theory result. We demonstrate the efficiency of this approach in ab initio simulations of liquid water and the reactive pro...
Steady state Ab-initio Theory of Lasers with Injected Signals
Cerjan, Alexander; Stone, A. Douglas
2013-01-01
We present an ab-initio treatment of the steady-state of lasers with injected signals that describes a regime, valid for micro lasers, in which the locking transition is dominated by cross-saturation and spatial hole-burning. The theory goes beyond standard approaches and treats multimode lasing with injected signals and finds the possibility of partially locked states and as well as repulsion of the free-running frequencies from the injected signal. The theory agrees well with exact integrat...
Tanha, Matteus; Cappiello, Alex; Gordon, Geoffrey J; Yaron, David J
2013-01-01
A means to take advantage of molecular similarity to lower the computational cost of electronic structure theory is proposed, in which parameters are embedded into a low-cost, low-level (LL) ab initio theory and adjusted to obtain agreement with a higher level (HL) ab initio theory. This approach is explored by training such a model on data for ethane and testing the resulting model on methane, propane and butane. The electronic distribution of the molecules is varied by placing them in strong electrostatic environments consisting of random charges placed on the corners of a cube. The results find that parameters embedded in HF/STO-3G theory can be adjusted to obtain agreement, to within about 2 kcal/mol, with results of HF/6-31G theory. Obtaining this level of agreement requires the use of parameters that are functions of the bond lengths, atomic charges, and bond orders within the molecules. The argument is made that this approach provides a well-controlled means to take advantage of molecular similarity in...
Paired-permanent approach for VB theory (II) -An ab initio spin-free VB program
Institute of Scientific and Technical Information of China (English)
无
2001-01-01
Paired-permanent approach for VB theory is extensively developed. Canonical expan sion of a paired-permanent is deduced. Furthermore, it is shown that a paired-permanent may be expressed in terms of the products of sub-paired-permanents of any given order and their corre sponding minors. An ab initio spin-free valence bond program, called Xiamen, is implemented by using paired-permanent approach. Test calculation shows that Xiamen package is more efficient than some other programs based on the traditional VB algorithm, and it provides a new practical tool for quantum chemistry.
Quantum chemistry the development of ab initio methods in molecular electronic structure theory
Schaefer III, Henry F
2004-01-01
This guide is guaranteed to prove of keen interest to the broad spectrum of experimental chemists who use electronic structure theory to assist in the interpretation of their laboratory findings. A list of 150 landmark papers in ab initio molecular electronic structure methods, it features the first page of each paper (which usually encompasses the abstract and introduction). Its primary focus is methodology, rather than the examination of particular chemical problems, and the selected papers either present new and important methods or illustrate the effectiveness of existing methods in predi
Ab-initio density functional theory study of a WO3 NH3-sensing mechanism
International Nuclear Information System (INIS)
WO3 bulk and various surfaces are studied by an ab-initio density functional theory technique. The band structures and electronic density states of WO3 bulk are investigated. The surface energies of different WO3 surfaces are compared and then the (002) surface with minimum energy is computed for its NH3 sensing mechanism which explains the results in the experiments. Three adsorption sites are considered. According to the comparisons of the energy and the charge change between before and after adsorption in the optimal adsorption site O1c, the NH3 sensing mechanism is obtained. (nuclear physics)
Steady-state ab initio theory of lasers with injected signals
Cerjan, Alexander; Stone, A. Douglas
2014-07-01
We present an ab initio treatment of the steady-state of lasers with injected signals that describes a regime, valid for microlasers, in which the locking transition is dominated by cross saturation and spatial hole burning. The theory goes beyond standard approaches and treats multimode lasing with injected signals and finds the possibility of partially locked states and as well as repulsion of the free-running frequencies from the injected signal. The theory agrees well with exact integration of the full wave and matter equations for the system. It can also describe accurately complex modern lasers structures and is applied to the example of deformed disk lasers. We show that in the case of a one-dimensional cavity in the locked or regenerative amplifier regime the theory reduces to an improved version of the Adler equations in the appropriate limit.
Embedding parameters in ab initio theory to develop approximations based on molecular similarity
Tanha, Matteus; Kaul, Shiva; Cappiello, Alexander; Gordon, Geoffrey J; Yaron, David J
2015-01-01
A means to take advantage of molecular similarity to lower the computational cost of electronic structure theory is explored, in which parameters are embedded into a low-cost, low-level (LL) ab initio model and adjusted to obtain agreement with results from a higher-level (HL) ab initio model. A parametrized LL (pLL) model is created by multiplying selected matrix elements of the Hamiltonian operators by scaling factors that depend on element types. Various schemes for applying the scaling factors are compared, along with the impact of making the scaling factors linear functions of variables related to bond lengths, atomic charges, and bond orders. The models are trained on ethane and ethylene, substituted with -NH2, -OH and -F, and tested on substituted propane, propylene and t-butane. Training and test datasets are created by distorting the molecular geometries and applying uniform electric fields. The fitted properties include changes in total energy arising from geometric distortions or applied fields, an...
Ab initio theory for current-induced molecular switching: Melamine on Cu(001)
Ohto, Tatsuhiko
2013-05-28
Melamine on Cu(001) is mechanically unstable under the current of a scanning tunneling microscope tip and can switch among configurations. However, these are not equally accessible, and the switching critical current depends on the bias polarity. In order to explain such rich phenomenology, we have developed a scheme to evaluate the evolution of the reaction paths and activation barriers as a function of bias, which is rooted in the nonequilibrium Green\\'s function method implemented within density functional theory. This, combined with the calculation of the inelastic electron tunneling spectroscopy signal, allows us to identify the vibrational modes promoting the observed molecular conformational changes. Finally, once our ab initio results are used within a resonance model, we are able to explain the details of the switching behavior, such as its dependence on the bias polarity, and the noninteger power relation between the reaction rate constants and both the bias voltage and the electric current. © 2013 American Physical Society.
Steady-state ab initio laser theory for complex gain media
Cerjan, Alexander; Stone, A Douglas
2014-01-01
We derive and test a generalization of Steady-State Ab Initio Laser Theory (SALT) to treat complex gain media. The generalized theory (C-SALT) is able to treat atomic and molecular gain media with diffusion and multiple lasing transitions, and semiconductor gain media in the free carrier approximation including fully the effect of Pauli blocking. The key assumption of the theory is stationarity of the level populations, which leads to coupled self-consistent equations for the populations and the lasing modes that fully include the effects of openness and non-linear spatial hole-burning. These equations can be solved efficiently for the steady-state lasing properties by a similar iteration procedure as in SALT, where a static gain medium with a single transition is assumed. The theory is tested by comparison to much less efficient Finite Difference Time Domain (FDTD) methods and excellent agreement is found. Using C-SALT to analyze the effects of varying gain diffusion constant we demonstrate a cross-over betw...
Nonlocal torque operators in ab initio theory of the Gilbert damping in random ferromagnetic alloys
Turek, I.; Kudrnovský, J.; Drchal, V.
2015-12-01
We present an ab initio theory of the Gilbert damping in substitutionally disordered ferromagnetic alloys. The theory rests on introduced nonlocal torques which replace traditional local torque operators in the well-known torque-correlation formula and which can be formulated within the atomic-sphere approximation. The formalism is sketched in a simple tight-binding model and worked out in detail in the relativistic tight-binding linear muffin-tin orbital method and the coherent potential approximation (CPA). The resulting nonlocal torques are represented by nonrandom, non-site-diagonal, and spin-independent matrices, which simplifies the configuration averaging. The CPA-vertex corrections play a crucial role for the internal consistency of the theory and for its exact equivalence to other first-principles approaches based on the random local torques. This equivalence is also illustrated by the calculated Gilbert damping parameters for binary NiFe and FeCo random alloys, for pure iron with a model atomic-level disorder, and for stoichiometric FePt alloys with a varying degree of L 10 atomic long-range order.
Marsalek, Ondrej; Markland, Thomas E.
2016-02-01
Path integral molecular dynamics simulations, combined with an ab initio evaluation of interactions using electronic structure theory, incorporate the quantum mechanical nature of both the electrons and nuclei, which are essential to accurately describe systems containing light nuclei. However, path integral simulations have traditionally required a computational cost around two orders of magnitude greater than treating the nuclei classically, making them prohibitively costly for most applications. Here we show that the cost of path integral simulations can be dramatically reduced by extending our ring polymer contraction approach to ab initio molecular dynamics simulations. By using density functional tight binding as a reference system, we show that our ring polymer contraction scheme gives rapid and systematic convergence to the full path integral density functional theory result. We demonstrate the efficiency of this approach in ab initio simulations of liquid water and the reactive protonated and deprotonated water dimer systems. We find that the vast majority of the nuclear quantum effects are accurately captured using contraction to just the ring polymer centroid, which requires the same number of density functional theory calculations as a classical simulation. Combined with a multiple time step scheme using the same reference system, which allows the time step to be increased, this approach is as fast as a typical classical ab initio molecular dynamics simulation and 35× faster than a full path integral calculation, while still exactly including the quantum sampling of nuclei. This development thus offers a route to routinely include nuclear quantum effects in ab initio molecular dynamics simulations at negligible computational cost.
Marsalek, Ondrej; Markland, Thomas E
2016-02-01
Path integral molecular dynamics simulations, combined with an ab initio evaluation of interactions using electronic structure theory, incorporate the quantum mechanical nature of both the electrons and nuclei, which are essential to accurately describe systems containing light nuclei. However, path integral simulations have traditionally required a computational cost around two orders of magnitude greater than treating the nuclei classically, making them prohibitively costly for most applications. Here we show that the cost of path integral simulations can be dramatically reduced by extending our ring polymer contraction approach to ab initio molecular dynamics simulations. By using density functional tight binding as a reference system, we show that our ring polymer contraction scheme gives rapid and systematic convergence to the full path integral density functional theory result. We demonstrate the efficiency of this approach in ab initio simulations of liquid water and the reactive protonated and deprotonated water dimer systems. We find that the vast majority of the nuclear quantum effects are accurately captured using contraction to just the ring polymer centroid, which requires the same number of density functional theory calculations as a classical simulation. Combined with a multiple time step scheme using the same reference system, which allows the time step to be increased, this approach is as fast as a typical classical ab initio molecular dynamics simulation and 35× faster than a full path integral calculation, while still exactly including the quantum sampling of nuclei. This development thus offers a route to routinely include nuclear quantum effects in ab initio molecular dynamics simulations at negligible computational cost. PMID:26851913
International Nuclear Information System (INIS)
Path integral molecular dynamics simulations, combined with an ab initio evaluation of interactions using electronic structure theory, incorporate the quantum mechanical nature of both the electrons and nuclei, which are essential to accurately describe systems containing light nuclei. However, path integral simulations have traditionally required a computational cost around two orders of magnitude greater than treating the nuclei classically, making them prohibitively costly for most applications. Here we show that the cost of path integral simulations can be dramatically reduced by extending our ring polymer contraction approach to ab initio molecular dynamics simulations. By using density functional tight binding as a reference system, we show that our ring polymer contraction scheme gives rapid and systematic convergence to the full path integral density functional theory result. We demonstrate the efficiency of this approach in ab initio simulations of liquid water and the reactive protonated and deprotonated water dimer systems. We find that the vast majority of the nuclear quantum effects are accurately captured using contraction to just the ring polymer centroid, which requires the same number of density functional theory calculations as a classical simulation. Combined with a multiple time step scheme using the same reference system, which allows the time step to be increased, this approach is as fast as a typical classical ab initio molecular dynamics simulation and 35× faster than a full path integral calculation, while still exactly including the quantum sampling of nuclei. This development thus offers a route to routinely include nuclear quantum effects in ab initio molecular dynamics simulations at negligible computational cost
Energy Technology Data Exchange (ETDEWEB)
Marsalek, Ondrej; Markland, Thomas E., E-mail: tmarkland@stanford.edu [Department of Chemistry, Stanford University, Stanford, California 94305 (United States)
2016-02-07
Path integral molecular dynamics simulations, combined with an ab initio evaluation of interactions using electronic structure theory, incorporate the quantum mechanical nature of both the electrons and nuclei, which are essential to accurately describe systems containing light nuclei. However, path integral simulations have traditionally required a computational cost around two orders of magnitude greater than treating the nuclei classically, making them prohibitively costly for most applications. Here we show that the cost of path integral simulations can be dramatically reduced by extending our ring polymer contraction approach to ab initio molecular dynamics simulations. By using density functional tight binding as a reference system, we show that our ring polymer contraction scheme gives rapid and systematic convergence to the full path integral density functional theory result. We demonstrate the efficiency of this approach in ab initio simulations of liquid water and the reactive protonated and deprotonated water dimer systems. We find that the vast majority of the nuclear quantum effects are accurately captured using contraction to just the ring polymer centroid, which requires the same number of density functional theory calculations as a classical simulation. Combined with a multiple time step scheme using the same reference system, which allows the time step to be increased, this approach is as fast as a typical classical ab initio molecular dynamics simulation and 35× faster than a full path integral calculation, while still exactly including the quantum sampling of nuclei. This development thus offers a route to routinely include nuclear quantum effects in ab initio molecular dynamics simulations at negligible computational cost.
An ab-initio coupled mode theory for near field radiative thermal transfer.
Chalabi, Hamidreza; Hasman, Erez; Brongersma, Mark L
2014-12-01
We investigate the thermal transfer between finite-thickness planar slabs which support surface phonon polariton modes (SPhPs). The thickness-dependent dispersion of SPhPs in such layered materials provides a unique opportunity to manipulate and enhance the near field thermal transfer. The key accomplishment of this paper is the development of an ab-initio coupled mode theory that accurately describes all of its thermal transfer properties. We illustrate how the coupled mode parameters can be obtained in a direct fashion from the dispersion relation of the relevant modes of the system. This is illustrated for the specific case of a semi-infinite SiC substrate placed in close proximity to a thin slab of SiC. This is a system that exhibits rich physics in terms of its thermal transfer properties, despite the seemingly simple geometry. This includes a universal scaling behavior of the thermal conductance with the slab thickness and spacing. The work highlights and further increases the value of coupled mode theories in rapidly calculating and intuitively understanding near-field transfer. PMID:25606933
Towards ab initio self-energy embedding theory in quantum chemistry
Lan, Tran Nguyen; Zgid, Dominika
2015-01-01
The self-energy embedding theory (SEET), in which the active space self-energy is embedded in the self-energy obtained from a perturbative method treating the non-local correlation effects, was recently developed in our group. In SEET the double counting problem does not appear and the accuracy can be improved either by increasing the perturbation order or by enlarging the active space. This method was first calibrated for the 2D Hubbard lattice showing promising results. In this paper, we report an extension of SEET to quantum chemical ab initio Hamiltonians for applications to molecular systems. The self-consistent second-order Green's function (GF2) method is used to describe the non-local correlations, while the full configuration interaction (FCI) method is carried out to capture strong correlation within the active space. Using few proof-of-concept examples, we show that SEET yields results of comparable quality to $n-$electron valence state second-order perturbation theory (NEVPT2) with the same active...
Communication: Towards ab initio self-energy embedding theory in quantum chemistry
Lan, Tran Nguyen; Kananenka, Alexei A.; Zgid, Dominika
2015-12-01
The self-energy embedding theory (SEET), in which the active space self-energy is embedded in the self-energy obtained from a perturbative method treating the non-local correlation effects, was recently developed in our group. In SEET, the double counting problem does not appear and the accuracy can be improved either by increasing the perturbation order or by enlarging the active space. This method was first calibrated for the 2D Hubbard lattice showing promising results. In this paper, we report an extension of SEET to quantum chemical ab initio Hamiltonians for applications to molecular systems. The self-consistent second-order Green's function method is used to describe the non-local correlations, while the full configuration interaction method is carried out to capture strong correlation within the active space. Using few proof-of-concept examples, we show that SEET yields results of comparable quality to n-electron valence state second-order perturbation theory with the same active space, and furthermore, the full active space can be split into smaller active spaces without further implementation. Moreover, SEET avoids intruder states and does not require any high-order reduced density matrices. These advantages show that SEET is a promising method to describe physical and chemical properties of challenging molecules requiring large active spaces.
Communication: Towards ab initio self-energy embedding theory in quantum chemistry
Energy Technology Data Exchange (ETDEWEB)
Lan, Tran Nguyen, E-mail: latran@umich.edu [Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109 (United States); Department of Physics, University of Michigan, Ann Arbor, Michigan 48109 (United States); Kananenka, Alexei A.; Zgid, Dominika [Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109 (United States)
2015-12-28
The self-energy embedding theory (SEET), in which the active space self-energy is embedded in the self-energy obtained from a perturbative method treating the non-local correlation effects, was recently developed in our group. In SEET, the double counting problem does not appear and the accuracy can be improved either by increasing the perturbation order or by enlarging the active space. This method was first calibrated for the 2D Hubbard lattice showing promising results. In this paper, we report an extension of SEET to quantum chemical ab initio Hamiltonians for applications to molecular systems. The self-consistent second-order Green’s function method is used to describe the non-local correlations, while the full configuration interaction method is carried out to capture strong correlation within the active space. Using few proof-of-concept examples, we show that SEET yields results of comparable quality to n-electron valence state second-order perturbation theory with the same active space, and furthermore, the full active space can be split into smaller active spaces without further implementation. Moreover, SEET avoids intruder states and does not require any high-order reduced density matrices. These advantages show that SEET is a promising method to describe physical and chemical properties of challenging molecules requiring large active spaces.
Communication: Towards ab initio self-energy embedding theory in quantum chemistry
International Nuclear Information System (INIS)
The self-energy embedding theory (SEET), in which the active space self-energy is embedded in the self-energy obtained from a perturbative method treating the non-local correlation effects, was recently developed in our group. In SEET, the double counting problem does not appear and the accuracy can be improved either by increasing the perturbation order or by enlarging the active space. This method was first calibrated for the 2D Hubbard lattice showing promising results. In this paper, we report an extension of SEET to quantum chemical ab initio Hamiltonians for applications to molecular systems. The self-consistent second-order Green’s function method is used to describe the non-local correlations, while the full configuration interaction method is carried out to capture strong correlation within the active space. Using few proof-of-concept examples, we show that SEET yields results of comparable quality to n-electron valence state second-order perturbation theory with the same active space, and furthermore, the full active space can be split into smaller active spaces without further implementation. Moreover, SEET avoids intruder states and does not require any high-order reduced density matrices. These advantages show that SEET is a promising method to describe physical and chemical properties of challenging molecules requiring large active spaces
360⁰ -View of Quantum Theory and Ab Initio Simulation at Extreme Conditions: 2014 Sanibel Symposium
Energy Technology Data Exchange (ETDEWEB)
Cheng, Hai-Ping [Univ. of Florida, Gainesville, FL (United States)
2016-09-02
The Sanibel Symposium 2014 was held February 16-21 2014 at the King and Prince, St. Simons Island, GA. It was successful in bringing condensed-matter physicists and quantum chemists together productively to drive the emergence of those specialties. The Symposium had a significant role in preparing a whole generation of quantum theorists. The 54th Sanibel meeting looked to the future in two ways. We had 360⁰-View sessions to honor the exceptional contributions of Rodney Bartlett (70), Bill Butler (70), Yngve Öhrn (80), Fritz Schaefer (70), and Malcolm Stocks (70). The work of these five has greatly impacted several generations of quantum chemists and condensed matter physicists. The “360⁰” is the sum of their ages. More significantly, it symbolizes a panoramic view of critical developments and accomplishments in theoretical and computational chemistry and physics oriented toward the future. Thus, two of the eight 360⁰-View sessions focused specifically on younger scientists. The 360⁰-View program was the major component of the 2014 Sanibel meeting. Another four sessions included a sub-symposium on ab initio Simulations at Extreme Conditions, with focus on getting past the barriers of present-day Born-Oppenheimer molecular dynamics by advances in finite-temperature density functional theory, orbital-free DFT, and new all-numerical approaches.
Patil, Amol Baliram; Bhanage, Bhalchandra Mahadeo
2016-06-21
The nature of bonding interactions between the cation and the anion of an ionic liquid is at the heart of understanding ionic liquid properties. A particularly interesting case is a special class of ionic liquids known as protic ionic liquids. The extent of proton transfer in protic ionic liquids has been observed to vary according to the interacting species. Back proton transfer renders protic ionic liquids volatile and to be considered as inferior ionic liquids. We try to address this issue by employing modern ab initio valence bond theory calculations. The results indicate that the bonding in the cation and the anion of a prototypical ionic liquid, ethylammonium nitrate, is fundamentally different. It is neither characteristic of covalent/polar covalent bonding nor ionic bonding but rather charge shift bonding as a resonance hybrid of two competing ionic molecular electronic structure configurations. An investigation of other analogous protic ionic liquids reveals that this charge shift bonding seems to be a typical characteristic of protic ionic liquids while the ionic solid analogue compound ammonium nitrate has less charge shift bonding character as compared to protic ionic liquids. Further the extent of charge shift bonding character has been found to be congruent with the trends in many physicochemical properties such as melting point, conductivity, viscosity, and ionicity of the studied ionic liquids indicating that percentage charge shift character may serve as a key descriptor for large scale computational screening of ionic liquids with desired properties. PMID:27229870
Red-Shifting versus Blue-Shifting Hydrogen Bonds: Perspective from Ab Initio Valence Bond Theory.
Chang, Xin; Zhang, Yang; Weng, Xinzhen; Su, Peifeng; Wu, Wei; Mo, Yirong
2016-05-01
Both proper, red-shifting and improper, blue-shifting hydrogen bonds have been well-recognized with enormous experimental and computational studies. The current consensus is that there is no difference in nature between these two kinds of hydrogen bonds, where the electrostatic interaction dominates. Since most if not all the computational studies are based on molecular orbital theory, it would be interesting to gain insight into the hydrogen bonds with modern valence bond (VB) theory. In this work, we performed ab initio VBSCF computations on a series of hydrogen-bonding systems, where the sole hydrogen bond donor CF3H interacts with ten hydrogen bond acceptors Y (═NH2CH3, NH3, NH2Cl, OH(-), H2O, CH3OH, (CH3)2O, F(-), HF, or CH3F). This series includes four red-shifting and six blue-shifting hydrogen bonds. Consistent with existing findings in literature, VB-based energy decomposition analyses show that electrostatic interaction plays the dominating role and polarization plays the secondary role in all these hydrogen-bonding systems, and the charge transfer interaction, which denotes the hyperconjugation effect, contributes only slightly to the total interaction energy. As VB theory describes any real chemical bond in terms of pure covalent and ionic structures, our fragment interaction analysis reveals that with the approaching of a hydrogen bond acceptor Y, the covalent state of the F3C-H bond tends to blue-shift, due to the strong repulsion between the hydrogen atom and Y. In contrast, the ionic state F3C(-) H(+) leads to the red-shifting of the C-H vibrational frequency, owing to the attraction between the proton and Y. Thus, the relative weights of the covalent and ionic structures essentially determine the direction of frequency change. Indeed, we find the correlation between the structural weights and vibrational frequency changes. PMID:27074500
Predictive Nuclear Many-Body Theory with Ab Initio Methods: A Brief Survey and A Look Ahead
Hergert, Heiko
2015-10-01
The reach of ab initio many-body techniques has increased tremendously in recent years, owing to new developments in many-body theory as well as advances in their numerical implementation. Coupled Cluster, Self-Consistent Green's Function, and In-Medium Similarity Renormalization Group (IM-SRG) calculations are routinely performed for isotopes in the A ~ 100 region. Moreover, these techniques have been extended to tackle open-shell nuclei, either directly or through the auxiliary step of deriving valence-space interactions for use with existing Shell Model technology. One of the most powerful aspects of ab initio methods is their capability to provide results for energies and other observables with systematic uncertainties. Together with new accurate nuclear forces (and operators) derived from Chiral Effective Field Theory, they provide a consistent framework--and a road map--for a predictive description of nuclei. This will have a critical impact on the search for the limits of nuclear existence, tests of fundamental symmetries (e.g., the search for neutrinoless double beta decay), our understanding of quenching and effective charges in phenomenological Shell Model calculations etc. Using the Multi-Reference IM-SRG as a representative example, I will survey the current capabilities of ab initio methods with an emphasis on uncertainty quantification, highlight successes in the description of ground-state properties and spectra, and preview upcoming developments like the construction of consistent transition operators.
Paired-permanent approach for VB theory (II) -An ab initio spin-free VB program
Institute of Scientific and Technical Information of China (English)
SONG; Lingchun
2001-01-01
［1］Gerratt. J., Orville-Thomas, W. J., Adavance in valence bond theory, J. Mol. Struct. (Theochem), 1991, 222: 1-290.［2］Cooper, K. L., Gerratt, J., Raimondi, M., Applications of spin-coupled valence bond theory, Chem. Rev., 1991, 91: 929-964.［3］Wu, W., McWeeny, R., Valence bond calculations of the potential energy surface for CH4→CH3+H, J. Chem. Phys., 1994,101: 4826-4831.［4］Mo, Y., Wu, W.. Zhang, Q., Theoretical resonance energies of benzene, cyclobutadiene and butadiene, J. Phys. Chem.,1994,98: 10048-10053.［5］Mo, Y.. Lin, Z., Wu, W. et al., Bond-distorted orbitals and effects of hybridization and resonance on C-C bond lengths, J. Phys. Chem.. 1996, 100:11569-11572.［6］Cao, Z.. Wu, W., Zhang, Q., Valence bond study on excited states of molecules- Bonding features of the low-lying states of molecule B2. Science in China, Series B, 1997, 40(5): 548-553.［7］Cao, Z., Xian, H., Wu, W. et al., Visual valence bond rules for chemical reactions, Theor. Chem. Acc., 1999, 101: 352-358.［8］Wu, W., Wu, A., Mo, Y. et al., Paired-permanent approach in VB theory, Science in China, Series B, 1996, 39(5): 456-467.［9］Wu, W., Wu, A., Mo, Y. et al., An efficient algorithm for the spin-free valence bond theory, I. New strategy and primary expressions, Int. J. Quantum Chem., 1998, 67: 287-297.［10］Wu, W., Song, L., Mo, Y. et al., Xiamen -- An ab initio spin-free valence bond (VB) program, Xiamen: Xiamen University, 1998.［11］Badenhoop, J. K., Weinhold, F., Natural steric analysis of luternal rotation barriers, Int. J. Q. Chem., 1999, 72: 269--280.［12］Kijkstra, F., van Lenthe, J. H., On the rapid evaluation of cofactors in the calculation of nonorthogonai matrix elements, Int. J. Quant. Chem., 1998, 67: 77-83.［13］Galbraith, J. M., Schreiner, P. R., Harris, N. et al., A valence bond study of the Bergmen cyclization: Geometric features,resonance energy, and nucleus-independent chemical shift (NICS) values, Chem. Eur. J., 2000, 6: 1446-1454.
Toth, Laszlo Daniel
2013-05-07
Disordered photonics is the study of light in random media. In a disordered photonic medium, multiple scattering of light and coherence, together with the fundamental principle of reciprocity, produce a wide range of interesting phenomena, such as enhanced backscattering and Anderson localization of light. They are also responsible for the existence of modes in these random systems. It is known that analogous processes to Bose-Einstein condensation can occur in classical wave systems, too. Classical condensation has been studied in several contexts in photonics: pulse formation in lasers, mode-locking theory and coherent emission of disordered lasers. All these systems have the common theme of possessing a large ensemble of waves or modes, together with nonlinearity, dispersion or gain. In this work, we study light condensation and its connection with light localization in a disordered, passive dielectric medium. We develop a theory for the modes inside the disordered resonator, which combines the Feshbach projection technique with spin-glass theory and statistical physics. In particular, starting from the Maxwell’s equations, we map the system to a spherical p-spin model with p = 2. The spins are replaced by modes and the temperature is related to the fluctuations in the environment. We study the equilibrium thermodynamics of the system in a general framework and show that two distinct phases exist: a paramagnetic phase, where all the modes are randomly oscillating and a condensed phase, where the energy condensates on a single mode. The thermodynamic quantities can be explicitly interpreted and can also be computed from the disorder-averaged time domain correlation function. We launch an ab initio simulation campaign using our own code and the Shaheen supercomputer to test the theoretical predictions. We construct photonic samples of varying disorder and find computationally relevant ways to obtain the thermodynamic quantities. We observe the phase transition
Ab initio, density functional theory and structural studies of 4-amino-2-methylquinoline
Arjunan, V.; Saravanan, I.; Ravindran, P.; Mohan, S.
2009-10-01
The Fourier transform infrared (FTIR) and FT-Raman spectra of 4-amino-2-methylquinoline (AMQ) have been recorded in the range 4000-400 and 4000-100 cm -1, respectively. The experimental vibrational frequency was compared with the wavenumbers obtained theoretically by ab initio HF and DFT-B3LYP gradient calculations employing the standard 6-31G** and high level 6-311++G** basis sets for optimised geometry of the compound. The complete vibrational assignment and analysis of the fundamental modes of the compounds were carried out using the experimental FTIR and FT-Raman data, and quantum mechanical studies. The geometry and normal modes of vibration obtained from the HF and DFT methods are in good agreement with the experimental data. The potential energy distribution of the fundamental modes was calculated with ab initio force fields utilising Wilson's FG matrix method. The NH -π interactions and the influence of amino and methyl groups on the skeletal modes are investigated.
International Nuclear Information System (INIS)
Graphical abstract: The electronic properties such as the static dipole polarizability, anisotropy of the polarizability, and dipole moment of yttrium bromide, YBr (X1Σ) have been theoretically studied. Highlights: → Conventional ab initio and density functional theory methods were employed to study linear optical properties of YBr molecule. → Properties derivatives and their level of theory dependence were studied. → Electron correlation effects and rovibrational corrections have also been discussed. - Abstract: We have employed conventional ab initio and density functional theory methods to study the electronic properties such as the mean static dipole polarizability, α-bar, anisotropy of the polarizability, Δα, and dipole moment, μ, of yttrium bromide. The bond length dependence of properties is determined at different levels of theory and appropriate expansions around experimental internuclear distance have been presented. Moreover, the first and second geometrical derivatives for each property are quantified and their level of theory dependence has been analyzed. To study the effect of molecular rotation and vibration on the electronic properties, the rovibrational corrections have also been carried out. It is found that these corrections are less pronounced for considered properties of YBr. In all calculations, the electron correlation effects have been considered and discussed. The obtained results show that the electron correlation is more significant in the calculation of the mean and the anisotropy of dipole polarizability.
Ab initio valence calculations in chemistry
Cook, D B
1974-01-01
Ab Initio Valence Calculations in Chemistry describes the theory and practice of ab initio valence calculations in chemistry and applies the ideas to a specific example, linear BeH2. Topics covered include the Schrödinger equation and the orbital approximation to atomic orbitals; molecular orbital and valence bond methods; practical molecular wave functions; and molecular integrals. Open shell systems, molecular symmetry, and localized descriptions of electronic structure are also discussed. This book is comprised of 13 chapters and begins by introducing the reader to the use of the Schrödinge
Ab initio joint density-functional theory of solvated electrodes, with model and explicit solvation
Arias, Tomas
2015-03-01
First-principles guided design of improved electrochemical systems has the potential for great societal impact by making non-fossil-fuel systems economically viable. Potential applications include improvements in fuel-cells, solar-fuel systems (``artificial photosynthesis''), supercapacitors and batteries. Economical fuel-cell systems would enable zero-carbon footprint transportation, solar-fuel systems would directly convert sunlight and water into hydrogen fuel for such fuel-cell vehicles, supercapacitors would enable nearly full recovery of energy lost during vehicle braking thus extending electric vehicle range and acceptance, and economical high-capacity batteries would be central to mitigating the indeterminacy of renewable resources such as wind and solar. Central to the operation of all of the above electrochemical systems is the electrode-electrolyte interface, whose underlying physics is quite rich, yet remains remarkably poorly understood. The essential underlying technical challenge to the first principles studies which could explore this physics is the need to properly represent simultaneously both the interaction between electron-transfer events at the electrode, which demand a quantum mechanical description, and multiscale phenomena in the liquid environment such as the electrochemical double layer (ECDL) and its associated shielding, which demand a statistical description. A direct ab initio approach to this challenge would, in principle, require statistical sampling and thousands of repetitions of already computationally demanding quantum mechanical calculations. This talk will begin with a brief review of a recent advance, joint density-functional theory (JDFT), which allows for a fully rigorous and, in principle, exact representation of the thermodynamic equilibrium between a system described at the quantum-mechanical level and a liquid environment, but without the need for costly sampling. We then shall demonstrate how this approach applies in
International Nuclear Information System (INIS)
Based on the ab initio molecular orbital theory at the HF/6-31G(d) level, the effect of hydration on the reduced partition function ratio(RPFR) of the monoborate anion (B(OH)4-) is evaluated in order to better understand boron isotope fractionation observed in aqueous systems. Aquoborate anions up to decaaquoborate anion, B(OH)4-(H2O)10, were considered and their geometry optimization and RPFR calculations were carried out. It was induced that hydration decreased the ln(RPFR) value of B(OH)4- by ca. 1.2%. (author)
Frandsen, Benjamin A.; Brunelli, Michela; Page, Katharine; Uemura, Yasutomo J.; Staunton, Julie B.; Billinge, Simon J. L.
2016-05-01
We present a temperature-dependent atomic and magnetic pair distribution function (PDF) analysis of neutron total scattering measurements of antiferromagnetic MnO, an archetypal strongly correlated transition-metal oxide. The known antiferromagnetic ground-state structure fits the low-temperature data closely with refined parameters that agree with conventional techniques, confirming the reliability of the newly developed magnetic PDF method. The measurements performed in the paramagnetic phase reveal significant short-range magnetic correlations on a ˜1 nm length scale that differ substantially from the low-temperature long-range spin arrangement. Ab initio calculations using a self-interaction-corrected local spin density approximation of density functional theory predict magnetic interactions dominated by Anderson superexchange and reproduce the measured short-range magnetic correlations to a high degree of accuracy. Further calculations simulating an additional contribution from a direct exchange interaction show much worse agreement with the data. The Anderson superexchange model for MnO is thus verified by experimentation and confirmed by ab initio theory.
Frandsen, Benjamin A; Brunelli, Michela; Page, Katharine; Uemura, Yasutomo J; Staunton, Julie B; Billinge, Simon J L
2016-05-13
We present a temperature-dependent atomic and magnetic pair distribution function (PDF) analysis of neutron total scattering measurements of antiferromagnetic MnO, an archetypal strongly correlated transition-metal oxide. The known antiferromagnetic ground-state structure fits the low-temperature data closely with refined parameters that agree with conventional techniques, confirming the reliability of the newly developed magnetic PDF method. The measurements performed in the paramagnetic phase reveal significant short-range magnetic correlations on a ∼1 nm length scale that differ substantially from the low-temperature long-range spin arrangement. Ab initio calculations using a self-interaction-corrected local spin density approximation of density functional theory predict magnetic interactions dominated by Anderson superexchange and reproduce the measured short-range magnetic correlations to a high degree of accuracy. Further calculations simulating an additional contribution from a direct exchange interaction show much worse agreement with the data. The Anderson superexchange model for MnO is thus verified by experimentation and confirmed by ab initio theory. PMID:27232042
Discovering chemistry with an ab initio nanoreactor
Wang, Lee-Ping; Titov, Alexey; McGibbon, Robert; Liu, Fang; Pande, Vijay S.; Martínez, Todd J.
2014-01-01
Chemical understanding is driven by the experimental discovery of new compounds and reactivity, and is supported by theory and computation that provides detailed physical insight. While theoretical and computational studies have generally focused on specific processes or mechanistic hypotheses, recent methodological and computational advances harken the advent of their principal role in discovery. Here we report the development and application of the ab initio nanoreactor – a highly accelerat...
International Nuclear Information System (INIS)
The excited states of small-diameter diamond nanoparticles in the gas phase are studied using the GW method and Bethe-Salpeter equation (BSE) within the ab initio many-body perturbation theory. The calculated ionization potentials and optical gaps are in agreement with experimental results, with the average error about 0.2 eV. The electron affinity is negative and the lowest unoccupied molecular orbital is rather delocalized. Precise determination of the electron affinity requires one to take the off-diagonal matrix elements of the self-energy operator into account in the GW calculation. BSE calculations predict a large exciton binding energy which is an order of magnitude larger than that in the bulk diamond
International Nuclear Information System (INIS)
Based on the ab initio molecular orbital theory at the HF/6-31G(d) level, the effect of hydration on the reduced partition function ratio (RPFR) of the boric acid molecule (B(OH)3) was evaluated in order to better understand boron isotope fractionation observed in aqueous systems. The B(OH)3(H2O)n species with n up to 12 were considered and their geometry optimization and RPFR calculations were carried out. It was induced that hydration decreased the RPFR of B(OH)3 and the degree of decrease in ln(RPFR) was ca. 1.5%, which was nearly equivalent to that of B(OH)4-. (author)
Energy Technology Data Exchange (ETDEWEB)
Masrour, R., E-mail: rachidmasrour@hotmail.com [Laboratory of Materials, Processes, Environment and Quality, Cady Ayyed University, National School of Applied Sciences, BP. 63, 46000 Safi (Morocco); LMPHE (URAC 12), Faculty of Science, Mohammed V-Agdal University, Rabat (Morocco); Hlil, E.K. [Institut Néel, CNRS et Université Joseph Fourier, BP 166, F-38042 Grenoble Cedex 9 (France); Hamedoun, M. [Institute of Nanomaterials and Nanotechnologies, MAScIR, Rabat (Morocco); Benyoussef, A. [LMPHE (URAC 12), Faculty of Science, Mohammed V-Agdal University, Rabat (Morocco); Institute of Nanomaterials and Nanotechnologies, MAScIR, Rabat (Morocco); Hassan II Academy of Science and Technology, Rabat (Morocco); Mounkachi, O.; El Moussaoui, H. [Institute of Nanomaterials and Nanotechnologies, MAScIR, Rabat (Morocco)
2014-06-01
Self-consistent ab initio calculations, based on DFT (Density Functional Theory) approach and using FLAPW (Full potential Linear Augmented Plane Wave) method, are performed to investigate both electronic and magnetic properties of the MnSe lattice. Polarized spin and spin–orbit coupling are included in calculations within the framework of the antiferromagnetic state between two adjacent Mn lattices. Magnetic moments considered to lie along (001) axes are computed. Obtained data from ab initio calculations are used as input for the high temperature series expansions (HTSEs) calculations to compute other magnetic parameters. The zero-field high temperature static susceptibility series of the spin −4.28 nearest-neighbor Ising model on face centered cubic (fcc) and lattices is thoroughly analyzed by means of a power series coherent anomaly method (CAM). The exchange interaction between the magnetic atoms and the Néel temperature are deduced using the mean filed and HTSEs theories. - Highlights: • Ab initio calculations are used to investigate both electronic and magnetic properties of the MnSe alloys. • Obtained data from ab initio calculations are used as input for the HTSEs. • The Néel temperature is obtained for MnSe alloys.
International Nuclear Information System (INIS)
Density functional theory and ab initio methods are employed to investigate decomposition pathways of 1,3,3-trinitroazetidine initiated by unimolecular loss of NO2 or HONO. Geometry optimizations are performed using M06/cc-pVTZ and coupled-cluster (CC) theory with single, double, and perturbative triple excitations, CCSD(T), is used to calculate accurate single-point energies for those geometries. The CCSD(T)/cc-pVTZ energies for NO2 elimination by N–N and C–N bond fission are, including zero-point energy (ZPE) corrections, 43.21 kcal/mol and 50.46 kcal/mol, respectively. The decomposition initiated by trans-HONO elimination can occur by a concerted H-atom and nitramine NO2 group elimination or by a concerted H-atom and nitroalkyl NO2 group elimination via barriers (at the CCSD(T)/cc-pVTZ level with ZPE corrections) of 47.00 kcal/mol and 48.27 kcal/mol, respectively. Thus, at the CCSD(T)/cc-pVTZ level, the ordering of these four decomposition steps from energetically most favored to least favored is: NO2 elimination by N–N bond fission, HONO elimination involving the nitramine NO2 group, HONO elimination involving a nitroalkyl NO2 group, and finally NO2 elimination by C–N bond fission
Institute of Scientific and Technical Information of China (English)
2008-01-01
Ionic dissociation of chlorosulfonic acid (HSO3Cl) in the molecular clusters HSO3Cl-(H2O)n (n = 1-4) and HSO3Cl-NH3-(H2O)n (n = 0-3) was investigated by density functional theory and ab initio molecular orbital theory. The equilibrium structures, binding energies, and thermodynamic properties, such as relative enthalpy and relative Gibbs free energy, and were calculated using the hybrid density func- tional (B3LYP) method and the second order M?ller-Plesset approximation (MP2) method with the 6-311++G** basis set. Chlorosulfonic acid was found to require a minimum of three water molecules for ionization to occur and at least one water molecule to protonate ammonia. The corresponding clusters with fewer water molecules were found to be strongly hydrogen-bonded. The related properties and acid strength of chlorosulfonic acid were discussed and compared to the acid strengths of perchloric acid and sulfuric acid in the context of clusters with ammonia and water. The relative stabilities of these clusters were also investigated.
Ab-initio study of thermoelectricity of layered tellurium compounds
Ibarra Hernández, Wilfredo
2015-01-01
In this thesis, we explore the electronic, dynamic and thermoelectric properties of different tellurium-based compounds. We perform ab-initio calculations within the Vienna Ab-initio Simulation Package (VASP) that works in the framework of Density Functional Theory (DFT). For the thermoelectric properties, we use the Boltztrap code that solves the Boltzmann Transport Equations (BTE) for electrons within the Constant Relaxation Time Approximation (CRTA). This computational pa...
Kühne, Thomas D
2012-01-01
Computer simulations and molecular dynamics in particular, is a very powerful method to provide detailed and essentially exact informations of classical many-body problems. With the advent of \\textit{ab-initio} molecular dynamics, where the forces are computed on-the-fly by accurate electronic structure calculations, the scope of either method has been greatly extended. This new approach, which unifies Newton's and Schr\\"odinger's equations, allows for complex simulations without relying on any adjustable parameter. This review is intended to outline the basic principles as well as a survey of the field. Beginning with the derivation of Born-Oppenheimer molecular dynamics, the Car-Parrinello method as well as novel hybrid scheme that unifies best of either approach are discussed. The predictive power is demonstrated by a series of applications ranging from insulators to semiconductors and even metals in condensed phases.
Ab Initio Molecular Dynamics: A Virtual Laboratory
Hobbi Mobarhan, Milad
2014-01-01
In this thesis, we perform ab initio molecular dynamics (MD) simulations at the Hartree-Fock level, where the forces are computed on-the-fly using the Born-Oppenheimer approximation. The theory behind the Hartree-Fock method is discussed in detail and an implementation of this method based on Gaussian basis functions is explained. We also demonstrate how to calculate the analytic energy derivatives needed for obtaining the forces acting on the nuclei. Hartree-Fock calculations on the ground s...
Polin, Daniel; Ziegler, Joshua; Malozovsky, Yuriy; Bagayoko, Diola
We present the findings of ab-initio calculations of electronic, transport, and structural properties of cubic sodium oxide (Na2O). These results were obtained using density functional theory (DFT), specifically a local density approximation (LDA) potential, and the linear combination of Gaussian orbitals (LCGO). Our implementation of LCGO followed the Bagayoko, Zhao, and Williams method as enhanced by the work of Ekuma and Franklin (BZW-EF). We describe the electronic band structure of Na2O with a direct band gap of 2.22 eV. Our results include predicted values for the electronic band structure and associated energy eigenvalues, the total and partial density of states (DOS and pDOS), the equilibrium lattice constant of Na2O, and the bulk modulus. We have also calculated the electron and holes effective masses in the Γ to L, Γ to X, and Γ to K directions. Acknowledgments: This work was funded in part by the National Science Foundation (NSF) and the Louisiana Board of Regents, through LASiGMA [Award Nos. EPS- 1003897, NSF (2010-15)-RII-SUBR] and NSF HRD-1002541, the US Department of Energy - National, Nuclear Security Administration (NNSA) (Award No. DE- NA0002630), LaSPACE, and LONI-SUBR.
Kornobis, Karina; Kumar, Neeraj; Wong, Bryan M.; Lodowski, Piotr; Jaworska, Maria; Andruniów, Tadeusz; Rudd, Kenneth; Kozlowski, Pawel M.
2011-01-01
Time-dependent density functional theory (TD-DFT) and correlated ab initio methods have been applied to the electronically excited states of vitamin B12 (cyanocobalamin or CNCbl). Different experimental techniques have been used to probe the excited states of CNCbl, revealing many issues that remain poorly understood from an electronic structure point of view. Due to its efficient scaling with size, TD-DFT emerges as one of the most practical tools that can be used to predict the electronic p...
Bacca, Sonia
2016-04-01
A brief review of models to describe nuclear structure and reactions properties is presented, starting from the historical shell model picture and encompassing modern ab initio approaches. A selection of recent theoretical results on observables for exotic light and medium-mass nuclei is shown. Emphasis is given to the comparison with experiment and to what can be learned about three-body forces and continuum properties.
Grassi, A; Forte, G; Angilella, G G N; Pucci, R; March, N H
2004-01-01
The present study of small molecules containing silicon has been motivated by (a) the considerable interest being shown currently in the kinetics and reactivity of such molecules, and (b) the biotechnological potential of silicon-derivate surfaces as substrates in the adsorption of, for instance, amino acids and proteins. Therefore, we have studied by (i) a semi-empirical approach and (ii) an ab initio procedure employing low-order Moller-Plesset perturbation theory, the molecular correlation energies of some neutral closed and open shell silicon-containing molecules in the series SiXnYm. Procedure (i) is shown to have particular merit for the correlation of the ionic members studied in the above series, while the ab initio procedures employed come into their own for neutral species.
Ab-initio Hamiltonian approach to light nuclei and to quantum field theory
Indian Academy of Sciences (India)
J P Vary; H Honkanen; Jun Li; P Maris; A M Shirokov; S J Brodsky; A Harindranath; G F De Teramond; E G Ng; C Yang; M Sosonkina
2010-07-01
Nuclear structure physics is on the threshold of confronting several long-standing problems such as the origin of shell structure from basic nucleon–nucleon and three-nucleon interactions. At the same time those interactions are being developed with increasing contact to QCD, the underlying theory of the strong interactions, using effective field theory. The motivation is clear – QCD offers the promise of great predictive power spanning phenomena on multiple scales from quarks and gluons to nuclear structure. However, new tools that involve non-perturbative methods are required to build bridges from one scale to the next. We present an overview of recent theoretical and computational progress with a Hamiltonian approach to build these bridges and provide illustrative results for the nuclear structure of light nuclei and quantum field theory.
Ab-Initio Hamiltonian Approach to Light Nuclei And to Quantum Field Theory
International Nuclear Information System (INIS)
Nuclear structure physics is on the threshold of confronting several long-standing problems such as the origin of shell structure from basic nucleon-nucleon and three-nucleon interactions. At the same time those interactions are being developed with increasing contact to QCD, the underlying theory of the strong interactions, using effective field theory. The motivation is clear - QCD offers the promise of great predictive power spanning phenomena on multiple scales from quarks and gluons to nuclear structure. However, new tools that involve non-perturbative methods are required to build bridges from one scale to the next. We present an overview of recent theoretical and computational progress with a Hamiltonian approach to build these bridges and provide illustrative results for the nuclear structure of light nuclei and quantum field theory.
Ab Initio Nuclear Theory - Progress and Prospects from Quarks to the Cosmos
International Nuclear Information System (INIS)
The vision of solving the nuclear many-body problem with fundamental interactions tied to QCD appears to approach reality. The goals are to preserve the predictive power of the underlying theory, to test fundamental symmetries with the nucleus as laboratory and to develop new understandings of the full range of complex nuclear phenomena. Recent progress includes the derivation, within chiral perturbation theory (ChPT), of the leading terms of the nucleon-nucleon (NN), three-nucleon (3N) and four nucleon (4N) potentials. Additional substantial progress includes solving nuclear structure and reactions in nuclei up to mass 16 and selected heavier nuclei around closed shells using these ChPT interactions. Advances in theoretical frameworks (renormalization and many-body methods) as well as in computational resources (new algorithms and leadership-class parallel computers) signal a new generation of theory simulations that will yield valuable insights into origins of nuclear shell structure, collective phenomena and complex reaction dynamics. I outline some recent achievements and present ambitious consensus plans for a coming decade of research that will strengthen the links between nuclear theory and nuclear experiment, between nuclear physics and astrophysics, and between nuclear physics and nuclear energy applications. (author)
Institute of Scientific and Technical Information of China (English)
周立新
2000-01-01
Results or ab initio selr-consistent-field ( SCF) and denityfunctional theory (DFr) calculations of the gas-phase structure, acidity (free energy of deprotona tion, △G°) and aroma ticity of tetraselenosquaric acid (3, 4-diseleny-3-cyclobutene-1,2-diselenone, H2C4Se4) are reported.The global minimu found on the potenial energy surface of tetraselenosquaric acid presents a planar conformation. The ZZ isomer was found to have the lowest energy among the three planar conformers and the ZZ and ZE is omers are very close in energy. The optimized geometric parameters exhibit a bond length equalization relative to reference compounds, cyclobutanediselenone, and cyclobutenediselenol. The computed aromaic stabilization energy(ASE)by homodesmotic reaction is -77.4 (MP2(fu)/6 - 311 + G* *//RHF/6 - 311 + G* * ) and - 54.8 kJ/mol (B3LYP/6 - 311 + G* * //B3LYP/6 - 311 + G* * ). The aromaticity of tetraselenosquaric acid is indicated by the calculated diamagnetic susceptibility exaltation (A) - 19.13 (CSGT(IGAIM) - RHF/6 - 311 + G**//RHF/6 - 311 + G* * and - 32.91(4π· 10-6 m3/mo l)(CSGT(IGA1M)-B3LYP/6 - 311 + G* *//B3LYP/6 - 311 + G* * ).Thus, tetraselenosquaric acid fulfils the geometric, energetic and magnetic criteria of aromaticity. The calculated gas-phase acidity is △G10(298K) = 1257.7 and △G20(298K) = 1617.1 kJ/mol. Hence, tetraselenosquarc acid is the stronest acid among the three squaric acids (3,4-dihydroxy-3-cyclobutene-1,2-dione, H2C4O4, 3,4-dithiohydroxy-3- cyclobutene-1,2-dithione, H2C4S4, 3,4- diselenyl-3- cyclobutene- 1,2-diselenone, H2C4Se4).
Ab Initio Path to Heavy Nuclei
Binder, Sven; Calci, Angelo; Roth, Robert
2014-01-01
We present the first ab initio calculations of nuclear ground states up into the domain of heavy nuclei, spanning the range from 16-O to 132-Sn based on two- plus three-nucleon interactions derived within chiral effective field theory. We employ the similarity renormalization group for preparing the Hamiltonian and use coupled-cluster theory to solve the many-body problem for nuclei with closed sub-shells. Through an analysis of theoretical uncertainties resulting from various truncations in this framework, we identify and eliminate the technical hurdles that previously inhibited the step beyond medium-mass nuclei, allowing for reliable validations of nuclear Hamiltonians in the heavy regime. Following this path we show that chiral Hamiltonians qualitatively reproduce the systematics of nuclear ground-state energies up to the neutron-rich Sn isotopes.
Ab initio thermal transport properties of nanostructures from density functional perturbation theory
International Nuclear Information System (INIS)
We present a comprehensive first-principles study of the thermal transport properties of low-dimensional nanostructures such as polymers and nanowires. An approach is introduced where the phonon quantum conductance is computed from the combination of accurate plane-wave density functional theory electronic structure calculations, the evaluation of the interatomic force constants through density functional perturbation theory for lattice dynamics, and the calculation of transport properties by a real-space Green’s function method based on the Landauer formalism. This approach is computationally very efficient, can be straightforwardly implemented as a post-processing step in a standard electronic structure calculation (Quantum ESPRESSO and WanT in the present implementation), and allows us to directly link the thermal transport properties of a device to the coupling, dimensionality, and atomistic structure of the system. It provides invaluable insight into the mechanisms that govern heat flow at the nanoscale and paves the way to the fundamental understanding of phonon engineering in nanostructures.
Molt, Robert W; Bartlett, Rodney J; Watson, Thomas; Bazanté, Alexandre P
2012-12-13
We have identified the major conformers of CL-20 explosive, otherwise known as 2,4,6,8,10,12-hexanitrohexaazaisowurtzitane, more formally known as 2,4,6,8,10,12-hexanitrohexaazatetracyclo[5.5.0.0]-dodecane, via Monte Carlo search in conformational space through molecular mechanics and subsequent quantum mechanical refinement using perturbation theory. Our search produced enough conformers to account for all of the various forms of CL-20 found in crystals. This suggests that our methodology will be useful in studying the conformational landscape of other nitramines. The energy levels of the conformers found are all within 0.25 eV of one another based on MBPT(2)/6-311G(d,p); consequently, without further refinement from a method such as coupled cluster theory, all conformers may reasonably be populated at STP in the gas phase. We also report the harmonic vibrational frequencies of conformers, including the implications on the mechanism of detonation. In particular, we establish that the weakest N-N nitramine of CL-20 is the cyclohexane equatorial nitramine. This preliminary mapping of the conformers of CL-20 makes it possible to study the mechanism of detonation of this explosive rigorously in future work. PMID:23136867
Ab initio bubble-driven denaturation of double-stranded DNA: Self-mechanical theory.
Kuetche, Victor K
2016-07-21
Among the different theoretical models of the open-site-driven DNA-denaturation found in the literature, very few interests are actually paid to the fundamental unzipping process of the double-stranded DNA within the vicinity of its ground state condensate. In this paper, we address an alternative to better understand the process of denaturation of such a macromolecule by investigating the onset of its dynamics around its equilibrium state. We show that from the initiation of the transcription bubble by the promoter to the termination state, the open-states of the strands evolve dynamically while generating some localized waveguide channels with elastic scattering properties. We properly discuss the nonlinear dynamics of these structures within the viewpoint of the self-mechanical theory while inferring to the physical structure of the findings and their potential issues. PMID:27113786
Ab initio theory of the electronic structure of nf-ions in solids and liquids
International Nuclear Information System (INIS)
Full text: In the books and papers we developed the self-consistent field theory of nl- ions in Me+n:[L]k- clusters, where k is number of ligands - L, by ion-ligands distance - R. The results which were obtained for all RE and AC ions for Me+n:[L]k, where L- F-, O-2 and so on ligands, are closely corresponded to experimental data. The expression for energy of the cluster may be write as: E = E0 + kE1 + k'(Ez + Ec + Ee + Eex), (Eq.1), where E0 and E1 are the energies of the free nl-ion and surrounding one; Ez , Ec and Ee are the energy of electrons interaction with 'strange' nucleus, Coulomb electron-electron interaction and exchange one. Eex is the energy of the interaction of electrons and nucleus with external field. The equations for the radial one-electron wave functions of the ions in the cluster were obtained by minimizing the Eq. 1 for the radial orbitals of the central ion and ligand one. We have received the general system of equations of the self-consistent field for cluster in liquids and solid states. The results of calculations of the energy structure of clusters and values of the standard radial integrals (spectroscopy parameters) for Ac-ions in 1-2-3 superconductors and RE-ions in garnet crystals by different values of R are qualitatively and quantitatively correct. We've got the best results for pressure dependence of Nd ions spectra, change of optical and X-Ray spectra after irradiation of garnets. We explained the nature of anomalous in SrTiO3 and separate lasers crystals by used of results of the calculations. In the framework of our approach we obtained the best accuracy for the energy of X- Ray lines for all nf- ions in solids and liquids
Clerc, Daryl G
2016-07-21
An ab initio approach was used to study the molecular-level interactions that connect gene-mutation to changes in an organism׳s phenotype. The study provides new insights into the evolutionary process and presents a simplification whereby changes in phenotypic properties may be studied in terms of the binding affinities of the chemical interactions affected by mutation, rather than by correlation to the genes. The study also reports the role that nonlinear effects play in the progression of organs, and how those effects relate to the classical theory of evolution. Results indicate that the classical theory of evolution occurs as a special case within the ab initio model - a case having two attributes. The first attribute: proteins and promoter regions are not shared among organs. The second attribute: continuous limiting behavior exists in the physical properties of organs as well as in the binding affinity of the associated chemical interactions, with respect to displacements in the chemical properties of proteins and promoter regions induced by mutation. Outside of the special case, second-order coupling contributions are significant and nonlinear effects play an important role, a result corroborated by analyses of published activity levels in binding and transactivation assays. Further, gradations in the state of perfection of an organ may be small or large depending on the type of mutation, and not necessarily closely-separated as maintained by the classical theory. Results also indicate that organs progress with varying degrees of interdependence, the likelihood of successful mutation decreases with increasing complexity of the affected chemical system, and differences between the ab initio model and the classical theory increase with increasing complexity of the organism. PMID:27029513
Guiding ab initio calculations by alchemical derivatives
to Baben, M.; Achenbach, J. O.; von Lilienfeld, O. A.
2016-03-01
We assess the concept of alchemical transformations for predicting how a further and not-tested change in composition would change materials properties. This might help to guide ab initio calculations through multidimensional property-composition spaces. Equilibrium volumes, bulk moduli, and relative lattice stability of fcc and bcc 4d transition metals Zr, Nb, Mo, Tc, Ru, Rh, Pd, and Ag are calculated using density functional theory. Alchemical derivatives predict qualitative trends in lattice stability while equilibrium volumes and bulk moduli are predicted with less than 9% and 28% deviation, respectively. Predicted changes in equilibrium volume and bulk moduli for binary and ternary mixtures of Rh-Pd-Ag are in qualitative agreement even for predicted bulk modulus changes as large as +100% or -50%. Based on these results, it is suggested that alchemical transformations could be meaningful for enhanced sampling in the context of virtual high-throughput materials screening projects.
Energy Technology Data Exchange (ETDEWEB)
Ribeiro, M., E-mail: ribeiro.jr@oorbit.com.br [Office of Operational Research for Business Intelligence and Technology, Principal Office, Buffalo, Wyoming 82834 (United States)
2015-06-21
Ab initio calculations of hydrogen-passivated Si nanowires were performed using density functional theory within LDA-1/2, to account for the excited states properties. A range of diameters was calculated to draw conclusions about the ability of the method to correctly describe the main trends of bandgap, quantum confinement, and self-energy corrections versus the diameter of the nanowire. Bandgaps are predicted with excellent accuracy if compared with other theoretical results like GW, and with the experiment as well, but with a low computational cost.
International Nuclear Information System (INIS)
Ab initio calculations of hydrogen-passivated Si nanowires were performed using density functional theory within LDA-1/2, to account for the excited states properties. A range of diameters was calculated to draw conclusions about the ability of the method to correctly describe the main trends of bandgap, quantum confinement, and self-energy corrections versus the diameter of the nanowire. Bandgaps are predicted with excellent accuracy if compared with other theoretical results like GW, and with the experiment as well, but with a low computational cost
Ab initio alpha-alpha scattering.
Elhatisari, Serdar; Lee, Dean; Rupak, Gautam; Epelbaum, Evgeny; Krebs, Hermann; Lähde, Timo A; Luu, Thomas; Meißner, Ulf-G
2015-12-01
Processes such as the scattering of alpha particles ((4)He), the triple-alpha reaction, and alpha capture play a major role in stellar nucleosynthesis. In particular, alpha capture on carbon determines the ratio of carbon to oxygen during helium burning, and affects subsequent carbon, neon, oxygen, and silicon burning stages. It also substantially affects models of thermonuclear type Ia supernovae, owing to carbon detonation in accreting carbon-oxygen white-dwarf stars. In these reactions, the accurate calculation of the elastic scattering of alpha particles and alpha-like nuclei--nuclei with even and equal numbers of protons and neutrons--is important for understanding background and resonant scattering contributions. First-principles calculations of processes involving alpha particles and alpha-like nuclei have so far been impractical, owing to the exponential growth of the number of computational operations with the number of particles. Here we describe an ab initio calculation of alpha-alpha scattering that uses lattice Monte Carlo simulations. We use lattice effective field theory to describe the low-energy interactions of protons and neutrons, and apply a technique called the 'adiabatic projection method' to reduce the eight-body system to a two-cluster system. We take advantage of the computational efficiency and the more favourable scaling with system size of auxiliary-field Monte Carlo simulations to compute an ab initio effective Hamiltonian for the two clusters. We find promising agreement between lattice results and experimental phase shifts for s-wave and d-wave scattering. The approximately quadratic scaling of computational operations with particle number suggests that it should be possible to compute alpha scattering and capture on carbon and oxygen in the near future. The methods described here can be applied to ultracold atomic few-body systems as well as to hadronic systems using lattice quantum chromodynamics to describe the interactions of
Ab initio alpha-alpha scattering
Elhatisari, Serdar; Lee, Dean; Rupak, Gautam; Epelbaum, Evgeny; Krebs, Hermann; Lähde, Timo A.; Luu, Thomas; Meißner, Ulf-G.
2015-12-01
Processes such as the scattering of alpha particles (4He), the triple-alpha reaction, and alpha capture play a major role in stellar nucleosynthesis. In particular, alpha capture on carbon determines the ratio of carbon to oxygen during helium burning, and affects subsequent carbon, neon, oxygen, and silicon burning stages. It also substantially affects models of thermonuclear type Ia supernovae, owing to carbon detonation in accreting carbon-oxygen white-dwarf stars. In these reactions, the accurate calculation of the elastic scattering of alpha particles and alpha-like nuclei—nuclei with even and equal numbers of protons and neutrons—is important for understanding background and resonant scattering contributions. First-principles calculations of processes involving alpha particles and alpha-like nuclei have so far been impractical, owing to the exponential growth of the number of computational operations with the number of particles. Here we describe an ab initio calculation of alpha-alpha scattering that uses lattice Monte Carlo simulations. We use lattice effective field theory to describe the low-energy interactions of protons and neutrons, and apply a technique called the ‘adiabatic projection method’ to reduce the eight-body system to a two-cluster system. We take advantage of the computational efficiency and the more favourable scaling with system size of auxiliary-field Monte Carlo simulations to compute an ab initio effective Hamiltonian for the two clusters. We find promising agreement between lattice results and experimental phase shifts for s-wave and d-wave scattering. The approximately quadratic scaling of computational operations with particle number suggests that it should be possible to compute alpha scattering and capture on carbon and oxygen in the near future. The methods described here can be applied to ultracold atomic few-body systems as well as to hadronic systems using lattice quantum chromodynamics to describe the interactions of
Ab initio theory of magnetic-field-induced odd-frequency two-band superconductivity in MgB2
Aperis, Alex; Maldonado, Pablo; Oppeneer, Peter M.
2015-08-01
We develop the anisotropic Eliashberg framework for superconductivity in the presence of an applied magnetic field. Using as input the ab initio calculated electron and phonon band structures and electron-phonon coupling, we solve self-consistently the anisotropic Eliashberg equations for the archetypal superconductor MgB2. We find two self-consistent solutions, time-even two-band superconductivity, as well as unconventional time-odd s -wave spin triplet two-band superconductivity emerging with applied field. We provide the full momentum, frequency, and spin-resolved dependence and magnetic field-temperature phase diagrams of the time-even and time-odd superconducting pair amplitudes and predict fingerprints of this novel odd-frequency state in tunneling experiments.
Kornobis, Karina; Wong, Bryan M; Lodowski, Piotr; Jaworska, Maria; Andruniów, Tadeusz; Rudd, Kenneth; Kozlowski, Pawel M; 10.1021/jp110914y
2011-01-01
Time-dependent density functional theory (TD-DFT) and correlated ab initio methods have been applied to the electronically excited states of vitamin B12 (cyanocobalamin or CNCbl). Different experimental techniques have been used to probe the excited states of CNCbl, revealing many issues that remain poorly understood from an electronic structure point of view. Due to its efficient scaling with size, TD-DFT emerges as one of the most practical tools that can be used to predict the electronic properties of these fairly complex molecules. However, the description of excited states is strongly dependent on the type of functional used in the calculations. In the present contribution, the choice of a proper functional for vitamin B12 was evaluated in terms of its agreement with both experimental results and correlated ab initio calculations. Three different functionals, i.e. B3LYP, BP86, and LC-BLYP, were tested. In addition, the effect of relative contributions of DFT and HF to the exchange-correlation functional ...
Molexpl: a tool for ab initio data exploration and visualization
Wang, Xueying; Onofrio, Nicolas,; Strachan, Alejandro
2015-01-01
Density functional theory (DFT) based on ab initio theory, is a powerful method to resolve the electronic structure of atoms, molecules and solids. However, in practical, DFT is limited to few hundreds of atoms. To overcome this limitation, researchers have developed empirical interatomic potentials implemented in molecular dynamics (MD) simulations. MD ignores the movements of electrons and describes bonding and non-bonding interaction as a function of the distance between atoms called force...
Changala, P Bryan
2016-01-01
We present a perturbative method for ab initio calculations of rotational and rovibrational effective Hamiltonians of both rigid and non-rigid molecules. Our approach is based on a curvilinear implementation of second order vibrational M{\\o}ller-Plesset perturbation theory (VMP2) extended to include rotational effects via a second order contact transformation. Though more expensive, this approach is significantly more accurate than standard second order vibrational perturbation theory (VPT2) for systems that are poorly described to zeroth order by rectilinear normal mode harmonic oscillators. We apply this method and demonstrate its accuracy on two molecules: Si$_2$C, a quasilinear triatomic with significant bending anharmonicity, and CH$_3$NO$_2$, which contains a completely unhindered methyl rotor. In addition to these two examples, we discuss several key technical aspects of the method, including an efficient implementation of Eckart and quasi-Eckart frame embedding that does not rely on numerical finite d...
Zimmermann, Bernd; Mavropoulos, Phivos; Long, Nguyen H.; Gerhorst, Christian-Roman; Blügel, Stefan; Mokrousov, Yuriy
2016-04-01
The Fermi surfaces and Elliott-Yafet spin-mixing parameter (EYP) of several elemental metals are studied by ab initio calculations. We focus first on the anisotropy of the EYP as a function of the direction of the spin-quantization axis [B. Zimmermann et al., Phys. Rev. Lett. 109, 236603 (2012), 10.1103/PhysRevLett.109.236603]. We analyze in detail the origin of the gigantic anisotropy in 5 d hcp metals as compared to 5 d cubic metals by band structure calculations and discuss the stability of our results against an applied magnetic field. We further present calculations of light (4 d and 3 d ) hcp crystals, where we find a huge increase of the EYP anisotropy, reaching colossal values as large as 6000 % in hcp Ti. We attribute these findings to the reduced strength of spin-orbit coupling, which promotes the anisotropic spin-flip hot loops at the Fermi surface. In order to conduct these investigations, we developed an adapted tetrahedron-based method for the precise calculation of Fermi surfaces of complicated shape and accurate Fermi-surface integrals within the full-potential relativistic Korringa-Kohn-Rostoker Green function method.
Ab initio potential for solids
DEFF Research Database (Denmark)
Chetty, N.; Stokbro, Kurt; Jacobsen, Karsten Wedel; Nørskov, Jens Kehlet
1992-01-01
A total-energy theory for a solid is presented. It is based on density-functional theory and consists of a succession of approximations. At the most accurate level, the theory consists of a systematic derivation of an ansatz for the electron density which is best suited for the Harris functional...
Ab initio two-component Ehrenfest dynamics
International Nuclear Information System (INIS)
We present an ab initio two-component Ehrenfest-based mixed quantum/classical molecular dynamics method to describe the effect of nuclear motion on the electron spin dynamics (and vice versa) in molecular systems. The two-component time-dependent non-collinear density functional theory is used for the propagation of spin-polarized electrons while the nuclei are treated classically. We use a three-time-step algorithm for the numerical integration of the coupled equations of motion, namely, the velocity Verlet for nuclear motion, the nuclear-position-dependent midpoint Fock update, and the modified midpoint and unitary transformation method for electronic propagation. As a test case, the method is applied to the dissociation of H2 and O2. In contrast to conventional Ehrenfest dynamics, this two-component approach provides a first principles description of the dynamics of non-collinear (e.g., spin-frustrated) magnetic materials, as well as the proper description of spin-state crossover, spin-rotation, and spin-flip dynamics by relaxing the constraint on spin configuration. This method also holds potential for applications to spin transport in molecular or even nanoscale magnetic devices
Ab initio mass tensor molecular dynamics
Tsuchida, Eiji
2010-01-01
Mass tensor molecular dynamics was first introduced by Bennett [J. Comput. Phys. 19, 267 (1975)] for efficient sampling of phase space through the use of generalized atomic masses. Here, we show how to apply this method to ab initio molecular dynamics simulations with minimal computational overhead. Test calculations on liquid water show a threefold reduction in computational effort without making the fixed geometry approximation. We also present a simple recipe for estimating the optimal ato...
Thiessen, P. A.; Treder, H.-J.
Jedes initium wird durch experimenta crucis zum eventus. Jedes theoretisch interpretierbare ex-eventu-Resultat führt auf ein neues Initium. Gerade dies ist die gemeinsame Aussage von Atomistik, Quantenmechanik und Relativitätstheorie.Translated AbstractAb initio vel ex eventu. IIEvery initium becomes an eventus by experimenta crucis. Every theoretically interpretable ex-eventu result leads to a new initium. Right this is the joint assertion of atomism, quantum mechanics, and relativity.
Energy Technology Data Exchange (ETDEWEB)
Barrett, B R; Navratil, P; Vary, J P
2011-04-11
A long-standing goal of nuclear theory is to determine the properties of atomic nuclei based on the fundamental interactions among the protons and neutrons (i.e., nucleons). By adopting nucleon-nucleon (NN), three-nucleon (NNN) and higher-nucleon interactions determined from either meson-exchange theory or QCD, with couplings fixed by few-body systems, we preserve the predictive power of nuclear theory. This foundation enables tests of nature's fundamental symmetries and offers new vistas for the full range of complex nuclear phenomena. Basic questions that drive our quest for a microscopic predictive theory of nuclear phenomena include: (1) What controls nuclear saturation; (2) How the nuclear shell model emerges from the underlying theory; (3) What are the properties of nuclei with extreme neutron/proton ratios; (4) Can we predict useful cross sections that cannot be measured; (5) Can nuclei provide precision tests of the fundamental laws of nature; and (6) Under what conditions do we need QCD to describe nuclear structure, among others. Along with other ab initio nuclear theory groups, we have pursued these questions with meson-theoretical NN interactions, such as CD-Bonn and Argonne V18, that were tuned to provide high-quality descriptions of the NN scattering phase shifts and deuteron properties. We then add meson-theoretic NNN interactions such as the Tucson-Melbourne or Urbana IX interactions. More recently, we have adopted realistic NN and NNN interactions with ties to QCD. Chiral perturbation theory within effective field theory ({chi}EFT) provides us with a promising bridge between QCD and hadronic systems. In this approach one works consistently with systems of increasing nucleon number and makes use of the explicit and spontaneous breaking of chiral symmetry to expand the strong interaction in terms of a dimensionless constant, the ratio of a generic small momentum divided by the chiral symmetry breaking scale taken to be about 1 GeV/c. The
International Nuclear Information System (INIS)
A long-standing goal of nuclear theory is to determine the properties of atomic nuclei based on the fundamental interactions among the protons and neutrons (i.e., nucleons). By adopting nucleon-nucleon (NN), three-nucleon (NNN) and higher-nucleon interactions determined from either meson-exchange theory or QCD, with couplings fixed by few-body systems, we preserve the predictive power of nuclear theory. This foundation enables tests of nature's fundamental symmetries and offers new vistas for the full range of complex nuclear phenomena. Basic questions that drive our quest for a microscopic predictive theory of nuclear phenomena include: (1) What controls nuclear saturation; (2) How the nuclear shell model emerges from the underlying theory; (3) What are the properties of nuclei with extreme neutron/proton ratios; (4) Can we predict useful cross sections that cannot be measured; (5) Can nuclei provide precision tests of the fundamental laws of nature; and (6) Under what conditions do we need QCD to describe nuclear structure, among others. Along with other ab initio nuclear theory groups, we have pursued these questions with meson-theoretical NN interactions, such as CD-Bonn and Argonne V18, that were tuned to provide high-quality descriptions of the NN scattering phase shifts and deuteron properties. We then add meson-theoretic NNN interactions such as the Tucson-Melbourne or Urbana IX interactions. More recently, we have adopted realistic NN and NNN interactions with ties to QCD. Chiral perturbation theory within effective field theory (χEFT) provides us with a promising bridge between QCD and hadronic systems. In this approach one works consistently with systems of increasing nucleon number and makes use of the explicit and spontaneous breaking of chiral symmetry to expand the strong interaction in terms of a dimensionless constant, the ratio of a generic small momentum divided by the chiral symmetry breaking scale taken to be about 1 GeV/c. The resulting NN
Highly scalable Ab initio genomic motif identification
Marchand, Benoît
2011-01-01
We present results of scaling an ab initio motif family identification system, Dragon Motif Finder (DMF), to 65,536 processor cores of IBM Blue Gene/P. DMF seeks groups of mutually similar polynucleotide patterns within a set of genomic sequences and builds various motif families from them. Such information is of relevance to many problems in life sciences. Prior attempts to scale such ab initio motif-finding algorithms achieved limited success. We solve the scalability issues using a combination of mixed-mode MPI-OpenMP parallel programming, master-slave work assignment, multi-level workload distribution, multi-level MPI collectives, and serial optimizations. While the scalability of our algorithm was excellent (94% parallel efficiency on 65,536 cores relative to 256 cores on a modest-size problem), the final speedup with respect to the original serial code exceeded 250,000 when serial optimizations are included. This enabled us to carry out many large-scale ab initio motiffinding simulations in a few hours while the original serial code would have needed decades of execution time. Copyright 2011 ACM.
Understanding phonon transport in thermoelectric materials using ab initio approaches
Broido, David
Good thermoelectric materials have low phonon thermal conductivity, kph. Accurate theories to describe kph are important components in developing predictive models of thermoelectric efficiency that can help guide synthesis and measurement efforts. We have developed ab initio approaches to calculate kph, in which phonon modes and phonon scattering rates are computed using interatomic force constants determined from density functional theory, and a full solution of the Boltzmann transport equation for phonons is implemented. A recent approach to calculate interatomic force constants using ab initio molecular dynamics has yielded a good description of the thermal properties of Bi2Te3. But, the complexity of new promising candidate thermoelectric materials introduces computational challenges in assessing their thermal properties. An example is germanane, a germanium based hydrogen-terminated layered semiconductor, which we will discuss in this talk.
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 transition to a metallic state. In contrast to ordinary liquid silicon, the new liquid is characterized by a high coordination number and a strong reduction of covalent bonding effects.
The density matrix renormalization group for ab initio quantum chemistry
Wouters, Sebastian
2015-01-01
During the past 15 years, the density matrix renormalization group (DMRG) has become increasingly important for ab initio quantum chemistry. It is used as a numerically exact solver for highly correlated regions in molecules. While the method works extremely well for one-dimensional systems, the correlated regions of interest are often far from one-dimensional. In this introductory talk, I will discuss the DMRG algorithm from a quantum information perspective, how quantum information theory h...
Ab-initio calculations for dilute magnetic semiconductors
Belhadji, Brahim
2008-01-01
This thesis focusses on ab-initio calculations for the electronic structure and the magnetic properties of dilute magnetic semiconductors (DMS). In particular we aim at the understanding of the complex exchange interactions in these systems. Our calculations are based on density functional theory, being ideally suited for a description of the material specific properties of the considered DMS. Moreover we use the KKR Green function method in connection with the coherent potential approximatio...
Ab Initio Calculations of Oxosulfatovanadates
DEFF Research Database (Denmark)
Frøberg, Torben; Johansen, Helge
1996-01-01
Restricted Hartree-Fock and multi-configurational self-consistent-field calculations together with secondorder perturbation theory have been used to study the geometry, the electron density, and the electronicspectrum of (VO2SO4)-. A bidentate sulphate attachment to vanadium was found to be stable...... with anO-V-O angle of 72.5 degrees . The calculated spectrum shows bands in reasonable agreement with anexperimental spectrum which has been attributed to (VO2SO4)-. The geometry and the electron density fortwo binuclear vanadium complexes proposed as intermediates in the vanadium catalyzed SO2...
Ab initio no core full configuration approach for light nuclei
Kim, Youngman; Shin, Ik Jae; Maris, Pieter; Vary, James P.; Forssén, Christian; Rotureau, Jimmy
2015-10-01
Comprehensive understanding of the structure and reactions of light nuclei poses theoretical and computational challenges. Still, a number of ab initio approaches have been developed to calculate the properties of atomic nuclei using fundamental interactions among nucleons. Among them, we work with the ab initio no core full configuration (NCFC) method and ab initio no core Gamow Shell Model (GSM). We first review these approaches and present some recent results.
Augmented wave ab initio EFG calculations: some methodological warnings
Energy Technology Data Exchange (ETDEWEB)
Errico, Leonardo A. [Departamento de Fisica-IFLP (CONICET), Facultad de Ciencias Exactas, Universidad Nacional de La Plata, CC67 (1900) La Plata (Argentina); Renteria, Mario [Departamento de Fisica-IFLP (CONICET), Facultad de Ciencias Exactas, Universidad Nacional de La Plata, CC67 (1900) La Plata (Argentina); Petrilli, Helena M. [Instituto de Fisica-DFMT, Universidade de Sao Paulo, C.P. 66318, 05315-970 Sao Paulo, SP (Brazil)]. E-mail: hmpetril@macbeth.if.usp.br
2007-02-01
We discuss some accuracy aspects inherent to ab initio electronic structure calculations in the understanding of nuclear quadrupole interactions. We use the projector augmented wave method to study the electric-field gradient (EFG) at both Sn and O sites in the prototype cases SnO and SnO{sub 2}. The term ab initio is used in the standard context of the also called first principles methods in the framework of the Density Functional Theory. As the main contributions of EFG calculations to problems in condensed matter physics are related to structural characterizations on the atomic scale, we discuss the 'state of the art' on theoretical EFG calculations and make a brief critical review on the subject, calling attention to some fundamental theoretical aspects.
Augmented wave ab initio EFG calculations: some methodological warnings
International Nuclear Information System (INIS)
We discuss some accuracy aspects inherent to ab initio electronic structure calculations in the understanding of nuclear quadrupole interactions. We use the projector augmented wave method to study the electric-field gradient (EFG) at both Sn and O sites in the prototype cases SnO and SnO2. The term ab initio is used in the standard context of the also called first principles methods in the framework of the Density Functional Theory. As the main contributions of EFG calculations to problems in condensed matter physics are related to structural characterizations on the atomic scale, we discuss the 'state of the art' on theoretical EFG calculations and make a brief critical review on the subject, calling attention to some fundamental theoretical aspects
Ab initio nuclear structure - the large sparse matrix eigenvalue problem
International Nuclear Information System (INIS)
The structure and reactions of light nuclei represent fundamental and formidable challenges for microscopic theory based on realistic strong interaction potentials. Several ab initio methods have now emerged that provide nearly exact solutions for some nuclear properties. The ab initio no core shell model (NCSM) and the no core full configuration (NCFC) method, frame this quantum many-particle problem as a large sparse matrix eigenvalue problem where one evaluates the Hamiltonian matrix in a basis space consisting of many-fermion Slater determinants and then solves for a set of the lowest eigenvalues and their associated eigenvectors. The resulting eigenvectors are employed to evaluate a set of experimental quantities to test the underlying potential. For fundamental problems of interest, the matrix dimension often exceeds 1010 and the number of nonzero matrix elements may saturate available storage on present-day leadership class facilities. We survey recent results and advances in solving this large sparse matrix eigenvalue problem. We also outline the challenges that lie ahead for achieving further breakthroughs in fundamental nuclear theory using these ab initio approaches.
DEFF Research Database (Denmark)
Wolf, T. J. A.; Kuhlman, Thomas Scheby; Schalk, O.;
2014-01-01
Time-resolved photoelectron spectroscopy and ab initio multiple spawning were applied to the ultrafast non-adiabatic dynamics of hexamethylcyclopentadiene. The high level of agreement between experiment and theory associates wavepacket motion with a distinct degree of freedom....
Germacrene D Cyclization: An Ab Initio Investigation
Directory of Open Access Journals (Sweden)
William N. Setzer
2008-01-01
Full Text Available Essential oils that contain large concentrations of germacrene D are typically accompanied by cadinane sesquiterpenoids. The acid-catalyzed cyclization of germacrene D to give cadinane and selinane sesquiterpenes has been computationally investigated using both density functional (B3LYP/6-31G* and post Hartree-Fock (MP2/6-31G** ab initio methods. The calculated energies are in general agreement with experimentally observed product distributions, both from acid-catalyzed cyclizations as well as distribution of the compounds in essential oils.
Ab initio calculations of material strength
Czech Academy of Sciences Publication Activity Database
Šob, Mojmír; Friák, Martin; Vitek, V.
Tokyo : The Japan Society of Mechanical Engineers, 2003, s. 467-475. [International Symposium on Micro-Mechanical Engineering - Heat Transfer, Fluid Dynamics, Reliability and Mechanotronics.. Tsuchiura and Tsukuba (JP), 01.12.2003-03.12.2003] R&D Projects: GA AV ČR IAA1041302; GA ČR GA202/03/1351; GA MŠk OC 523.90 Institutional research plan: CEZ:AV0Z2041904 Keywords : ab initio calculations * electronic structure * theoretical tensile strength Subject RIV: BM - Solid Matter Physics ; Magnetism
Ab initio non-relativistic spin dynamics
Energy Technology Data Exchange (ETDEWEB)
Ding, Feizhi; Goings, Joshua J.; Li, Xiaosong, E-mail: xsli@uw.edu [Department of Chemistry, University of Washington, Seattle, Washington 98195 (United States); Frisch, Michael J. [Gaussian, Inc., 340 Quinnipiac St, Bldg 40, Wallingford, Connecticut 06492 (United States)
2014-12-07
Many magnetic materials do not conform to the (anti-)ferromagnetic paradigm where all electronic spins are aligned to a global magnetization axis. Unfortunately, most electronic structure methods cannot describe such materials with noncollinear electron spin on account of formally requiring spin alignment. To overcome this limitation, it is necessary to generalize electronic structure methods and allow each electron spin to rotate freely. Here, we report the development of an ab initio time-dependent non-relativistic two-component spinor (TDN2C), which is a generalization of the time-dependent Hartree-Fock equations. Propagating the TDN2C equations in the time domain allows for the first-principles description of spin dynamics. A numerical tool based on the Hirshfeld partitioning scheme is developed to analyze the time-dependent spin magnetization. In this work, we also introduce the coupling between electron spin and a homogenous magnetic field into the TDN2C framework to simulate the response of the electronic spin degrees of freedom to an external magnetic field. This is illustrated for several model systems, including the spin-frustrated Li{sub 3} molecule. Exact agreement is found between numerical and analytic results for Larmor precession of hydrogen and lithium atoms. The TDN2C method paves the way for the ab initio description of molecular spin transport and spintronics in the time domain.
Knyazev, D. V.; Levashov, P. R.
2014-01-01
This work is devoted to the investigation of transport and optical properties of liquid aluminum in the two-temperature case. At first optical properties, static electrical and thermal conductivities were obtained in the \\textit{ab initio} calculation. The \\textit{ab initio} calculation is based on the quantum molecular dynamics, density functional theory and the Kubo-Greenwood formula. The semiempirical approximation was constructed based on the results of the \\textit{ab initio} caculation. ...
Kopplung von Dichtefunktional- und ab-initio-Methoden
Goll, Erich
2008-01-01
Im Rahmen der Doktorarbeit wurde untersucht, inwieweit die Kopplung von Dichtefunktionalmethoden und ab-initio-Korrelationsmethoden der Quantenchemie eine Verbesserung bezüglich beider Grenzmethoden erbringt. Die Kopplung erfolgt durch eine Aufspaltung des interelektronischen Hamiltonoperators (abstoßende Coulombwechselwirkung). Die kurzreichweitige Wechselwirkung wird mit Dichtefunktionaltheorie behandelt, die langreichweitige mit Hilfe von ab-initio-Methoden. Diese Aufteilung soll dazu dien...
Realistic modelling of water/solid interfaces from ab Initio molecular dynamics
Tocci, G.
2014-01-01
Water/solid interfaces are of utmost importance to a number of technological processes. Theoretical studies, based on ab initio approaches are suitable to unveil processes occurring at water/solid interfaces and can therefore be instrumental to delineate guidelines to improve the efficiency of these processes. In this thesis we study several systems of current interest using ab initio methods based on density functional theory (DFT). By going often beyond the use of standard DFT methods and a...
International Nuclear Information System (INIS)
Highlights: • The lattice parameters changes by K doping is explained by ionic charge and the metal–metal bonding scenario. • Creation of a larger peak near the Fermi level and increase the DOS at the Fermi level by K doping. • The effects of K on DOS has been related to the magnetic transition, change of the lattice parameters and FeAs4 tetrahedron. • Investigation of the number of Fermi level crossed bands by K doping and the band changes near the Fermi level. - Abstract: The crystal and electronic structures of normal phase of Ba1−xKxFe2As2 for x = 0.0, 0.5 and 1.0 have been studied by using pseudopotential Quantum Espresso code based on ab-initio density functional theory. Effects of K doping on the crystal structure and lattice parameters have been calculated and compared with the experimental and computational reported data for similar compounds. The metal–metal bonding scenario was used to explain the changes of lattice parameters by K doping. The electronic structure of this system including of density of states and band structure have been calculated and investigated by K doping. One of the interesting results is that a larger peak is appeared near the Fermi level by increasing of the K doping. These changes could produce a potential for creation of the superconducting state in this system
Reyes-Lillo, Sebastian E.; Rangel, Tonatiuh; Bruneval, Fabien; Neaton, Jeffrey B.
2016-07-01
The Ruddlesden-Popper (RP) homologous series Srn +1TinO3 n +1 provides a useful template for the study and control of the effects of dimensionality and quantum confinement on the excited state properties of the complex oxide SrTiO3. We use ab initio many-body perturbation theory within the G W approximation and the Bethe-Salpeter equation approach to calculate quasiparticle energies and absorption spectra of Srn +1TinO3 n +1 for n =1 -5 and ∞ . Our computed direct and indirect optical gaps are in excellent agreement with spectroscopic measurements. The calculated optical spectra reproduce the main experimental features and reveal excitonic structure near the gap edge. We find that electron-hole interactions are important across the series, leading to significant exciton binding energies that increase for small n and reach a value of 330 meV for n =1 , a trend attributed to increased quantum confinement. We find that the lowest-energy singlet exciton of Sr2TiO4 (n =1 ) localizes in the two-dimensional plane defined by the TiO2 layer, and we explain the origin of its localization.
Discovering chemistry with an ab initio nanoreactor
Martinez, Todd
Traditional approaches for modeling chemical reaction networks such as those involved in combustion have focused on identifying individual reactions and using theoretical approaches to explore the underlying mechanisms. Recent advances involving graphical processing units (GPUs), commodity products developed for the videogaming industry, have made it possible to consider a distinct approach wherein one attempts to discover chemical reactions and mechanisms. We provide a brief summary of these developments and then discuss the concept behind the ``ab initio nanoreactor'' which explores the space of possible chemical reactions and molecular species for a given stoichiometry. The nanoreactor concept is exemplified with an example to the Urey-Miller reaction network which has been previously advanced as a potential model for prebiotic chemistry. We briefly discuss some of the future directions envisioned for the development of this nanoreactor concept.
Ab initio molar volumes and Gaussian radii.
Parsons, Drew F; Ninham, Barry W
2009-02-12
Ab initio molar volumes are calculated and used to derive radii for ions and neutral molecules using a spatially diffuse model of the electron distribution with Gaussian spread. The Gaussian radii obtained can be used for computation of nonelectrostatic ion-ion dispersion forces that underlie Hofmeister specific ion effects. Equivalent hard-sphere radii are also derived, and these are in reasonable agreement with crystalline ionic radii. The Born electrostatic self-energy is derived for a Gaussian model of the electronic charge distribution. It is shown that the ionic volumes used in electrostatic calculations of strongly hydrated cosmotropic ions ought best to include the first hydration shell. Ionic volumes for weakly hydrated chaotropic metal cations should exclude electron overlap (in electrostatic calculations). Spherical radii are calculated as well as nonisotropic ellipsoidal radii for nonspherical ions, via their nonisotropic static polarizability tensors. PMID:19140766
Equations of state of heavy metals: ab initio approaches
International Nuclear Information System (INIS)
The determination of equations of states of heavy metals through ab initio calculation, i.e. without any adjustable parameter, allows to access to pressure and temperature thermodynamic conditions sometimes inaccessible to experiment. To perform such calculations, density functional theory (DFT) is a good starting point: when electronic densities are homogeneous enough, the local density approximation (LDA) remarkably accounts for thermodynamic properties of heavy metals, such as tantalum, or the light actinides, as well for static properties - equilibrium volume, elastic constants - as for dynamical quantities like phonon spectra. For heavier elements, like neptunium or plutonium, relativistic effects and strong electronic interactions must be taken into account, which requires more sophisticated theoretical approaches. (authors)
Ab initio calculation of tensile strength in iron
Czech Academy of Sciences Publication Activity Database
Friák, Martin; Šob, Mojmír; Vitek, V.
2003-01-01
Roč. 83, 31-34 (2003), s. 3529-3537. ISSN 1478-6435. [Multiscale Materials Modelling: Working Theory for Industry /1./. London, 17.06.2002-20.06.2002] R&D Projects: GA AV ČR IAA1041302; GA ČR GA202/03/1351; GA MŠk OC 523.90 Institutional research plan: CEZ:AV0Z2041904 Keywords : ab initio calculations * electronic structure * theoretical tensile strength Subject RIV: BM - Solid Matter Physics ; Magnetism
Ab initio vibrational and dielectric properties of Y V O
Vali, R.
2009-10-01
For the yttrium orthovanadate Y V O with a tetragonal zircon-type structure, the first complete set of Raman-active and IR-active phonon modes has been calculated using ab initio density functional perturbation theory. The calculated IR reflectivity spectra are in good agreement with available experimental data. We report the calculated frequencies of three Raman-active modes that could not be detected experimentally and a new assignment of the experimental Raman data. The contributions of each IR-active phonon modes to static dielectric tensor have been determined.
International Nuclear Information System (INIS)
Graphical abstract: ThO–He potential energy surface has a peculiar topology with the global minimum in the bent He–OTh configuration, local minimum in the collinear He–ThO arrangement and two saddle points separating the minima. Vibrational wave functions reflect increasing delocalization of He atom within the complex. Complexes with Ne and Ar have similar potentials, but are increasingly more rigid. Highlights: ► CCSD(T): ThO–RG – floppy van der Waals complexes with bent equilibrium structure. ► Rovibrational energy levels: spatial RG localization decreases from He to Ar. ► SAPT qualitative: delicate balance between exchange and dispersion forces. ► SAPT quantitative: problems with intramonomer correlation and core potentials. ► Long-range interactions: peculiar reduction of the dispersion anisotropy. - Abstract: Two-dimensional interaction potential energy surfaces for the ground-state ThO complexes with RG atoms from He to Ar are calculated ab initio at the coupled cluster CCSD(T) level of theory. The global minimum for all complexes is related to a bent geometry, with the RG atom closer to the oxygen end. Parallel symmetry adapted perturbation theory (SAPT) calculations for the ThO–He complex showed that this configuration is favored by the exchange interaction that slightly prevails over the dispersion and induction contributions which prefer the collinear arrangement. Variational calculations of rovibrational energy levels provided the dissociation energies of 9, 15 and 184 cm−1, for the 3He, Ne and Ar complexes, respectively. The He and Ne complexes in the ground state resemble linear molecules as their zero-point energies exceed the barrier at the linear RG–OTh arrangement. Vibrationally-averaged structure of the Ar–ThO complex better reflects the bent geometry of the equilibrium point.
On the hierarchical parallelization of ab initio simulations
Ruiz-Barragan, Sergi; Shiga, Motoyuki
2016-01-01
A hierarchical parallelization has been implemented in a new unified code PIMD-SMASH for ab initio simulation where the replicas and the Born-Oppenheimer forces are parallelized. It is demonstrated that ab initio path integral molecular dynamics simulations can be carried out very efficiently for systems up to a few tens of water molecules. The code was then used to study a Diels-Alder reaction of cyclopentadiene and butenone by ab initio string method. A reduction in the reaction energy barrier is found in the presence of hydrogen-bonded water, in accordance with experiment.
Loay A. Elalfy; Wael N. Akl; Hassan, Walid M. I.
2013-01-01
Density functional theory calculations using B3LYP/3-21G level of theory have been implemented on 6 carbon nanotubes (CNTs) structures (3 zigzag and 3 armchair CNTs) to study the energetics of the reverse osmosis during water desalination process. Calculations of the band gap, interaction energy, highest occupied molecular orbital, lowest unoccupied molecular orbital, electronegativity, hardness, and pressure of the system are discussed. The calculations showed that the water molecule that ex...
Ricca, Alessandra; Bauschlicher, Charles W.; Langhoff, Stephen R. (Technical Monitor)
1994-01-01
Density functional theory (DFT) is found to give a better description of the geometries and vibrational frequencies of FeL and FeL(sup +) systems than second order Moller Plesset perturbation theory (MP2). Namely, the DFT correctly predicts the shift in the CO vibrational frequency between free CO and the Sigma(sup -) state of FeCO and yields a good result for the Fe-C distance in the quartet states of FeCH4(+) 4 These are properties where the MP2 results are unsatisfactory. Thus DFT appears to be an excellent approach for optimizing the geometries and computing the zero-point energies of systems containing first transition row atoms. Because the DFT approach is biased in favor of the 3d(exp 7) occupation, whereas the more traditional approaches are biased in favor of the 3d(exp 6) occupation, differences are found in the relative ordering of states. It is shown that if the dissociation is computed to the most appropriate atomic asymptote and corrected to the ground state asymptote using the experimental separations, the DFT results are in good agreement with high levels of theory. The energetics at the DFT level are much superior to the MP2 and in most cases in good agreement with high levels of theory.
Khanniche, Sarah; Louis, Florent; Cantrel, Laurent; Černušák, Ivan
2016-03-17
To get an insight into the possible reactivity between iodine oxides and CO, a first step was to study the thermochemical properties and kinetic parameters of the reaction between IO and CO using theoretical chemistry tools. All stationary points involved were optimized using the Becke's three-parameter hybrid exchange functional coupled with the Lee-Yang-Parr nonlocal correlation functional (B3LYP) and the Møller-Plesset second-order perturbation theory (MP2). Single-point energy calculations were performed using the coupled cluster theory with the iterative inclusion of singles and doubles and the perturbative estimation for triple excitations (CCSD(T)) and the aug-cc-pVnZ (n = T, Q, and 5) basis sets on geometries previously optimized at the aug-cc-pVTZ level. The energetics was then recalculated using the one-component DK-CCSD(T) approach with the relativistic ANO basis sets. The spin-orbit coupling for the iodine containing species was calculated a posteriori using the restricted active space state interaction method in conjunction with the multiconfigurational perturbation theory (CASPT2/RASSI) employing the complete active space (CASSCF) wave function as the reference. The CCSD(T) energies were also corrected for BSSE for molecular complexes and refined with the extrapolation to CBS limit while the DK-CCSD(T) values were refined with the extrapolation to FCI. The exploration of the potential energy surface revealed a two-steps mechanism with a trans and a cis pathway. The rate constants for the direct and complex mechanism were computed as a function of temperature (250-2500 K) using the canonical transition state theory. The three-parameter Arrhenius expressions obtained for the direct and indirect mechanism at the DK-CCSD(T)-cf level of theory is 1.49 × 10(-17) × T(1.77) exp(-47.4 (kJ mol(-1))/RT). PMID:26908233
Directory of Open Access Journals (Sweden)
Loay A. Elalfy
2013-01-01
Full Text Available Density functional theory calculations using B3LYP/3-21G level of theory have been implemented on 6 carbon nanotubes (CNTs structures (3 zigzag and 3 armchair CNTs to study the energetics of the reverse osmosis during water desalination process. Calculations of the band gap, interaction energy, highest occupied molecular orbital, lowest unoccupied molecular orbital, electronegativity, hardness, and pressure of the system are discussed. The calculations showed that the water molecule that exists inside the CNT is about 2-3 Å away from its wall. The calculations have proven that the zigzag CNTs are more efficient for reverse osmosis water desalination process than armchair CNTs as the reverse osmosis process requires pressure of approximately 200 MPa for armchair CNTs, which is consistent with the values used in molecular dynamics simulations, while that needed when using zigzag CNTs was in the order of 60 MPa.
An investigation of ab initio shell-model interactions derived by no-core shell model
Wang, XiaoBao; Dong, GuoXiang; Li, QingFeng; Shen, CaiWan; Yu, ShaoYing
2016-09-01
The microscopic shell-model effective interactions are mainly based on the many-body perturbation theory (MBPT), the first work of which can be traced to Brown and Kuo's first attempt in 1966, derived from the Hamada-Johnston nucleon-nucleon potential. However, the convergence of the MBPT is still unclear. On the other hand, ab initio theories, such as Green's function Monte Carlo (GFMC), no-core shell model (NCSM), and coupled-cluster theory with single and double excitations (CCSD), have made many progress in recent years. However, due to the increasing demanding of computing resources, these ab initio applications are usually limited to nuclei with mass up to A = 16. Recently, people have realized the ab initio construction of valence-space effective interactions, which is obtained through a second-time renormalization, or to be more exactly, projecting the full-manybody Hamiltonian into core, one-body, and two-body cluster parts. In this paper, we present the investigation of such ab initio shell-model interactions, by the recent derived sd-shell effective interactions based on effective J-matrix Inverse Scattering Potential (JISP) and chiral effective-field theory (EFT) through NCSM. In this work, we have seen the similarity between the ab initio shellmodel interactions and the interactions obtained by MBPT or by empirical fitting. Without the inclusion of three-body (3-bd) force, the ab initio shell-model interactions still share similar defects with the microscopic interactions by MBPT, i.e., T = 1 channel is more attractive while T = 0 channel is more repulsive than empirical interactions. The progress to include more many-body correlations and 3-bd force is still badly needed, to see whether such efforts of ab initio shell-model interactions can reach similar precision as the interactions fitted to experimental data.
Pham, Tuan Anh
2015-03-01
Photoelectrochemical cells offer a promising avenue for hydrogen production from water and sunlight. The efficiency of these devices depends on the electronic structure of the interface between the photoelectrode and liquid water, including the alignment between the semiconductor band edges and the water redox potential. In this talk, we will present the results of first principles calculations of semiconductor-water interfaces that are obtained with a combination of density functional theory (DFT)-based molecular dynamics simulations and many-body perturbation theory (MBPT). First, we will discuss the development of an MBPT approach that is aimed at improving the efficiency and accuracy of existing methodologies while still being applicable to complex heterogeneous interfaces consisting of hundreds of atoms. We will then present studies of the electronic structure of liquid water and aqueous solutions using MBPT, which represent an essential step in establishing a quantitative framework for computing the energy alignment at semiconductor-water interfaces. Finally, using a combination of DFT-based molecular dynamics simulations and MBPT, we will describe the relationship between interfacial structure, electronic properties of semiconductors and their reactivity in aqueous solutions through a number of examples, including functionalized Si surfaces and GaP/InP surfaces in contact with liquid water. T.A.P was supported by the U.S. Department of Energy at the Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 and by the Lawrence Fellowship Program.
Ab initio gene identification in metagenomic sequences.
Zhu, Wenhan; Lomsadze, Alexandre; Borodovsky, Mark
2010-07-01
We describe an algorithm for gene identification in DNA sequences derived from shotgun sequencing of microbial communities. Accurate ab initio gene prediction in a short nucleotide sequence of anonymous origin is hampered by uncertainty in model parameters. While several machine learning approaches could be proposed to bypass this difficulty, one effective method is to estimate parameters from dependencies, formed in evolution, between frequencies of oligonucleotides in protein-coding regions and genome nucleotide composition. Original version of the method was proposed in 1999 and has been used since for (i) reconstructing codon frequency vector needed for gene finding in viral genomes and (ii) initializing parameters of self-training gene finding algorithms. With advent of new prokaryotic genomes en masse it became possible to enhance the original approach by using direct polynomial and logistic approximations of oligonucleotide frequencies, as well as by separating models for bacteria and archaea. These advances have increased the accuracy of model reconstruction and, subsequently, gene prediction. We describe the refined method and assess its accuracy on known prokaryotic genomes split into short sequences. Also, we show that as a result of application of the new method, several thousands of new genes could be added to existing annotations of several human and mouse gut metagenomes. PMID:20403810
International Nuclear Information System (INIS)
Electronic and optical properties of the various nitrogen-doped Si43C44H76 nanoparticles investigated with time-dependent density functional theory. The binding energies of the nitrogen atom as an impurity in the silicon, carbon substitutional defects and also for the interstitial defects were analyzed. The changes in the energy gap between highest occupied molecular orbital and lowest unoccupied molecular orbital were discussed for various doped systems. Finally the optical excitation and exciton binding energy of the pure and stable nitrogen-doped Si43C44H76 nanoparticles derived from time-dependent density functional calculation. - Highlights: • Nitrogen-doped SiC nanocrystal can form thermodynamically stable structure. • Surface carbon substitutional defects are interesting for binding the nitrogen impurity. • The nitrogen impurity can significantly change the optical absorption
Institute of Scientific and Technical Information of China (English)
FENG Hong-Jian; LIU Fa-Min
2008-01-01
The coupling between magnetism and structural distortions in BiFeO3 (BFO) is investigated using density functional theory by considering the spin-orbit effect.Computational results show that the resulting magnetization M is rotated by reversal of sense of rotation of the oxygen octahedra in the double cell.The resulting magnetization is determined by the antiferrodistortive (AFD) distortions and ferroelectric (FE) displacements.This work clarifies the previous view that magnetism is only coupled with,and determined by,FE displacements.The excellent ferroelectricity is attributed significantly to the anomaly of Born effective charge of Bi,which is caused by the stereochemically active long pair of Bi 6s.
Coccia, Emanuele; Guidoni, Leonardo
2014-01-01
In this letter we report the singlet ground state structure of the full carotenoid peridinin by means of variational Monte Carlo (VMC) calculations. The VMC relaxed geometry has an average bond length alternation of 0.1165(10) {\\AA}, larger than the values obtained by DFT (PBE, B3LYP and CAM-B3LYP) and shorter than that calculated at the Hartree-Fock (HF) level. TDDFT and EOM-CCSD calculations on a reduced peridinin model confirm the HOMO-LUMO major contribution of the Bu+-like (S2) bright excited state. Many Body Green's Function Theory (MBGFT) calculations of the vertical excitation energy of the Bu+-like state for the VMC structure (VMC/MBGFT) provide excitation energy of 2.62 eV, in agreement with experimental results in n-hexane (2.72 eV). The dependence of the excitation energy on the bond length alternation in the MBGFT and TDDFT calculations with different functionals is discussed.
Ab Initio Thermodynamic Model for Magnesium Carbonates and Hydrates
Energy Technology Data Exchange (ETDEWEB)
Chaka, Anne M.; Felmy, Andrew R.
2014-03-28
An ab initio thermodynamic framework for predicting properties of hydrated magnesium carbonate minerals has been developed using density-functional theory linked to macroscopic thermodynamics through the experimental chemical potentials for MgO, water, and CO2. Including semiempirical dispersion via the Grimme method and small corrections to the generalized gradient approximation of Perdew, Burke, and Ernzerhof for the heat of formation yields a model with quantitative agreement for the benchmark minerals brucite, magnesite, nesquehonite, and hydromagnesite. The model shows how small differences in experimental conditions determine whether nesquehonite, hydromagnesite, or magnesite is the result of laboratory synthesis from carbonation of brucite, and what transformations are expected to occur on geological time scales. Because of the reliance on parameter-free first principles methods, the model is reliably extensible to experimental conditions not readily accessible to experiment and to any mineral composition for which the structure is known or can be hypothesized, including structures containing defects, substitutions, or transitional structures during solid state transformations induced by temperature changes or processes such as water, CO2, or O2 diffusion. Demonstrated applications of the ab initio thermodynamic framework include an independent means to evaluate differences in thermodynamic data for lansfordite, predicting the properties of Mg analogs of Ca-based hydrated carbonates monohydrocalcite and ikaite which have not been observed in nature, and an estimation of the thermodynamics of barringtonite from the stoichiometry and a single experimental observation.
Unified ab initio approaches to nuclear structure and reactions
Navrátil, Petr; Quaglioni, Sofia; Hupin, Guillaume; Romero-Redondo, Carolina; Calci, Angelo
2016-05-01
The description of nuclei starting from the constituent nucleons and the realistic interactions among them has been a long-standing goal in nuclear physics. In addition to the complex nature of the nuclear forces, with two-, three- and possibly higher many-nucleon components, one faces the quantum-mechanical many-nucleon problem governed by an interplay between bound and continuum states. In recent years, significant progress has been made in ab initio nuclear structure and reaction calculations based on input from QCD-employing Hamiltonians constructed within chiral effective field theory. After a brief overview of the field, we focus on ab initio many-body approaches—built upon the no-core shell model—that are capable of simultaneously describing both bound and scattering nuclear states, and present results for resonances in light nuclei, reactions important for astrophysics and fusion research. In particular, we review recent calculations of resonances in the 6He halo nucleus, of five- and six-nucleon scattering, and an investigation of the role of chiral three-nucleon interactions in the structure of 9Be. Further, we discuss applications to the 7Be {({{p}},γ )}8{{B}} radiative capture. Finally, we highlight our efforts to describe transfer reactions including the 3H{({{d}},{{n}})}4He fusion.
Unified ab initio approaches to nuclear structure and reactions
Navratil, Petr; Hupin, Guillaume; Romero-Redondo, Carolina; Calci, Angelo
2016-01-01
The description of nuclei starting from the constituent nucleons and the realistic interactions among them has been a long-standing goal in nuclear physics. In addition to the complex nature of the nuclear forces, with two-, three- and possibly higher many-nucleon components, one faces the quantum-mechanical many-nucleon problem governed by an interplay between bound and continuum states. In recent years, significant progress has been made in {\\em ab initio} nuclear structure and reaction calculations based on input from QCD-employing Hamiltonians constructed within chiral effective field theory. After a brief overview of the field, we focus on ab initio many-body approaches - built upon the No-Core Shell Model - that are capable of simultaneously describing both bound and scattering nuclear states, and present results for resonances in light nuclei, reactions important for astrophysics and fusion research. In particular, we review recent calculations of resonances in the $^6$He halo nucleus, of five- and six...
Ab initio electronic stopping power of protons in bulk materials
Shukri, Abdullah Atef; Bruneval, Fabien; Reining, Lucia
2016-01-01
The electronic stopping power is a crucial quantity for ion irradiation: it governs the deposited heat, the damage profile, and the implantation depth. Whereas experimental data are readily available for elemental solids, the data are much more scarce for compounds. Here we develop a fully ab initio computational scheme based on linear response time-dependent density-functional theory to predict the random electronic stopping power (RESP) of materials without any empirical fitting. We show that the calculated RESP compares well with experimental data, when at full convergence, with the inclusion of the core states and of the exchange correlation. We evaluate the unexpectedly limited magnitude of the nonlinear terms in the RESP by comparing with other approaches based on the time propagation of time-dependent density-functional theory. Finally, we check the validity of a few empirical rules of thumbs that are commonly used to estimate the electronic stopping power.
de Melo, Pedro Miguel M. C.; Marini, Andrea
2016-04-01
We present a full ab initio description of the coupled out-of-equilibrium dynamics of photons, phonons, and electrons. In the present approach, the quantized nature of the electromagnetic field as well as of the nuclear oscillations is fully taken into account. The result is a set of integrodifferential equations, written on the Keldysh contour, for the Green's functions of electrons, phonons, and photons where the different kinds of interactions are merged together. We then concentrate on the electronic dynamics in order to reduce the problem to a computationally feasible approach. By using the generalized Baym-Kadanoff ansatz and the completed collision approximation, we introduce a series of efficient but controllable approximations. In this way, we reduce all equations to a set of decoupled equations for the density matrix that describe all kinds of static and dynamical correlations. The final result is a coherent, general, and inclusive scheme to calculate several physical quantities: carrier dynamics, transient photoabsorption, and light emission, all of which include, at the same time, electron-electron, electron-phonon, and electron-photon interactions. We further discuss how all these observables can be easily calculated within the present scheme using a fully atomistic ab initio approach.
Ab initio study of II-(VI){sub 2} dichalcogenides
Energy Technology Data Exchange (ETDEWEB)
Olsson, P; Vidal, J; Lincot, D, E-mail: polsson@kth.se [Institut de R and D sur l' energie photovoltaique (IRDEP), UMR 7174-EDF-CNRS-ENSCP, 6 quai Watier, 78401 Chatou Cedex (France)
2011-10-12
The structural stabilities of the (Zn,Cd)(S,Se,Te){sub 2} dichalcogenides have been determined ab initio. These compounds are shown to be stable in the pyrite phase, in agreement with available experiments. Structural parameters for the ZnTe{sub 2} pyrite semiconductor compound proposed here are presented. The opto-electronic properties of these dichalcogenide compounds have been calculated using quasiparticle GW theory. Bandgaps, band structures and effective masses are proposed as well as absorption coefficients and refraction indices. The compounds are all indirect semiconductors with very flat conduction band dispersion and high absorption coefficients. The work functions and surface properties are predicted. The Te and Se based compounds could be of interest as absorber materials in photovoltaic applications. (paper)
Ab initio calculations of grain boundaries in bcc metals
Scheiber, Daniel; Pippan, Reinhard; Puschnig, Peter; Romaner, Lorenz
2016-03-01
In this study, we compute grain boundary (GB) properties for a large set of GBs in bcc transition metals with a special focus on W, Mo and Fe using ab initio density functional theory (DFT) and semi-empirical second nearest neighbour modified embedded atom method (2NN-MEAM) potentials. The GB properties include GB energies, surface energies, GB excess volume and work of separation, which we analyse and then compare to experimental data. We find that the used 2NN-MEAM potentials can predict general trends of GB properties, but do not always reproduce the GB ground state structure and energy found with DFT. In particular, our results explain the experimental finding that W and Mo prefer intergranular fracture, while other bcc metals prefer transgranular cleavage.
Ab initio study of the transition-metal carbene cations
Institute of Scientific and Technical Information of China (English)
李吉海; 冯大诚; 冯圣玉
1999-01-01
The geometries and bonding characteristics of the first-row transition-metal carbene cations MCH2+ were investigated by ab initio molecular orbital theory （HF/LANL2DZ）. All of MCH2+ are coplanar. In the closed shell structures the C bonds to M with double bonds; while in the open shell structures the partial double bonds are formed, because one of the σ and π orbitals is singly occupied. It is mainly the π-type overlap between the 2px orbital of C and 4px, 3dxz, orbitals of M+ that forms the π orbitals. The dissociation energies of C—M bond appear in periodic trend from Sc to Cu. Most of the calculated bond dissociation energies are close to the experimental ones.
High-throughput ab-initio dilute solute diffusion database.
Wu, Henry; Mayeshiba, Tam; Morgan, Dane
2016-01-01
We demonstrate automated generation of diffusion databases from high-throughput density functional theory (DFT) calculations. A total of more than 230 dilute solute diffusion systems in Mg, Al, Cu, Ni, Pd, and Pt host lattices have been determined using multi-frequency diffusion models. We apply a correction method for solute diffusion in alloys using experimental and simulated values of host self-diffusivity. We find good agreement with experimental solute diffusion data, obtaining a weighted activation barrier RMS error of 0.176 eV when excluding magnetic solutes in non-magnetic alloys. The compiled database is the largest collection of consistently calculated ab-initio solute diffusion data in the world. PMID:27434308
Ab-initio melting curve and principal Hugoniot of tantalum
International Nuclear Information System (INIS)
We report first principles calculations of the melting curve and principal Hugoniot (P - V curve) of body centered cubic (bcc) tantalum in the pressure range 0-300 GPa. A description of lattice dynamics and thermal properties of bcc Ta using finite temperature density functional theory (DFT) is presented. The approach works within the projector augmented wave (PAW) implementation of DFT and explicitly treats in valence the 5p, 6s and 5d electrons. The principal Hugoniot (P - V curve), obtained using the Rankine-Hugoniot equation, is investigated using the generalized gradient approximations (GGA). Very good agreement with the shock experiments is obtained with GGA in all the range of pressure. We also report the temperature-pressure relation on the shock Hugoniot and the full ab-initio melting curve of Ta
Ab initio electronic structure and optical conductivity of bismuth tellurohalides
Schwalbe, Sebastian; Starke, Ronald; Schober, Giulio A H; Kortus, Jens
2016-01-01
We investigate the electronic structure, dielectric and optical properties of bismuth tellurohalides BiTeX (X = I, Cl, Br) by means of all-electron density functional theory. In particular, we present the ab initio conductivities and dielectric tensors calculated over a wide frequency range, and compare our results with the recent measurements by Akrap et al. , Makhnev et al. , and Rusinov et al. . We show how the low-frequency branch of the optical conductivity can be used to identify characteristic intra- and interband transitions between the Rashba spin-split bands in all three bismuth tellurohalides. We further calculate the refractive indices and dielectric constants, which in turn are systematically compared to previous predictions and measurements. We expect that our quantitative analysis will contribute to the general assessment of bulk Rashba materials for their potential use in spintronics devices.
Ab initio potential energy surface and rovibrational states of HBO
Ha, Tae-Kyu; Makarewicz, Jan
1999-01-01
The potential energy surface describing the large-amplitude motion of H around the BO core in the HBO molecule has been determined from ab initio calculations. This surface has been sampled by a set of 170 grid points from a two-dimensional space defined by the stretching and the bending coordinates of the H nucleus. At each grid point, the BO bond length has been optimized using the second-order Møller-Plesset perturbation theory with the basis set aug-cc-pVTZ. The surface has a local minimum for the linear as well as the bent configuration of HBO. A low energy barrier to the linear configuration BOH causes a large-amplitude motion and a strong rovibrational interaction in the molecule. Its rovibrational dynamics is different from the dynamics in bent or quasilinear triatomic molecules.
An Efficient Approach to Ab Initio Monte Carlo Simulation
Leiding, Jeff
2013-01-01
We present a Nested Markov Chain Monte Carlo (NMC) scheme for building equilibrium averages based on accurate potentials such as density functional theory. Metropolis sampling of a reference system, defined by an inexpensive but approximate potential, is used to substantially decorrelate configurations at which the potential of interest is evaluated, thereby dramatically reducing the number needed to build ensemble averages at a given level of precision. The efficiency of this procedure is maximized on-the-fly through variation of the reference system thermodynamic state (characterized here by its inverse temperature \\beta^0), which is otherwise unconstrained. Local density approximation (LDA) results are presented for shocked states in argon at pressures from 4 to 60 GPa. Depending on the quality of the reference potential, the acceptance probability is enhanced by factors of 1.2-28 relative to unoptimized NMC sampling, and the procedure's efficiency is found to be competitive with that of standard ab initio...
Interatomic Coulombic decay widths of helium trimer: Ab initio calculations
Energy Technology Data Exchange (ETDEWEB)
Kolorenč, Přemysl, E-mail: kolorenc@mbox.troja.mff.cuni.cz [Charles University in Prague, Faculty of Mathematics and Physics, Institute of Theoretical Physics, V Holešovičkách 2, 180 00 Prague (Czech Republic); Sisourat, Nicolas [Sorbonne Universités, UPMC Univ Paris 06, UMR 7614, Laboratoire de Chimie Physique Matière et Rayonnement, F-75005 Paris (France); CNRS, UMR 7614, Laboratoire de Chimie Physique Matière et Rayonnement, F-75005 Paris (France)
2015-12-14
We report on an extensive study of interatomic Coulombic decay (ICD) widths in helium trimer computed using a fully ab initio method based on the Fano theory of resonances. Algebraic diagrammatic construction for one-particle Green’s function is utilized for the solution of the many-electron problem. An advanced and universal approach to partitioning of the configuration space into discrete states and continuum subspaces is described and employed. Total decay widths are presented for all ICD-active states of the trimer characterized by one-site ionization and additional excitation of an electron into the second shell. Selected partial decay widths are analyzed in detail, showing how three-body effects can qualitatively change the character of certain relaxation transitions. Previously unreported type of three-electron decay processes is identified in one class of the metastable states.
Ab Initio Studies of Stratospheric Ozone Depletion Chemistry
Lee, Timothy J.; Head-Gordon, Martin; Langhoff, Stephen R. (Technical Monitor)
1995-01-01
An overview of the current understanding of ozone depletion chemistry, particularly with regards the formation of the so-called Antarctic ozone hole, will be presented together with an outline as to how ab initio quantum chemistry can be used to further our understanding of stratospheric chemistry. The ability of modern state-of-the art ab initio quantum chemical techniques to characterize reliably the gas-phase molecular structure, vibrational spectrum, electronic spectrum, and thermal stability of fluorine, chlorine, bromine and nitrogen oxide species will be demonstrated by presentation of some example studies. The ab initio results will be shown to be in excellent agreement with the available experimental data, and where the experimental data are either not known or are inconclusive, the theoretical results are shown to fill in the gaps and to resolve experimental controversies. In addition, ab initio studies in which the electronic spectra and the characterization of excited electronic states of halogen oxide species will also be presented. Again where available, the ab initio results are compared to experimental observations, and are used to aid in the interpretation of experimental studies.
Ab initio simulation of transport phenomena in rarefied gases.
Sharipov, Felix; Strapasson, José L
2012-09-01
Ab initio potentials are implemented into the direct simulation Monte Carlo (DSMC) method. Such an implementation allows us to model transport phenomena in rarefied gases without any fitting parameter of intermolecular collisions usually extracted from experimental data. Applying the method proposed by Sharipov and Strapasson [Phys. Fluids 24, 011703 (2012)], the use of ab initio potentials in the DSMC requires the same computational efforts as the widely used potentials such as hard spheres, variable hard sphere, variable soft spheres, etc. At the same time, the ab initio potentials provide more reliable results than any other one. As an example, the transport coefficients of a binary mixture He-Ar, viz., viscosity, thermal conductivity, and thermal diffusion factor, have been calculated for several values of the mole fraction. PMID:23030889
Use of ab initio quantum chemical methods in battery technology
Energy Technology Data Exchange (ETDEWEB)
Deiss, E. [Paul Scherrer Inst. (PSI), Villigen (Switzerland)
1997-06-01
Ab initio quantum chemistry can nowadays predict physical and chemical properties of molecules and solids. An attempt should be made to use this tool more widely for predicting technologically favourable materials. To demonstrate the use of ab initio quantum chemistry in battery technology, the theoretical energy density (energy per volume of active electrode material) and specific energy (energy per mass of active electrode material) of a rechargeable lithium-ion battery consisting of a graphite electrode and a nickel oxide electrode has been calculated with this method. (author) 1 fig., 1 tab., 7 refs.
P-V Relation for Mercuric Calcogenides: Ab Initio Method
Directory of Open Access Journals (Sweden)
G. Misra
2011-01-01
Full Text Available Mercuric Calcogenides found many applications in electronic and optical devices as semiconducting materials. An equation of state provides useful information about the relationship between pressure (P, volume (V and temperature (T that helps to understand the behaviour of materials under the effect of high pressure and high temperature. The present paper sheds light on the electronic structure of Mercuric Calcogenides by simulating its electronic properties through ab initio method. This ab initio method is extended to derive the equation of state for Mercuric Calcogenides. The present equation of state has also been tested for the prediction of End Point. The computed results compare well with Quantum statistical data.
Recent achievements in ab initio modelling of liquid water
Khaliullin, Rustam Z
2013-01-01
The application of newly developed first-principle modeling techniques to liquid water deepens our understanding of the microscopic origins of its unusual macroscopic properties and behaviour. Here, we review two novel ab initio computational methods: second-generation Car-Parrinello molecular dynamics and decomposition analysis based on absolutely localized molecular orbitals. We show that these two methods in combination not only enable ab initio molecular dynamics simulations on previously inaccessible time and length scales, but also provide unprecedented insights into the nature of hydrogen bonding between water molecules. We discuss recent applications of these methods to water clusters and bulk water.
DEFF Research Database (Denmark)
Wolf, T. J. A.; Kuhlman, Thomas Scheby; Schalk, O.;
2014-01-01
comparing time-resolved photoelectron spectroscopy (TRPES) with ab initio multiple spawning (AIMS) simulations on the MS-MR-CASPT2 level of theory. We disentangle the relationship between two phenomena that dominate the immediate molecular response upon light absorption: a spectrally dependent delay of the...
Ab initio molecular crystal structures, spectra, and phase diagrams.
Hirata, So; Gilliard, Kandis; He, Xiao; Li, Jinjin; Sode, Olaseni
2014-09-16
Conspectus Molecular crystals are chemists' solids in the sense that their structures and properties can be understood in terms of those of the constituent molecules merely perturbed by a crystalline environment. They form a large and important class of solids including ices of atmospheric species, drugs, explosives, and even some organic optoelectronic materials and supramolecular assemblies. Recently, surprisingly simple yet extremely efficient, versatile, easily implemented, and systematically accurate electronic structure methods for molecular crystals have been developed. The methods, collectively referred to as the embedded-fragment scheme, divide a crystal into monomers and overlapping dimers and apply modern molecular electronic structure methods and software to these fragments of the crystal that are embedded in a self-consistently determined crystalline electrostatic field. They enable facile applications of accurate but otherwise prohibitively expensive ab initio molecular orbital theories such as Møller-Plesset perturbation and coupled-cluster theories to a broad range of properties of solids such as internal energies, enthalpies, structures, equation of state, phonon dispersion curves and density of states, infrared and Raman spectra (including band intensities and sometimes anharmonic effects), inelastic neutron scattering spectra, heat capacities, Gibbs energies, and phase diagrams, while accounting for many-body electrostatic (namely, induction or polarization) effects as well as two-body exchange and dispersion interactions from first principles. They can fundamentally alter the role of computing in the studies of molecular crystals in the same way ab initio molecular orbital theories have transformed research practices in gas-phase physical chemistry and synthetic chemistry in the last half century. In this Account, after a brief summary of formalisms and algorithms, we discuss applications of these methods performed in our group as compelling
Gonzalez, Luis E.; Gonzalez, David J
2006-01-01
We have performed orbital free ab initio molecular dynamics simulations in order to study the thermal behaviour of two open surfaces of solid metallic systems, namely the (110) face of fcc Al and the (10-10) face of hcp Mg. Our results reproduce qualitatively both the experimental measurements and previous ab initio calculations performed with the more costly Kohn-Sham approach of Density Functional Theory. These calculations can be viewed as a validation test of the orbital free method for s...
Ab-initio chemical potentials of solid and liquid solutions and the chemistry of the Earth's core
Alfe, D.; Gillan, M. J.; Price, G. D.
2001-01-01
A general set of methods is presented for calculating chemical potentials in solid and liquid mixtures using {\\em ab initio} techniques based on density functional theory (DFT). The methods are designed to give an {\\em ab initio} approach to treating chemical equilibrium between coexisting solid and liquid solutions, and particularly the partitioning ratio of solutes between such solutions. For the liquid phase, the methods are based on the general technique of thermodynamic integration, appl...
Ab initio calculations of mechanical properties: Methods and applications
Czech Academy of Sciences Publication Activity Database
Pokluda, J.; Černý, Miroslav; Šob, Mojmír; Umeno, Y.
2015-01-01
Roč. 73, AUG (2015), s. 127-158. ISSN 0079-6425 R&D Projects: GA ČR(CZ) GAP108/12/0311 Institutional support: RVO:68081723 Keywords : Ab initio methods * Elastic moduli * Intrinsic hardness * Stability analysis * Theoretical strength * Intrinsic brittleness/ductility Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 27.417, year: 2014
Cyanogen Azide. Ionization Potentials and Ab Initio SCF MO Calculation
DEFF Research Database (Denmark)
Bak, Börge; Jansen, Peter; Stafast, Herbert
1975-01-01
The Ne(I) and He(I) photoelectron(PE) spectra of cyanogen azide, NCN3, have been recorded at high resolution. Their interpretation is achieved by comparison with the PE spectrum of HN3 and an ab initio LCGO SCF MO calculation. Deviations from Koopmans' theorem of quite different magnitudes are fo...
Relaxation of Small Molecules: an ab initio Study
Institute of Scientific and Technical Information of China (English)
CAO Yi-Gang; JIAO Zheng-Kuan; A. Antons; K. Schroeder; S. Blügel2
2002-01-01
Using an ab initio total energy and force method, we have relaxed several group IV and group V elementalclusters, in detail the arsenic and antimony dimers, silicon, phosphorus, arsenic and antimony tetramers. The obtainedbond lengths and cohesive energies are more accurate than other calculating methods, and in excellent agreement withthe experimental results.
Ab initio study of phase equilibria in TiCx
DEFF Research Database (Denmark)
Korzhavyi, P.A.; Pourovskii, L.V.; Hugosson, H.W.;
2002-01-01
The phase diagram for the vacancy-ordered structures in the substoichiometric TiCx (x = 0.5-1.0) has been established from Monte Carlo simulations with the long-range pair and multisite effective interactions obtained from ab initio calculations. Three ordered superstructures of vacancies (Ti2C, ...
Ab InitioStudy of Hot Carriers in the First Picosecond after Sunlight Absorption in Silicon
Bernardi, Marco; Vigil-Fowler, Derek; Lischner, Johannes; Neaton, Jeffrey B.; Louie, Steven G.
2014-01-01
Hot carrier thermalization is a major source of efficiency loss in solar cells. Because of the subpicosecond time scale and complex physics involved, a microscopic characterization of hot carriers is challenging even for the simplest materials. We develop and apply an ab initio approach based on density functional theory and many-body perturbation theory to investigate hot carriers in semiconductors. Our calculations include electron-electron and electron-phonon interactions, and require no e...
Accurate ab initio vibrational energies of methyl chloride
International Nuclear Information System (INIS)
Two new nine-dimensional potential energy surfaces (PESs) have been generated using high-level ab initio theory for the two main isotopologues of methyl chloride, CH335Cl and CH337Cl. The respective PESs, CBS-35 HL, and CBS-37 HL, are based on explicitly correlated coupled cluster calculations with extrapolation to the complete basis set (CBS) limit, and incorporate a range of higher-level (HL) additive energy corrections to account for core-valence electron correlation, higher-order coupled cluster terms, scalar relativistic effects, and diagonal Born-Oppenheimer corrections. Variational calculations of the vibrational energy levels were performed using the computer program TROVE, whose functionality has been extended to handle molecules of the form XY 3Z. Fully converged energies were obtained by means of a complete vibrational basis set extrapolation. The CBS-35 HL and CBS-37 HL PESs reproduce the fundamental term values with root-mean-square errors of 0.75 and 1.00 cm−1, respectively. An analysis of the combined effect of the HL corrections and CBS extrapolation on the vibrational wavenumbers indicates that both are needed to compute accurate theoretical results for methyl chloride. We believe that it would be extremely challenging to go beyond the accuracy currently achieved for CH3Cl without empirical refinement of the respective PESs
Accurate ab initio vibrational energies of methyl chloride
Energy Technology Data Exchange (ETDEWEB)
Owens, Alec, E-mail: owens@mpi-muelheim.mpg.de [Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470 Mülheim an der Ruhr (Germany); Department of Physics and Astronomy, University College London, Gower Street, WC1E 6BT London (United Kingdom); Yurchenko, Sergei N.; Yachmenev, Andrey; Tennyson, Jonathan [Department of Physics and Astronomy, University College London, Gower Street, WC1E 6BT London (United Kingdom); Thiel, Walter [Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470 Mülheim an der Ruhr (Germany)
2015-06-28
Two new nine-dimensional potential energy surfaces (PESs) have been generated using high-level ab initio theory for the two main isotopologues of methyl chloride, CH{sub 3}{sup 35}Cl and CH{sub 3}{sup 37}Cl. The respective PESs, CBS-35{sup HL}, and CBS-37{sup HL}, are based on explicitly correlated coupled cluster calculations with extrapolation to the complete basis set (CBS) limit, and incorporate a range of higher-level (HL) additive energy corrections to account for core-valence electron correlation, higher-order coupled cluster terms, scalar relativistic effects, and diagonal Born-Oppenheimer corrections. Variational calculations of the vibrational energy levels were performed using the computer program TROVE, whose functionality has been extended to handle molecules of the form XY {sub 3}Z. Fully converged energies were obtained by means of a complete vibrational basis set extrapolation. The CBS-35{sup HL} and CBS-37{sup HL} PESs reproduce the fundamental term values with root-mean-square errors of 0.75 and 1.00 cm{sup −1}, respectively. An analysis of the combined effect of the HL corrections and CBS extrapolation on the vibrational wavenumbers indicates that both are needed to compute accurate theoretical results for methyl chloride. We believe that it would be extremely challenging to go beyond the accuracy currently achieved for CH{sub 3}Cl without empirical refinement of the respective PESs.
Ab-initio calculations for dilute magnetic semiconductors
Energy Technology Data Exchange (ETDEWEB)
Belhadji, Brahim
2008-03-03
This thesis focusses on ab-initio calculations for the electronic structure and the magnetic properties of dilute magnetic semiconductors (DMS). In particular we aim at the understanding of the complex exchange interactions in these systems. Our calculations are based on density functional theory, being ideally suited for a description of the material specific properties of the considered DMS. Moreover we use the KKR Green function method in connection with the coherent potential approximation (CPA), which allows to include the random substitutional disorder in a mean field-like approximation for the electronic structure. Finally we calculate the exchange coupling constants J{sub ij} between two impurities in a CPA medium by using the Lichtenstein formula and from this calculate the Curie temperature by a numerically exact Monte Carlo method. Based on this analysis we found and investigated four different exchange mechanisms being of importance in DMS systems: Double exchange, p-d exchange, antiferromagnetic superexchanges, and ferromagnetic superexchange. A second topic we have investigated in this thesis is the pressure dependence of the exchange interactions and the Curie temperatures in (Ga,Mn)As and (In,Mn)As, using the LDA and the LDA+U approximations. Exact calculations of T{sub C} by Monte Carlo simulations show a somehow different behavior. (orig.)
Exploring the free energy surface using ab initio molecular dynamics
Samanta, Amit; Morales, Miguel A.; Schwegler, Eric
2016-04-01
Efficient exploration of configuration space and identification of metastable structures in condensed phase systems are challenging from both computational and algorithmic perspectives. In this regard, schemes that utilize a set of pre-defined order parameters to sample the relevant parts of the configuration space [L. Maragliano and E. Vanden-Eijnden, Chem. Phys. Lett. 426, 168 (2006); J. B. Abrams and M. E. Tuckerman, J. Phys. Chem. B 112, 15742 (2008)] have proved useful. Here, we demonstrate how these order-parameter aided temperature accelerated sampling schemes can be used within the Born-Oppenheimer and the Car-Parrinello frameworks of ab initio molecular dynamics to efficiently and systematically explore free energy surfaces, and search for metastable states and reaction pathways. We have used these methods to identify the metastable structures and reaction pathways in SiO2 and Ti. In addition, we have used the string method [W. E, W. Ren, and E. Vanden-Eijnden, Phys. Rev. B 66, 052301 (2002); L. Maragliano et al., J. Chem. Phys. 125, 024106 (2006)] within the density functional theory to study the melting pathways in the high pressure cotunnite phase of SiO2 and the hexagonal closed packed to face centered cubic phase transition in Ti.
Exploring the free energy surface using ab initio molecular dynamics.
Samanta, Amit; Morales, Miguel A; Schwegler, Eric
2016-04-28
Efficient exploration of configuration space and identification of metastable structures in condensed phase systems are challenging from both computational and algorithmic perspectives. In this regard, schemes that utilize a set of pre-defined order parameters to sample the relevant parts of the configuration space [L. Maragliano and E. Vanden-Eijnden, Chem. Phys. Lett. 426, 168 (2006); J. B. Abrams and M. E. Tuckerman, J. Phys. Chem. B 112, 15742 (2008)] have proved useful. Here, we demonstrate how these order-parameter aided temperature accelerated sampling schemes can be used within the Born-Oppenheimer and the Car-Parrinello frameworks of ab initio molecular dynamics to efficiently and systematically explore free energy surfaces, and search for metastable states and reaction pathways. We have used these methods to identify the metastable structures and reaction pathways in SiO2 and Ti. In addition, we have used the string method [W. E, W. Ren, and E. Vanden-Eijnden, Phys. Rev. B 66, 052301 (2002); L. Maragliano et al., J. Chem. Phys. 125, 024106 (2006)] within the density functional theory to study the melting pathways in the high pressure cotunnite phase of SiO2 and the hexagonal closed packed to face centered cubic phase transition in Ti. PMID:27131525
Ab initio simulations on rutile-based titania nanowires
Zhukovskii, Yu F.; Evarestov, R. A.
2012-08-01
The rod symmetry groups for monoperiodic (1D) nanostructures have been applied for construction of models for bulk-like TiO2 nanowires (NWs) cut from a rutile-based 3D crystal along the chosen [001] and [110] directions of crystallographic axes. In this study, we have considered nanowires described by both the Ti-atom centered rotation axes as well as the hollow site centered axes passing through the interstitial positions between the Ti and O atoms closest to the axes. The most stable [001]-oriented TiO2 NWs with rhombic cross sections are found to display the energetically preferable {110} facets only while the nanowires with quasi-square sections across the [110] axis are formed by the alternating { 1bar 10 } and {001} facets. For simulations on rutile-based nanowires possessing different diameters for each NW type, we have performed large-scale ab initio Density Functional Theory (DFT) and hybrid DFT-Hartree Fock (DFT-HF) calculations with total geometry optimization within the Generalized Gradient Approximation (GGA) in the form of the Perdew-Becke-Ernzenhof (PBE) exchange-correlation functionals (PBE and PBE0, respectively), using the formalism of linear combination of localized atomic functions (LCAO). We have simulated both structural and electronic properties of TiO2 NWs depending both on orientation and position of symmetry axes as well as on diameter and morphology of nanowires.
Serine Proteases an Ab Initio Molecular Dynamics Study
De Santis, L
1999-01-01
In serine proteases (SP's), the H-bond between His-57 and Asp-102, and that between Gly-193 and the transition state intermediate play a crucial role for enzymatic function. To shed light on the nature of these interactions, we have carried out ab initio molecular dynamics simulations on complexes representing adducts between the reaction intermediate and elastase (one protein belonging to the SP family). Our calculations indicate the presence of a low--barrier H-bond between His-57 and Asp-102, in complete agreement with NMR experiments on enzyme--transition state analog complexes. Comparison with an ab initio molecular dynamics simulation on a model of the substrate--enzyme adduct indicates that the Gly-193--induced strong stabilization of the intermediate is accomplished by charge/dipole interactions and not by H-bonding as previously suggested. Inclusion of the protein electric field in the calculations does not affect significantly the charge distribution.
The density matrix renormalization group for ab initio quantum chemistry
Wouters, Sebastian
2014-01-01
During the past 15 years, the density matrix renormalization group (DMRG) has become increasingly important for ab initio quantum chemistry. Its underlying wavefunction ansatz, the matrix product state (MPS), is a low-rank decomposition of the full configuration interaction tensor. The virtual dimension of the MPS, the rank of the decomposition, controls the size of the corner of the many-body Hilbert space that can be reached with the ansatz. This parameter can be systematically increased until numerical convergence is reached. The MPS ansatz naturally captures exponentially decaying correlation functions. Therefore DMRG works extremely well for noncritical one-dimensional systems. The active orbital spaces in quantum chemistry are however often far from one-dimensional, and relatively large virtual dimensions are required to use DMRG for ab initio quantum chemistry (QC-DMRG). The QC-DMRG algorithm, its computational cost, and its properties are discussed. Two important aspects to reduce the computational co...
Ab-initio calculations on melting of thorium
Mukherjee, D.; Sahoo, B. D.; Joshi, K. D.; Kaushik, T. C.; Gupta, Satish C.
2016-05-01
Ab-initio molecular dynamics study has been performed on face centered cubic structured thorium to determine its melting temperature at room pressure. The ion-electron interaction potential energy calculated as a function of temperature for three volumes (a0)3 and (1.02a0)3 and (1.04a0)3 increases gradually with temperature and undergoes a sharp jump at ~2200 K, ~2100 K and ~1800 K, respectively. Here, a0 = 5.043 Å is the equilibrium lattice parameter at 0 K obtained from ab-initio calculations. These jumps in interaction energy are treated as due to the onset of melting and corresponding temperatures as melting point. The melting point of 2100 K is close to the experimental value of 2023K. Further, the same has been verified by plotting the atomic arrangement evolved at various temperatures and corresponding pair correlation functions.
Ab initio no-core shell model with continuum
Navratil, Petr
2008-04-01
The ab initio no-core shell model (NCSM) is a many-body approach to nuclear structure of light nuclei. The NCSM adopts an effective interaction theory to transform fundamental inter-nucleon interactions into effective interactions for a specified nucleus in a selected harmonic oscillator basis space [1]. The method is capable of predicting nuclear structure from inter-nucleon forces derived from quantum chromodynamics by means of chiral effective field theory [2]. NCSM extensions to the microscopic description of nuclear reactions are now under development. In my talk, I will first discuss our recent calculations of the ^4He total photo-absorption cross section using two- and three-nucleon interactions from chiral effective field theory [3]. I will then outline our effort to augment the NCSM by the resonating group method (RGM) technique to develop a new method capable of describing simultaneously both bound states and nuclear reactions on light nuclei [4]. This approach, which preserves translational symmetry and the Pauli principle, will allow us to calculate cross sections of reactions important for astrophysics and describe weakly-bound systems from first principles. I will present our first phase shift results for neutron scattering off ^3H, ^4He and ^7Li and proton scattering off ^3He, ^4He and ^7Be using realistic nucleon-nucleon potentials. 3mm [1] P. Navr'atil, J. P. Vary and B. R. Barrett, Phys. Rev. C 62, 054311 (2000). [2] P. Navr'atil and V. G. Gueorguiev and J. P. Vary, W. E. Ormand and A. Nogga, Phys. Rev. Lett. 99, 042501 (2007). [3] S. Quaglioni and P. Navr'atil, Phys. Lett. B 652, 370 (2007). [4] S. Quaglioni and P. Navr'atil, arXiv:0712.0855.
Ab-initio molecular dynamics for metallic systems
International Nuclear Information System (INIS)
This thesis deals with the problem of performing first-principles electronic structure calculations in metallic systems, with the goal of bringing ab-initio quantum-mechanical molecular dynamics simulations for these systems to the same level of computational cost, efficiency and accuracy that are now obtained for semiconductors and insulators. The problem is first reviewed from the theoretical and methodological point of view, with a presentation of the current state of research. In particular, the formulation of the electronic structure problem as a Density-Functional-Theory constrained minimization is examined in detail, as well as the description of metallic systems via generalized electronic free energy functionals. A novel reformulation of the problem is here proposed, using the language of Ensemble Density Functional Theory, and a variational realization of it is developed and implemented. The dramatic improvement in the efficiency for the convergence to the electronic ground-state is discussed and explained. The role of the fictitious electronic temperature is examined, as well as its contribution to controlling the errors originating from inadequate sampling of the Brillouin Zone. The associated systematic errors are also examined, and non-selfconsistent and self-consistent estimates for these errors in the energies and the ionic forces are made explicit. The novel technique of cold smearing is introduced. The new method of Ensemble Density Functional Theory, in conjunction with the cold smearing, is shown to reach the proposed goal of greatly improving our current efficiency and accuracy for molecular dynamics simulations, making them affordable at the level of currently available computational power. The method is applied to the study of the finite temperature properties of bulk aluminium and aluminium surfaces, to identify the microscopical processes that give rise to the premelting of the (110) surface and to show evidence for the different phase
Towards an ab initio description of magnetism in ionic solids
Illas, F.; Casanovas, J.; García-Bach, M. A.; Caballol, R.; Castell, O.
1993-11-01
The physical contributions to the KNiF3 magnetic exchange coupling integral have been obtained from specially designed ab initio cluster model calculations. Three important mechanisms have been identified. These are the delocalization of the magnetic orbitals into the anion ``p'' band, the variational contribution of the second-order interactions, and the many-body terms ``hidden'' in the two-body operator and the Heisenberg Hamiltonian.
Ab initio simulation of helium inside carbon nanotubes
International Nuclear Information System (INIS)
In present work we consider the complex behaviour of quantum liquids like liquid He-4 inside carbon nanotubes. Interactions between helium atoms and carbon atoms of the short-length atomistic model and model with periodical boundary conditions of carbon nanotube were studied via ab initio quantum simulations. Effects of geometrical confinement of the tube on the He behaviour inside CNT (13,0) have been explored. Nanotubes with typical average diameter of 10 angstroms are under consideration.
Ab Initio Modeling of Ecosystems with Artificial Life
Adami, C.
2002-01-01
Artificial Life provides the opportunity to study the emergence and evolution of simple ecosystems in real time. We give an overview of the advantages and limitations of such an approach, as well as its relation to individual-based modeling techniques. The Digital Life system Avida is introduced and prospects for experiments with ab initio evolution (evolution "from scratch"), maintenance, as well as stability of ecosystems are discussed.
P-V Relation for Mercuric Calcogenides: Ab Initio Method
G. Misra; S. Tenguria; Gautam, M.
2011-01-01
Mercuric Calcogenides found many applications in electronic and optical devices as semiconducting materials. An equation of state provides useful information about the relationship between pressure (P), volume (V) and temperature (T) that helps to understand the behaviour of materials under the effect of high pressure and high temperature. The present paper sheds light on the electronic structure of Mercuric Calcogenides by simulating its electronic properties through ab initio method. This a...
Structure models: from shell model to ab initio methods
Bacca, Sonia
2016-01-01
A brief review of models to describe nuclear structure and reactions properties is presented, starting from the historical shell model picture and encompassing modern ab initio approaches. A selection of recent theoretical results on observables for exotic light and medium-mass nuclei is shown. Emphasis is given to the comparison with experiment and to what can be learned about three-body forces and continuum properties.
GAUSSIAN 76: An ab initio Molecular Orbital Program
Binkley, J. S.; Whiteside, R.; Hariharan, P. C.; Seeger, R.; Hehre, W. J.; Lathan, W. A.; Newton, M. D.; Ditchfield, R.; Pople, J. A.
1978-01-01
Gaussian 76 is a general-purpose computer program for ab initio Hartree-Fock molecular orbital calculations. It can handle basis sets involving s, p and d-type Gaussian functions. Certain standard sets (STO-3G, 4-31G, 6-31G*, etc.) are stored internally for easy use. Closed shell (RHF) or unrestricted open shell (UHF) wave functions can be obtained. Facilities are provided for geometry optimization to potential minima and for limited potential surface scans.
GAUSSIAN 76: an ab initio molecular orbital program
Energy Technology Data Exchange (ETDEWEB)
Binkley, J. S.; Whiteside, R.; Hariharan, P. C.; Seeger, R.; Hehre, W. J.; Lathan, W. A.; Newton, M. D.; Ditchfield, R.; Pople, J. A.
1978-06-01
Gaussian 76 is a general-purpose computer program for ab initio Hartree-Fock molecular orbital calculations. It can handle basis sets involving s, p and d-type gaussian functions. Certain standard sets (STO-3G, 4-31G, 6-31G*, etc.) are stored internally for easy use. Closed shell (RHF) or unrestricted open shell (UHF) wave functions can be obtained. Facilities are provided for geometry optimization to potential minima and for limited potential surface scans.
Der halbunendliche Kristall - Elektronische und optische Eigenschaften ab-initio
Brodersen, Sven
2002-01-01
Es werden die elektronischen und optischen Eigenschaften eines kristallinen Festkörpers unter Berücksichtigung der Oberfläche mit ab-initio Methoden berechnet. Die Behandlung der Oberflächeneffekte in einer Halbraum-Geometrie erzwingt die Darstellung der Wellenfunktionen und der Dielektrischen Funktion (DK) in einer lokalen Basis. Anhand von Volumenkristallen wird die Effizienz von LCAO- Basisfunktionen demonstriert. Die Erweiterung der atomaren Orbitale mit unbesetzten Orbitalen und 'off-sit...
Ab Initio Nuclear Structure and Reaction Calculations for Rare Isotopes
International Nuclear Information System (INIS)
We have developed a novel ab initio symmetry-adapted no-core shell model (SA-NCSM), which has opened the intermediate-mass region for ab initio investigations, thereby providing an opportunity for first-principle symmetry-guided applications to nuclear structure and reactions for nuclear isotopes from the lightest p-shell systems to intermediate-mass nuclei. This includes short-lived proton-rich nuclei on the path of X-ray burst nucleosynthesis and rare neutron-rich isotopes to be produced by the Facility for Rare Isotope Beams (FRIB). We have provided ab initio descriptions of high accuracy for low-lying (including collectivity-driven) states of isotopes of Li, He, Be, C, O, Ne, Mg, Al, and Si, and studied related strong- and weak-interaction driven reactions that are important, in astrophysics, for further understanding stellar evolution, X-ray bursts and triggering of s, p, and rp processes, and in applied physics, for electron and neutrino-nucleus scattering experiments as well as for fusion ignition at the National Ignition Facility (NIF).
Ab Initio Nuclear Structure and Reaction Calculations for Rare Isotopes
Energy Technology Data Exchange (ETDEWEB)
Draayer, Jerry P. [Louisiana State Univ., Baton Rouge, LA (United States)
2014-09-28
We have developed a novel ab initio symmetry-adapted no-core shell model (SA-NCSM), which has opened the intermediate-mass region for ab initio investigations, thereby providing an opportunity for first-principle symmetry-guided applications to nuclear structure and reactions for nuclear isotopes from the lightest p-shell systems to intermediate-mass nuclei. This includes short-lived proton-rich nuclei on the path of X-ray burst nucleosynthesis and rare neutron-rich isotopes to be produced by the Facility for Rare Isotope Beams (FRIB). We have provided ab initio descriptions of high accuracy for low-lying (including collectivity-driven) states of isotopes of Li, He, Be, C, O, Ne, Mg, Al, and Si, and studied related strong- and weak-interaction driven reactions that are important, in astrophysics, for further understanding stellar evolution, X-ray bursts and triggering of s, p, and rp processes, and in applied physics, for electron and neutrino-nucleus scattering experiments as well as for fusion ignition at the National Ignition Facility (NIF).
Ab initio calculations for industrial materials engineering: successes and challenges
Energy Technology Data Exchange (ETDEWEB)
Wimmer, Erich; Freeman, Clive; Christensen, Mikael; Wolf, Walter; Saxe, Paul [Materials Design, Inc., PO Box 2000, Angel Fire, NM 87710 (United States); Najafabadi, Reza; Young Jr, George A; Ballard, Jake D; Angeliu, Thomas M; Vollmer, James [Knolls Atomic Power Laboratory, PO Box 1072, Schenectady, NY 12301-1072 (United States); Chambers, James J; Niimi, Hiroaki; Shaw, Judy B, E-mail: ewimmer@materialsdesign.co [Advanced CMOS, Texas Instruments Incorporated, Dallas, TX 75243 (United States)
2010-09-29
Computational materials science based on ab initio calculations has become an important partner to experiment. This is demonstrated here for the effect of impurities and alloying elements on the strength of a Zr twist grain boundary, the dissociative adsorption and diffusion of iodine on a zirconium surface, the diffusion of oxygen atoms in a Ni twist grain boundary and in bulk Ni, and the dependence of the work function of a TiN-HfO{sub 2} junction on the replacement of N by O atoms. In all of these cases, computations provide atomic-scale understanding as well as quantitative materials property data of value to industrial research and development. There are two key challenges in applying ab initio calculations, namely a higher accuracy in the electronic energy and the efficient exploration of large parts of the configurational space. While progress in these areas is fueled by advances in computer hardware, innovative theoretical concepts combined with systematic large-scale computations will be needed to realize the full potential of ab initio calculations for industrial applications.
Ab initio calculations for industrial materials engineering: successes and challenges
International Nuclear Information System (INIS)
Computational materials science based on ab initio calculations has become an important partner to experiment. This is demonstrated here for the effect of impurities and alloying elements on the strength of a Zr twist grain boundary, the dissociative adsorption and diffusion of iodine on a zirconium surface, the diffusion of oxygen atoms in a Ni twist grain boundary and in bulk Ni, and the dependence of the work function of a TiN-HfO2 junction on the replacement of N by O atoms. In all of these cases, computations provide atomic-scale understanding as well as quantitative materials property data of value to industrial research and development. There are two key challenges in applying ab initio calculations, namely a higher accuracy in the electronic energy and the efficient exploration of large parts of the configurational space. While progress in these areas is fueled by advances in computer hardware, innovative theoretical concepts combined with systematic large-scale computations will be needed to realize the full potential of ab initio calculations for industrial applications.
Ab initio multiple cloning algorithm for quantum nonadiabatic molecular dynamics
International Nuclear Information System (INIS)
We present a new algorithm for ab initio quantum nonadiabatic molecular dynamics that combines the best features of ab initio Multiple Spawning (AIMS) and Multiconfigurational Ehrenfest (MCE) methods. In this new method, ab initio multiple cloning (AIMC), the individual trajectory basis functions (TBFs) follow Ehrenfest equations of motion (as in MCE). However, the basis set is expanded (as in AIMS) when these TBFs become sufficiently mixed, preventing prolonged evolution on an averaged potential energy surface. We refer to the expansion of the basis set as “cloning,” in analogy to the “spawning” procedure in AIMS. This synthesis of AIMS and MCE allows us to leverage the benefits of mean-field evolution during periods of strong nonadiabatic coupling while simultaneously avoiding mean-field artifacts in Ehrenfest dynamics. We explore the use of time-displaced basis sets, “trains,” as a means of expanding the basis set for little cost. We also introduce a new bra-ket averaged Taylor expansion (BAT) to approximate the necessary potential energy and nonadiabatic coupling matrix elements. The BAT approximation avoids the necessity of computing electronic structure information at intermediate points between TBFs, as is usually done in saddle-point approximations used in AIMS. The efficiency of AIMC is demonstrated on the nonradiative decay of the first excited state of ethylene. The AIMC method has been implemented within the AIMS-MOLPRO package, which was extended to include Ehrenfest basis functions
Ab initio calculations of reactions with light nuclei
Quaglioni, Sofia; Hupin, Guillaume; Calci, Angelo; Navrátil, Petr; Roth, Robert
2016-03-01
An ab initio (i.e., from first principles) theoretical framework capable of providing a unified description of the structure and low-energy reaction properties of light nuclei is desirable to further our understanding of the fundamental interactions among nucleons, and provide accurate predictions of crucial reaction rates for nuclear astrophysics, fusion-energy research, and other applications. In this contribution we review ab initio calculations for nucleon and deuterium scattering on light nuclei starting from chiral two- and three-body Hamiltonians, obtained within the framework of the ab initio no-core shell model with continuum. This is a unified approach to nuclear bound and scattering states, in which square-integrable energy eigenstates of the A-nucleon system are coupled to (A-a)+a target-plus-projectile wave functions in the spirit of the resonating group method to obtain an efficient description of the many-body nuclear dynamics both at short and medium distances and at long ranges.
A Complete and Accurate Ab Initio Repeat Finding Algorithm.
Lian, Shuaibin; Chen, Xinwu; Wang, Peng; Zhang, Xiaoli; Dai, Xianhua
2016-03-01
It has become clear that repetitive sequences have played multiple roles in eukaryotic genome evolution including increasing genetic diversity through mutation, changes in gene expression and facilitating generation of novel genes. However, identification of repetitive elements can be difficult in the ab initio manner. Currently, some classical ab initio tools of finding repeats have already presented and compared. The completeness and accuracy of detecting repeats of them are little pool. To this end, we proposed a new ab initio repeat finding tool, named HashRepeatFinder, which is based on hash index and word counting. Furthermore, we assessed the performances of HashRepeatFinder with other two famous tools, such as RepeatScout and Repeatfinder, in human genome data hg19. The results indicated the following three conclusions: (1) The completeness of HashRepeatFinder is the best one among these three compared tools in almost all chromosomes, especially in chr9 (8 times of RepeatScout, 10 times of Repeatfinder); (2) in terms of detecting large repeats, HashRepeatFinder also performed best in all chromosomes, especially in chr3 (24 times of RepeatScout and 250 times of Repeatfinder) and chr19 (12 times of RepeatScout and 60 times of Repeatfinder); (3) in terms of accuracy, HashRepeatFinder can merge the abundant repeats with high accuracy. PMID:26272474
Ab initio multiple cloning algorithm for quantum nonadiabatic molecular dynamics
Makhov, Dmitry V.; Glover, William J.; Martinez, Todd J.; Shalashilin, Dmitrii V.
2014-08-01
We present a new algorithm for ab initio quantum nonadiabatic molecular dynamics that combines the best features of ab initio Multiple Spawning (AIMS) and Multiconfigurational Ehrenfest (MCE) methods. In this new method, ab initio multiple cloning (AIMC), the individual trajectory basis functions (TBFs) follow Ehrenfest equations of motion (as in MCE). However, the basis set is expanded (as in AIMS) when these TBFs become sufficiently mixed, preventing prolonged evolution on an averaged potential energy surface. We refer to the expansion of the basis set as "cloning," in analogy to the "spawning" procedure in AIMS. This synthesis of AIMS and MCE allows us to leverage the benefits of mean-field evolution during periods of strong nonadiabatic coupling while simultaneously avoiding mean-field artifacts in Ehrenfest dynamics. We explore the use of time-displaced basis sets, "trains," as a means of expanding the basis set for little cost. We also introduce a new bra-ket averaged Taylor expansion (BAT) to approximate the necessary potential energy and nonadiabatic coupling matrix elements. The BAT approximation avoids the necessity of computing electronic structure information at intermediate points between TBFs, as is usually done in saddle-point approximations used in AIMS. The efficiency of AIMC is demonstrated on the nonradiative decay of the first excited state of ethylene. The AIMC method has been implemented within the AIMS-MOLPRO package, which was extended to include Ehrenfest basis functions.
Ab initio multiple cloning algorithm for quantum nonadiabatic molecular dynamics
Energy Technology Data Exchange (ETDEWEB)
Makhov, Dmitry V.; Shalashilin, Dmitrii V. [Department of Chemistry, University of Leeds, Leeds LS2 9JT (United Kingdom); Glover, William J.; Martinez, Todd J. [Department of Chemistry and The PULSE Institute, Stanford University, Stanford, California 94305, USA and SLAC National Accelerator Laboratory, Menlo Park, California 94025 (United States)
2014-08-07
We present a new algorithm for ab initio quantum nonadiabatic molecular dynamics that combines the best features of ab initio Multiple Spawning (AIMS) and Multiconfigurational Ehrenfest (MCE) methods. In this new method, ab initio multiple cloning (AIMC), the individual trajectory basis functions (TBFs) follow Ehrenfest equations of motion (as in MCE). However, the basis set is expanded (as in AIMS) when these TBFs become sufficiently mixed, preventing prolonged evolution on an averaged potential energy surface. We refer to the expansion of the basis set as “cloning,” in analogy to the “spawning” procedure in AIMS. This synthesis of AIMS and MCE allows us to leverage the benefits of mean-field evolution during periods of strong nonadiabatic coupling while simultaneously avoiding mean-field artifacts in Ehrenfest dynamics. We explore the use of time-displaced basis sets, “trains,” as a means of expanding the basis set for little cost. We also introduce a new bra-ket averaged Taylor expansion (BAT) to approximate the necessary potential energy and nonadiabatic coupling matrix elements. The BAT approximation avoids the necessity of computing electronic structure information at intermediate points between TBFs, as is usually done in saddle-point approximations used in AIMS. The efficiency of AIMC is demonstrated on the nonradiative decay of the first excited state of ethylene. The AIMC method has been implemented within the AIMS-MOLPRO package, which was extended to include Ehrenfest basis functions.
DEFF Research Database (Denmark)
Berg, Rolf W.; Riisager, Anders; Nguyen van Buu, Olivier;
2010-01-01
The salt 1,1,3,3-tetramethylguanidinium bromide, [((CH3)2N)2C=NH2]+Br- or [tmgH]Br, was found to melt at 135(5) °C, forming what may be referred to as a moderate temperature ionic liquid. The chemistry was studied and compared with the corresponding chloride compound. We present X-ray diffraction...... = 104.31(3)°, Z = 2, based on 11769 reflections, measured from θ = 2.71-28.00° on a small colorless needle crystal. Raman and IR spectra are presented and assigned. When heated, both the chloride and the bromide salts form vapor phases. The Raman spectra of the vapors are surprisingly alike, showing...... initio molecular orbital density functional theory type calculations. It is not likely that both the bromide and chloride should have identical spectra. As explanation, the formation of 1,1-dimethylcyanamide gas is proposed, by decomposition of [tmgH]X leaving dimethylammonium halogenide (X = Cl, Br...
Ab Initio Computation of Dynamical Properties: Pressure Broadening
Wiesenfeld, Laurent; Drouin, Brian
2014-06-01
Rotational spectroscopy of polar molecules is the main observational tool in many areas of astrophysics, for gases of low densities (n ˜ 102 - 108 cm-3). Spectral line shapes in astrophysical media are largely dominated by turbulence-induced Doppler effects and natural line broadening are negligible. However line broadening remains an important tool for denser gases, like planetary high atmospheres. Understanding the excitation schemes of polar molecules requires the knowledge of excitation transfer rate due to collisional excitation, between the polar molecule and the ambient gas, usually H2. Transport properties in ionized media also require a precise knowledge of momentum transfer rates by elastic collisions. In order to assess the theoretically computed cross section and energy/momentum transfer rates, direct absolute experiments are scarce. The best way is to measure not individual scattering events but rather the global effect of the buffer gas, thanks to the pressure broadening cross sections, whose magnitude can be measured without any scaling parameters. At low temperatures, both elastic and inelastic scattering amplitudes are tested. At higher temperature, depending on the interaction strength, only inelastic scattering cross section are shown to play a significant role 1 ,2. Thanks to the advances of computer capabilities, it has become practical to compute spectral line parameters fromab initio quantum chemistry. In particular, the theory of rotational line broadening is readily incorporated into scattering quantum dynamical theory, like close-coupling schemes. The only approximations used in the computation are the isolated collision/isolated line approximations. We compute the non-binding interaction potential with high precision quantum chemistry and fit the resulting ab initio points onto a suitable functional. We have recently computed several such systems, for molecules in H2 buffer gas: H2O,3 H2CO,4 HCO+ .5 Detailed computations taking into
Energy Technology Data Exchange (ETDEWEB)
Bernard, St
1998-12-31
The quest for metallic hydrogen is a major goal for both theoretical and experimental condensed matter physics. Hydrogen and deuterium have been compressed up to 200 GPa in diamond anvil cells, without any clear evidence for a metallic behaviour. Loubeyere has recently suggested that hydrogen could metallize, at pressures within experimental range, in a new Van der Waals compound: Ar(H{sub 2}){sub 2} which is characterized at ambient pressure by an open and anisotropic sublattice of hydrogen molecules, stabilized by an argon skeleton. This thesis deals with a detailed ab initio investigation, by Car-Parrinello molecular dynamics methods, of the evolution under pressure of this compound. In a last chapter, we go to much higher pressures and temperatures, in order to compare orbital and orbital free ab initio methods for the dense hydrogen plasma. (author) 109 refs.
Cosmic-ray modulation: an ab initio approach
International Nuclear Information System (INIS)
A better understanding of cosmic-ray modulation in the heliosphere can only be gained through a proper understanding of the effects of turbulence on the diffusion and drift of cosmic rays. We present an ab initio model for cosmic-ray modulation, incorporating for the first time the results yielded by a two-component turbulence transport model. This model is solved for periods of minimum solar activity, utilizing boundary values chosen so that model results are in fair to good agreement with spacecraft observations of turbulence quantities, not only in the solar ecliptic plane but also along the out-of-ecliptic trajectory of the Ulysses spacecraft. These results are employed as inputs for modelled slab and 2D turbulence energy spectra. The latter spectrum is chosen based on physical considerations, with a drop-off at the very lowest wavenumbers commencing at the 2D outerscale. There currently exist no models or observations for this quantity, and it is the only free parameter in this study. The modelled turbulence spectra are used as inputs for parallel mean free path expressions based on those derived from quasi-linear theory and perpendicular mean free paths from extended nonlinear guiding center theory. Furthermore, the effects of turbulence on cosmic-ray drifts are modelled in a self-consistent way, employing a recently developed model for drift along the wavy current sheet. The resulting diffusion coefficients and drift expressions are applied to the study of galactic cosmic-ray protons and antiprotons using a three dimensional, steady-state cosmic-ray modulation code, and sample solutions in fair agreement with multiple spacecraft observations are presented. (author)
Ab initio work function of elemental metals
DEFF Research Database (Denmark)
Skriver, Hans Lomholt; Rosengaard, N. M.
1992-01-01
We have used a recently developed self-consistent Green’s-function technique based on tight-binding linear-muffin-tin-orbital theory to calculate the work function for the close-packed surfaces of 37 elemental metals. The results agree with the limited experimental data obtained from single...
Fertitta, E.; Paulus, B.; Barcza, G.; Legeza, Ö.
2014-01-01
We have studied the Metal-Insulator like Transition (MIT) in lithium and beryllium ring-shaped clusters through ab initio Density Matrix Renormalization Group (DMRG) method. Performing accurate calculations for different interatomic distances and using Quantum Information Theory (QIT) we investigated the changes occurring in the wavefunction between a metallic-like state and an insulating state built from free atoms. We also discuss entanglement and relevant excitations among the molecular or...
Ab-initio simulation and experimental validation of beta-titanium alloys
Raabe, D.; Sander, B.; Friák, M.; Ma, D.; Neugebauer, J.
2008-01-01
In this progress report we present a new approach to the ab-initio guided bottom up design of beta-Ti alloys for biomedical applications using a quantum mechanical simulation method in conjunction with experiments. Parameter-free density functional theory calculations are used to provide theoretical guidance in selecting and optimizing Ti-based alloys with respect to three constraints: (i) the use of non-toxic alloy elements; (ii) the stabilization of the body centered cubic beta phase at roo...
Knyazev, D. V.; Levashov, P. R.
2013-01-01
This work is devoted to the \\textit{ab initio} calculation of transport and optical properties of aluminum. The calculation is based on the quantum molecular dynamics simulation, density functional theory and the Kubo-Greenwood formula. Mainly the calculations are performed for liquid aluminum at near-normal densities for the temperatures from melting up to 20000 K. The results on dynamic electrical conductivity, static electrical conductivity and thermal conductivity are obtained and compare...
Belousov, Roman; Prencipe, Mauro
2014-01-01
The isothermal compression of magnesium perovskite and postperovskite is examined through the F-f plot and the diagnostic plot of Vinet universal model theoretically from the ab initio quantum-mechanical calculations at the hybrid Hartree-Fock / Density Functional Theory level. A purely numerical approach, first time applied in this paper, shows that the discrepancies largely observed between studies on the perovskite and criticized in geophysical applications are due to the inadequate choice...
Ab initio lattice dynamics of complex structures
DEFF Research Database (Denmark)
Voss, Johannes
2008-01-01
In this thesis, density functional theory is applied in a study of thermodynamic properties of so-called complex metal hydrides, which are promising materials for hydrogen storage applications. Since the unit cells of these crystals can be relatively large with many symmetrically inequivalent...... atomic coordinates, we have developed a new numerical optimization scheme, which allows for a fast convergence of the coordinate relaxation. Moreover, a method for the efficient calculation of phonon frequencies has been developed, which is based on a combination of density functional theory calculations...... have been studied, showing that the mobility of hydrogen is limited by high energetic barriers in the intermediate decomposition product Na3AlH6 in particular, and that the effect of titanium as a dopant on the dynamics is negligible. The presented methods and studies demonstrate possibilities for a...
Ab initio vibrations in nonequilibrium nanowires
International Nuclear Information System (INIS)
We review recent results on electronic and thermal transport in two different quasi one-dimensional systems: Silicon nanowires (SiNW) and atomic gold chains. For SiNW's we compute the ballistic electronic and thermal transport properties on equal footing, allowing us to make quantitative predictions for the thermoelectric properties, while for the atomic gold chains we evaluate microscopically the damping of the vibrations, due to the coupling of the chain atoms to the modes in the bulk contacts. Both approaches are based on the combination of density-functional theory, and nonequilibrium Green's functions.
Hydrogen adsorption in ZIF-7: A DFT and ab-initio molecular dynamics study
Dixit, Mudit; Major, Dan Thomas; Pal, Sourav
2016-05-01
Primary H2 adsorption sites in a zeolitic imidazolate framework, ZIF-7, are identified using ab-initio density functional theory (DFT) based molecular dynamics annealing simulations. The simulations suggest several low energy adsorption sites. The effect of light transition metal decoration on hydrogen storage properties was studied. Our ab-intio DFT calculations illustrate that decorating the ZIF with Sc increases both the number of H2 molecules, as well as the H2 binding energy. The binding energy (∼25 kJ/mol per H2) at 8H2 loading in the pore, suggests that Sc-ZIFs can be potential candidates for hydrogen storage.
Challenges for ab initio defect modeling
Energy Technology Data Exchange (ETDEWEB)
Deak, Peter [U. Bremen, Bremen Center for Computational Materials Science, POB 330440, D-28334 Bremen (Germany)], E-mail: deak@bccms.uni-bremen.de; Aradi, Balint; Frauenheim, Thomas [U. Bremen, Bremen Center for Computational Materials Science, POB 330440, D-28334 Bremen (Germany); Gali, Adam [Budapest University of Technology and Economics, Dept. Atomic Physics, H-1521 Budapest (Hungary)
2008-12-05
Supercell calculations using density functional theory with local or semi-local exchange functionals have proved to be a very successful tool in defect engineering, apparently leading to some degree of overconfidence lately. With a case study on selected defects, we demonstrate that the approximations involved in these functionals lead not only to an underestimation of the gap but also to related errors in the total energy difference of two defect configurations (be these in the same or different charge states). Sometimes the error is so serious, that even the common expectation: 'DFT provides the correct ground state' is refuted. We also demonstrate, that semi-empirical hybrid exchange functionals, which reproduce the correct band gap, give good total energy differences even in these cases. In such calculations, the position of Kohn-Sham gap levels with respect to the band edges gives a good estimation of the vertical ionization energy. Based on that, we propose a simple correction scheme for checking the necessity of extended total energy calculations with a hybrid functional.
Ab initio quantum transport calculations using plane waves
Garcia-Lekue, A.; Vergniory, M. G.; Jiang, X. W.; Wang, L. W.
2015-08-01
We present an ab initio method to calculate elastic quantum transport at the nanoscale. The method is based on a combination of density functional theory using plane wave nonlocal pseudopotentials and the use of auxiliary periodic boundary conditions to obtain the scattering states. The method can be applied to any applied bias voltage and the charge density and potential profile can either be calculated self-consistently, or using an approximated self-consistent field (SCF) approach. Based on the scattering states one can straightforwardly calculate the transmission coefficients and the corresponding electronic current. The overall scheme allows us to obtain accurate and numerically stable solutions for the elastic transport, with a computational time similar to that of a ground state calculation. This method is particularly suitable for calculations of tunneling currents through vacuum, that some of the nonequilibrium Greens function (NEGF) approaches based on atomic basis sets might have difficulty to deal with. Several examples are provided using this method from electron tunneling, to molecular electronics, to electronic devices: (i) On a Au nanojunction, the tunneling current dependence on the electrode-electrode distance is investigated. (ii) The tunneling through field emission resonances (FERs) is studied via an accurate description of the surface vacuum states. (iii) Based on quantum transport calculations, we have designed a molecular conformational switch, which can turn on and off a molecular junction by applying a perpendicular electric field. (iv) Finally, we have used the method to simulate tunnel field-effect transistors (TFETs) based on two-dimensional transition-metal dichalcogenides (TMDCs), where we have studied the performance and scaling limits of such nanodevices and proposed atomic doping to enhance the transistor performance.
Ab initio molecular dynamics simulation of liquid water by quantum Monte Carlo
International Nuclear Information System (INIS)
Although liquid water is ubiquitous in chemical reactions at roots of life and climate on the earth, the prediction of its properties by high-level ab initio molecular dynamics simulations still represents a formidable task for quantum chemistry. In this article, we present a room temperature simulation of liquid water based on the potential energy surface obtained by a many-body wave function through quantum Monte Carlo (QMC) methods. The simulated properties are in good agreement with recent neutron scattering and X-ray experiments, particularly concerning the position of the oxygen-oxygen peak in the radial distribution function, at variance of previous density functional theory attempts. Given the excellent performances of QMC on large scale supercomputers, this work opens new perspectives for predictive and reliable ab initio simulations of complex chemical systems
Ab initio calculation of valley splitting in monolayer δ-doped phosphorus in silicon.
Drumm, Daniel W; Budi, Akin; Per, Manolo C; Russo, Salvy P; L Hollenberg, Lloyd C
2013-01-01
: The differences in energy between electronic bands due to valley splitting are of paramount importance in interpreting transport spectroscopy experiments on state-of-the-art quantum devices defined by scanning tunnelling microscope lithography. Using vasp, we develop a plane-wave density functional theory description of systems which is size limited due to computational tractability. Nonetheless, we provide valuable data for the benchmarking of empirical modelling techniques more capable of extending this discussion to confined disordered systems or actual devices. We then develop a less resource-intensive alternative via localised basis functions in siesta, retaining the physics of the plane-wave description, and extend this model beyond the capability of plane-wave methods to determine the ab initio valley splitting of well-isolated δ-layers. In obtaining an agreement between plane-wave and localised methods, we show that valley splitting has been overestimated in previous ab initio calculations by more than 50%. PMID:23445785
Emergent properties of nuclei from ab initio coupled-cluster calculations
Hagen, G; Hjorth-Jensen, M; Papenbrock, T
2016-01-01
Emergent properties such as nuclear saturation and deformation, and the effects on shell structure due to the proximity of the scattering continuum and particle decay channels are fascinating phenomena in atomic nuclei. In recent years, ab initio approaches to nuclei have taken the first steps towards tackling the computational challenge of describing these phenomena from Hamiltonians with microscopic degrees of freedom. This endeavor is now possible due to ideas from effective field theories, novel optimization strategies for nuclear interactions, ab initio methods exhibiting a soft scaling with mass number, and ever-increasing computational power. This paper reviews some of the recent accomplishments. We also present new results. The recently optimized chiral interaction NNLO$_{\\rm sat}$ is shown to provide an accurate description of both charge radii and binding energies in selected light- and medium-mass nuclei up to $^{56}$Ni. We derive an efficient scheme for including continuum effects in coupled-clust...
Ab initio calculations on twisted graphene/hBN: Electronic structure and STM image simulation
Correa, J. D.; Cisternas, E.
2016-09-01
By performing ab initio calculations we obtained theoretical scanning tunneling microscopy (STM) images and studied the electronic properties of graphene on a hexagonal boron-nitrite (hBN) layer. Three different stack configurations and four twisted angles were considered. All calculations were performed using density functional theory, including van der Waals interactions as implemented in the SIESTA ab initio package. Our results show that the electronic structure of graphene is preserved, although some small changes are induced by the interaction with the hBN layer, particularly in the total density of states at 1.5 eV under the Fermi level. When layers present a twisted angle, the density of states shows several van Hove singularities under the Fermi level, which are associated to moiré patterns observed in theoretical STM images.
Properties of metals during the heating by intense laser irradiation using ab initio simulations
Holst, Bastian; Recoules, Vanina; Torrent, Marc; Mazevet, Stephane
2011-10-01
Ultrashort laser pulses irradiating a target heat the electrons to very high temperatures. In contrast, the ionic lattice is unaffected on the time scale of the laser pulse since the heat capacity of electrons is much smaller than that of the lattice. This non-equilibrium system can be described as a composition of two subsystems: one consisting of hot electrons and the other of an ionic lattice at low temperature. We studied the effect of this intense electronic excitations on the optical properties of gold using ab initio simulations. We additionally use ab initio linear response to compute the phonon spectrum and the electron-phonon coupling constant within Density Functional Theory for several electronic temperatures of few eV. LULI, Ecole Polytechnique, CNRS, CEA, UPMC, 91128 Palaiseau, France.
Ab-initio molecular dynamics simulation of liquid water by Quantum Monte Carlo
Zen, Andrea; Mazzola, Guglielmo; Guidoni, Leonardo; Sorella, Sandro
2014-01-01
Despite liquid water is ubiquitous in chemical reactions at roots of life and climate on earth, the prediction of its properties by high-level ab initio molecular dynamics simulations still represents a formidable task for quantum chemistry. In this article we present a room temperature simulation of liquid water based on the potential energy surface obtained by a many-body wave function through quantum Monte Carlo (QMC) methods. The simulated properties are in excellent agreement with recent neutron scattering and X-ray experiments, particularly concerning the position of the oxygen-oxygen peak in the radial distribution function, at variance of previous Density Functional Theory attempts. Given the excellent performances of QMC on large scale supercomputers, this work opens new perspectives for predictive and reliable ab-initio simulations of complex chemical systems.
Ab initio molecular dynamics simulation of liquid water by quantum Monte Carlo
Energy Technology Data Exchange (ETDEWEB)
Zen, Andrea, E-mail: a.zen@ucl.ac.uk [Dipartimento di Fisica, “La Sapienza” - Università di Roma, piazzale Aldo Moro 5, 00185 Rome (Italy); London Centre for Nanotechnology, University College London, London WC1E 6BT (United Kingdom); Luo, Ye, E-mail: xw111luoye@gmail.com; Mazzola, Guglielmo, E-mail: gmazzola@phys.ethz.ch; Sorella, Sandro, E-mail: sorella@sissa.it [SISSA–International School for Advanced Studies, Via Bonomea 26, 34136 Trieste (Italy); Democritos Simulation Center CNR–IOM Istituto Officina dei Materiali, 34151 Trieste (Italy); Guidoni, Leonardo, E-mail: leonardo.guidoni@univaq.it [Dipartimento di Fisica, “La Sapienza” - Università di Roma, piazzale Aldo Moro 5, 00185 Rome (Italy); Dipartimento di Scienze Fisiche e Chimiche, Università degli Studi dell’ Aquila, via Vetoio, 67100 L’ Aquila (Italy)
2015-04-14
Although liquid water is ubiquitous in chemical reactions at roots of life and climate on the earth, the prediction of its properties by high-level ab initio molecular dynamics simulations still represents a formidable task for quantum chemistry. In this article, we present a room temperature simulation of liquid water based on the potential energy surface obtained by a many-body wave function through quantum Monte Carlo (QMC) methods. The simulated properties are in good agreement with recent neutron scattering and X-ray experiments, particularly concerning the position of the oxygen-oxygen peak in the radial distribution function, at variance of previous density functional theory attempts. Given the excellent performances of QMC on large scale supercomputers, this work opens new perspectives for predictive and reliable ab initio simulations of complex chemical systems.
{\\it Ab initio} nuclear structure - the large sparse matrix eigenvalue problem
Vary, James P; Ng, Esmond; Yang, Chao; Sosonkina, Masha
2009-01-01
The structure and reactions of light nuclei represent fundamental and formidable challenges for microscopic theory based on realistic strong interaction potentials. Several {\\it ab initio} methods have now emerged that provide nearly exact solutions for some nuclear properties. The {\\it ab initio} no core shell model (NCSM) and the no core full configuration (NCFC) method, frame this quantum many-particle problem as a large sparse matrix eigenvalue problem where one evaluates the Hamiltonian matrix in a basis space consisting of many-fermion Slater determinants and then solves for a set of the lowest eigenvalues and their associated eigenvectors. The resulting eigenvectors are employed to evaluate a set of experimental quantities to test the underlying potential. For fundamental problems of interest, the matrix dimension often exceeds $10^{10}$ and the number of nonzero matrix elements may saturate available storage on present-day leadership class facilities. We survey recent results and advances in solving t...
Ab initio modelling of the behaviour of point defects and fission products in nuclear fuel
International Nuclear Information System (INIS)
The aim of this work is to determine precisely the mechanisms of formation and migration of defects and fission products as well as the associated energies. Examples on uranium dioxide UO2 (standard nuclear fuel) and on uranium carbide UC (potential fuel for new generation reactors) are given. The obtained results are discussed and compared with the experimental results carried out. The ab initio method used is the Projector Augmented-Wave (PAW) method based on the density functional theory. The particular electronic properties of actinides are especially studied because, on account of their 5f orbitals more or less localized around the nucleus, it is difficult to model the actinide compounds by the DFT method. In particular, the modelling of the exchange-correlation interaction of the 5f electrons of UO2 requires approximations (as GGA+U) beyond those more currently used in ab initio calculations (LDA or GGA). (O.M.)
Monte Carlo methods in AB initio quantum chemistry quantum Monte Carlo for molecules
Lester, William A; Reynolds, PJ
1994-01-01
This book presents the basic theory and application of the Monte Carlo method to the electronic structure of atoms and molecules. It assumes no previous knowledge of the subject, only a knowledge of molecular quantum mechanics at the first-year graduate level. A working knowledge of traditional ab initio quantum chemistry is helpful, but not essential.Some distinguishing features of this book are: Clear exposition of the basic theory at a level to facilitate independent study. Discussion of the various versions of the theory: diffusion Monte Carlo, Green's function Monte Carlo, and release n
In pursuit of the ab initio limit for conformational energy prototypes
Császár, Attila G.; Allen, Wesley D.; Schaefer, Henry F.
1998-06-01
The convergence of ab initio predictions to the one- and n-particle limits has been systematically explored for several conformational energy prototypes: the inversion barriers of ammonia, water, and isocyanic acid, the torsional barrier of ethane, the E/Z rotamer separation of formic acid, and the barrier to linearity of silicon dicarbide. Explicit ab initio results were obtained with atomic-orbital basis sets as large as [7s6p5d4f3g2h1i/6s5p4d3f2g1h] and electron correlation treatments as extensive as fifth-order Møller-Plesset perturbation theory (MP5), the full coupled-cluster method through triple excitations (CCSDT), and Brueckner doubles theory including perturbational corrections for both triple and quadruple excitations [BD(TQ)]. Subsequently, basis set and electron correlation extrapolation schemes were invoked to gauge any further variations in arriving at the ab initio limit. Physical effects which are tacitly neglected in most theoretical work have also been quantified by computations of non-Born-Oppenheimer (BODC), relativistic, and core correlation shifts of relative energies. Instructive conclusions are drawn for the pursuit of spectroscopic accuracy in theoretical conformational analyses, and precise predictions for the key energetic quantities of the molecular prototypes are advanced.
Ab-initio study of napthelene based conducting polymer
Energy Technology Data Exchange (ETDEWEB)
Ruhela, Ankur [Advanced Materials Research Group, Computational Nanoscience and Technology Lab (CNTL), ABV-Indian Institute of Information Technology and Management, Gwalior -474010, India and Amity Institute of Nanotechnology, Amity University, Noida-201303 (India); Kanchan, Reena, E-mail: reena.kanchan1977@gmail.com [Department of Chemistry, Jiwaji University, Gwalior-474001 (India); Srivastava, Anurag [Advanced Materials Research Group, Computational Nanoscience and Technology Lab (CNTL), ABV-Indian Institute of Information Technology and Management, Gwalior -474010 (India); Sinha, O. P. [Amity Institute of Nanotechnology, Amity University, Noida-201303 (India)
2014-04-24
In this paper, we have identified structural and electronic properties of conducting polymers by using DFT based ATK-VNL ab-initio tool. Naphthalene derivative structures were stabilized by varying the bond length between two atoms of the molecule C-N and C-C. We have also studied the molecular energy spectrum of naphthalene derivatives and found the HOMOLUMO for the same. A comparison of structural and electronic properties of naphthalene derivatives by attaching the functional group of amine, have been performed and found that they show good semi conducting properties.
Ab-initio study of napthelene based conducting polymer
International Nuclear Information System (INIS)
In this paper, we have identified structural and electronic properties of conducting polymers by using DFT based ATK-VNL ab-initio tool. Naphthalene derivative structures were stabilized by varying the bond length between two atoms of the molecule C-N and C-C. We have also studied the molecular energy spectrum of naphthalene derivatives and found the HOMOLUMO for the same. A comparison of structural and electronic properties of naphthalene derivatives by attaching the functional group of amine, have been performed and found that they show good semi conducting properties
Ab initio study of neutron drops with chiral Hamiltonians
Directory of Open Access Journals (Sweden)
H.D. Potter
2014-12-01
Full Text Available We report ab initio calculations for neutron drops in a 10 MeV external harmonic-oscillator trap using chiral nucleon–nucleon plus three-nucleon interactions. We present total binding energies, internal energies, radii and odd–even energy differences for neutron numbers N=2–18 using the no-core shell model with and without importance truncation. Furthermore, we present total binding energies for N=8,16,20,28,40,50 obtained in a coupled-cluster approach. Comparisons with quantum Monte Carlo results, where available, using Argonne v8′ with three-nucleon interactions reveal important dependences on the chosen Hamiltonian.
Tailoring magnetoresistance at the atomic level: An ab initio study
Tao, Kun
2012-01-05
The possibility of manipulating the tunneling magnetoresistance (TMR) of antiferromagnetic nanostructures is predicted in the framework of ab initio calculations. By the example of a junction composed of an antiferromagnetic dimer and a spin-polarized scanning tunneling microscopy tip we show that the TMR can be tuned and even reversed in sign by lateral and vertical movements of the tip. Moreover, our finite-bias calculations demonstrate that the magnitude and the sign of the TMR can also be tuned by an external voltage. © 2012 American Physical Society.
Ab initio study of neutron drops with chiral Hamiltonians
International Nuclear Information System (INIS)
We report ab initio calculations for neutron drops in a 10 MeV external harmonic-oscillator trap using chiral nucleon–nucleon plus three-nucleon interactions. We present total binding energies, internal energies, radii and odd–even energy differences for neutron numbers N=2–18 using the no-core shell model with and without importance truncation. Furthermore, we present total binding energies for N=8,16,20,28,40,50 obtained in a coupled-cluster approach. Comparisons with quantum Monte Carlo results, where available, using Argonne v8′ with three-nucleon interactions reveal important dependences on the chosen Hamiltonian
Ab initio and kinetic modeling studies of formic acid oxidation
DEFF Research Database (Denmark)
Marshall, Paul; Glarborg, Peter
2015-01-01
A detailed chemical kinetic model for oxidation of formic acid (HOCHO) in flames has been developed, based on theoretical work and data from literature. Ab initio calculations were used to obtain rate coefficients for reactions of HOCHO with H, O, and HO2. Modeling predictions with the mechanism...... on calculations with the kinetic model. Formic acid is consumed mainly by reaction with OH, yielding OCHO, which dissociates rapidly to CO2 + H, and HOCO, which may dissociate to CO + OH or CO2 + H, or react with H, OH, or O2 to form more stable products. The branching fraction of the HOCHO + OH...
Accelerating Ab Initio Nuclear Physics Calculations with GPUs
Potter, Hugh; Maris, Pieter; Sosonkina, Masha; Vary, James; Binder, Sven; Calci, Angelo; Langhammer, Joachim; Roth, Robert; Çatalyürek, Ümit; Saule, Erik
2014-01-01
This paper describes some applications of GPU acceleration in ab initio nuclear structure calculations. Specifically, we discuss GPU acceleration of the software package MFDn, a parallel nuclear structure eigensolver. We modify the matrix construction stage to run partly on the GPU. On the Titan supercomputer at the Oak Ridge Leadership Computing Facility, this produces a speedup of approximately 2.2x - 2.7x for the matrix construction stage and 1.2x - 1.4x for the entire run.
Ab initio structure determination via powder X-ray diffraction
Indian Academy of Sciences (India)
Digamber G Porob; T N Guru Row
2001-10-01
Structure determination by powder X-ray diffraction data has gone through a recent surge since it has become important to get to the structural information of materials which do not yield good quality single crystals. Although the method of structure completion when once the starting model is provided is facile through the Rietveld refinement technique, the structure solution ab initio os still not push-button technology. In this article a survey of the recent development in this area is provided with an illustration of the structure determination of -NaBi3V2O10.
Ab initio studies of magnetism in the iron chalcogenides FeTe and FeSe
International Nuclear Information System (INIS)
The iron chalcogenides FeTe and FeSe belong to the family of iron-based superconductors. We study the magnetism in these compounds in the normal state using the ab initio downfolding scheme developed for strongly correlated electron systems. In deriving ab initio low-energy effective models, we employ the constrained GW method to eliminate the double counting of electron correlations originating from the exchange correlations already taken into account in the density functional theory. By solving the derived ab initio effective models, we reveal that the elimination of the double counting is important in reproducing the bicollinear antiferromagnetic order in FeTe, as is observed in experiments. We also show that the elimination of the double counting induces a unique degeneracy of several magnetic orders in FeSe, which may explain the absence of the magnetic ordering. We discuss the relationship between the degeneracy and the recently found puzzling phenomena in FeSe as well as the magnetic ordering found under pressure. (author)
AB INITIO SIMULATIONS FOR MATERIAL PROPERTIES ALONG THE JUPITER ADIABAT
International Nuclear Information System (INIS)
We determine basic thermodynamic and transport properties of hydrogen-helium-water mixtures for the extreme conditions along Jupiter's adiabat via ab initio simulations, which are compiled in an accurate and consistent data set. In particular, we calculate the electrical and thermal conductivity, the shear and longitudinal viscosity, and diffusion coefficients of the nuclei. We present results for associated quantities like the magnetic and thermal diffusivity and the kinematic shear viscosity along an adiabat that is taken from a state-of-the-art interior structure model. Furthermore, the heat capacities, the thermal expansion coefficient, the isothermal compressibility, the Grüneisen parameter, and the speed of sound are calculated. We find that the onset of dissociation and ionization of hydrogen at about 0.9 Jupiter radii marks a region where the material properties change drastically. In the deep interior, where the electrons are degenerate, many of the material properties remain relatively constant. Our ab initio data will serve as a robust foundation for applications that require accurate knowledge of the material properties in Jupiter's interior, e.g., models for the dynamo generation.
Three-cluster dynamics within an ab initio framework
Quaglioni, S; Navrátil, P
2013-01-01
We introduce a fully antisymmetrized treatment of three-cluster dynamics within the ab initio framework of the no-core shell model/resonating-group method (NCSM/RGM). Energy-independent non-local interactions among the three nuclear fragments are obtained from realistic nucleon-nucleon interactions and consistent ab initio many-body wave functions of the clusters. The three-cluster Schr\\"odinger equation is solved with bound-state boundary conditions by means of the hyperspherical-harmonic method on a Lagrange mesh. We discuss the formalism in detail and give algebraic expressions for systems of two single nucleons plus a nucleus. Using a soft similarity-renormalization-group evolved chiral nucleon-nucleon potential, we apply the method to an $^4$He+$n+n$ description of $^6$He and compare the results to experiment and to a six-body diagonalization of the Hamiltonian performed within the harmonic-oscillator expansions of the NCSM. Differences between the two calculations provide a measure of core ($^4$He) pola...
Ab Initio Protein Structure Prediction Using Pathway Models
Directory of Open Access Journals (Sweden)
Christopher Bystroff
2006-04-01
Full Text Available Ab initio prediction is the challenging attempt to predict protein structures based only on sequence information and without using templates. It is often divided into two distinct sub-problems: (a the scoring function that can distinguish native, or native-like structures, from non-native ones; and (b the method of searching the conformational space. Currently, there is no reliable scoring function that can always drive a search to the native fold, and there is no general search method that can guarantee a significant sampling of near-natives. Pathway models combine the scoring function and the search. In this short review, we explore some of the ways pathway models are used in folding, in published works since 2001, and present a new pathway model, HMMSTR-CM, that uses a fragment library and a set of nucleation/propagation-based rules. The new method was used for ab initio predictions as part of CASP5. This work was presented at the Winter School in Bioinformatics, Bologna, Italy, 10Ã¢Â€Â“14 February 2003.
Ab initio dynamics of the cytochrome P450 hydroxylation reaction
Energy Technology Data Exchange (ETDEWEB)
Elenewski, Justin E.; Hackett, John C, E-mail: jchackett@vcu.edu [Department of Physiology and Biophysics and The Massey Cancer Center, School of Medicine, Virginia Commonwealth University, 401 College Street, Richmond, Virginia 23219-1540 (United States)
2015-02-14
The iron(IV)-oxo porphyrin π-cation radical known as Compound I is the primary oxidant within the cytochromes P450, allowing these enzymes to affect the substrate hydroxylation. In the course of this reaction, a hydrogen atom is abstracted from the substrate to generate hydroxyiron(IV) porphyrin and a substrate-centered radical. The hydroxy radical then rebounds from the iron to the substrate, yielding the hydroxylated product. While Compound I has succumbed to theoretical and spectroscopic characterization, the associated hydroxyiron species is elusive as a consequence of its very short lifetime, for which there are no quantitative estimates. To ascertain the physical mechanism underlying substrate hydroxylation and probe this timescale, ab initio molecular dynamics simulations and free energy calculations are performed for a model of Compound I catalysis. Semiclassical estimates based on these calculations reveal the hydrogen atom abstraction step to be extremely fast, kinetically comparable to enzymes such as carbonic anhydrase. Using an ensemble of ab initio simulations, the resultant hydroxyiron species is found to have a similarly short lifetime, ranging between 300 fs and 3600 fs, putatively depending on the enzyme active site architecture. The addition of tunneling corrections to these rates suggests a strong contribution from nuclear quantum effects, which should accelerate every step of substrate hydroxylation by an order of magnitude. These observations have strong implications for the detection of individual hydroxylation intermediates during P450 catalysis.
Silvestrelli, P.-L.; Alavi, A.; Parrinello, M.; Frenkel, D.
1997-01-01
The method of ab initio molecular dynamics, based on finite-temperature density-functional theory, is used to simulate laser heating of crystalline silicon. We found that a high concentration of excited electrons dramatically weakens the covalent bonding. As a result the system undergoes a melting t
Energy Technology Data Exchange (ETDEWEB)
Ulian, Gianfranco; Valdrè, Giovanni, E-mail: giovanni.valdre@unibo.it [Dipartimento di Scienze Biologiche e Geologico-Ambientali, Centro di Ricerca Interdisciplinare di Biomineralogia, Cristallografia e Biomateriali, Università di Bologna “Alma Mater Studiorum” Piazza di Porta San Donato 1, 40126 Bologna (Italy); Tosoni, Sergio [Departament de Química Física and Institut de Química Teòrica i Computacional (IQTCUB), Universitat de Barcelona, C/ Martí i Franquès 1, E-08028 Barcelona (Spain)
2013-11-28
The quantum chemical characterization of solid state systems is conducted with many different approaches, among which the adoption of periodic boundary conditions to deal with three-dimensional infinite condensed systems. This method, coupled to the Density Functional Theory (DFT), has been proved successful in simulating a huge variety of solids. Only in relatively recent years this ab initio quantum-mechanic approach has been used for the investigation of layer silicate structures and minerals. In the present work, a systematic comparison of different DFT functionals (GGA-PBEsol and hybrid B3LYP) and basis sets (plane waves and all-electron Gaussian-type orbitals) on the geometry, energy, and phonon properties of a model layer silicate, talc [Mg{sub 3}Si{sub 4}O{sub 10}(OH){sub 2}], is presented. Long range dispersion is taken into account by DFT+D method. Results are in agreement with experimental data reported in literature, with minimal deviation given by the GTO/B3LYP-D* method regarding both axial lattice parameters and interaction energy and by PW/PBE-D for the unit-cell volume and angular values. All the considered methods adequately describe the experimental talc infrared spectrum.
Ulian, Gianfranco; Tosoni, Sergio; Valdrè, Giovanni
2013-11-01
The quantum chemical characterization of solid state systems is conducted with many different approaches, among which the adoption of periodic boundary conditions to deal with three-dimensional infinite condensed systems. This method, coupled to the Density Functional Theory (DFT), has been proved successful in simulating a huge variety of solids. Only in relatively recent years this ab initio quantum-mechanic approach has been used for the investigation of layer silicate structures and minerals. In the present work, a systematic comparison of different DFT functionals (GGA-PBEsol and hybrid B3LYP) and basis sets (plane waves and all-electron Gaussian-type orbitals) on the geometry, energy, and phonon properties of a model layer silicate, talc [Mg3Si4O10(OH)2], is presented. Long range dispersion is taken into account by DFT+D method. Results are in agreement with experimental data reported in literature, with minimal deviation given by the GTO/B3LYP-D* method regarding both axial lattice parameters and interaction energy and by PW/PBE-D for the unit-cell volume and angular values. All the considered methods adequately describe the experimental talc infrared spectrum.
International Nuclear Information System (INIS)
The quantum chemical characterization of solid state systems is conducted with many different approaches, among which the adoption of periodic boundary conditions to deal with three-dimensional infinite condensed systems. This method, coupled to the Density Functional Theory (DFT), has been proved successful in simulating a huge variety of solids. Only in relatively recent years this ab initio quantum-mechanic approach has been used for the investigation of layer silicate structures and minerals. In the present work, a systematic comparison of different DFT functionals (GGA-PBEsol and hybrid B3LYP) and basis sets (plane waves and all-electron Gaussian-type orbitals) on the geometry, energy, and phonon properties of a model layer silicate, talc [Mg3Si4O10(OH)2], is presented. Long range dispersion is taken into account by DFT+D method. Results are in agreement with experimental data reported in literature, with minimal deviation given by the GTO/B3LYP-D* method regarding both axial lattice parameters and interaction energy and by PW/PBE-D for the unit-cell volume and angular values. All the considered methods adequately describe the experimental talc infrared spectrum
Energy Technology Data Exchange (ETDEWEB)
Sandoghchi, M. [Advanced Materials Laboratory (AML), Department of Physics, Sharif University of Technology, P.O. Box 11365-9161, Tehran (Iran, Islamic Republic of); Khosroabadi, H., E-mail: khosroabadi@sharif.edu [Advanced Materials Laboratory (AML), Department of Physics, Sharif University of Technology, P.O. Box 11365-9161, Tehran (Iran, Islamic Republic of); Magnet Research Laboratory (MRL), Department of Physics, Sharif University of Technology, P.O. Box 11365-916, Tehran (Iran, Islamic Republic of); Akhavan, M. [Magnet Research Laboratory (MRL), Department of Physics, Sharif University of Technology, P.O. Box 11365-916, Tehran (Iran, Islamic Republic of)
2014-12-15
Highlights: • The lattice parameters changes by K doping is explained by ionic charge and the metal–metal bonding scenario. • Creation of a larger peak near the Fermi level and increase the DOS at the Fermi level by K doping. • The effects of K on DOS has been related to the magnetic transition, change of the lattice parameters and FeAs{sub 4} tetrahedron. • Investigation of the number of Fermi level crossed bands by K doping and the band changes near the Fermi level. - Abstract: The crystal and electronic structures of normal phase of Ba{sub 1−x}K{sub x}Fe{sub 2}As{sub 2} for x = 0.0, 0.5 and 1.0 have been studied by using pseudopotential Quantum Espresso code based on ab-initio density functional theory. Effects of K doping on the crystal structure and lattice parameters have been calculated and compared with the experimental and computational reported data for similar compounds. The metal–metal bonding scenario was used to explain the changes of lattice parameters by K doping. The electronic structure of this system including of density of states and band structure have been calculated and investigated by K doping. One of the interesting results is that a larger peak is appeared near the Fermi level by increasing of the K doping. These changes could produce a potential for creation of the superconducting state in this system.
Ab Initio Studies of Shock-Induced Chemical Reactions of Inter-Metallics
Zaharieva, Roussislava; Hanagud, Sathya
2009-06-01
Shock-induced and shock assisted chemical reactions of intermetallic mixtures are studied by many researchers, using both experimental and theoretical techniques. The theoretical studies are primarily at continuum scales. The model frameworks include mixture theories and meso-scale models of grains of porous mixtures. The reaction models vary from equilibrium thermodynamic model to several non-equilibrium thermodynamic models. The shock-effects are primarily studied using appropriate conservation equations and numerical techniques to integrate the equations. All these models require material constants from experiments and estimates of transition states. Thus, the objective of this paper is to present studies based on ab initio techniques. The ab inito studies, to date, use ab inito molecular dynamics. This paper presents a study that uses shock pressures, and associated temperatures as starting variables. Then intermetallic mixtures are modeled as slabs. The required shock stresses are created by straining the lattice. Then, ab initio binding energy calculations are used to examine the stability of the reactions. Binding energies are obtained for different strain components super imposed on uniform compression and finite temperatures. Then, vibrational frequencies and nudge elastic band techniques are used to study reactivity and transition states. Examples include Ni and Al.
Accelerating Ab Initio Path Integral Simulations via Imaginary Multiple-Timestepping.
Cheng, Xiaolu; Herr, Jonathan D; Steele, Ryan P
2016-04-12
This work investigates the use of multiple-timestep schemes in imaginary time for computationally efficient ab initio equilibrium path integral simulations of quantum molecular motion. In the simplest formulation, only every n(th) path integral replica is computed at the target level of electronic structure theory, whereas the remaining low-level replicas still account for nuclear motion quantum effects with a more computationally economical theory. Motivated by recent developments for multiple-timestep techniques in real-time classical molecular dynamics, both 1-electron (atomic-orbital basis set) and 2-electron (electron correlation) truncations are shown to be effective. Structural distributions and thermodynamic averages are tested for representative analytic potentials and ab initio molecular examples. Target quantum chemistry methods include density functional theory and second-order Møller-Plesset perturbation theory, although any level of theory is formally amenable to this framework. For a standard two-level splitting, computational speedups of 1.6-4.0x are observed when using a 4-fold reduction in time slices; an 8-fold reduction is feasible in some cases. Multitiered options further reduce computational requirements and suggest that quantum mechanical motion could potentially be obtained at a cost not significantly different from the cost of classical simulations. PMID:26966920
Chen, J; Ren, X.; Li, X Z; Alfè, D.; Wang, E
2014-01-01
The finite-temperature phase diagram of hydrogen in the region of phase IV and its neighborhood was studied using the ab initio molecular dynamics (MD) and the ab initio path-integral molecular dynamics (PIMD). The electronic structures were analyzed using the density-functional theory (DFT), the random-phase approximation, and the diffusion Monte Carlo (DMC) methods. Taking the state-of-the-art DMC results as benchmark, comparisons of the energy differences between structures generated from ...
Ab initio quantum dynamics using coupled-cluster
Kvaal, Simen
2012-01-01
The curse of dimensionality (COD) limits the current state-of-the-art {\\it ab initio} propagation methods for non-relativistic quantum mechanics to relatively few particles. For stationary structure calculations, the coupled-cluster (CC) method overcomes the COD in the sense that the method scales polynomially with the number of particles while still being size-consistent and extensive. We generalize the CC method to the time domain while allowing the single-particle functions to vary in an adaptive fashion as well, thereby creating a highly flexible, polynomially scaling approximation to the time-dependent Schr\\"odinger equation. The method inherits size-consistency and extensivity from the CC method. The method is dubbed orbital-adaptive time-dependent coupled-cluster (OATDCC), and is a hierarchy of approximations to the now standard multi-configurational time-dependent Hartree method for fermions. A numerical experiment is also given.
Transport coefficients in diamond from ab-initio calculations
Löfâs, Henrik; Grigoriev, Anton; Isberg, Jan; Ahuja, Rajeev
2013-03-01
By combining the Boltzmann transport equation with ab-initio electronic structure calculations, we obtain transport coefficients for boron-doped diamond. We find the temperature dependence of the resistivity and the hall coefficients in good agreement with experimental measurements. Doping in the samples is treated via the rigid band approximation and scattering is treated in the relaxation time approximation. In contrast to previous results, the acoustic phonon scattering is the dominating scattering mechanism for the considered doping range. At room temperature, we find the thermopower, S, in the range 1-1.6 mV/K and the power factor, S2σ, in the range 0.004-0.16 μW /cm K2.
Ab initio methods for electron-molecule collisions
International Nuclear Information System (INIS)
This review concentrates on the recent advances in treating the electronic aspect of the electron-molecule interaction and leaves to other articles the description of the rotational and vibrational motions. Those methods which give the most complete treatment of the direct, exchange, and correlation effects are focused on. Such full treatments are generally necessary at energies below a few Rydbergs (≅ 60 eV). This choice unfortunately necessitates omission of those active and vital areas devoted to the development of model potentials and approximate scattering formulations. The ab initio and model approaches complement each other and are both extremely important to the full explication of the electron-scattering process. Due to the rapid developments of recent years, the approaches that provide the fullest treatment are concentrated on. 81 refs
Relativistic ab initio calculations for ion-atom collisions
International Nuclear Information System (INIS)
Within the independent particle model we solve the time---dependent single-particle equation using ab initio SCF-DIRAC-FOCK-SLATER wavefunctions as a basis. To reinstate the many-particle aspect of the collision system we use the inclusive probability formalism to answer experimental questions. As an example we show an application to the case of S15+ on Ar where experimental data on the K-K charge transfer are available for a wide range of impact energies from 4.7 to 90 MeV. Our molecular adiabatic calculations and the evaluation using the inclusive probability formalism show good results in the low energy range from 4.7 to 16 MeV impact energy
Ab Initio Calculations of Co Shielding in Model Complexes
Directory of Open Access Journals (Sweden)
Elaine A. Moore
2002-08-01
Full Text Available Abstract: Recent ab initio calculations of cobalt NMR shielding show that DFT-GIAO calculations using hybrid functionals are found to reproduce experimental values well. This method is used to calculate the variation of the cobalt NMR shielding tensor of sqaure pyramidal nitrosyl complexes with respect to the CoNO geometry and to differing basal ligands. The isotropic shielding is shown to have a large negative derivative with respect to CoX distance where X is a ligating atom.; the derivative with respect to NO distance is smaller but still significant. The zz component where z is along the CoN(NO bond is more sensitive to the basal ligands but the other two principal components are sensitive to the CoNO geometry.
Ab initio H2O in realistic hydrophilic confinement.
Allolio, Christoph; Klameth, Felix; Vogel, Michael; Sebastiani, Daniel
2014-12-15
A protocol for the ab initio construction of a realistic cylindrical pore in amorphous silica, serving as a geometric nanoscale confinement for liquids and solutions, is presented. Upon filling the pore with liquid water at different densities, the structure and dynamics of the liquid inside the confinement can be characterized. At high density, the pore introduces long-range oscillations into the water density profile, which makes the water structure unlike that of the bulk across the entire pore. The tetrahedral structure of water is also affected up to the second solvation shell of the pore wall. Furthermore, the effects of the confinement on hydrogen bonding and diffusion, resulting in a weakening and distortion of the water structure at the pore walls and a slowdown in diffusion, are characterized. PMID:25208765
Ab initio design of laser pulse for controlling photochemical reactions
International Nuclear Information System (INIS)
With high level ab initio description of molecule-field interaction, we have developed an optimal control algorithm for manipulating molecular transformation and quantum populations. High order molecule-field interactions are fully taken into account through the use of electric-nuclear Born-Oppenheimer (ENBO) approximation. The present algorithm is demonstrated on the control of molecular post-pulse (transient)alignment and orientation. High degrees of alignment and orientation are achieved in a vibrationally selective manner by optimized infrared laser pulses of duration on the order one rotational period of molecule. To reveal the control mechanism behind the complicated optimal pulses, an analytical pulse design method is developed within the ENBO approximation, which is based on a two-state treatment of the dynamics in a Floquet picture. This analytical method is also illustrated on the control of the alignment of homonuclear diatomics. (author)
Highly anisotropic thermal conductivity of arsenene: An ab initio study
Zeraati, Majid; Vaez Allaei, S. Mehdi; Abdolhosseini Sarsari, I.; Pourfath, Mahdi; Donadio, Davide
2016-02-01
Elemental two-dimensional (2D) materials exhibit intriguing heat transport and phononic properties. Here we have investigated the lattice thermal conductivity of newly proposed arsenene, the 2D honeycomb structure of arsenic, using ab initio calculations. Solving the Boltzmann transport equation for phonons, we predict a highly anisotropic thermal conductivity of 30.4 and 7.8 W/mK along the zigzag and armchair directions, respectively, at room temperature. Our calculations reveal that phonons with mean free paths between 20 nm and 1 μ m provide the main contribution to the large thermal conductivity in the zigzag direction; mean free paths of phonons contributing to heat transport in the armchair directions range between 20 and 100 nm. The obtained anisotropic thermal conductivity and feasibility of synthesis, in addition to high electron mobility reported elsewhere, make arsenene a promising material for nanoelectronic applications and thermal management.
Efficient Ab initio Modeling of Random Multicomponent Alloys
Jiang, Chao; Uberuaga, Blas P.
2016-03-01
We present in this Letter a novel small set of ordered structures (SSOS) method that allows extremely efficient ab initio modeling of random multicomponent alloys. Using inverse II-III spinel oxides and equiatomic quinary bcc (so-called high entropy) alloys as examples, we demonstrate that a SSOS can achieve the same accuracy as a large supercell or a well-converged cluster expansion, but with significantly reduced computational cost. In particular, because of this efficiency, a large number of quinary alloy compositions can be quickly screened, leading to the identification of several new possible high-entropy alloy chemistries. The SSOS method developed here can be broadly useful for the rapid computational design of multicomponent materials, especially those with a large number of alloying elements, a challenging problem for other approaches.
Molecular ion LiHe+: ab initio study
International Nuclear Information System (INIS)
Highlights: ► Excited electronic states of LiHe+ are studied. ► Potential energy curves of thirteen states are calculated. ► Dipole moment and transition dipole moment functions are determined. ► Basic spectroscopic properties of the electronic states are derived. - Abstract: High level ab initio calculations are performed on the molecular ion LiHe+. Potential energy curves for the low-lying singlet and triplet electronic states are calculated using the multi-reference configuration interaction and single-reference coupled cluster methods with large basis sets. The corresponding dipole moments and transition dipole moments functions are also determined. The basic spectroscopic properties and excitation energies of the electronic states are derived from rovibrational bound state calculations.
Ab initio quantum dynamics using coupled-cluster.
Kvaal, Simen
2012-05-21
The curse of dimensionality (COD) limits the current state-of-the-art ab initio propagation methods for non-relativistic quantum mechanics to relatively few particles. For stationary structure calculations, the coupled-cluster (CC) method overcomes the COD in the sense that the method scales polynomially with the number of particles while still being size-consistent and extensive. We generalize the CC method to the time domain while allowing the single-particle functions to vary in an adaptive fashion as well, thereby creating a highly flexible, polynomially scaling approximation to the time-dependent Schrödinger equation. The method inherits size-consistency and extensivity from the CC method. The method is dubbed orbital-adaptive time-dependent coupled-cluster, and is a hierarchy of approximations to the now standard multi-configurational time-dependent Hartree method for fermions. A numerical experiment is also given. PMID:22612082
A Review on Ab Initio Approaches for Multielectron Dynamics
Ishikawa, Kenichi L
2015-01-01
In parallel with the evolution of femtosecond and attosecond laser as well as free-electron laser technology, a variety of theoretical methods have been developed to describe the behavior of atoms, molecules, clusters, and solids under the action of those laser pulses. Here we review major ab initio wave-function-based numerical approaches to simulate multielectron dynamics in atoms and molecules driven by intense long-wavelength and/or ultrashort short-wavelength laser pulses. Direct solution of the time-dependent Schr\\"odinger equation (TDSE), though its applicability is limited to He, ${\\rm H}_2$, and Li, can provide an exact description and has been greatly contributing to the understanding of dynamical electron-electron correlation. Multiconfiguration self-consistent-field (MCSCF) approach offers a flexible framework from which a variety of methods can be derived to treat both atoms and molecules, with possibility to systematically control the accuracy. The equations of motion of configuration interactio...
Ab initio engineering of materials with stacked hexagonal tin frameworks
Shao, Junping; Beaufils, Clément; Kolmogorov, Aleksey N.
2016-01-01
The group-IV tin has been hypothesized to possess intriguing electronic properties in an atom-thick hexagonal form. An attractive pathway of producing sizable 2D crystallites of tin is based on deintercalation of bulk compounds with suitable tin frameworks. Here, we have identified a new synthesizable metal distannide, NaSn2, with a 3D stacking of flat hexagonal layers and examined a known compound, BaSn2, with buckled hexagonal layers. Our ab initio results illustrate that despite being an exception to the 8-electron rule, NaSn2 should form under pressures easily achievable in multi-anvil cells and remain (meta)stable under ambient conditions. Based on calculated Z2 invariants, the predicted NaSn2 may display topologically non-trivial behavior and the known BaSn2 could be a strong topological insulator. PMID:27387140
Ab initio Potential Energy Surface for H-H2
Partridge, Harry; Bauschlicher, Charles W., Jr.; Stallcop, James R.; Levin, Eugene
1993-01-01
Ab initio calculations employing large basis sets are performed to determine an accurate potential energy surface for H-H2 interactions for a broad range of separation distances. At large distances, the spherically averaged potential determined from the calculated energies agrees well with the corresponding results determined from dispersion coefficients; the van der Waals well depth is predicted to be 75 +/- (mu)E(sub h). Large basis sets have also been applied to reexamine the accuracy of theoretical repulsive potential energy surfaces. Multipolar expansions of the computed H-H2 potential energy surface are reported for four internuclear separation distances (1.2, 1.401, 1.449, and 1.7a(sub 0) of the hydrogen molecule. The differential elastic scattering cross section calculated from the present results is compared with the measurements from a crossed beam experiment.
Energy Technology Data Exchange (ETDEWEB)
Liu, Hanchao; Wang, Yimin; Bowman, Joel M. [Cherry L. Emerson Center for Scientific Computation and Department of Chemistry, Emory University, Atlanta, Georgia 30322 (United States)
2015-05-21
The calculation and characterization of the IR spectrum of liquid water have remained a challenge for theory. In this paper, we address this challenge using a combination of ab initio approaches, namely, a quantum treatment of IR spectrum using the ab initio WHBB water potential energy surface and a refined ab initio dipole moment surface. The quantum treatment is based on the embedded local monomer method, in which the three intramolecular modes of each embedded H{sub 2}O monomer are fully coupled and also coupled singly to each of six intermolecular modes. The new dipole moment surface consists of a previous spectroscopically accurate 1-body dipole moment surface and a newly fitted ab initio intrinsic 2-body dipole moment. A detailed analysis of the new dipole moment surface in terms of the coordinate dependence of the effective atomic charges is done along with tests of it for the water dimer and prism hexamer double-harmonic spectra against direct ab initio calculations. The liquid configurations are taken from previous molecular dynamics calculations of Skinner and co-workers, using the TIP4P plus E3B rigid monomer water potential. The IR spectrum of water at 300 K in the range of 0–4000 cm{sup −1} is calculated and compared with experiment, using the ab initio WHBB potential and new ab initio dipole moment, the q-TIP4P/F potential, which has a fixed-charged description of the dipole moment, and the TTM3-F potential and dipole moment surfaces. The newly calculated ab initio spectrum is in very good agreement with experiment throughout the above spectral range, both in band positions and intensities. This contrasts to results with the other potentials and dipole moments, especially the fixed-charge q-TIP4P/F model, which gives unrealistic intensities. The calculated ab initio spectrum is analyzed by examining the contribution of various transitions to each band.
Precise Ab-initio prediction of terahertz vibrational modes in crystalline systems
DEFF Research Database (Denmark)
Jepsen, Peter Uhd; Clark, Stewart J.
2007-01-01
We use a combination of experimental THz time-domain spectroscopy and ab-initio density functional perturbative theory to accurately predict the terahertz vibrational spectrum of molecules in the crystalline phase. Our calculations show that distinct vibrational modes found in solid-state materials...... are best described as phonon modes with strong coupling to the intramolecular degrees of freedom. Hence a computational method taking the periodicity of the crystal lattice as well as intramolecular motion into account is a prerequisite for the correct prediction of vibrational modes in such materials....
Pham, Thi Nu; Ono, Shota; Ohno, Kaoru
2016-04-01
Doing ab initio molecular dynamics simulations, we demonstrate a possibility of hydrogenation of carbon monoxide producing methanol step by step. At first, the hydrogen atom reacts with the carbon monoxide molecule at the excited state forming the formyl radical. Formaldehyde was formed after adding one more hydrogen atom to the system. Finally, absorption of two hydrogen atoms to formaldehyde produces methanol molecule. This study is performed by using the all-electron mixed basis approach based on the time dependent density functional theory within the adiabatic local density approximation for an electronic ground-state configuration and the one-shot GW approximation for an electronic excited state configuration.
Structure and lattice dynamics of PrFe3(BO3)4: Ab initio calculation
Chernyshev, V. A.; Nikiforov, A. E.; Petrov, V. P.
2016-06-01
The crystal structure and phonon spectrum of PrFe3(BO3)4 are ab initio calculated in the context of the density functional theory. The ion coordinates in the unit cell of a crystal and the lattice parameters are evaluated from the calculations. The types and frequencies of the fundamental vibrations, as well as the line intensities of the IR spectrum, are determined. The elastic constants of the crystal are calculated. A "seed" frequency of the vibration strongly interacting with the electron excitation on the praseodymium ion is obtained for low-frequency A 2 mode. The calculated results are in agreement with the known experimental data.
Ab initio adiabatic and quasidiabatic potential energy surfaces of H++ CN system
Indian Academy of Sciences (India)
Bhargava Anusuri; Sanjay Kumar
2016-02-01
We present restricted geometry (collinear and perpendicular approaches of proton) ab initio three dimensional potential energy surfaces for H++ CN system. The calculations were performed at the internally contracted multi-reference configuration interaction level of theory using Dunning’s correlation consistent polarized valence triple zeta basis set. Adiabatic and quasidiabatic surfaces have been computed for the ground and the first excited electronic states. Nonadiabatic effects arising from radial coupling have been analyzed in terms of nonadiabatic coupling matrix elements and coupling potentials.
Site occupancy trend of Co in Ni2MnIn: Ab initio approach
International Nuclear Information System (INIS)
The trend of site occupation of Co at Ni sites of Ni2MnIn system is studied in austenitic phase having L21 structure by ab initio density functional theory (DFT) calculation. The Co atoms prefer to be at Ni sites rather than Mn site and are ferromagetically coupled with Ni and Mn. The ground state has tetragonal structure for Ni1.5Co0.5MnIn and Ni1.25Co0.75MnIn. The Co tends to form cluster
Phonon spectrum of lead oxychloride Pb3O2Cl2: Ab initio calculation and experiment
Zakir'yanov, D. O.; Chernyshev, V. A.; Zakir'yanova, I. D.
2016-02-01
IR and Raman spectra of Pb3O2Cl2 in the range of 50-600 cm-1 have been detected for the first time. Ab initio calculations of the crystal structure and the phonon spectrum of Pb3O2Cl2 in the framework of LCAO approach have been performed by the Hartree-Fock method and in the framework of the density functional theory with the use of hybrid functionals. The results of calculations have made it possible to interpret the experimental vibration spectra and reveal silent modes, which do not manifest themselves in these spectra but influence the optical properties of the crystal.
Ab initio study of the EFG at the N sites in imidazole
International Nuclear Information System (INIS)
We study the nuclear quadrupole interaction at the nitrogen sites in the molecular and crystalline phases of the imidazole compound. We use PAW which is a state-of-the-art method to calculate the electronic structure and electric field gradient at the nucleus in the framework of the density functional theory. The quadrupole frequencies at both imino and amino N sites are in excellent agreement with measurements. This is the first time that the electric field gradient at crystalline imidazole is correctly treated by an ab initio theoretical approach.
Experimental and ab initio DFT calculated Raman Spectrum of Sudan I, a Red Dye
DEFF Research Database (Denmark)
Kunov-Kruse, Andreas Jonas; Kristensen, Steffen Buus; Liu, Chuan;
2011-01-01
. Comparison was made with other azo dyes in the literature on natural, abundant plant pigments. The results show that there is a possibility in foodstuff analysis to distinguish Sudan I from other dyes by using Raman spectroscopy with more than one laser wavelength for resonance enhancement of the different......The red dye Sudan I was investigated by Raman spectroscopy using different excitation wavelengths (1064, 532 and 244 nm). A calculation of the Raman spectrum based on quantum mechanical ab initio density functional theory (DFT) was made using the RB3LYP method with the 3-21G and 6-311+G(d,p) basis...
Ab-initio GMR and current-induced torques in Au/Cr multilayers
Haney, P. M.; Waldron, D; Duine, R. A.; Nunez, A. S.; Guo, H; MacDonald, A.H.
2006-01-01
We report on an {\\em ab-initio} study of giant magnetoresistance (GMR) and current-induced-torques (CITs) in Cr/Au multilayers that is based on non-equilibrium Green's functions and spin density functional theory. We find substantial GMR due primarily to a spin-dependent resonance centered at the Cr/Au interface and predict that the CITs are strong enough to switch the antiferromagnetic order parameter at current-densities $\\sim 100$ times smaller than typical ferromagnetic metal circuit swit...
Sahoo, B K
2006-01-01
We present a general approach within the relativistic coupled-cluster theory framework to calculate exactly the first order wave functions due to any rank perturbation operators. Using this method, we calculate the static dipole and quadrupole polarizabilities in some alkali atoms and alkaline earth-metal ions. This may be a good test of the present theory for different rank and parity interaction operators. This shows a wide range of applications including precise calculations of both parity and CP violating amplitudes due to rank zero and rank one weak interaction Hamiltonians. We also give contributions from correlation effects and discuss them in terms of lower order many-body perturbation theory.
Emergence of rotational bands in ab initio no-core configuration interaction calculations
Caprio, M A; Vary, J P; Smith, R
2015-01-01
Rotational bands have been observed to emerge in ab initio no-core configuration interaction (NCCI) calculations for p-shell nuclei, as evidenced by rotational patterns for excitation energies, electromagnetic moments, and electromagnetic transitions. We investigate the ab initio emergence of nuclear rotation in the Be isotopes, focusing on 9Be for illustration, and make use of basis extrapolation methods to obtain ab initio predictions of rotational band parameters for comparison with experiment. We find robust signatures for rotational motion, which reproduce both qualitative and quantitative features of the experimentally observed bands.
Ab initio theory of native defects in alloys: application to charged N vacancies in AlxGa1-xN
International Nuclear Information System (INIS)
We present calculations of the electronic and atomic structures of neutral and charged nitrogen vacancies in AlxGa1-xN alloys using a combination of first-principles methods. The treatment of the alloys is based on the generalized quasichemical approach to disorder and composition effects and a cluster expansion to account for the various configurations. The point defects are modelled by supercells which are multiples of the alloy clusters. The total-energy and electronic-structure calculations are performed within the density functional theory and the local spin density approximation. We study the atomic structure, the energetics and the charge-dependent vacancy states for the different clusters versus cation numbers and for the alloys versus composition x. (author)
Lan, Tran Nguyen; Zgid, Dominika
2016-01-01
We present a detailed discussion of self-energy embedding theory (SEET) which is a quantum embedding scheme allowing us to describe a chosen subsystem very accurately while keeping the description of the environment at a lower cost. We apply SEET to molecular examples where commonly our chosen subsystem is made out of a set of strongly correlated orbitals while the weakly correlated orbitals constitute an environment. Such a self-energy separation is very general and to make this procedure applicable to multiple systems a detailed and practical procedure for the evaluation of the system and environment self-energy is necessary. We list all the intricacies for one of the possible procedures while focusing our discussion on many practical implementation aspects such as the choice of best orbital basis, impurity solver, and many steps necessary to reach chemical accuracy. Finally, on a set of carefully chosen molecular examples, we demonstrate that SEET which is a controlled, systematically improvable Green's fu...
Isegawa, Miho; Neese, Frank; Pantazis, Dimitrios A
2016-05-10
The calculation of redox potentials involves large energetic terms arising from gas phase ionization energies, thermodynamic contributions, and solvation energies of the reduced and oxidized species. In this work we study the performance of a wide range of wave function and density functional theory methods for the prediction of ionization energies and aqueous one-electron oxidation potentials of a set of 19 organic molecules. Emphasis is placed on evaluating methods that employ the computationally efficient local pair natural orbital (LPNO) approach, as well as several implementations of coupled cluster theory and explicitly correlated F12 methods. The electronic energies are combined with implicit solvation models for the solvation energies. With the exception of MP2 and its variants, which suffer from enormous errors arising at least partially from the poor Hartree-Fock reference, ionization energies can be systematically predicted with average errors below 0.1 eV for most of the correlated wave function based methods studies here, provided basis set extrapolation is performed. LPNO methods are the most efficient way to achieve this type of accuracy. DFT methods show in general larger errors and suffer from inconsistent behavior. The only exception is the M06-2X functional which is found to be competitive with the best LPNO-based approaches for ionization energies. Importantly, the limiting factor for the calculation of accurate redox potentials is the solvation energy. The errors in the predicted solvation energies by all continuum solvation models tested in this work dominate the final computed reduction potential, resulting in average errors typically in excess of 0.3 V and hence obscuring the gains that arise from choosing a more accurate electronic structure method. PMID:27065224
Energy Technology Data Exchange (ETDEWEB)
Arghavani Nia, Borhan, E-mail: b.arghavani@gmail.com [Department of Physics, Kermanshah Branch, Islamic Azad University, Kermanshah (Iran, Islamic Republic of); Sedighi, Matin [Department of Physics, Kermanshah Branch, Islamic Azad University, Kermanshah (Iran, Islamic Republic of); Shahrokhi, Masoud [Young Researchers and Elite Club, Kermanshah Branch, Islamic Azad University, Kermanshah (Iran, Islamic Republic of); Moradian, Rostam [Nano-Science and Nano-Technology Research Center, Razi University, Kermanshah (Iran, Islamic Republic of); Computational Physics Science Research Laboratory, Department of Nano-Science, Institute for Studies in Theoretical Physics and Mathematics (IPM), P.O. Box 19395-1795, Tehran (Iran, Islamic Republic of)
2013-11-15
A density functional theory study of structural, electronical and optical properties of Ca{sub 3}Sb{sub 2} compound in hexagonal and cubic phases is presented. In the exchange–correlation potential, generalized gradient approximation (PBE-GGA) has been used to calculate lattice parameters, bulk modulus, cohesive energy, dielectric function and energy loss spectra. The electronic band structure of this compound has been calculated using the above two approximations as well as another form of PBE-GGA, proposed by Engle and Vosko (EV-GGA). It is found that the hexagonal phase of Ca{sub 3}Sb{sub 2} has an indirect gap in the Γ→N direction; while in the cubic phase there is a direct-gap at the Γ point in the PBE-GGA and EV-GGA. Effects of applying pressure on the band structure of the system studied and optical properties of these systems were calculated. - Graphical abstract: A density functional theory study of structural, electronic and optical properties of Ca{sub 3}Sb{sub 2} compound in hexagonal and cubic phases is presented. Display Omitted - Highlights: • Physical properties of Ca{sub 3}Sb{sub 2} in hexagonal and cubic phases are investigated. • It is found that the hexagonal phase is an indirect gap semiconductor. • Ca{sub 3}Sb{sub 2} is a direct-gap semiconductor at the Γ point in the cubic phase. • By increasing pressure the semiconducting band gap and anti-symmetry gap are decreased.
Ab Initio Study of Hot Carriers in the First Picosecond after Sunlight Absorption in Silicon
Bernardi, Marco; Vigil-Fowler, Derek; Lischner, Johannes; Neaton, Jeffrey B.; Louie, Steven G.
2014-06-01
Hot carrier thermalization is a major source of efficiency loss in solar cells. Because of the subpicosecond time scale and complex physics involved, a microscopic characterization of hot carriers is challenging even for the simplest materials. We develop and apply an ab initio approach based on density functional theory and many-body perturbation theory to investigate hot carriers in semiconductors. Our calculations include electron-electron and electron-phonon interactions, and require no experimental input other than the structure of the material. We apply our approach to study the relaxation time and mean free path of hot carriers in Si, and map the band and k dependence of these quantities. We demonstrate that a hot carrier distribution characteristic of Si under solar illumination thermalizes within 350 fs, in excellent agreement with pump-probe experiments. Our work sheds light on the subpicosecond time scale after sunlight absorption in Si, and constitutes a first step towards ab initio quantification of hot carrier dynamics in materials.
International Nuclear Information System (INIS)
With the final goal to elucidate boron isotope fractionation observed experimentally, molecular orbital calculations were performed on boric acid and borate monomers and dimers. The geometries of B(OH)3 and B(OH)4- were first optimized and their vibrational frequencies were calculated at their optimized structures. The estimated 11B-to-10B isotopic reduced partition function ratios (RPFRs) of B(OH)3 and B(OH)4- and the calculated equilibrium constant of the boron isotope exchange reaction between the two boron species revealed that a more advanced molecular orbital theory with a higher level basis set did not necessarily yield better results. It was concluded that HF/6-31G(d) calculations were most appropriate for the present purpose. The RPFRs of the dimers, H4B2O5, H5B2O6- and H6B2O72-, estimated from the RPFRs of the monomers by the use of additivity of the logarithm of RPFRs agreed with those calculated using the frequencies of the dimers within a margin of 1%. This error corresponded to the error of 5% on ln (RPFR). The equilibrium constant of boron isotope exchange reaction between two boron species among the monomer and dimers at 25degC varied from 1.0207 to 1.0360, indicating the importance of accurate estimation of the RPFRs of polyboric acids and polyborates in real systems. (author)
International Nuclear Information System (INIS)
Analyses of the local crystal and electronic structure in the vicinity of Fe3+ centers in perovskite KMgF3 crystal have been carried out in a comprehensive manner. A combination of density functional theory (DFT) and a semi-empirical superposition model (SPM) is used for a complete analysis of all Fe3+ centers in this study for the first time. Some quantitative information has been derived from the DFT calculations on both the electronic structure and the local geometry around Fe3+ centers. All of the trigonal (K-vacancy case, K-Li substitution case, and normal trigonal Fe3+ center case), FeF5O cluster, and tetragonal (Mg-vacancy and Mg-Li substitution cases) centers have been taken into account based on the previously suggested experimental and theoretical inferences. The collaboration between the experimental data and the results of both DFT and SPM calculations provides us to understand most probable structural model for Fe3+ centers in KMgF3
Energy Technology Data Exchange (ETDEWEB)
Javaid, Saqib [EMMG, Physics Division, PINSTECH, P.O. Nilore, Islamabad (Pakistan); National Centre of Physics, Islamabad (Pakistan); Javed Akhtar, M., E-mail: javedakhtar6@gmail.com [EMMG, Physics Division, PINSTECH, P.O. Nilore, Islamabad (Pakistan)
2015-07-28
We have employed density functional theory to study the C60/ZnPc interface with face-on orientation, which has recently been tailored experimentally. For this purpose, adsorption of ZnPc on C60 has been studied, while taking into account different orientations of C60. Out of various adsorption sites investigated, 6:6 C-C bridge position in apex configuration of C60 has been found energetically the most favourable one with C60-ZnPc adsorption distance of ∼2.77 Å. The adsorption of ZnPc on C60 ensues both charge re-organization and charge transfer at the interface, resulting in the formation of interface dipole. Moreover, by comparing results with that of C60/CuPc interface, we show that the direction of interface dipole can be tuned by the change of the central atom of the phthalocyanine molecule. These results highlight the complexity of electronic interactions present at the C60/Phthalocyanine interface.
International Nuclear Information System (INIS)
We have employed density functional theory to study the C60/ZnPc interface with face-on orientation, which has recently been tailored experimentally. For this purpose, adsorption of ZnPc on C60 has been studied, while taking into account different orientations of C60. Out of various adsorption sites investigated, 6:6 C-C bridge position in apex configuration of C60 has been found energetically the most favourable one with C60-ZnPc adsorption distance of ∼2.77 Å. The adsorption of ZnPc on C60 ensues both charge re-organization and charge transfer at the interface, resulting in the formation of interface dipole. Moreover, by comparing results with that of C60/CuPc interface, we show that the direction of interface dipole can be tuned by the change of the central atom of the phthalocyanine molecule. These results highlight the complexity of electronic interactions present at the C60/Phthalocyanine interface
Hydrogen adsorption on boron doped graphene: an {\\it ab initio} study
Miwa, R. H.; Martins, T B; Fazzio, A.
2007-01-01
The electronic and structural properties of (i) boron doped graphene sheets, and (ii) the chemisorption processes of hydrogen adatoms on the boron doped graphene sheets have been examined by {\\it ab initio} total energy calculations.
Ab Initio Studies of Halogen and Nitrogen Oxide Species of Interest in Stratospheric Chemistry
Lee, Timothy J.; Langhoff, Stephen R. (Technical Monitor)
1995-01-01
The ability of modern state-of-the art ab initio quantum chemical techniques to characterize reliably the gas-phase molecular structure, vibrational spectrum, electronic spectrum, and thermal stability of fluorine, chlorine, bromine and nitrogen oxide species will be demonstrated by presentation of some example studies. The ab initio results are shown to be in excellent agreement with the available experimental data, and where the experimental data are either not known or are inconclusive, the theoretical results are shown to fill in the gaps and to resolve experimental controversies. In addition, ab initio studies in which the electronic spectra and the characterization of excited electronic states of halogen oxide species will also be presented. Again where available, the ab initio results are compared to experimental observations, and are used to aid in the interpretation of experimental studies.
Ab initio evaluations of the He solubility in liquid Li
Energy Technology Data Exchange (ETDEWEB)
Sedano, Luis A. [EURATOM-CIEMAT Assoc., Materials for Fusion Program, Bd. 43 P0.04, Avda. Complutense 22, 28040 Madrid (Spain)]. E-mail: luis.sedano@ciemat.es; Hassanein, Ahmed [Argonne Nat. Lab, 9700 South Class Av., Argonne, IL (United States)]. E-mail: hassanein@anl.gov; Sanz, Javier [ETSII-UNED, c/Juan del Rosal, 12, 28040 Madrid (E) (Spain)]. E-mail: jsanz@ind.UNED.es
2005-11-15
Modified embedding atom methods (MEAM) are developed to have predictions of the partial molar heat of solution (-H{sub s}) by direct simulation of metal cohesion, He-metal and He-He interaction. Transitions from crystalline Li to configurations, having the liquid Li structure's factors (h-bar (q)), are simulated ab initio. Once h-bar (q) reproduced, He atoms are added, one by one, to the Li system. Parallel lines for each case, with slopes clearly independent on the number of He atoms in the system, are obtained for energy versus pressure at given temperatures. Average differences between two adjacent parallels at zero pressure, once kinetic energy of the system discounted, represents the energy gained by an He atom when added to the Li system, related to the solution energy -H{sub s}. The molar excess entropy of gas in solution (S-bar {sub l}{sup ex}) is previously evaluated following diverse fundamental approaches: a 'thermodynamic liquid-hole' (TL-H) model for alkali liquids and a statistical-mechanics (Neff and McQuarrie's) model (SMM). Between 600 and 900 deg. C, a typical range of interest for the use of Li in fusion technology, the computed values for the (He) Henry's constant in Li range from 8x10{sup -14} to 10{sup -13} at. fr. Pa{sup -1}.
Volumic omit maps in ab initio dual-space phasing.
Oszlányi, Gábor; Sütő, András
2016-07-01
Alternating-projection-type dual-space algorithms have a clear construction, but are susceptible to stagnation and, thus, inefficient for solving the phase problem ab initio. To improve this behaviour new omit maps are introduced, which are real-space perturbations applied periodically during the iteration process. The omit maps are called volumic, because they delete some predetermined subvolume of the unit cell without searching for atomic regions or analysing the electron density in any other way. The basic algorithms of positivity, histogram matching and low-density elimination are tested by their solution statistics. It is concluded that, while all these algorithms based on weak constraints are practically useless in their pure forms, appropriate volumic omit maps can transform them to practically useful methods. In addition, the efficiency of the already useful reflector-type charge-flipping algorithm can be further improved. It is important that these results are obtained by using non-sharpened structure factors and without any weighting scheme or reciprocal-space perturbation. The mathematical background of volumic omit maps and their expected applications are also discussed. PMID:27357850
Engineering Room-temperature Superconductors Via ab-initio Calculations
Gulian, Mamikon; Melkonyan, Gurgen; Gulian, Armen
The BCS, or bosonic model of superconductivity, as Little and Ginzburg have first argued, can bring in superconductivity at room temperatures in the case of high-enough frequency of bosonic mode. It was further elucidated by Kirzhnitset al., that the condition for existence of high-temperature superconductivity is closely related to negative values of the real part of the dielectric function at finite values of the reciprocal lattice vectors. In view of these findings, the task is to calculate the dielectric function for real materials. Then the poles of this function will indicate the existence of bosonic excitations which can serve as a "glue" for Cooper pairing, and if the frequency is high enough, and the dielectric matrix is simultaneously negative, this material is a good candidate for very high-Tc superconductivity. Thus, our approach is to elaborate a methodology of ab-initio calculation of the dielectric function of various materials, and then point out appropriate candidates. We used the powerful codes (TDDF with the DP package in conjunction with ABINIT) for computing dielectric responses at finite values of the wave vectors in the reciprocal lattice space. Though our report is concerned with the particular problem of superconductivity, the application range of the data processing methodology is much wider. The ability to compute the dielectric function of existing and still non-existing (though being predicted!) materials will have many more repercussions not only in fundamental sciences but also in technology and industry.
Ab initio simulations of peptide-mineral interactions
Hug, Susanna; Hunter, Graeme K.; Goldberg, Harvey; Karttunen, Mikko
We performed Car-Parrinello Molecular Dynamics (CPMD) simulations of two amino acids, aspartic acid (Asp) and phophoserine (pSer), on a calcium oxalate monohydrate (COM) surface as a model of the interactions of phosphoproteins with biominerals. In our earlier work using in vitro experiments and classical Molecular Dynamics (MD) simulations we have demonstrated the importance of phosphorylation of serine on the interactions of osteopontin (OPN) with COM. We used configurations from our previous classical MD simulations as a starting point for the ab initio simulations. In the case of Asp we found that the α-carboxyl and amine groups form temporary close contacts with the surface. For the dipeptide Asp-pSer the carboxyl groups form permanent close contacts with the surface and the distances of its other functional groups do not vary much. We show how the interaction of carboxyl groups with COM crystal is established and confirm the importance of phosphorylation in mediating the interactions between COM surfaces and OPN.
Ab initio transport across bismuth selenide surface barriers
Narayan, Awadhesh
2014-11-24
© 2014 American Physical Society. We investigate the effect of potential barriers in the form of step edges on the scattering properties of Bi2Se3(111) topological surface states by means of large-scale ab initio transport simulations. Our results demonstrate the suppression of perfect backscattering, while all other scattering processes, which do not entail a complete spin and momentum reversal, are allowed. Furthermore, we find that the spin of the surface state develops an out-of-plane component as it traverses the barrier. Our calculations reveal the existence of quasibound states in the vicinity of the surface barriers, which appear in the form of an enhanced density of states in the energy window corresponding to the topological state. For double barriers we demonstrate the formation of quantum well states. To complement our first-principles results we construct a two-dimensional low-energy effective model and illustrate its shortcomings. Our findings are discussed in the context of a number of recent experimental works.
Ab initio no-core solutions for $^6$Li
Shin, Ik Jae; Maris, Pieter; Vary, James P; Forssén, Christian; Rotureau, Jimmy; Michel, Nicolas
2016-01-01
We solve for properties of $^6$Li in the ab initio No-Core Full Configuration approach and we separately solve for its ground state and $J^{\\pi}=2_{2}^{+}$ resonance with the Gamow Shell Model in the Berggren basis. We employ both the JISP16 and chiral NNLO$_{opt}$ realistic nucleon-nucleon interactions and investigate the ground state energy, excitation energies, point proton root-mean-square radius and a suite of electroweak observables. We also extend and test methods to extrapolate the ground state energy, point proton root-mean-square radius, and electric quadrupole moment. We attain improved estimates of these observables in the No-Core Full Configuration approach by using basis spaces up through N$_{max}$=18 that enable more definitive comparisons with experiment. Using the Density Matrix Renormalization Group approach with the JISP16 interaction, we find that we can significantly improve the convergence of the Gamow Shell Model treatment of the $^6$Li ground state and $J^{\\pi}=2_{2}^{+}$ resonance by ...
Ab initio studies of phoshorene island single electron transistor.
Ray, S J; Venkata Kamalakar, M; Chowdhury, R
2016-05-18
Phosphorene is a newly unveiled two-dimensional crystal with immense potential for nanoelectronic and optoelectronic applications. Its unique electronic structure and two dimensionality also present opportunities for single electron devices. Here we report the behaviour of a single electron transistor (SET) made of a phosphorene island, explored for the first time using ab initio calculations. We find that the band gap and the charging energy decrease monotonically with increasing layer numbers due to weak quantum confinement. When compared to two other novel 2D crystals such as graphene and MoS2, our investigation reveals larger adsorption energies of gas molecules on phosphorene, which indicates better a sensing ability. The calculated charge stability diagrams show distinct changes in the presence of an individual molecule which can be applied to detect the presence of different molecules with sensitivity at a single molecular level. The higher charging energies of the molecules within the SET display operational viability at room temperature, which is promising for possible ultra sensitive detection applications. PMID:27093536
Ab initio studies of phosphorene island single electron transistor
Ray, S. J.; Venkata Kamalakar, M.; Chowdhury, R.
2016-05-01
Phosphorene is a newly unveiled two-dimensional crystal with immense potential for nanoelectronic and optoelectronic applications. Its unique electronic structure and two dimensionality also present opportunities for single electron devices. Here we report the behaviour of a single electron transistor (SET) made of a phosphorene island, explored for the first time using ab initio calculations. We find that the band gap and the charging energy decrease monotonically with increasing layer numbers due to weak quantum confinement. When compared to two other novel 2D crystals such as graphene and MoS2, our investigation reveals larger adsorption energies of gas molecules on phosphorene, which indicates better a sensing ability. The calculated charge stability diagrams show distinct changes in the presence of an individual molecule which can be applied to detect the presence of different molecules with sensitivity at a single molecular level. The higher charging energies of the molecules within the SET display operational viability at room temperature, which is promising for possible ultra sensitive detection applications.
Ab initio study of MoS2 nanotube bundles
Verstraete, Matthieu; Charlier, Jean-Christophe
2003-07-01
Recently, the synthesis of a new phase of MoS2I1/3 stoichiometry was reported [M. Remskar, A. Mrzel, Z. Skraba, A. Jesih, M. Ceh, J. Demšar, P. Stadelmann, F. Lévy, and D. Mihailovic, Science 292, 479 (2001)]. Electron microscope images and diffraction data were interpreted to indicate bundles of sub-nanometer-diameter single-wall MoS2 nanotubes. After experimental characterization, the structure was attributed to an assembly of “armchair” nanotubes with interstitial iodine. Using first-principles total-energy calculations, bundles of MoS2 nanotubes with different topologies and stoichiometries are investigated. All of the systems are strongly metallic. Configurations with “zigzag” structures are found to be more stable energetically than the “armchair” ones, though all of the structures have similar stabilities. After relaxation, there remain several candidates which give a lattice parameter in relative agreement with experiment. Further, spin-polarized calculations indicate that a structure with armchair tubes iodine atoms in their center acquires a very large spontaneous magnetic moment of 12μB, while the other structures are nonmagnetic. Our ab initio calculations show that in most of the other structures, the tubes are very strongly bound together, and that the compounds should be considered as a crystal, rather than as a bundle of tubes in the habitual sense.
Ab initio molecular dynamics calculations of ion hydration free energies
International Nuclear Information System (INIS)
We apply ab initio molecular dynamics (AIMD) methods in conjunction with the thermodynamic integration or 'λ-path' technique to compute the intrinsic hydration free energies of Li+, Cl-, and Ag+ ions. Using the Perdew-Burke-Ernzerhof functional, adapting methods developed for classical force field applications, and with consistent assumptions about surface potential (φ) contributions, we obtain absolute AIMD hydration free energies (ΔGhyd) within a few kcal/mol, or better than 4%, of Tissandier et al.'s [J. Phys. Chem. A 102, 7787 (1998)] experimental values augmented with the SPC/E water model φ predictions. The sums of Li+/Cl- and Ag+/Cl- AIMD ΔGhyd, which are not affected by surface potentials, are within 2.6% and 1.2 % of experimental values, respectively. We also report the free energy changes associated with the transition metal ion redox reaction Ag++Ni+→Ag+Ni2+ in water. The predictions for this reaction suggest that existing estimates of ΔGhyd for unstable radiolysis intermediates such as Ni+ may need to be extensively revised.
Ab initio study of optical excitations in VO2
Coulter, John; Gali, Adam; Manousakis, Efstratios
2014-03-01
Motivated by recent experimental efforts to fabricate p-n junctions from transition metal oxides (TMOs) and a recent theoretical study claiming TMOs to be good absorbers and promising materials for efficient carrier multiplication, we study the optical properties of a prototypical TMO, the insulator M1 phase of vanadium dioxide (VO2), by ab initio methods. We applied the Bethe-Salpeter equations (BSE) to calculate the optical properties, starting from self-consistent GW quasi-particle energy levels and states. In contrast to expectations, the exciton binding energy obtained by BSE is in good agreement with the experiment. We find that the electron-electron interaction is very strong which makes this material promising for efficient carrier multiplication that might lead to an enhanced efficiency in photo-voltaics applications. To illustrate this more quantitatively, we calculated the impact ionization rate within the independent quasiparticle approximation, and find that the rate is significantly higher than silicon in the region of highest solar intensity, due to the strong multiple carrier excitations.
Development of materials science by Ab initio powder diffraction analysis
International Nuclear Information System (INIS)
Crystal structure is most important information to understand properties and behavior of target materials. Technique to analyze unknown crystal structures from powder diffraction data (ab initio powder diffraction analysis) enables us to reveal crystal structures of target materials even we cannot obtain a single crystal. In the present article, three examples are introduced to show the power of this technique in the field of materials sciences. The first example is dehydration/hydration of the pharmaceutically relevant material erythrocycin A. In this example, crystal structures of two anhydrous phases were determined from synchrotron X-ray powder diffraction data and their different dehydration/hydration properties were understood from the crystal structures. In the second example, a crystal structure of a three dimensional metal-organic-framework prepared by a mechanochemical reaction was determined from laboratory X-ray powder diffraction data and the reaction scheme has been revealed. In the third example, a crystal structure of a novel oxide-ion conductor of a new structure family was determined from synchrotron X-ray and neutron powder diffraction data which gave an important information to understand the mechanism of the oxide-ion conduction. (author)
Ab initio design of elastically isotropic TiZrNbMoVx high-entropy alloys
International Nuclear Information System (INIS)
Highlights: • The refractory high-entropy alloys are studied with ab initio theory. • We study the effect of alloying elements on the elastic parameters. • We propose an criterion of elastically isotropic refractory high-entropy alloys. - Abstract: The TiZrVNb and TiZrNbMoVx (x = 0–1.5) high-entropy alloys (HEAs) are single-phase solid solutions having the body centered cubic crystallographic structure. Here we use the ab initio exact muffin-tin orbitals method in combination with the coherent potential approximation to study the equilibrium bulk properties of the above refractory HEAs. We provide a detailed investigation of the effect of alloying elements on the electronic structure and elastic parameters. Our results indicate that vanadium enhances the anisotropy of TiZrNbMoVx. As an application of the present theoretical database, we verify the often quoted correlation between the valence electron concentration (VEC) and the micro-mechanical properties in the case of multi-component alloys. Furthermore, we predict that the present HEAs become elastically isotropic for VEC∼4.72
International Nuclear Information System (INIS)
We have calculated the intermolecular interaction energies of the chloroform dimer in 12 orientations using the second-order Møller-Plesset perturbation theory. Single point energies of important geometries were calibrated by the coupled cluster with single and double and perturbative triple excitation method. Dunning's correlation consistent basis sets up to aug-cc-pVQZ have been employed in extrapolating the interaction energies to the complete basis set limit values. With the ab initio potential data we constructed a 5-site force field model for molecular dynamics simulations. We compared the simulation results with recent experiments and obtained quantitative agreements for the detailed atomwise radial distribution functions. Our results were also consistent with previous results using empirical force fields with polarization effects. Moreover, the calculated diffusion coefficients reproduced the experimental data over a wide range of thermodynamic conditions. To the best of our knowledge, this is the first ab initio force field which is capable of competing with existing empirical force fields for liquid chloroform
Boese, A D; Martin, J M L; Marx, D; Chandra, Amalendu; Martin, Jan M.L.; Marx, Dominik
2003-01-01
The ammonia dimer (NH3)2 has been investigated using high--level ab initio quantum chemistry methods and density functional theory (DFT). The structure and energetics of important isomers is obtained to unprecedented accuracy without resorting to experiment. The global minimum of eclipsed C_s symmetry is characterized by a significantly bent hydrogen bond which deviates from linearity by about 20 degrees. In addition, the so-called cyclic C_{2h} structure is extremely close in energy on an overall flat potential energy surface. It is demonstrated that none of the currently available (GGA, meta--GGA, and hybrid) density functionals satisfactorily describe the structure and relative energies of this nonlinear hydrogen bond. We present a novel density functional, HCTH/407+, designed to describe this sort of hydrogen bond quantitatively on the level of the dimer, contrary to e.g. the widely used BLYP functional. This improved functional is employed in Car-Parrinello ab initio molecular dynamics simulations of liq...
Arjunan, V.; Mohan, S.; Ravindran, P.; Mythili, C. V.
2009-05-01
The Fourier transform infrared (FTIR) and FT-Raman spectra of 7-bromo-5-chloro-8-hydroxyquinoline (BCHQ) have been measured in the range 4000-400 and 4000-100 cm -1, respectively. Complete vibrational assignment and analysis of the fundamental modes of the compound were carried out using the observed FTIR and FT-Raman data. The geometry was optimised without any symmetry constrains using the DFT/B3LYP and HF methods with 6-31G** basis set. The vibrational frequencies which were determined experimentally are compared with those obtained theoretically from ab initio HF and density functional theory (DFT) gradient calculations employing the HF/6-31G** and B3LYP/6-31G** methods for the optimised geometry of the compound. The structural parameters and normal modes of vibration obtained from HF and DFT methods are in good agreement with the experimental data. Normal coordinate analysis was also carried out with ab initio force fields utilising Wilson's FG matrix method.
Ab initio modeling of plasticity in HCP metals: pure zirconium and titanium and effect of oxygen
International Nuclear Information System (INIS)
We performed atomistic simulations to determine screw dislocations properties in pure zirconium and titanium and to explain the hardening effect attributed to oxygen alloying in both hexagonal close-packed transition metals. We used two energetic models: ab initio calculations based on the density functional theory and calculations with an empirical potential. The complete energetic profile of the screw dislocation when gliding in the different slip planes is obtained in pure Zr. Our calculations reveal the existence of a metastable configuration of the screw dislocation partially spread in the first order pyramidal plane. This configuration is responsible for the cross slip of screw dislocations from prismatic planes, the easiest glide planes, to pyramidal or basal planes. This energy profile is affected by oxygen addition. Ab initio calculations reveal two main effects: oxygen enhances pyramidal cross slip by modifying the dislocation core structure, and pins the dislocation in its metastable sessile configuration. The same modeling approach is applied to titanium. In pure Ti, the same configurations of the screw dislocation in Zr are obtained, but with different energy levels. This leads to a different gliding mechanism. The same way as in Zr, oxygen enhances pyramidal glide in Ti by modifying the dislocation core structure. Besides, oxygen atom lowers the energy of the metastable configuration but not enough to pin the dislocation in this sessile configuration. (author)
Emergent properties of nuclei from ab initio coupled-cluster calculations
Hagen, G.; Hjorth-Jensen, M.; Jansen, G. R.; Papenbrock, T.
2016-06-01
Emergent properties such as nuclear saturation and deformation, and the effects on shell structure due to the proximity of the scattering continuum and particle decay channels are fascinating phenomena in atomic nuclei. In recent years, ab initio approaches to nuclei have taken the first steps towards tackling the computational challenge of describing these phenomena from Hamiltonians with microscopic degrees of freedom. This endeavor is now possible due to ideas from effective field theories, novel optimization strategies for nuclear interactions, ab initio methods exhibiting a soft scaling with mass number, and ever-increasing computational power. This paper reviews some of the recent accomplishments. We also present new results. The recently optimized chiral interaction NNLO{}{{sat}} is shown to provide an accurate description of both charge radii and binding energies in selected light- and medium-mass nuclei up to 56Ni. We derive an efficient scheme for including continuum effects in coupled-cluster computations of nuclei based on chiral nucleon–nucleon and three-nucleon forces, and present new results for unbound states in the neutron-rich isotopes of oxygen and calcium. The coupling to the continuum impacts the energies of the {J}π =1/{2}-,3/{2}-,7/{2}-,3/{2}+ states in {}{17,23,25}O, and—contrary to naive shell-model expectations—the level ordering of the {J}π =3/{2}+,5/{2}+,9/{2}+ states in {}{53,55,61}Ca. ).
Ab initio many-body calculations of light nuclei neutron and proton scattering
Quaglioni, Sofia
2008-10-01
One of the greatest challenges of nuclear physics today is the development of a quantitative microscopic theory of low-energy reactions on light nuclei. At the same time, technical progress on the theoretical front is urgent to match the major experimental advances in the study of exotic nuclei at the radioactive beam facilities. We build a new ab initio many-body approachootnotetextS. Quaglioni and P. Navratil, arXiv:0804.1560. capable of describing simultaneously both bound and scattering states in light nuclei, by combining the resonating-group methodootnotetextY. C. Tang et al., Phys. Rep. 47, 167 (1978); K. Langanke and H. Friedrich, Advances in Nuclear Physics, chapter 4., Plenum, New York, 1987. with the ab initio no-core shell model.ootnotetextP. Navratil, J. P. Vary, and B. R. Barrett, Phys. Rev. Lett. 84, 5728 (2000); Phys. Rev. C 62, 054311 (2000).. In this way, we complement a microscopic-cluster technique with the use of realistic interactions, and a microscopic and consistent description of the nucleon clusters, while preserving Pauli principle and translational symmetry. I will present results for neutron and proton scattering on light nuclei, including n- and p-^4He phase shifts, and low-lying states of one-neutron halo p-shell nuclei, obtained using realistic nucleon-nucleon potentials. In particular, I will address the parity inversion of the ^11Be ground state.
Recent progress with large-scale ab initio calculations: the CONQUEST code
Bowler, D. R.; Choudhury, R.; Gillan, M. J.; Miyazaki, T.
While the success of density functional theory (DFT) has led to its use in a wide variety of fields such as physics, chemistry, materials science and biochemistry, it has long been recognised that conventional methods are very inefficient for large complex systems, because the memory requirements scale as N 2 and the cpu requirements as N 3 (where N is the number of atoms). The principles necessary to develop methods with linear scaling of the cpu and memory requirements with system size (O(N ) methods) have been established for more than ten years, but only recently have practical codes showing this scaling for DFT started to appear. We report recent progress in the development of the Conquest code, which performs O(N ) DFT calculations on parallel computers, and has a demonstrated ability to handle systems of over 10000 atoms. The code can be run at different levels of precision, ranging from empirical tight-binding, through ab initio tight-binding, to full ab initio , and techniques for calculating ionic forces in a consistent way at all levels of precision will be presented. Illustrations are given of practical Conquest calculations in the strained Ge/Si(001) system.
Ground state analytical ab initio intermolecular potential for the Cl{sub 2}-water system
Energy Technology Data Exchange (ETDEWEB)
Hormain, Laureline; Monnerville, Maurice, E-mail: maurice.monnerville@univ-lille1.fr; Toubin, Céline; Duflot, Denis; Pouilly, Brigitte; Briquez, Stéphane [Laboratoire de Physique des Lasers Atomes et Molécules, Unité Mixte de Recherche (UMR) 8523, Université Lille I, Bât. P5, 59655 Villeneuve d’Ascq Cedex (France); Bernal-Uruchurtu, Margarita I.; Hernández-Lamoneda, Ramón [Centro de Investigaciones Químicas, Universidad Autónoma del Estado de Morelos, Cuernavaca 62209, México (Mexico)
2015-04-14
The chlorine/water interface is of crucial importance in the context of atmospheric chemistry. Modeling the structure and dynamics at this interface requires an accurate description of the interaction potential energy surfaces. We propose here an analytical intermolecular potential that reproduces the interaction between the Cl{sub 2} molecule and a water molecule. Our functional form is fitted to a set of high level ab initio data using the coupled-cluster single double (triple)/aug-cc-p-VTZ level of electronic structure theory for the Cl{sub 2} − H{sub 2}O complex. The potential fitted to reproduce the three minima structures of 1:1 complex is validated by the comparison of ab initio results of Cl{sub 2} interacting with an increasing number of water molecules. Finally, the model potential is used to study the physisorption of Cl{sub 2} on a perfectly ordered hexagonal ice slab. The calculated adsorption energy, in the range 0.27 eV, shows a good agreement with previous experimental results.
Silvestrelli, P.-L.; Alavi, A; Parrinello, M.; Frenkel, D
1997-01-01
The method of ab initio molecular dynamics, based on finite-temperature density-functional theory, is used to simulate laser heating of crystalline silicon. We found that a high concentration of excited electrons dramatically weakens the covalent bonding. As a result the system undergoes a melting transition to a metallic state. We studied several structural, dynamical, electronic, and bonding properties of this phase of silicon. In contrast to ordinary liquid silicon, this liquid is characte...
DEFF Research Database (Denmark)
Pedersen, Kim Vestergaard; Christensen, Henrik; Shim, Irene;
2004-01-01
Trichlorothioacetyl chloride 1, tetrachlorothiirane 2, and trichloroethenesulfenyl chloride 3 and the equilibria between them have been investigated by ab initio Hartree-Fock (HF), Møller-Plesset second order perturbation (MP2) calculations, and by Gaussian-3 theory, G3(MP2). The transition state...... of the isomerization reactions have been identified. Also investigated were possible reactions leading to the isomers and their possible decomposition products. The results show that the unobserved isomerization reactions are feasible....
Shukla, Alok
1999-01-01
In this paper we present a fully ab initio Hartree-Fock approach aimed at calculating the static structure factor of crystalline insulators at arbitrary values of momentum transfer. In particular, we outline the computation of the incoherent scattering function, the component of the structure factor which governs the incoherent x-ray scattering from solids. The presented theory is applied to crystalline LiF to obtain benchmark Hartree-Fock values for its incoherent scattering function. Benchm...
Palummo, Maurizia; Hogan, Conor; Sottile, Francesco; Bagalá, Paolo; Rubio, Angel
2009-08-28
We present a theoretical investigation of electronic and optical properties of free-base porphyrins based on density functional theory and many-body perturbation theory. The electronic levels of free-base porphine (H(2)P) and its phenyl derivative, free-base tetraphenylporphyrin (H(2)TPP) are calculated using the ab initio GW approximation for the self-energy. The approach is found to yield results that compare favorably with the available photoemission spectra. The excitonic nature of the optical peaks is revealed by solving the Bethe-Salpeter equation, which provides an accurate description of the experimental absorption spectra. The lowest triplet transition energies are in good agreement with the measured values. PMID:19725603
Relaxation of the excited -(2-hydroxy benzylidene) aniline molecule: An ab initio and TD DFT study
Indian Academy of Sciences (India)
Biswajit Chowdhury; Rina De; Pinaky Sett; Joydeep Chowdhury
2010-11-01
The photophysical behaviour of N-(2-hydroxy benzylidene) aniline or most commonly known as salicylideneaniline (SA) has been investigated using the ab initio and DFT levels of theory. The quantum chemical calculations show that the optimized non planar enol (1) form of the SA molecule is the most stable conformer in the ground state and is marked by the twisting of the phenolic and anilino rings of the molecule. The geometry optimizations and the subsequent frequency calculations of the excited singlet electronic states of the various tautomeric forms of SA molecule were performed with the CIS level of theory. A detail theoretical investigation on the relaxation dynamics of the SA molecule has been presented. Possible explanation on the excitation wavelength dependence of the photochromic yield of the molecule is also reported.
Barbosa, Marcelo
A review about the nuclear properties, namely the nuclear moments (magnetic dipole moment and electric quadrupole moment) and their interaction with electromagnetic fields external to the nucleus (hyperfine interactions), as well as the angular distribution of radiation produced by $\\gamma$-decay, is presented. A detailed description about the theory of Perturbed Angular Correlations was done, including the comparison between $\\gamma-\\gamma$- correlations and $e^{-}- \\gamma$ correlations. For dynamic nuclear interactions, an introduction to the theory of stochastic states in PAC was performed. We focused on ab-initio implementation of observables for analyzing fluctuating quadrupole hyperfine interactions on time dependent perturbed angular correlations experiments. The development of computacional codes solving the full problem, adapted to fit data obtained on single crystals or polycrystals for two-state transient fields with any axial symmetry and orientation was the main purpose of this work. The final pa...
Kang, Youngho; Jeon, Sang Ho; Cho, Youngmi; Han, Seungwu
2016-01-01
We investigate the vertical ionization potential (IP) and electron affinity (EA) of organic semiconductors in the solid state that govern the optoelectrical property of organic devices using a fully ab initio way. The present method combines the density functional theory and many-body perturbation theory based on G W approximations. To demonstrate the accuracy of this approach, we carry out calculations on several prototypical organic molecules. Since IP and EA depend on the molecular orientation at the surface, the molecular geometry of the surface is explicitly considered through the slab model. The computed IP and EA are in reasonable and consistent agreements with spectroscopic data on organic surfaces with various molecular arrangements. However, the transport gaps are slightly underestimated in calculations, which can be explained by different screening effects between surface and bulk regions.
An ab initio study of plutonium oxides surfaces
International Nuclear Information System (INIS)
By means of first-principles calculations, we have studied the atomic structure as well as the thermodynamic stability of various plutonium dioxide surfaces in function of their environment (in terms of oxygen partial pressure and temperature). All these simulations have been performed with the ABINIT code. It is well known that DFT fails to describe correctly plutonium-based materials since 5f electrons in such systems are strongly correlated. In order to go beyond DFT, we have treated PuO2 and β-Pu2O3 in a DFT+U framework. We show that the couple of parameters (U,J) that works well for pure Pu is also well designed for describing ground state (GS) properties of these two oxides. The major improvement with respect with DFT is that we are able to predict an insulating GS in agreement with experiments. The presence of a gap in the DOS (Density of States) of plutonium oxides should play a significant role in the predicted surface reactivity. However, performing DFT+U calculations on surfaces of plutonium oxide from scratch was too ambitious. That is why we decided, as a first step, to study the stability of the (100), (110) and (111) surfaces of PuO2 in a DFT-GGA framework. For each of these orientations, we considered various terminations. These ab initio results have been introduced in a thermodynamic model which allows us to predict the relative stability of the different terminations as a function of temperature and oxygen partial pressure (pO2). We conclude that at room temperature and for pO2∼10 atm., the polar O2-(100) termination is favoured. The stabilization of such a polar stoichiometric surface is surprising and should be confirmed by DFT+U calculations before any final conclusion. (authors)
Lithium Insertion In Silicon Nanowires: An ab Initio Study
Zhang, Qianfan
2010-09-08
The ultrahigh specific lithium ion storage capacity of Si nanowires (SiNWs) has been demonstrated recently and has opened up exciting opportunities for energy storage. However, a systematic theoretical study on lithium insertion in SiNWs remains a challenge, and as a result, understanding of the fundamental interaction and microscopic dynamics during lithium insertion is still lacking. This paper focuses on the study of single Li atom insertion into SiNWs with different sizes and axis orientations by using full ab initio calculations. We show that the binding energy of interstitial Li increases as the SiNW diameter grows. The binding energies at different insertion sites, which can be classified as surface, intermediate, and core sites, are quite different. We find that surface sites are energetically the most favorable insertion positions and that intermediate sites are the most unfavorable insertion positions. Compared with the other growth directions, the [110] SiNWs with different diameters always present the highest binding energies on various insertion locations, which indicates that [110] SiNWs are more favorable by Li doping. Furthermore, we study Li diffusion inside SiNWs. The results show that the Li surface diffusion has a much higher chance to occur than the surface to core diffusion, which is consistent with the experimental observation that the Li insertion in SiNWs is layer by layer from surface to inner region. After overcoming a large barrier crossing surface-to-intermediate region, the diffusion toward center has a higher possibility to occur than the inverse process. © 2010 American Chemical Society.
Energy Technology Data Exchange (ETDEWEB)
Freyss, M.; Dorado, B.; Durinck, J. [CEA Cadarache (DEN/DEC/SESC/LLCC), 13 - Saint-Paul-lez-Durance (France). Dept. d' Etudes des Combustibles
2008-07-01
The aim of this work is to determine precisely the mechanisms of formation and migration of defects and fission products as well as the associated energies. Examples on uranium dioxide UO{sub 2} (standard nuclear fuel) and on uranium carbide UC (potential fuel for new generation reactors) are given. The obtained results are discussed and compared with the experimental results carried out. The ab initio method used is the Projector Augmented-Wave (PAW) method based on the density functional theory. The particular electronic properties of actinides are especially studied because, on account of their 5f orbitals more or less localized around the nucleus, it is difficult to model the actinide compounds by the DFT method. In particular, the modelling of the exchange-correlation interaction of the 5f electrons of UO{sub 2} requires approximations (as GGA+U) beyond those more currently used in ab initio calculations (LDA or GGA). (O.M.)
Directory of Open Access Journals (Sweden)
Hirokazu Takaki
2014-01-01
Full Text Available We present an efficient computation technique for ab-initio electron transport calculations based on density functional theory and the nonequilibrium Green’s function formalism for application to heterostructures with two-dimensional (2D interfaces. The computational load for constructing the Green’s functions, which depends not only on the energy but also on the 2D Bloch wave vector along the interfaces and is thus catastrophically heavy, is circumvented by parallel computational techniques with the message passing interface, which divides the calculations of the Green’s functions with respect to energy and wave vectors. To demonstrate the computational efficiency of the present code, we perform ab-initio electron transport calculations of Al(100-Si(100-Al(100 heterostructures, one of the most typical metal-semiconductor-metal systems, and show their transmission spectra, density of states (DOSs, and dependence on the thickness of the Si layers.
Ab initio global optimization of the structures of SinH, n=4-10, using parallel genetic algorithms
International Nuclear Information System (INIS)
The results of ab initio global optimizations of the structures of SinH, n=4-10, atomic clusters using a parallel genetic algorithm are presented. Driving the global search with the parallel implementation of the genetic algorithm are presented and using the density functional theory as implemented in the Carr-Parinello molecular dynamics code to calculate atomic cluster energies and perform the local optimization of their structures, we have been able to demonstrate that it is possible to perform global optimizations of the structure of atomic clusters using ab initio methods. The results show that this approach is able to find many structures that were not previously reported in the literature. Moreover, in most cases the new structures have considerable lower energies than those previously known. The results clearly demonstrate that these calculations are now possible and in spite of their larger computational demands provide more reliable results
An ab initio study of plutonium oxides surfaces; Etude ab initio des surfaces d'oxydes de Pu
Energy Technology Data Exchange (ETDEWEB)
Jomard, G.; Bottin, F.; Amadon, B
2007-07-01
By means of first-principles calculations, we have studied the atomic structure as well as the thermodynamic stability of various plutonium dioxide surfaces in function of their environment (in terms of oxygen partial pressure and temperature). All these simulations have been performed with the ABINIT code. It is well known that DFT fails to describe correctly plutonium-based materials since 5f electrons in such systems are strongly correlated. In order to go beyond DFT, we have treated PuO{sub 2} and {beta}-Pu{sub 2}O{sub 3} in a DFT+U framework. We show that the couple of parameters (U,J) that works well for pure Pu is also well designed for describing ground state (GS) properties of these two oxides. The major improvement with respect with DFT is that we are able to predict an insulating GS in agreement with experiments. The presence of a gap in the DOS (Density of States) of plutonium oxides should play a significant role in the predicted surface reactivity. However, performing DFT+U calculations on surfaces of plutonium oxide from scratch was too ambitious. That is why we decided, as a first step, to study the stability of the (100), (110) and (111) surfaces of PuO{sub 2} in a DFT-GGA framework. For each of these orientations, we considered various terminations. These ab initio results have been introduced in a thermodynamic model which allows us to predict the relative stability of the different terminations as a function of temperature and oxygen partial pressure (p{sub O{sub 2}}). We conclude that at room temperature and for p{sub O{sub 2}}{approx}10 atm., the polar O{sub 2}-(100) termination is favoured. The stabilization of such a polar stoichiometric surface is surprising and should be confirmed by DFT+U calculations before any final conclusion. (authors)
Ab initio intermolecular potential energy surface and thermophysical properties of nitrous oxide
International Nuclear Information System (INIS)
We present an analytical intermolecular potential energy surface (PES) for two rigid nitrous oxide (N2O) molecules derived from high-level quantum-chemical ab initio calculations. Interaction energies for 2018 N2O–N2O configurations were computed utilizing the counterpoise-corrected supermolecular approach at the CCSD(T) level of theory using basis sets up to aug-cc-pVQZ supplemented with bond functions. A site-site potential function with seven sites per N2O molecule was fitted to the pair interaction energies. We validated our PES by computing the second virial coefficient as well as shear viscosity and thermal conductivity in the dilute-gas limit. The values of these properties are substantiated by the best experimental data
Ab initio study of the electronic properties of the planar Ga5N5 cluster
Institute of Scientific and Technical Information of China (English)
Zheng Hao-Ping; Hao Jing-An
2005-01-01
The first-principles, all electron, ab initio calculations have been performed for an the amazing stable planar structure of Ga5N5 cluster based on the density functional theory. Electronic structure, Electron affinity, ionization potential, and binding energy are obtained. No spin magnetic moment is found. The results show that the planar structure of the Ga5N5 cluster is stable. It is found that for the planar structure of Ga5N5 cluster, three nitrogen atoms in the N3 subunit bind together with large electon transfer although no free N3 can exist. This may be important to the stability of the planar structure of the Ga5N5 cluster which has the lowest ground-state energy.
High-pressure physical properties of magnesium silicate post-perovskite from ab initio calculations
Indian Academy of Sciences (India)
Zi-Jiang Liu; Xiao-Wei Sun; Cai-Rong Zhang; Jian-Bu Hu; Ling-Cang Cai; Qi-Feng Chen
2012-08-01
The structure, thermodynamic and elastic properties of magnesium silicate (MgSiO3) post-perovskite at high pressure are investigated with quasi-harmonic Debye model and ab initio method based on the density functional theory (DFT). The calculated structural parameters of MgSiO3 post-perovskite are consistent with the available experimental results and the recent theoretical results. The Debye temperature, heat capacity and thermal expansion coefficient at high pressures and temperatures are predicted using the quasi-harmonic Debye model. The elastic constants are calculated using stress–strain relations. A complete elastic tensor of MgSiO3 post-perovskite is determined in the wide pressure range. The calculated elastic anisotropic factors and directional bulk modulus show that MgSiO3 post-perovskite possesses high elastic anisotropy.
Directory of Open Access Journals (Sweden)
Karl-Heinz Böhm
2014-04-01
Full Text Available We present ab-initio calculations of secondary isotope effects on NMR chemical shieldings. The change of the NMR chemical shift of a certain nucleus that is observed if another nucleus is replaced by a different isotope can be calculated by computing vibrational corrections on the NMR parameters using electronic structure methods. We demonstrate that the accuracy of the computational results is sufficient to even distinguish different conformers. For this purpose, benchmark calculations for fluoro(2-2Hethane in gauche and antiperiplanar conformation are carried out at the HF, MP2 and CCSD(T level of theory using basis sets ranging from double- to quadruple-zeta quality. The methodology is applied to the secondary isotope shifts for 2-fluoronorbornane in order to resolve an ambiguity in the literature on the assignment of endo- and exo-2-fluoronorbornanes with deuterium substituents in endo-3 and exo-3 positions, also yielding insight into mechanistic details of the corresponding synthesis.
Ab initio study of one-dimensional disorder on III-V semiconductor surfaces
Energy Technology Data Exchange (ETDEWEB)
Romanyuk, O. [Institute of Physics, Academy of Sciences of the Czech Republic, Cukrovarnicka 10, 162 00 Prague 6 (Czech Republic); Grosse, F.; Braun, W. [Paul-Drude-Institut fuer Festkoerperelektronik, Hausvogteiplatz 5-7, 10117 Berlin (Germany)
2010-02-15
Atomic disorder on GaSb(001) and GaAs(001) surfaces is studied by ab initio calculations within density functional theory (DFT). Surface energies are computed for GaSb(001) {beta} (4 x 3), {beta} (4 x 3)A{sub 1}, (4 x 6), and GaAs(001) {beta} 2(2 x 4), c (4 x 4) reconstructions. The computed energy differences do not exceed 1 meV / (1 x 1) which is in agreement with observed one-dimensional disorder on GaSb(001) and GaAs(001) surfaces at elevated temperatures for {beta} and {beta} 2 stoichiometries, respectively. Deviations in bond lengths due to disorder with respect to the ordered ground state phases are calculated (copyright 2010 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)
Determination of absolute configuration using ab initio calculation of optical rotation.
Stephens, P J; Devlin, F J; Cheeseman, J R; Frisch, M J; Bortolini, O; Besse, P
2003-01-01
Ab initio Density Functional Theory (DFT) calculations of transparent spectral region, discrete frequency specific rotations were used to assign the absolute configurations (ACs) of: 1, 2H-naphtho[1,8-bc]thiophene 1-oxide; 2, m-F-phenyl glycidic acid methyl ester; 3, o-Br-phenyl glycidic acid methyl ester; 4, p-CH(3)-phenyl glycidic acid methyl ester; 5, 2-(1-hydroxyethyl)-chromen-4-one; and 6, 6-Br-2-(1-hydroxyethyl)-chromen-4-one. The ACs of 5 and 6 were previously determined via X-ray crystallography to be: 5, R(-)/S(+); 6, R(+)/S(-). The ACs obtained using [alpha](D) are the same for both 5 and 6: R(+)/S(-). We conclude that the previously reported AC of 5 is incorrect. PMID:12884375
International Nuclear Information System (INIS)
C and N atoms are the most frequent foreign interstitial atoms (FIAs), and often incorporated into the surface layers of steels to enhance their properties by thermochemical treatments. Al, Si, Ti, V, Cr, Mn, Co, Ni, Cu, Nb and Mo are the most common alloying elements in steels, also can be called foreign substitutional atoms (FSAs). The FIA and FSA interactions play an important role in the diffusion of C and N atoms, and the microstructures and mechanical properties of surface modified layers. Ab initio calculations based on the density functional theory are carried out to investigate FIA interactions with FSA in ferromagnetic bcc iron. The FIA–FSA interactions are analyzed systematically from five aspects, including interaction energies, density of states (DOS), bond populations, electron density difference maps and local magnetic moments
Ab initio intermolecular potential energy surface and thermophysical properties of nitrous oxide
Energy Technology Data Exchange (ETDEWEB)
Crusius, Johann-Philipp, E-mail: johann-philipp.crusius@uni-rostock.de; Hassel, Egon [Lehrstuhl für Technische Thermodynamik, Universität Rostock, 18059 Rostock (Germany); Hellmann, Robert, E-mail: robert.hellmann@uni-rostock.de; Bich, Eckard [Institut für Chemie, Universität Rostock, 18059 Rostock (Germany)
2015-06-28
We present an analytical intermolecular potential energy surface (PES) for two rigid nitrous oxide (N{sub 2}O) molecules derived from high-level quantum-chemical ab initio calculations. Interaction energies for 2018 N{sub 2}O–N{sub 2}O configurations were computed utilizing the counterpoise-corrected supermolecular approach at the CCSD(T) level of theory using basis sets up to aug-cc-pVQZ supplemented with bond functions. A site-site potential function with seven sites per N{sub 2}O molecule was fitted to the pair interaction energies. We validated our PES by computing the second virial coefficient as well as shear viscosity and thermal conductivity in the dilute-gas limit. The values of these properties are substantiated by the best experimental data.
Ab initio intermolecular potential energy surface and thermophysical properties of nitrous oxide.
Crusius, Johann-Philipp; Hellmann, Robert; Hassel, Egon; Bich, Eckard
2015-06-28
We present an analytical intermolecular potential energy surface (PES) for two rigid nitrous oxide (N2O) molecules derived from high-level quantum-chemical ab initio calculations. Interaction energies for 2018 N2O-N2O configurations were computed utilizing the counterpoise-corrected supermolecular approach at the CCSD(T) level of theory using basis sets up to aug-cc-pVQZ supplemented with bond functions. A site-site potential function with seven sites per N2O molecule was fitted to the pair interaction energies. We validated our PES by computing the second virial coefficient as well as shear viscosity and thermal conductivity in the dilute-gas limit. The values of these properties are substantiated by the best experimental data. PMID:26133428
Minimal parameter implicit solvent model for ab initio electronic structure calculations
Dziedzic, Jacek; Skylaris, Chris-Kriton; Mostofi, Arash A; Payne, Mike C
2011-01-01
We present an implicit solvent model for ab initio electronic structure calculations which is fully self-consistent and is based on direct solution of the nonhomogeneous Poisson equation. The solute cavity is naturally defined in terms of an isosurface of the electronic density according to the formula of Fattebert and Gygi (J. Comp. Chem. 23, 6 (2002)). While this model depends on only two parameters, we demonstrate that by using appropriate boundary conditions and dispersion-repulsion contributions, solvation energies obtained for an extensive test set including neutral and charged molecules show dramatic improvement compared to existing models. Our approach is implemented in, but not restricted to, a linear-scaling density functional theory (DFT) framework, opening the path for self-consistent implicit solvent DFT calculations on systems of unprecedented size, which we demonstrate with calculations on a 2615-atom protein-ligand complex.
An ab initio variationally computed room-temperature line list for SO3
Underwood, Daniel S; Yurchenko, Sergei N
2013-01-01
{\\it Ab initio} potential energy and dipole moment surfaces are computed for sulfur trioxide (SO$_3$) at the CCSD(T)-F12b level of theory and appropriate triple-zeta basis sets. These surfaces are fitted and used, with a slight correction for the equilibrium S--O distance, to compute pure rotational and rotation-vibraton spectra of \\sothree\\ using the variational nuclear motion program TROVE. The calculations considered transitions in the region 0--4000 \\cm\\ with rotational states up to J=85. The resulting line list of 349~348~513 transitions is appropriate for modelling room temperature SO3 spectra. Good agreement is found with the observed infrared absorption spectra and the calculations are used to place the measured relative intensities on an absolute scale. A list of 10~878 experimental transitions is provided in a form suitable for inclusion in standard atmospheric and planetary spectroscopic databases.
An ab initio variationally computed room-temperature line list for (32)S(16)O3.
Underwood, Daniel S; Tennyson, Jonathan; Yurchenko, Sergei N
2013-07-01
Ab initio potential energy and dipole moment surfaces are computed for sulfur trioxide (SO3) at the CCSD(T)-F12b level of theory with appropriate triple-zeta basis sets. The analytical representations of these surfaces are used, with a slight correction, to compute pure rotational and rotation-vibration spectra of (32)S(16)O3 using the variational nuclear motion program TROVE. The calculations considered transitions in the region 0-4000 cm(-1) with rotational states up to J = 85. The resulting line list of 174,674,257 transitions is appropriate for modelling room temperature (32)S(16)O3 spectra. Good agreement is found with the observed infrared absorption spectra and the calculations are used to place the measured relative intensities on an absolute scale. A list of 10,878 experimental transitions is provided in a form suitable for inclusion in standard atmospheric and planetary spectroscopic databases. PMID:23579443
Knyazev, D V
2013-01-01
This work is devoted to the \\textit{ab initio} calculation of transport and optical properties of aluminum. The calculation is based on the quantum molecular dynamics simulation, density functional theory and the Kubo-Greenwood formula. Mainly the calculations are performed for liquid aluminum at near-normal densities for the temperatures from melting up to 20000 K. The results on dynamic electrical conductivity, static electrical conductivity and thermal conductivity are obtained and compared with available reference and experimental data and the calculations of other authors. The influence of the technical parameters on the results is investigated in detail. The error of static electrical conductivity calculation is estimated to be about 20%; more accurate results require bigger number of atoms.
Global ab initio ground-state potential energy surface of N4
Paukku, Yuliya; Yang, Ke R.; Varga, Zoltan; Truhlar, Donald G.
2013-07-01
We present a global ground-state potential energy surface for N4 suitable for treating high-energy vibrational-rotational energy transfer and collision-induced dissociation in N2-N2 collisions. To obtain the surface, complete active space second-order perturbation theory calculations were performed for the ground singlet state with an active space of 12 electrons in 12 orbitals and the maug-cc-pVTZ triple zeta basis set. About 17 000 ab initio data points have been calculated for the N4 system, distributed along nine series of N2 + N2 geometries and three series of N3 + N geometries. The six-dimensional ground-state potential energy surface is fitted using least-squares fits to the many-body component of the electronic energies based on permutationally invariant polynomials in bond order variables.
Electronic Curves Crossing in Methyl Iodide by Spin–Orbit Ab Initio Calculation
International Nuclear Information System (INIS)
An ab initio investigation of electronic curve crossing in a methyl iodide molecule is carried out using Spin–Orbit multiconfigurational quasidegenerate perturbation theory. The one-dimensional rigid potential curves and optimized effective curves of low-lying states, including Spin–Orbit coupling and relativistic effects, are calculated. The Spin–Orbit electronic curve crossing between 3Q0+and 1Q1, and the shadow minimum in potential energy curve of 3Q0+ at large internuclear distance are found in both sets of the curves according to the present calculations. The crossing position is in the range of RC–I = 0.2370 ± 00001 nm. Comparisons with other reports are presented. (atomic and molecular physics)
Ab Initio Calculation of Structure and Thermodynamic Properties of Zintl Aluminide SrAl2
Fu, Zhi-Jian; Jia, Li-Jun; Xia, Ji-Hong; Tang, Ke; Li, Zhao-Hong; Sun, Xiao-Wei; Chen, Qi-Feng
2015-12-01
The structural and thermodynamic properties of the orthorhombic and cubic structure SrAl2 at pressure and temperature are investigated by using the ab initio plane-wave pseudopotential density functional theory methodwithin the generalised gradient approximation (GGA). The calculated lattice parameters are in agreement with the available experimental data and other theoretical results. The phase transition predicted takes place at 0.5 GPa from the orthorhombic to the cubic structure at zero temperature. The thermodynamic properties of the zinc-blende structure SrAl2 are calculated by the quasi-harmonic Debye model. The pressure-volume relationship and the variations inthe thermal expansion α are obtained systematically in the pressure and temperature ranges of 0-5 GPa and 0-500 K, respectively.
Ab initio and phenomenological studies of the static response of neutron matter
Buraczynski, Mateusz
2016-01-01
We investigate the problem of periodically modulated strongly interacting neutron matter. We carry out ab initio non-perturbative auxiliary-field diffusion Monte Carlo calculations using an external sinusoidal potential in addition to phenomenological two- and three-nucleon interactions. Several choices for the wave function ansatz are explored and special care is taken to extrapolate finite-sized results to the thermodynamic limit. We perform calculations at various densities as well as at different strengths and periodicities of the one-body potential. Our microscopic results are then used to constrain the isovector term from energy-density functional theories of nuclei at many different densities, while making sure to separate isovector contributions from bulk properties. Lastly, we use our results to extract the static density-density linear response function of neutron matter at different densities. Our findings provide insights into inhomogeneous neutron matter and are related to the physics of neutron-...
The {\\it ab initio} calculation of spectra of open shell diatomic molecules
Tennyson, Jonathan; McKemmish, Laura K; Yurchenko, Sergei N
2016-01-01
The spectra (rotational, rotation-vibrational or electronic) of diatomic molecules due to transitions involving only closed-shell ($^1\\Sigma$) electronic states follow very regular, simple patterns and their theoretical analysis is usually straightforward. On the other hand, open-shell electronic states lead to more complicated spectral patterns and, moreover, often appear as a manifold of closely lying electronic states, leading to perturbations with even larger complexity. This is especially true when at least one of the atoms is a transition metal. Traditionally these complex cases have been analysed using approaches based on perturbation theory, with semi-empirical parameters determined by fitting to spectral data. Recently the needs of two rather diverse scientific areas have driven the demand for improved theoretical models of open-shell diatomic systems based on an \\emph{ab initio} approach, these areas are ultracold chemistry and the astrophysics of "cool" stars, brown dwarfs and most recently extraso...
Boron and nitrogen impurities in SiC nanoribbons: an ab initio investigation.
Costa, C D; Morbec, J M
2011-05-25
Using ab initio calculations based on density-functional theory we have performed a theoretical investigation of substitutional boron and nitrogen impurities in silicon carbide (SiC) nanoribbons. We have considered hydrogen terminated SiC ribbons with zigzag and armchair edges. In both systems we verify that the boron and nitrogen atoms energetically prefer to be localized at the edges of the nanoribbons. However, while boron preferentially substitutes a silicon atom, nitrogen prefers to occupy a carbon site. In addition, our electronic-structure calculations indicate that (i) substitutional boron and nitrogen impurities do not affect the semiconducting character of the armchair SiC nanoribbons, and (ii) the half-metallic behavior of the zigzag nanoribbons is maintained in the presence of substitutional boron impurities. In contrast, nitrogen atoms occupying edge carbon sites transform half-metallic zigzag nanoribbons into metallic systems. PMID:21540516
Rosenow, Phil
2016-01-01
The extent of hydrogen coverage of the Si(001)c(4x2) surface in the presence of hydrogen gas has been studied with dispersion corrected density functional theory. Electronic energy contributions are well described using a hybrid functional. The temperature dependence of the coverage in thermodynamic equilibrium was studied computing the phonon spectrum in a supercell approach. As an approximation to these demanding computations, an interpolated phonon approach was found to give comparable accuracy. The simpler ab initio thermodynamic approach is not accurate enough for the system studied, even if corrections by the Einstein model for surface vibrations are considered. The on-set of H2 desorption from the fully hydrogenated surface is predicted to occur at temperatures around 750 K. Strong changes in hydrogen coverage are found between 1000 and 1200 K in good agreement with previous reflectance anisotropy spectroscopy experiments. These findings allow a rational choice for the surface state in the computationa...
Femtosecond Laser Processing of Germanium: An Ab Initio Molecular Dynamics Study
Ji, Pengfei
2016-01-01
An ab initio molecular dynamics study of femtosecond laser processing of germanium is presented in this paper. The method based on the finite temperature density functional theory is adopted to probe the structural change, thermal motion of the atoms, dynamic property of the velocity autocorrelation, and the vibrational density of states. Starting from a cubic system at room temperature (300 K) containing 64 germanium atoms with an ordered arrangement of 1.132 nm in each dimension, the femtosecond laser processing is simulated by imposing the Nose Hoover thermostat to the electronic subsystem lasting for ~100 fs and continuing with microcanonical ensemble simulation of ~200 fs. The simulation results show solid, liquid and gas phases of germanium under adjusted intensities of the femtosecond laser irradiation. We find the irradiated germanium distinguishes from the usual germanium crystal by analyzing their melting and dynamic properties.
Atomic and Electronic Structures of C_60+BN Nanopeapods from ab initio Pseudopotential Calculations
Trave, Andrea; Ribeiro, Filipe; Louie, Steven G.; Cohen, Marvin L.
2004-03-01
Nanopeapods are structures of nanometric size consisting of an external carbon nanotube encapsulating a chain or complex array of fullerenes. Recent calculations and experiments have proven that nanopeapods can be obtained assembling fullerenes within boron nitride nanotubes, creating novel materials of possible interest for electronic transport applications. To improve the understanding of the properties of these composite systems, as compared to empty nanotubes and carbon nanopeapods, ab-initio total energy calculations have been performed within the pseudopotential Density Functional Theory in local density approximation. Results of these calculations on the energetics and geometrical deformations involved in the encapsulation will be presented, followed by a discussion of the consequences on the electronic structures of these systems, with particular focus on aspects relevant to electronic transport phenomena. This work is supported by NFS (Grant DMR00-87088) and DOE (Contract DE-AC03-76SF00098), using computational resources at NERSC and NPACI.
Lattice thermal conductivity of UO2 using ab-initio and classical molecular dynamics
International Nuclear Information System (INIS)
We applied the non-equilibrium ab-initio molecular dynamics and predict the lattice thermal conductivity of the pristine uranium dioxide for up to 2000 K. We also use the equilibrium classical molecular dynamics and heat-current autocorrelation decay theory to decompose the lattice thermal conductivity into acoustic and optical components. The predicted optical phonon transport is temperature independent and small, while the acoustic component follows the Slack relation and is in good agreement with the limited single-crystal experimental results. Considering the phonon grain-boundary and pore scatterings, the effective lattice thermal conductivity is reduced, and we show it is in general agreement with the sintered-powder experimental results. The charge and photon thermal conductivities are also addressed, and we find small roles for electron, surface polaron, and photon in the defect-free structures and for temperatures below 1500 K
Ab-initio calculations of electronic, transport, and structural properties of boron phosphide
Energy Technology Data Exchange (ETDEWEB)
Ejembi, J. I.; Nwigboji, I. H.; Franklin, L.; Malozovsky, Y.; Zhao, G. L.; Bagayoko, D., E-mail: diola-bagayoko@subr.edu [Department of Physics, Southern University and A and M College, Baton Rouge, Louisiana 70813 (United States)
2014-09-14
We present results from ab-initio, self-consistent density functional theory calculations of electronic and related properties of zinc blende boron phosphide (zb-BP). We employed a local density approximation potential and implemented the linear combination of atomic orbitals formalism. This technique follows the Bagayoko, Zhao, and Williams method, as enhanced by the work of Ekuma and Franklin. The results include electronic energy bands, densities of states, and effective masses. The calculated band gap of 2.02 eV, for the room temperature lattice constant of a=4.5383 Å, is in excellent agreement with the experimental value of 2.02±0.05 eV. Our result for the bulk modulus, 155.7 GPa, agrees with experiment (152–155 GPa). Our predictions for the equilibrium lattice constant and the corresponding band gap, for very low temperatures, are 4.5269 Å and 2.01 eV, respectively.
Tokizaki, Chihiro; Yoshida, Takahiko; Takayanagi, Toshiyuki
2016-05-01
The cyclooctatetraene (COT) anion has a stable D4h structure that is similar to the transition state configurations of the neutral C-C bond-alternation (D4h ↔ D8h ↔ D4h) and ring-inversion (D2d ↔ D4h ↔ D2d) unimolecular reactions. The previously measured photodetachment spectrum of COT- revealed the reaction dynamics in the vicinity of the two transition states on the neutral potential energy surface. In this work, the photodetachment spectrum is calculated quantum mechanically on ab initio-level potential energy surfaces within a three degree-of-freedom reduced-dimensionality model. Very good agreement has been obtained between theory and experiment, providing reliable interpretations for the experimental spectrum. A detailed picture of the reactive molecular dynamics of the COT unimolecular reaction in the transition state region is also discussed.
Ab initio simulations for the ion-ion structure factor of warm dense aluminum.
Rüter, Hannes R; Redmer, Ronald
2014-04-11
We perform ab initio simulations based on finite-temperature density functional theory in order to determine the static and dynamic ion-ion structure factor in aluminum. We calculate the dynamic structure factor via the intermediate scattering function and extract the dispersion relation for the collective excitations. The results are compared with available experimental x-ray scattering data. Very good agreement is obtained for the liquid metal domain. In addition we perform simulations for warm dense aluminum in order to obtain the ion dynamics in this strongly correlated quantum regime. We determine the sound velocity for both liquid and warm dense aluminum which can be checked experimentally using narrow-bandwidth free electron laser radiation. PMID:24765982
Pressure-induced semimetallic behavior of calcium from ab initio calculations
International Nuclear Information System (INIS)
A loss of metallic properties in fcc calcium under high pressure is studied ab initio using the density functional theory (DFT) and GW approximation. It is found that a more correct description of many-electron effects given by GW method does not provide significant changes in the behavior of electronic spectrum in comparison with DFT approach. We note that the obtained width of (pseudo)gap is highly sensitive to the k-point sampling used for density of states calculation. The analysis of fcc calcium's band structure at p ∼ 20 GPa shows that the crossing of bands at the Fermi level is removed if the spin-orbit coupling is taken into account.
Ab initio calculation of molecular energies including parity violating interactions
International Nuclear Information System (INIS)
A new approach, RHF-CIS, based on the perturbation of the ground state RHF wave function by the CIS excitations, has been implemented for evaluation of energy of parity violating interaction in molecules, Epv. The earlier approach, RHF-SDE, was based on the perturbation of the RHF ground states by the single-determinant ''excitations'' (SDE). The results obtained show the dramatic difference between Epv values in the RHF-CIS framework and those in the RHF-SDE framework: the Epv values of the RHF-CIS formalism are more than one order of magnitude greater compared to the RHF-SDE formalism as well as the corresponding tensor components. The maximal total value obtained for hydrogen peroxide in the RHF-CIS framework is 3.661 X 10-19 EH (DZ** basis set) while the maximal Epv value for the RHF-SDE formalism is just 3.635 X 10-20 EH (TZ basis set). It is remarkable that both in the RFH-CIS and in the RHF-SDE approaches the diagonal tensor components of Epv strictly follow the geometry of a molecule and are always different from zero at chiral conformations. The zeros of the total Epv at chiral geometries are now found to be the results of the interplay between the diagonal tensor components values. We have carried out exhaustive analysis of the RHF-SDE formalism and found that it is not sufficiently accurate for studies of Epv. To this end, we have completely reproduced the previous work, which has been done in the RHF-SDE frame-work, and developed it further, studying how the RHF-SDE results vary when changing size and quality of basis sets. This last resource does not save the RHF-SDE formalism for evaluations of Epv from the general failure. Packages of FORTRAN routines called ENWEAK/RHFSDE-93 and ENWEAK/RHFCIS-94 have been developed which run on top of an ab initio MO package. We used 6-31G and 6-31G**, DZ and DZ**, TZ and TZ**, and (10s, 6p,**) basis sets. We will discuss the importance of the present results for possible measurement of the parity violating energy
Improved parametric fits for the HeH2 ab initio energy surface
International Nuclear Information System (INIS)
A brief history of the development of ab initio calculations for the HeH2 quasi-molecule energy surface, and the parametric fits to these ab initio calculations, is presented. The concept of 'physical reasonableness' of the parametric fit is discussed. Several new improved parametric fits for the energy surface, meeting these requirements, are then proposed. One fit extends the Russek-Garcia parametric fit for the deep repulsion region to include r-dependent parameters, resulting in a more physically reasonable fit with smaller average error. This improved surface fit is applied to quasi-elastic collisions of He on H2 in the impulse approximation. Previous classical calculations of the scaled inelastic vibrorotational excitation energy distributions are improved with this more accurate parametric fit of the energy surface and with the incorporation of quantum effects in vibrational excitation. It is shown that Sigmund's approach in developing his scaling law is incomplete in the contribution of the three-body interactions to vibrational excitation of the H2 molecule is concerned. The Sigmund theory is extended to take into account for r-dependency of three-body interactions. A parametric fit for the entire energy surface from essentially 0 ≤R≤∞ and 1.2≤r≤1.6 a.u., where R is the intermolecular spacing and r is the hydrogen bonding length, is also presented. This fit is physically reasonable in all asymptotic limits. This first, full surface parametric fit is based primarily upon a composite of ab initio studies by Russek and Garcia and Meyer, Hariharan and Kutzelnigg. Parametric fits for the H2(1sσg)2, H2+(1sσg), H2+(2pσu) and (LiH2)+ energy surfaces are also presented. The new parametric fits for H2, H2+(1sσg) are shown to be improvements over the well-known Morse potentials for these surfaces
Meisel, David D.; Szasz, Csilla; Kero, Johan
2008-06-01
The Arecibo UHF radar is able to detect the head-echos of micron-sized meteoroids up to velocities of 75 km/s over a height range of 80 140 km. Because of their small size there are many uncertainties involved in calculating their above atmosphere properties as needed for orbit determination. An ab initio model of meteor ablation has been devised that should work over the mass range 10-16 kg to 10-7 kg, but the faint end of this range cannot be observed by any other method and so direct verification is not possible. On the other hand, the EISCAT UHF radar system detects micrometeors in the high mass part of this range and its observations can be fit to a “standard” ablation model and calibrated to optical observations (Szasz et al. 2007). In this paper, we present a preliminary comparison of the two models, one observationally confirmable. Among the features of the ab initio model that are different from the “standard” model are: (1) uses the experimentally based low pressure vaporization theory of O’Hanlon (A users’s guide to vacuum technology, 2003) for ablation, (2) uses velocity dependent functions fit from experimental data on heat transfer, luminosity and ionization efficiencies measured by Friichtenicht and Becker (NASA Special Publication 319: 53, 1973) for micron sized particles, (3) assumes a density and temperature dependence of the micrometeoroids and ablation product specific heats, (4) assumes a density and size dependent value for the thermal emissivity and (5) uses a unified synthesis of experimental data for the most important meteoroid elements and their oxides through least square fits (as functions of temperature, density, and/or melting point) of the tables of thermodynamic parameters given in Weast (CRC Handbook of Physics and Chemistry, 1984), Gray (American Institute of Physics Handbook, 1972), and Cox (Allen’s Astrophysical Quantities 2000). This utilization of mostly experimentally determined data is the main reason for
Comparative studies of the spectroscopy of CuCl2: DFT versus standard ab initio approaches.
Ramírez-Solís, A; Poteau, R; Vela, A; Daudey, J P
2005-04-22
The X2Pi g-2Sigma g+, X2Pi g-2Delta g, X2Pi g-2Sigma u+, X2Pi g-2Pi u transitions on CuCl2 have been studied using several exchange-correlation functionals from the various types of density functional theory (DFT) approaches like local density approximation (LDA), generalized gradient approximation (GGA), hybrid and meta-GGA. The results are compared with the experience and with those coming from the most sophisticated nondynamic and dynamic electronic correlation treatments using the same relativistic effective core potentials and especially developed basis sets to study the electronic structure of the five lowest states and the corresponding vertical and adiabatic transition energies. The calculated transition energies for three of the hybrid functionals (B3LYP, B97-2, and PBE0) are in very good agreement with the benchmark ab initio results and experimental figures. All of the other functionals largely overestimate the X2Pi g-2Sigma g+ and X2Pi g-2Delta g transition energies, many of them even placing the 2Delta g ligand field state above the charge transfer 2Pi u and 2Sigma u+ states. The relative weight of the Hartree-Fock exchange in the definition of the functional used appears to play a key role in the accurate description of the LambdaSSigma density defined by the orientation of the 3d hole (sigma, pi, or delta) on Cu in the field of both chlorine atoms, but no simple connection of this weight with the quality of the spectra has been found. Mulliken charges and spin densities are carefully analyzed; a possible link between the extent of spin density on the metal for the X2Pi g state and the performance of the various functionals was observed, suggesting that those that lead to the largest values (close to 0.65) are the ones that best reproduce these four transitions. Most functionals lead to a remarkably low ionicity for the three ligand field states even for the best performing functionals, compared to the complete active space (SCF) (21, 14) ab initio
Vlahos, Vasilios; Booske, John H.; Morgan, Dane
2010-02-01
Microwave, x-ray, and radio-frequency radiation sources require a cathode emitting electrons into vacuum. Thermionic B-type dispenser cathodes consist of BaxOz coatings on tungsten (W), where the surface coatings lower the W work function and enhance electron emission. The new and promising class of scandate cathodes modifies the B-type surface through inclusion of Sc, and their superior emissive properties are also believed to stem from the formation of a low work function surface alloy. In order to better understand these cathode systems, density-functional theory (DFT)-based ab initio modeling is used to explore the stability and work function of BaxScyOz on W(001) monolayer-type surface structures. It is demonstrated how surface depolarization effects can be calculated easily using ab initio calculations and fitted to an analytic depolarization equation. This approach enables the rapid extraction of the complete depolarization curve (work function versus coverage relation) from relatively few DFT calculations, useful for understanding and characterizing the emitting properties of novel cathode materials. It is generally believed that the B-type cathode has some concentration of Ba-O dimers on the W surface, although their structure is not known. Calculations suggest that tilted Ba-O dimers are the stable dimer surface configuration and can explain the observed work function reduction corresponding to various dimer coverages. Tilted Ba-O dimers represent a new surface coating structure not previously proposed for the activated B-type cathode. The thermodynamically stable phase of Ba and O on the W surface was identified to be the Ba0.25O configuration, possessing a significantly lower Φ value than any of the Ba-O dimer configurations investigated. The identification of a more stable Ba0.25O phase implies that if Ba-O dimers cover the surface of emitting B-type cathodes, then a nonequilibrium steady state must dominate the emitting surface. The identification of
Realization of prediction of materials properties by ab initio computer simulation
Indian Academy of Sciences (India)
Yoshiyuki Kawazoe
2003-01-01
Ab initio treatment is becoming realistic to predict physical, chemical, and even mechanical properties of academically and industrially interesting materials. There is, however, some limitation in size and time of the system up to the order of several hundred atoms and ∼ 1 pico second, even if we use the fastest supercomputer efficiently. Therefore, it is very difficult to simulate realistic materials with grain boundaries and important reactions like diffusion in materials. To improve this situation, two ways have been invented. One way is to upgrade approximations to match the necessary levels according to inhomogeneous electron gas theory beyond the present day standard, i.e. local density approximation (LDA). The reason is simply that the system we are interested in is composed of many particles interacting with Coulomb forces governed by quantum mechanics. (Complete knowledge is available, and only what we should do is to make better approximations to explain the phenomena!). Another is to extract the necessary parameters from the ab initio calculations on systems with limited number of atoms, and apply these results into cluster variation, direct, or any other sophisticated methods based on classical concepts such as statistical mechanics. In this paper, several typical examples recently worked out by our research group are introduced to indicate that these methodologies are actually possible to be successfully used to predict materials properties before experiments based on the present day state-of-art supercomputing systems. It includes scientific visualization of the results of ab initio molecular dynamics simulation on atom insertion process to C60 and to carbon nanotube, tight-binding calculation of single electron conductance properties in nanotube to create nano-scale diode virtually by computer, which will be a base of future nanoscale electric device in nanometer size, Li + H reaction without Born–Oppenheimer approximation, structural phase
Efficient ab initio free energy calculations by classically assisted trajectory sampling
Wilson, Hugh F.
2015-12-01
A method for efficiently performing ab initio free energy calculations based on coupling constant thermodynamic integration is demonstrated. By the use of Boltzmann-weighted sums over states generated from a classical ensemble, the free energy difference between the classical and ab initio ensembles is readily available without the need for time-consuming integration over molecular dynamics trajectories. Convergence and errors in this scheme are discussed and characterised in terms of a quantity representing the degree of misfit between the classical and ab initio systems. Smaller but still substantial efficiency gains over molecular dynamics are also demonstrated for the calculation of average properties such as pressure and total energy for systems in equilibrium.
Energy Technology Data Exchange (ETDEWEB)
Sahli, Beat [Integrated Systems Laboratory, ETH Zurich, Gloriastrasse 35, 8092 Zurich (Switzerland)], E-mail: sahli@iis.ee.ethz.ch; Vollenweider, Kilian [Integrated Systems Laboratory, ETH Zurich, Gloriastrasse 35, 8092 Zurich (Switzerland); Zographos, Nikolas; Zechner, Christoph [Synopsys Switzerland LLC, Affolternstrasse 52, 8050 Zurich (Switzerland)
2008-12-05
We present the results of extensive ab initio simulations for phosphorus clusters, arsenic clusters and mixed phosphorus/arsenic clusters in silicon. The specific defects and the parameters that are investigated are selected according to the needs of state-of-the-art diffusion and activation models, taking into account the availability of experimental data, the capabilities of current ab initio methods and the requirements for advanced technology development. The calculated binding energies are used to determine a good starting point for the calibration of a new clustering model implemented in an atomistic process simulator. The defect species V, I, P, PV, PI, As, AsV, AsI and clusters containing up to four dopant atoms and up to one V or I are considered in all relevant charge states. The ab initio results are discussed as well as the challenges arising in the transfer of this information into the process simulation model.
Electrical resistivity of NaPb compound-forming liquid alloy using ab initio pseudopotentials
Indian Academy of Sciences (India)
Anil Thakur; N S Negi; P K Ahluwalla
2005-08-01
The study of electrical resistivity of compound-forming liquid alloy, NaPb, is presented as a function of concentration. Hard sphere diameters of Na and Pb are obtained through the interionic pair potentials evaluated using Troullier and Martins ab initio pseudopotential, which have been used to calculate the partial structure factors (). Considering the liquid alloy to be a ternary mixture, Ziman formula, modified for complex formation has been used for calculating resistivity of binary liquid alloys. Form factors are calculated using ab initio pseudopotentials. The results suggest that Ziman formalism, when used with ab initio pseudopotentials, are quite successful in explaining the electrical resistivity data of compound-forming binary liquid alloys.
An Efficient Time-Stepping Scheme for Ab Initio Molecular Dynamics Simulations
Tsuchida, Eiji
2016-08-01
In ab initio molecular dynamics simulations of real-world problems, the simple Verlet method is still widely used for integrating the equations of motion, while more efficient algorithms are routinely used in classical molecular dynamics. We show that if the Verlet method is used in conjunction with pre- and postprocessing, the accuracy of the time integration is significantly improved with only a small computational overhead. We also propose several extensions of the algorithm required for use in ab initio molecular dynamics. The validity of the processed Verlet method is demonstrated in several examples including ab initio molecular dynamics simulations of liquid water. The structural properties obtained from the processed Verlet method are found to be sufficiently accurate even for large time steps close to the stability limit. This approach results in a 2× performance gain over the standard Verlet method for a given accuracy. We also show how to generate a canonical ensemble within this approach.
Chan, Garnet Kin-Lic; Nakatani, Naoki; Li, Zhendong; White, Steven R
2016-01-01
Current descriptions of the ab initio DMRG algorithm use two superficially different languages: an older language of the renormalization group and renormalized operators, and a more recent language of matrix product states and matrix product operators. The same algorithm can appear dramatically different when written in the two different vocabularies. In this work, we carefully describe the translation between the two languages in several contexts. First, we describe how to efficiently implement the ab-initio DMRG sweep using a matrix product operator based code, and the equivalence to the original renormalized operator implementation. Next we describe how to implement the general matrix product operator/matrix product state algebra within a pure renormalized operator-based DMRG code. Finally, we discuss two improvements of the ab initio DMRG sweep algorithm motivated by matrix product operator language: Hamiltonian compression, and a sum over operators representation that allows for perfect computational par...
Energy Technology Data Exchange (ETDEWEB)
Keegan, Ronan M. [STFC Rutherford Appleton Laboratory, Didcot OX11 0FA (United Kingdom); Bibby, Jaclyn; Thomas, Jens [University of Liverpool, Liverpool L69 7ZB (United Kingdom); Xu, Dong [Sanford-Burnham Medical Research Institute, 10901 North Torrey Pines Road, La Jolla, CA 92037 (United States); Zhang, Yang [University of Michigan, Ann Arbor, MI 48109 (United States); Mayans, Olga [University of Liverpool, Liverpool L69 7ZB (United Kingdom); Winn, Martyn D. [Science and Technology Facilities Council Daresbury Laboratory, Warrington WA4 4AD (United Kingdom); Rigden, Daniel J., E-mail: drigden@liv.ac.uk [University of Liverpool, Liverpool L69 7ZB (United Kingdom); STFC Rutherford Appleton Laboratory, Didcot OX11 0FA (United Kingdom)
2015-02-01
Two ab initio modelling programs solve complementary sets of targets, enhancing the success of AMPLE with small proteins. AMPLE clusters and truncates ab initio protein structure predictions, producing search models for molecular replacement. Here, an interesting degree of complementarity is shown between targets solved using the different ab initio modelling programs QUARK and ROSETTA. Search models derived from either program collectively solve almost all of the all-helical targets in the test set. Initial solutions produced by Phaser after only 5 min perform surprisingly well, improving the prospects for in situ structure solution by AMPLE during synchrotron visits. Taken together, the results show the potential for AMPLE to run more quickly and successfully solve more targets than previously suspected.
Knyazev, D V
2014-01-01
This work is devoted to the investigation of transport and optical properties of liquid aluminum in the two-temperature case. At first optical properties, static electrical and thermal conductivities were obtained in the \\textit{ab initio} calculation. The \\textit{ab initio} calculation is based on the quantum molecular dynamics, density functional theory and the Kubo-Greenwood formula. The semiempirical approximation was constructed based on the results of the \\textit{ab initio} caculation. The approximation yields the dependences $\\sigma_{1_\\mathrm{DC}}\\propto1/T_i^{0.25}$ and $K\\propto T_e/T_i^{0.25}$ for the static electrical conductivity and thermal conductivity, respectively. The approximation is valid for liquid aluminum at $\\rho=2.70$~g/cm$^3$, 3~kK~$\\leq T_i\\leq T_e\\leq20$~kK. Our results are well described by the Drude model with the effective relaxation time $\\tau\\propto T_i^{-0.25}$. We have compared our results with a number of other models. They are all reduced in the low-temperature limit to th...
John, Christopher; Spura, Thomas; Habershon, Scott; Kühne, Thomas D.
2016-04-01
We present a simple and accurate computational method which facilitates ab initio path-integral molecular dynamics simulations, where the quantum-mechanical nature of the nuclei is explicitly taken into account, at essentially no additional computational cost in comparison to the corresponding calculation using classical nuclei. The predictive power of the proposed quantum ring-polymer contraction method is demonstrated by computing various static and dynamic properties of liquid water at ambient conditions using density functional theory. This development will enable routine inclusion of nuclear quantum effects in ab initio molecular dynamics simulations of condensed-phase systems.
Estudo da geometria da uréia por métodos ab initio e simulação computacional de líquidos
Directory of Open Access Journals (Sweden)
Cirino José Jair Vianna
2002-01-01
Full Text Available A study was carried out on the urea geometries using ab initio calculation and Monte Carlo computational simulation of liquids. The ab initio calculated results showed that urea has a non-planar conformation in the gas phase in which the hydrogen atoms are out of the plane formed by the heavy atoms. Free energies associated to the rotation of the amino groups of urea in water were obtained using the Monte Carlo method in which the thermodynamic perturbation theory is implemented. The magnitude of the free energy obtained from this simulation did not permit us to conclude that urea is non-planar in water.
Londero, Elisa; Bijeikytė, Monika; Maze, Jeromino R; Alkauskas, Audrius; Gali, Adam
2016-01-01
Understanding optical excitation spectra of point defects is still a scientific challenge. We demonstrate by \\emph{ab initio} calculations that a prominent sharp feature in the photoluminescence (PL) spectrum of the negatively charged silicon-vacancy defect in diamond can be only explained within the Herzberg-Teller approximation that goes beyond the commonly applied Franck-Condon approximation. The effect of the dynamic Jahn-Teller effect on the PL spectrum is also discussed. Our implementation of Herzberg-Teller theory paves the way for full \\emph{ab initio} description of the optical excitation spectrum of point defects in solids
Gellé, A.; Varignon, J.; Lepetit, M.-B.
2009-11-01
We propose a new ab initio method designed for the accurate calculation of effective exchange integrals between atoms with numerous open shells. This method applies to ferromagnetic as well as antiferromagnetic exchange, direct or ligand-mediated exchange. Test calculations on high spin transition metal oxides such as KNiF3, Ba2CoS3 or YMnO3 exhibit a very good accuracy compared either to the best ab initio calculations —when those are feasible— and with experimental evaluations.
Ab initio calculations versus polarized neutron diffraction for the spin density of free radicals
International Nuclear Information System (INIS)
The determination of the magnetization distribution using polarized neutron diffraction has played a key role during the last twenty years in the field of molecular magnetism. This distribution can also be obtained by first principle ab initio calculations. Such calculations always rely on approximations and the question that arises is to know whether the obtained results are reliable enough to represent accurately the properties of these molecules. The comparison between polarized neutron experimental results and ab initio calculations has turned to provide stringent tests for these methods. In the resent article a comparison between experimental and theoretical results is made and is illustrated by examples based on magnetic free radicals. (author)
Ab initio simulations of doped single-walled carbon nanotube sensors
International Nuclear Information System (INIS)
Graphical abstract: Ab initio calculations used to study O-doped and N-doped SWNT within the framework of the DFT. We introduced new type of gas sensor that can detect the presence of H2, Cl2, NO, and CO molecules. Band-gap narrowing occurs when the nanotube is doped with oxygen or nitrogen atoms. We proposed a new methodology of designing reliable, sensitive, and selective nanotube sensors. Highlights: ►Ab initio calculations used to study O-doped and N-doped SWNT within the framework of the DFT. ► We introduced new type of gas sensor that can detect the presence of H2, Cl2, NO, and CO molecules. ►Band-gap narrowing occurs when the nanotube is doped with oxygen or nitrogen atoms. ► We proposed a new methodology of designing reliable, sensitive, and selective nanotube sensors. - Abstract: The interactions between oxygen and nitrogen atoms with single-walled carbon nanotubes were investigated for nanotubes with two different geometrical configurations using first-principle calculations within the framework of the density functional theory. We introduced a new type of toxic gas sensor that can detect the presence of H2, Cl2, CO, and NO molecules. We also demonstrated that the sensitivity of this device can be controlled by the concentration of the dopants on the surface of the nanotube. In addition, the transport properties of the doped nanotube were studied for different concentrations of oxygen or nitrogen atoms that were randomly distributed on the surface of the single-walled carbon nanotube. We observed that small amounts of dopants can modify the electronic and transport properties of the nanotube and can lend metallic properties to the nanotube. Band-gap narrowing occurs when the nanotube is doped with either oxygen or nitrogen atoms.
Energy Technology Data Exchange (ETDEWEB)
Ganster, P
2004-10-15
A calcium aluminosilicate glass of molar composition 67 % SiO{sub 2} - 12 % Al{sub 2}O{sub 3} - 21 % CaO was modelled by classical and ab initio molecular dynamics. The size effect study in classical MD shows that the systems of 100 atoms are more ordered than the larger ones. These effects are mainly due to the 3-body terms in the empirical potentials. Nevertheless, these effects are small and the structures generated are in agreement with experimental data. In such kind of glass, we denote an aluminium avoidance and an excess of non bridging oxygens which can be compensated by tri coordinated oxygens. When the dynamics of systems of 100 and 200 atoms is followed by ab initio MD, some local arrangements occurs (bond length, angular distributions). Thus, more realistic vibrational properties are obtained in ab initio MD. The modelling of thin films shows that aluminium atoms extend to the most external part of the surface and they are all tri-coordinated. Calcium atoms are set in the sub layer part of the surface and they produce a depolymerization of the network. In classical MD, tri-coordinated aluminium atoms produce an important electric field above the surface. With non bridging oxygens, they constitute attractive sites for single water molecules. (author)
Senthil kumar, J.; Jeyavijayan, S.; Arivazhagan, M.
2015-02-01
The vibrational spectral analysis is carried out using FT-Raman and FT-IR spectroscopy in the range 3500-50 cm-1 and 4000-400 cm-1, respectively, for 6-nitrochromone (6NC). The molecular structure, fundamental vibrational frequencies and intensity of the vibrational bands are interpreted with the aid of structure optimization and normal coordinates force field calculation based on ab initio HF and DFT gradient calculations employing the HF/6-311++G(d,p) and B3LYP/6-311++G(d,p) basis set. Stability of the molecule has been analyzed using NBO analysis. The calculated HOMO and LUMO energies show that charge transfer occurs within the molecule. Thermodynamic properties like entropy, heat capacity, zero-point energy and Mulliken's charge analysis have been calculated for the 6NC. The complete assignments were performed on the basis of total energy distribution (TED) of the vibrational modes with scaled quantum mechanical (SQM) method. The MEP map shows the negative potential sites are on oxygen atoms as well as the positive potential sites are around the hydrogen atoms.
Unified ab initio formulation of flexoelectricity and strain-gradient elasticity
Stengel, Massimiliano
2016-06-01
The theory of flexoelectricity and that of nonlocal elasticity are closely related, and are often considered together when modeling strain-gradient effects in solids. Here I show, based on a first-principles lattice-dynamical analysis, that their relationship is much more intimate than previously thought, and their consistent simultaneous treatment is crucial for obtaining correct physical answers. In particular, I identify a gauge invariance in the theory, whereby the energies associated to strain-gradient elasticity and flexoelectrically induced electric fields are individually reference dependent, and only when summed up they yield a well-defined result. To illustrate this, I construct a minimal thermodynamic functional incorporating strain-gradient effects, and establish a formal link between the continuum description and ab initio phonon dispersion curves to calculate the relevant tensor quantities. As a practical demonstration, I apply such a formalism to bulk SrTiO3, where I find an unusually strong contribution of nonlocal elasticity, mediated by the interaction between the ferroelectric soft mode and the transverse acoustic branches. These results have important implications towards the construction of well-defined thermodynamic theories where flexoelectricity and ferroelectricity coexist. More generally, they open exciting new avenues for the implementation of hierarchical multiscale concepts in the first-principles simulation of crystalline insulators.
Ab-initio modeling of an anion $C_{60}^-$ pseudopotential for fullerene-based compounds
Vrubel, I I; Ivanov, V K
2015-01-01
A pseudopotential of $C_{60}^-$ has been constructed from ab-initio quantum-mechanical calculations. Since the obtained pseudopotential can be easily fitted by rather simple analytical approximation it can be effectively used both in classical and quantum molecular dynamics of fullerene-based compounds.
Ab initio Calculations of Charge Symmetry Breaking in the A=4 Hypernuclei
Czech Academy of Sciences Publication Activity Database
Gazda, Daniel; Gal, A.
2016-01-01
Roč. 116, č. 12 (2016), s. 122501. ISSN 0031-9007 R&D Projects: GA ČR(CZ) GA15-04301S Institutional support: RVO:61389005 Keywords : ab initio * shell model * four-body calculations Subject RIV: BE - Theoretical Physics Impact factor: 7.512, year: 2014
All-electron ab initio investigations of the electronic states of the NiC molecule
DEFF Research Database (Denmark)
Shim, Irene; Gingerich, Karl. A.
The low-lying electronic states of NiC are investigated by all-electron ab initio multi-configuration self-consistent-field (CASSCF) calculations including relativistic corrections. The electronic structure of NiC is interpreted as perturbed antiferromagnetic couplings of the localized angular...
Ab initio charge-carrier mobility model for amorphous molecular semiconductors
Massé, Andrea; Friederich, Pascal; Symalla, Franz; Liu, Feilong; Nitsche, Robert; Coehoorn, Reinder; Wenzel, Wolfgang; Bobbert, Peter A.
2016-05-01
Accurate charge-carrier mobility models of amorphous organic molecular semiconductors are essential to describe the electrical properties of devices based on these materials. The disordered nature of these semiconductors leads to percolative charge transport with a large characteristic length scale, posing a challenge to the development of such models from ab initio simulations. Here, we develop an ab initio mobility model using a four-step procedure. First, the amorphous morphology together with its energy disorder and intermolecular charge-transfer integrals are obtained from ab initio simulations in a small box. Next, the ab initio information is used to set up a stochastic model for the morphology and transfer integrals. This stochastic model is then employed to generate a large simulation box with modeled morphology and transfer integrals, which can fully capture the percolative charge transport. Finally, the charge-carrier mobility in this simulation box is calculated by solving a master equation, yielding a mobility function depending on temperature, carrier concentration, and electric field. We demonstrate the procedure for hole transport in two important molecular semiconductors, α -NPD and TCTA. In contrast to a previous study, we conclude that spatial correlations in the energy disorder are unimportant for α -NPD. We apply our mobility model to two types of hole-only α -NPD devices and find that the experimental temperature-dependent current density-voltage characteristics of all devices can be well described by only slightly decreasing the simulated energy disorder strength.
Optical Spectroscopy of the Bulk and Interfacial Hydrated Electron from Ab Initio Calculations
Czech Academy of Sciences Publication Activity Database
Uhlig, Frank; Herbert, J. M.; Coons, M. P.; Jungwirth, Pavel
2014-01-01
Roč. 118, č. 35 (2014), s. 7507-7515. ISSN 1089-5639 R&D Projects: GA ČR GBP208/12/G016 Institutional support: RVO:61388963 Keywords : hydrated electron * optical spectrum * ab initio molecular dynamics Subject RIV: CF - Physical ; Theoretical Chemistry Impact factor: 2.693, year: 2014
Mechanical properties of carbynes investigated by ab initio total-energy calculations
DEFF Research Database (Denmark)
Castelli, Ivano E.; Salvestrini, Paolo; Manini, Nicola
2012-01-01
As sp carbon chains (carbynes) are relatively rigid molecular objects, can we exploit them as construction elements in nanomechanics? To answer this question, we investigate their remarkable mechanical properties by ab initio total-energy simulations. In particular, we evaluate their linear...
Ab initio calculation of the lifetimes of 4p and 3d levels of Ca+
International Nuclear Information System (INIS)
We have done an ab initio calculation based on the Brueckner approximation for the lifetimes of 4p2P and 3d2D levels of Ca+. The results of the Brueckner approximation differ from experiment by 2.5%. With leading third-order corrections included, our results agree with the latest accurate experiment within a 1% difference
Atomic carbon chains as spin-transmitters: An ab initio transport study
DEFF Research Database (Denmark)
Fürst, Joachim Alexander; Brandbyge, Mads; Jauho, Antti-Pekka
2010-01-01
An atomic carbon chain joining two graphene flakes was recently realized in a ground-breaking experiment by Jin et al. (Phys. Rev. Lett., 102 (2009) 205501). We present ab initio results for the electron transport properties of such chains and demonstrate complete spin-polarization of the transmi...
Ab initio and work function and surface energy anisotropy of LaB6
Uijttewaal, M. A.; de Wijs, G. A.; de Groot, R. A.
2006-01-01
Lanthanum hexaboride is one of the cathode materials most used in high-power electronics technology, but the many experimental results do not provide a consistent picture of the surface properties. Therefore, we report the first ab initio calculations of the work functions and surface energies of th
Ab initio study of long-period superstructures in close-packed A3B compounds
DEFF Research Database (Denmark)
Rosengaard, N. M.; Skriver, Hans Lomholt
1994-01-01
We have performed ab initio calculations of the stability of one-dimensional long-period superstructures in Cu3Pd, Cu3Al, and Ag3Mg by means of an interface Green's function technique based on the linear-muffin-tin-orbitals method within the tight-binding and atomic-sphere approximations. The...
Czech Academy of Sciences Publication Activity Database
Meliá, C.; Ferrer, S.; Řezáč, Jan; Parisel, O.; Reinaud, O.; Moliner, V.; de la Lande, A.
2013-01-01
Roč. 19, č. 51 (2013), s. 17328-17337. ISSN 0947-6539 Institutional support: RVO:61388963 Keywords : ab initio calculations * copper * electron transfer * enzymes * molecular dynamics * reaction mechanisms Subject RIV: CC - Organic Chemistry Impact factor: 5.696, year: 2013
Ab initio molecular dynamics approach to a quantitative description of ion pairing in water
Czech Academy of Sciences Publication Activity Database
Pluhařová, Eva; Maršálek, Ondřej; Schmidt, B.; Jungwirth, Pavel
2013-01-01
Roč. 4, č. 23 (2013), s. 4177-4181. ISSN 1948-7185 R&D Projects: GA ČR GBP208/12/G016 Institutional support: RVO:61388963 Keywords : ion pairing * charge transfer * water * ab initio molecular dynamics Subject RIV: CF - Physical ; Theoretical Chemistry Impact factor: 6.687, year: 2013
Stabilization of Ab Initio Molecular Dynamics Simulations at Large Time Steps
Tsuchida, Eiji
2014-01-01
The Verlet method is still widely used to integrate the equations of motion in ab initio molecular dynamics simulations. We show that the stability limit of the Verlet method may be significantly increased by setting an upper limit on the kinetic energy of each atom with only a small loss in accuracy. The validity of this approach is demonstrated for molten lithium fluoride.
Ab initio Defect Energetics in LaBO3 Perovskite Solid Oxide Fuel Cell Materials
DEFF Research Database (Denmark)
Lee, Yueh-Lin; Morgan, Dane; Kleis, Jesper;
2009-01-01
Perovskite materials of the form ABO3 are a promising family of compounds for use in solid oxide fuel cell (SOFC) cathodes. Study of the physics of these compounds under SOFC conditions with ab initio methods is particularly challenging due to high temperatures, exchange of oxygen with O2 gas, and...
Czech Academy of Sciences Publication Activity Database
Sandlöbes, S.; Pei, Z.; Friák, Martin; Zhu, L.-F.; Wang, F.; Zaefferer, S.; Raabe, D.; Neugebauer, J.
2014-01-01
Roč. 70, MAY (2014), s. 92-104. ISSN 1359-6454 Grant ostatní: GA MŠk(CZ) LM2010005 Institutional support: RVO:68081723 Keywords : Magnesium * Rare-earth elements * Ductility * Modeling * Ab initio Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 4.465, year: 2014
Ab initio I-V characteristics of short C-20 chains
DEFF Research Database (Denmark)
Roland, C.; Larade, B.; Taylor, Jeremy Philip;
2002-01-01
We have calculated the I-V characteristics of short chains of C-20 molecular cages between Al and Au leads with an ab initio formalism. The results indicate that a linear chain of such molecules acts primarily as metallic nanowires. The transmission, however, depends sensitively both on the...
Ab initio electronic properties of dual phosphorus monolayers in silicon
DEFF Research Database (Denmark)
Drumm, Daniel W.; Per, Manolo C.; Budi, Akin;
2014-01-01
In the midst of the epitaxial circuitry revolution in silicon technology, we look ahead to the next paradigm shift: effective use of the third dimension - in particular, its combination with epitaxial technology. We perform ab initio calculations of atomically thin epitaxial bilayers in silicon, ...
Raman and ab initio studies of simple and binary 1-alkyl-3-methylimidazolium ionic liquids
DEFF Research Database (Denmark)
Berg, R.W.; Deetlefs, M.; Seddon, K.R.;
2005-01-01
Raman spectra of the ionic liquids, 1-butyl-3-methylimidazolium hexafluorophosphate ([C(4)mim][PF6]), 1-hexyl-3-methylimidazolium chloride ([C(6)mim]Cl), and 1-hexyl-3-methylimidazolium hexafluorophosphate ([C(6)mim][PF6]), and binary mixtures thereof, have been assigned using ab initio MP2...
Czech Academy of Sciences Publication Activity Database
Ma, D.; Friák, Martin; von Pezold, J.; Raabe, D.; Neugebauer, J.
2015-01-01
Roč. 85, FEB (2015), s. 53-66. ISSN 1359-6454 Institutional support: RVO:68081723 Keywords : Solid-solution strengthening * DFT * Peierls–Nabarro model * Ab initio * Al alloy s Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 4.465, year: 2014
Czech Academy of Sciences Publication Activity Database
Dračínský, Martin; Möller, H. M.; Exner, T. E.
2013-01-01
Roč. 9, č. 8 (2013), s. 3806-3815. ISSN 1549-9618 R&D Projects: GA ČR GA13-24880S Institutional support: RVO:61388963 Keywords : ab initio molecular dynamics * NMR spectroscopy * DFT calculations * hydration Subject RIV: CF - Physical ; Theoretical Chemistry Impact factor: 5.310, year: 2013
Ab Initio Calculations of Deuterium Isotope Effects on Chemical Shifts of Salt-Bridged Lysines
DEFF Research Database (Denmark)
Ullah, Saif; Ishimoto, Takayoshi; Williamson, Mike P.;
2011-01-01
Deuterium isotope effects measure the change in chemical shift on substitution of a proton by deuterium. They have been calculated by direct treatment of the H/D nuclear quantum effect using a multicomponent ab initio molecular orbital method based on a non-Born−Oppenheimer approximation. This me...
Study on the surface hydroxyl group on solid breeding materials by ab-initio calculations
Energy Technology Data Exchange (ETDEWEB)
Tanaka, Satoru; Taniguchi, Masaki [Tokyo Univ. (Japan). Faculty of Engineering
1996-10-01
The nature of -OH on the surface of Li{sub 2}O was analyzed with the ab-initio quantum chemical calculation technique. Calculation results showed that the stretching vibration of O-H is affected by the chemical species around the -OH. (author)
Lee, Timothy J.; Langhoff, Stephen R. (Technical Monitor)
1995-01-01
The ability of modern state-of-the art ab initio quantum chemical techniques to characterize reliably the gas-phase molecular structure, vibrational spectrum, electronic spectrum, and thermal stability of chlorine oxide and nitrogen oxide species will be demonstrated by presentation of some example studies. In particular the geometrical structures, vibrational spectra, and heats of formation Of ClNO2, CisClONO, and trans-ClONO are shown to be in excellent agreement with the available experimental data, and where the experimental data are either not known or are inconclusive, the ab initio results are shown to fill in the gaps and to resolve the experimental controversy. In addition, ab initio studies in which the electronic spectra and the characterization of excited electronic states of ClONO2, HONO2, ClOOC17 ClOOH, and HOOH will also be presented. Again where available, the ab initio results are compared to experimental observations, and are used to aid in the interpretation of the experimental studies.
DEFF Research Database (Denmark)
Cybulski, Hubert; Fernandez, Berta; Henriksen, Christian; Felker, Peter M.
2012-01-01
We evaluate the phenylacetylene-argon intermolecular potential energy surface by fitting a representative number of ab initio interaction energies to an analytic function. These energies are calculated at a grid of intermolecular geometries, using the CCSD(T) method and the aug-cc-pVDZ basis set...
Predicting materials for solar energy conversion: ab-initio spectroscopy of heterogeneous interfaces
Galli, Giulia
We will discuss some progress in predicting materials for solar energy conversion using ab initio calculations, in particular we will focus on heterogeneous interfaces between photo-electrodes and water and between nanocomposites. We will also address the problem of building much needed tighter connections between computational and laboratory experiments.
Ab Initio Investigations of the C2F4S Isomers and of Their Interconversions
DEFF Research Database (Denmark)
Shim, Irene; Vallano-Lorenzo, Sandra; Lisbona-Martin, Pilar;
2003-01-01
The transition states and the activation energies for the unobserved isomerization reactions between the three possible C2F4S isomers with divalent sulfur, trifluorothioacetyl fluoride 1, tetrafluorothiirane 2, and trifluoroethenesulfenyl fluoride 3, have been determined by ab initio Hartree-Fock...
Chan, Garnet Kin-Lic; Keselman, Anna; Nakatani, Naoki; Li, Zhendong; White, Steven R
2016-07-01
Current descriptions of the ab initio density matrix renormalization group (DMRG) algorithm use two superficially different languages: an older language of the renormalization group and renormalized operators, and a more recent language of matrix product states and matrix product operators. The same algorithm can appear dramatically different when written in the two different vocabularies. In this work, we carefully describe the translation between the two languages in several contexts. First, we describe how to efficiently implement the ab initio DMRG sweep using a matrix product operator based code, and the equivalence to the original renormalized operator implementation. Next we describe how to implement the general matrix product operator/matrix product state algebra within a pure renormalized operator-based DMRG code. Finally, we discuss two improvements of the ab initio DMRG sweep algorithm motivated by matrix product operator language: Hamiltonian compression, and a sum over operators representation that allows for perfect computational parallelism. The connections and correspondences described here serve to link the future developments with the past and are important in the efficient implementation of continuing advances in ab initio DMRG and related algorithms. PMID:27394094
Chan, Garnet Kin-Lic; Keselman, Anna; Nakatani, Naoki; Li, Zhendong; White, Steven R.
2016-07-01
Current descriptions of the ab initio density matrix renormalization group (DMRG) algorithm use two superficially different languages: an older language of the renormalization group and renormalized operators, and a more recent language of matrix product states and matrix product operators. The same algorithm can appear dramatically different when written in the two different vocabularies. In this work, we carefully describe the translation between the two languages in several contexts. First, we describe how to efficiently implement the ab initio DMRG sweep using a matrix product operator based code, and the equivalence to the original renormalized operator implementation. Next we describe how to implement the general matrix product operator/matrix product state algebra within a pure renormalized operator-based DMRG code. Finally, we discuss two improvements of the ab initio DMRG sweep algorithm motivated by matrix product operator language: Hamiltonian compression, and a sum over operators representation that allows for perfect computational parallelism. The connections and correspondences described here serve to link the future developments with the past and are important in the efficient implementation of continuing advances in ab initio DMRG and related algorithms.
Ab initio molecular dynamics simulation of hydrogen fluoride at several thermodynamic states
DEFF Research Database (Denmark)
Kreitmeir, M.; Bertagnolli, H.; Mortensen, Jens Jørgen;
2003-01-01
Liquid hydrogen fluoride is a simple but interesting system for studies of the influence of hydrogen bonds on physical properties. We have performed ab initio molecular dynamics simulations of HF at several thermodynamic states, where we examine the microscopic structure of the liquid as well as...
Timko, Jeff; Kuyucak, Serdar
2012-11-01
Polarization is an important component of molecular interactions and is expected to play a particularly significant role in inhomogeneous environments such as pores and interfaces. Here we investigate the effects of polarization in the gramicidin A ion channel by performing quantum mechanics/molecular mechanics molecular dynamics (MD) simulations and comparing the results with those obtained from classical MD simulations with non-polarizable force fields. We consider the dipole moments of backbone carbonyl groups and channel water molecules as well as a number of structural quantities of interest. The ab initio results show that the dipole moments of the carbonyl groups and water molecules are highly sensitive to the hydrogen bonds (H-bonds) they participate in. In the absence of a K+ ion, water molecules in the channel are quite mobile, making the H-bond network highly dynamic. A central K+ ion acts as an anchor for the channel waters, stabilizing the H-bond network and thereby increasing their average dipole moments. In contrast, the K+ ion has little effect on the dipole moments of the neighboring carbonyl groups. The weakness of the ion-peptide interactions helps to explain the near diffusion-rate conductance of K+ ions through the channel. We also address the sampling issue in relatively short ab initio MD simulations. Results obtained from a continuous 20 ps ab initio MD simulation are compared with those generated by sampling ten windows from a much longer classical MD simulation and running each window for 2 ps with ab initio MD. Both methods yield similar results for a number of quantities of interest, indicating that fluctuations are fast enough to justify the short ab initio MD simulations.
Ab initio study of the optical phonons in 1D antiferromagnet Ca2CuO3
Hoang, Nam Nhat; Nguyen, Thu Hang; Nguyen, Chau
2008-01-01
In the spin 1/2 antiferromagnetic Ca2CuO3 the optical excitation along 1D a Cu-O chain showed the enrichment of forbidden peaks that could not be explained by the group theory. We present the cluster-model ab initio study of these optical phonons based on the Hartree-Fock SCF calculation with the 3-21G basic set. The obtained results showed very good agreement with the observed data. The Cu-O resonances generally showed the lower shifts in Ca2CuO3 than in pure CuO and were primarily composed ...
Directory of Open Access Journals (Sweden)
R.M. Balabai
2016-06-01
Full Text Available Within the methods of density functional theory and ab initio pseudopotential, we have obtained the spatial distributions of the density of valence electron and the electronic energy spectrum for the small clusters from the atoms of Cu, Ni, Co, O, Si with the aim to determine the mechanisms of their high catalytic activity. Electron’s levels of catalyst guide course of chemical reaction. We explored, that the organization of electronic states of nanocatalysts on the basis of transition metals possible control by changing the spatial organization of clusters and adding electronegative atoms.
Ab initio study of ferromagnetic La0.5Ba0.5CoO3
Indian Academy of Sciences (India)
Umesh V Waghmare
2003-10-01
We study structure and magnetic properties of La0.5Ba0.5CoO3 (LBCO) using ab initio density functional theory (DFT) method based on pseudopotentials and a plane-wave basis. We find the cubic structure of LBCO is ferromagnetic and lower in energy than the nonmagnetic rhombohedral structure. Through the calculation of -point phonons of LBCO in the cubic structure, we determine its structural instabilities and find that they correspond to the structural phase transition accompanying a para-ferromagnetic transition observed recently.
International Nuclear Information System (INIS)
Highlights: ► Halon 1113, potential ozone depleting gas, vibrational eigenstates and intensity. ► FT-IR experimental and theoretical study of chlorotrifluoroethene. ► Ab initio calculations at MP2 and CCSD(T) levels with cc-pVTZ and ANO basis sets. ► Equilibrium geometry and harmonic force field. ► Full CCSD(T) and hybrid anharmonic force fields. - Abstract: Halon 1113 (chlorotrifluoroethene), used in the synthesis of fluorocarbon-based polymers, has been recently detected in the atmosphere and it is a potential source of chlorine atoms. In this work, the vibrational properties of chlorotrifluoroethene are studied in the 125–5000 cm−1 region by coupling Fourier-transform infrared spectroscopy and high-level ab initio calculations. The vibrational analysis is performed over the whole spectral range and band intensities are obtained in the range 400–3100 cm−1. Ab initio calculations of the anharmonic force field are performed at the coupled cluster level of theory employing either cc-pVTZ or ANO basis sets. Vibration perturbation theory is applied to obtain spectroscopic parameters from the computed anharmonic force fields. The present results provide a solid interpretation of chlorotrifluoroethene vibrational spectrum, and they represent a significant reference for future studies on this molecule, being also the first published data on absorption cross sections and ab initio calculations.
Determination of a silane intermolecular force field potential model from an ab initio calculation
International Nuclear Information System (INIS)
Intermolecular interaction potentials of the silane dimer in 12 orientations have been calculated by using the Hartree-Fock (HF) self-consistent theory and the second-order Moeller-Plesset (MP2) perturbation theory. We employed basis sets from Pople's medium-size basis sets [up to 6-311++G(3df, 3pd)] and Dunning's correlation consistent basis sets (up to the triply augmented correlation-consistent polarized valence quadruple-zeta basis set). We found that the minimum energy orientations were the G and H conformers. We have suggested that the Si-H attractions, the central silicon atom size, and electronegativity play essential roles in weakly binding of a silane dimer. The calculated MP2 potential data were employed to parametrize a five-site force field for molecular simulations. The Si-Si, Si-H, and H-H interaction parameters in a pairwise-additive, site-site potential model for silane molecules were regressed from the ab initio energies.
Ab initio search for global minimum structures of neutral and anionic hydrogenated Li5 clusters
International Nuclear Information System (INIS)
Highlights: • Stochastic search method is used to obtain global minimum of hydrogenated clusters. • The anionic hydrogenated Li5 clusters are firstly studied. • The fragmentation channels and energies of H atom and H2 dimer are investigated. • In hydrogenated Li5 clusters are easier to fragmentation the H2 dimer than H atom. • Li5Hn clusters is too high for the reversible hydrogen storage systems. - Abstract: The structure and some electronic properties of neutral and anionic Li5Hn (n = 0–6) clusters have been studied by using the stochastic search method with the B3LYP/6-31G level of theory. After searching possible isomers, first few isomers with the lowest energy have been recalculated by the B3LYP/6-311G++(2d,2p) and CCSD(T)/6-311G++(2d,2p) level of theory. The method used in this study has been compared with the previously reported ab initio calculations, and its reliability has been confirmed. The anionic Li5Hn (n = 0–6) clusters are reported in this study for the first time. Our results show that in general, stability increases with increasing number of hydrogen atoms. The fragmentation energies of hydrogenated Li5 clusters are easier to fragmentation the two hydrogen atoms than one hydrogen atom in hydrogenated clusters, and it is too high for the reversible hydrogen storage systems
Ab initio kinetics studies of hydrogen atom abstraction from methyl propanoate.
Tan, Ting; Yang, Xueliang; Ju, Yiguang; Carter, Emily A
2016-02-14
The kinetics of hydrogen abstraction by five radicals (H, CH3, O((3)P), OH, and HO2) from a biodiesel surrogate, methyl propanoate (MP), is theoretically investigated. We employ high-level ab initio quantum chemistry methods, coupled-cluster singles and doubles with perturbative triples correction (CCSD(T)) and multi-reference singles and doubles configuration interaction (MRSDCI) with the Davidson-Silver (DS) correction, and obtain chemically accurate reaction energetics. Overall, MRSDCI + DS predicts comparable energetics to CCSD(T) for MP + H/CH3/O/OH. The rate constants are computed using transition state theory (TST-Rice-Ramsperger-Kassel-Marcus theory) in conjunction with the separable-hindered-rotor approximation, variable reaction coordinate TST, and the multi-structure all-structure (MS-AS) approach. A simplified method, semi-multi-structure, is also employed for MP + OH/HO2, and the rate coefficients with this less expensive method are in good agreement with the results obtained with the MS-AS method. The fitted modified Arrhenius expressions are provided over a temperature range of 250 to 2000 K. The predicted rate coefficients for MP + OH agree remarkably well with experimental data over a wide temperature range. Branching ratio analysis of all the studied reactions shows that abstractions of the secondary H atoms within MP are expected to dominate the consumption of fuel at low temperatures, and the contributions of abstractions from the two methyl groups increase with temperature for all abstracting radicals. PMID:26796249
Brooks, M. S. S.; Gasche, T.; Auluck, S.; Nordström, L.; Severin, L.; Trygg, J.; Johansson, B.
1991-11-01
The interaction, KRM, between the rare-earth 4f moment and the transition-metal 3d moments in rare-earth transition-metal intermetallics is shown to depend upon the R-5d moment, which is due to 3d-5d hybridization, and local 4f-5d exchange integrals. Both the R-5d moment and KRM may be calculated ab initio from the local spin-density approximation to density functional theory in self-consistent energy-band calculations with the localized 4f-moments fixed at their Russel-Saunders values. Detailed examples are given for the RFe2 (R=Gd-Yb) series. The exchange integrals are similar to those entering into the density functional version of Stoner theory and their energy dependence must be treated carefully. The calculated local exchange integrals are shown to be related to the molecular fields derived from spin Hamiltonians, hence to the spin-wave spectra. Reasonable agreement with values of the molecular fields extracted from inelastic neutron scattering and high field susceptibility measurements is obtained.
PSI3: an open-source Ab Initio electronic structure package.
Crawford, T Daniel; Sherrill, C David; Valeev, Edward F; Fermann, Justin T; King, Rollin A; Leininger, Matthew L; Brown, Shawn T; Janssen, Curtis L; Seidl, Edward T; Kenny, Joseph P; Allen, Wesley D
2007-07-15
PSI3 is a program system and development platform for ab initio molecular electronic structure computations. The package includes mature programming interfaces for parsing user input, accessing commonly used data such as basis-set information or molecular orbital coefficients, and retrieving and storing binary data (with no software limitations on file sizes or file-system-sizes), especially multi-index quantities such as electron repulsion integrals. This platform is useful for the rapid implementation of both standard quantum chemical methods, as well as the development of new models. Features that have already been implemented include Hartree-Fock, multiconfigurational self-consistent-field, second-order Møller-Plesset perturbation theory, coupled cluster, and configuration interaction wave functions. Distinctive capabilities include the ability to employ Gaussian basis functions with arbitrary angular momentum levels; linear R12 second-order perturbation theory; coupled cluster frequency-dependent response properties, including dipole polarizabilities and optical rotation; and diagonal Born-Oppenheimer corrections with correlated wave functions. This article describes the programming infrastructure and main features of the package. PSI3 is available free of charge through the open-source, GNU General Public License. PMID:17420978
The role of anharmonicity in the ab-initio phase diagram of calcium
di Gennaro, Marco; Saha, Srijan Kumar; Verstraete, Matthieu Jean
2013-03-01
In the 32-119 GPa pressure range and at room temperature, a simple cubic phase was reported for calcium in many different experiments. Standard linear response theory, both within density functional perturbation theory and frozen phonon calculations, presents dynamical instabilities for simple cubic in the whole pressure range. Many other possible candidate phases, as well as several possible stabilization mechanisms for simple cubic phase, have been proposed as the result of ab-initio predictions but the role of temperature on the relative stability of the different phases has not been investigated systematically. We revisit the stability of three candidate phases of calcium for the intermediate pressure range and for various value of temperatures, taking explicitly into account thermal corrections relative to electronic as well as phononic entropy and anharmonic contributions. This corrects the discrepancies among previous theoretical results and experiments, and presents a different picture of the temperature driven phase transition, which results from dynamical anharmonic stabilization of simple cubic and de-stabilization of the tetragonal phase. Transport quantities are calculated in the stabilized phases, to provide additional points of comparison with experiments.
Role of Dynamical Instability in the Ab Initio Phase Diagram of Calcium
Di Gennaro, Marco; Saha, Srijan Kumar; Verstraete, Matthieu J.
2013-07-01
In the 32-119 GPa pressure range and at room temperature, a simple cubic phase was reported for calcium in many different experiments. Standard linear response theory, both within density functional perturbation theory and frozen phonon calculations, presents dynamical instabilities for the simple cubic structure in the whole pressure range. Many other possible candidate phases, as well as several possible stabilization mechanisms for the simple cubic phase, have been proposed as the result of ab initio predictions but the role of temperature on the relative stability of the different phases has not been systematically investigated. We revisit the stability of the three most important candidate phases of calcium for the intermediate pressure range and for various temperatures, taking explicitly into account thermal corrections relative to electronic as well as phononic entropy and anharmonic contributions. This corrects the discrepancies among previous theoretical results and experiments and presents a different picture of the temperature driven phase transition, which results from dynamical anharmonic stabilization of simple cubic and destabilization of the tetragonal phase.
Relaxation of Small Molecules：an ab initio Study
Institute of Scientific and Technical Information of China (English)
CAOYi－Gang; A.Antons; 等
2002-01-01
Using an ab inito total energy and force method,we have relaxed several group IV and group V elemental clusters,in detail the arsenic and antimony dimers,silicon,phosphorus,arsenic and antimony tetraners,The obtained bond lengths and cohesive energies are more accurate than other calculating methods,and in excellent agreement with the experimental results.
Bruneval, Fabien; Hamed, Samia M.; Neaton, Jeffrey B.
2015-01-01
The predictive power of the ab initio Bethe-Salpeter equation (BSE) approach, rigorously based on many-body Green's function theory but incorporating information from density functional theory, has already been demonstrated for the optical gaps and spectra of solid-state systems. Interest in photoactive hybrid organic/inorganic systems has recently increased, and so has the use of the BSE for computing neutral excitations of organic molecules. However, no systematic benchmarks of the BSE for ...
Energy Technology Data Exchange (ETDEWEB)
Wagner, Jan-Martin
2004-10-14
In this dissertation, ab-initio investigations of the strain influence on vibrational properties of GaN and AlN as well as of short-period GaN/AlN superlattices are presented. Based on densityfunctional theory and density-functional perturbation theory, for differently strained structures complete phonon spectra and related properties are calculated using the local-density approximation and norm-conserving pseudopotentials. (orig.)
Allen, B. Danette; Alexandrov, Natalia
2016-01-01
Incremental approaches to air transportation system development inherit current architectural constraints, which, in turn, place hard bounds on system capacity, efficiency of performance, and complexity. To enable airspace operations of the future, a clean-slate (ab initio) airspace design(s) must be considered. This ab initio National Airspace System (NAS) must be capable of accommodating increased traffic density, a broader diversity of aircraft, and on-demand mobility. System and subsystem designs should scale to accommodate the inevitable demand for airspace services that include large numbers of autonomous Unmanned Aerial Vehicles and a paradigm shift in general aviation (e.g., personal air vehicles) in addition to more traditional aerial vehicles such as commercial jetliners and weather balloons. The complex and adaptive nature of ab initio designs for the future NAS requires new approaches to validation, adding a significant physical experimentation component to analytical and simulation tools. In addition to software modeling and simulation, the ability to exercise system solutions in a flight environment will be an essential aspect of validation. The NASA Langley Research Center (LaRC) Autonomy Incubator seeks to develop a flight simulation infrastructure for ab initio modeling and simulation that assumes no specific NAS architecture and models vehicle-to-vehicle behavior to examine interactions and emergent behaviors among hundreds of intelligent aerial agents exhibiting collaborative, cooperative, coordinative, selfish, and malicious behaviors. The air transportation system of the future will be a complex adaptive system (CAS) characterized by complex and sometimes unpredictable (or unpredicted) behaviors that result from temporal and spatial interactions among large numbers of participants. A CAS not only evolves with a changing environment and adapts to it, it is closely coupled to all systems that constitute the environment. Thus, the ecosystem that
Guo, Xun; Zhang, Xitong; Zhao, Shijun; Huang, Qing; Xue, Jianming
2016-01-01
Density functional theory (DFT) calculation is employed to study the adsorption properties of Pb and Cu on recently synthesized two-dimensional materials MXenes, including Ti3C2, V2C1 and Ti2C1. The influence of surface decoration with functional groups such as H, OH and F have also been investigated. Most of these studied MXenes exhibit excellent capability to adsorb Pb and Cu, especially the adsorption capacity of Pb on Ti2C1 is as high as 2560 mg g(-1). Both the binding energies and the adsorption capacities are sensitive to the functional groups attached to the MXenes' surface. Ab initio molecular dynamics (ab-init MD) simulation confirms that Ti2C1 remains stable at room temperature after adsorbing Pb atoms. Our calculations imply that these newly emerging two-dimensional MXenes are promising candidates for wastewater treatment and ion separation. PMID:26602974
International Nuclear Information System (INIS)
The finite-temperature phase diagram of hydrogen in the region of phase IV and its neighborhood was studied using the ab initio molecular dynamics (MD) and the ab initio path-integral molecular dynamics (PIMD). The electronic structures were analyzed using the density-functional theory (DFT), the random-phase approximation, and the diffusion Monte Carlo (DMC) methods. Taking the state-of-the-art DMC results as benchmark, comparisons of the energy differences between structures generated from the MD and PIMD simulations, with molecular and dissociated hydrogens, respectively, in the weak molecular layers of phase IV, indicate that standard functionals in DFT tend to underestimate the dissociation barrier of the weak molecular layers in this mixed phase. Because of this underestimation, inclusion of the quantum nuclear effects (QNEs) in PIMD using electronic structures generated with these functionals leads to artificially dissociated hydrogen layers in phase IV and an error compensation between the neglect of QNEs and the deficiencies of these functionals in standard ab initio MD simulations exists. This analysis partly rationalizes why earlier ab initio MD simulations complement so well the experimental observations. The temperature and pressure dependencies for the stability of phase IV were also studied in the end and compared with earlier results
Chen, Ji; Ren, Xinguo; Li, Xin-Zheng; Alfè, Dario; Wang, Enge
2014-07-14
The finite-temperature phase diagram of hydrogen in the region of phase IV and its neighborhood was studied using the ab initio molecular dynamics (MD) and the ab initio path-integral molecular dynamics (PIMD). The electronic structures were analyzed using the density-functional theory (DFT), the random-phase approximation, and the diffusion Monte Carlo (DMC) methods. Taking the state-of-the-art DMC results as benchmark, comparisons of the energy differences between structures generated from the MD and PIMD simulations, with molecular and dissociated hydrogens, respectively, in the weak molecular layers of phase IV, indicate that standard functionals in DFT tend to underestimate the dissociation barrier of the weak molecular layers in this mixed phase. Because of this underestimation, inclusion of the quantum nuclear effects (QNEs) in PIMD using electronic structures generated with these functionals leads to artificially dissociated hydrogen layers in phase IV and an error compensation between the neglect of QNEs and the deficiencies of these functionals in standard ab initio MD simulations exists. This analysis partly rationalizes why earlier ab initio MD simulations complement so well the experimental observations. The temperature and pressure dependencies for the stability of phase IV were also studied in the end and compared with earlier results. PMID:25028021
Ab initio STM and STS simulations on magnetic and nonmagnetic metallic surfaces
Energy Technology Data Exchange (ETDEWEB)
Dick, Alexey
2008-04-14
The aim of this work was to provide an in-depth understanding of a new generation of scan- ning tunneling microscopy experiments, performed employing different regimes of the STM: the spectroscopy-mode (the so-called Fourier Transformed STM, FT-STM), and the spin-sensitive mode (the so-called spin-polarized STM, SP-STM). In the present thesis ab initio tools are proposed that are based on DFT calculations to theoretically predict and analyze such types of the STM. The first part of this thesis focusses on the simulation of FT-STM, the mode that allows to probe local dispersion properties of the electrons at the surface. In order to provide the theoretical counterpart of the experimental FT-STM spectra we have introduced a new implicit approach that is derived from Tersoff-Hamann theory of the STM. The importance of an accurate description of surface wavefunctions at 5-15 A above the surface as well as the spurious quantum- size effects have been discussed in detail together with approaches to obtain converged FT-STM images. We applied our method to FT-STM experiments performed on Ag(110) surfaces. In the second part of the thesis we discuss the modeling of the spin-resolved STM, the mode that allows to characterize the magnetic structure of a surface. As a case system we studied here the magnetically-ordered transition-metal nitride surface Mn{sub 3}N{sub 2}(010). Because SP-STM experiments did not allow a conclusive understanding of the surface structure, we have first employed ab initio thermodynamics to figure out the most stable magnetic and atomic configuration of the surface that are consistent with experiments. To simulate SP-STM images on the most stable Mn{sub 3}N{sub 2}(010) surface we have employed the spin-generalized transfer-Hamiltonian formalism, assuming that the tip wavefunctions have dominant radial symmetry (s-like tip). (orig.)
Trevisanutto, Paolo E; Vignale, Giovanni
2016-05-28
Ab initio electronic structure calculations of two-dimensional layered structures are typically performed using codes that were developed for three-dimensional structures, which are periodic in all three directions. The introduction of a periodicity in the third direction (perpendicular to the layer) is completely artificial and may lead in some cases to spurious results and to difficulties in treating the action of external fields. In this paper we develop a new approach, which is "native" to quasi-2D materials, making use of basis function that are periodic in the plane, but atomic-like in the perpendicular direction. We show how some of the basic tools of ab initio electronic structure theory - density functional theory, GW approximation and Bethe-Salpeter equation - are implemented in the new basis. We argue that the new approach will be preferable to the conventional one in treating the peculiarities of layered materials, including the long range of the unscreened Coulomb interaction in insulators, and the effects of strain, corrugations, and external fields. PMID:27250294
International Nuclear Information System (INIS)
Tensor factorization of the 2-electron integral matrix is a well-known technique for reducing the computational scaling of ab initio electronic structure methods toward that of Hartree-Fock and density functional theories. The simplest factorization that maintains the positive semidefinite character of the 2-electron integral matrix is the Cholesky factorization. In this paper, we introduce a family of positive semidefinite factorizations that generalize the Cholesky factorization. Using an implementation of the factorization within the parametric 2-RDM method [D. A. Mazziotti, Phys. Rev. Lett. 101, 253002 (2008)], we study several inorganic molecules, alkane chains, and potential energy curves and find that this generalized factorization retains the accuracy and size extensivity of the Cholesky factorization, even in the presence of multi-reference correlation. The generalized family of positive semidefinite factorizations has potential applications to low-scaling ab initio electronic structure methods that treat electron correlation with a computational cost approaching that of the Hartree-Fock method or density functional theory
Energy Technology Data Exchange (ETDEWEB)
Hoy, Erik P.; Mazziotti, David A., E-mail: damazz@uchicago.edu [Department of Chemistry and The James Franck Institute, The University of Chicago, Chicago, Illinois 60637 (United States)
2015-08-14
Tensor factorization of the 2-electron integral matrix is a well-known technique for reducing the computational scaling of ab initio electronic structure methods toward that of Hartree-Fock and density functional theories. The simplest factorization that maintains the positive semidefinite character of the 2-electron integral matrix is the Cholesky factorization. In this paper, we introduce a family of positive semidefinite factorizations that generalize the Cholesky factorization. Using an implementation of the factorization within the parametric 2-RDM method [D. A. Mazziotti, Phys. Rev. Lett. 101, 253002 (2008)], we study several inorganic molecules, alkane chains, and potential energy curves and find that this generalized factorization retains the accuracy and size extensivity of the Cholesky factorization, even in the presence of multi-reference correlation. The generalized family of positive semidefinite factorizations has potential applications to low-scaling ab initio electronic structure methods that treat electron correlation with a computational cost approaching that of the Hartree-Fock method or density functional theory.
Sanson, Andrea; Giarola, Marco; Rossi, Barbara; Mariotto, Gino; Cazzanelli, Enzo; Speghini, Adolfo
2012-12-01
The vibrational properties of yttrium orthovanadate (YVO4) single crystals, with tetragonal zircon structure, have been investigated by means of polarized micro-Raman spectroscopy and ab initio calculations. Raman spectra were taken at different polarizations and orientations carefully set by the use of a micromanipulator, so that all of the twelve Raman-active modes, expected on the basis of the group theory, were selected in turn and definitively assigned in wave number and symmetry. In particular the Eg(4) mode, assigned incorrectly in previous literature, has been observed at 387 cm-1. Moreover, the very weak Eg(1) mode, peaked at about 137 cm-1, was clearly observed only under some excitation wavelengths, and its peculiar Raman excitation profile was measured within a wide region of the visible. Finally, ab initio calculations based on density-functional theory have been performed in order to determine both Raman and infrared vibrational modes and to corroborate the experimental results. The rather good agreement between computational and experimental frequencies is slightly better than in previous computational works and supports our experimental symmetry assignments.
Hoy, Erik P; Mazziotti, David A
2015-08-14
Tensor factorization of the 2-electron integral matrix is a well-known technique for reducing the computational scaling of ab initio electronic structure methods toward that of Hartree-Fock and density functional theories. The simplest factorization that maintains the positive semidefinite character of the 2-electron integral matrix is the Cholesky factorization. In this paper, we introduce a family of positive semidefinite factorizations that generalize the Cholesky factorization. Using an implementation of the factorization within the parametric 2-RDM method [D. A. Mazziotti, Phys. Rev. Lett. 101, 253002 (2008)], we study several inorganic molecules, alkane chains, and potential energy curves and find that this generalized factorization retains the accuracy and size extensivity of the Cholesky factorization, even in the presence of multi-reference correlation. The generalized family of positive semidefinite factorizations has potential applications to low-scaling ab initio electronic structure methods that treat electron correlation with a computational cost approaching that of the Hartree-Fock method or density functional theory. PMID:26277123
A Comparative Study of Ab-Initio Thermal Conductivity Approaches: The Case of Cubic Boron Nitride
Mukhopadhyay, Saikat; Lindsay, Lucas; Broido, David; Stewart, Derek
2013-03-01
Given its high strength and large thermal conductivity, cubic boron nitride (cBN) provides an important complement to diamond films for heat spreading applications. However, cBN, in contrast to diamond, is a polar material with significant LO-TO splitting in the phonon dispersion. In this talk, we examine the lattice thermal conductivity of cBN using several approaches based on first principles calculations. These approaches include: (1) an analytic modified Callaway-Debye model that relies on parameters from ab-initio harmonic force constants, (2) a fully self-consistent calculation of the thermal conductivity that links an iterative solution of the phonon Boltzmann transport equation (BTE) with harmonic and anharmonic interatomic force constants. The force constants for the BTE are calculated using two approaches: density functional perturbation theory and a real-space supercell approach. We will compare the results from these approaches, highlight the role of normal phonon-phonon scattering, and also examine the impact of optical modes and LO-TO splitting. In addition, we will discuss how isotope scattering affects thermal conductivity and compare this to other boron nitride structures (hexagonal BN, BN sheets and BN nanotubes).
International Nuclear Information System (INIS)
Utilizing a combination of ab initio density-functional theory and thermodynamics formalism, we have established the microscopic mechanisms for oxidation of the binary and ternary alloy surfaces and provided a clear explanation for the experimental results of the oxidation. We construct three-dimensional surface phase diagrams (SPDs) for oxygen adsorption on three different Nb-X(110) (X = Ti, Al or Si) binary alloy surfaces. On the basis of the obtained SPDs, we conclude a general microscopic mechanism for the thermodynamic oxidation, that is, under O-rich conditions, a uniform single-phase SPD (type I) and a nonuniform double-phase SPD (type II) correspond to the sustained complete selective oxidation and the non-sustained partial selective oxidation by adding the X element, respectively. Furthermore, by revealing the framework of thermodynamics for the oxidation mechanism of ternary alloys through the comparison of the surface energies of two separated binary alloys, we provide an understanding for the selective oxidation behavior of the Nb ternary alloy surfaces. Using these general microscopic mechanisms, one could predict the oxidation behavior of any binary and multi-component alloy surfaces based on thermodynamics considerations
Ab initio many-body calculations of nucleon-4He scattering with three-nucleon forces
Hupin, Guillaume; Navrátil, Petr; Quaglioni, Sofia; Calci, Angelo; Roth, Robert
2013-01-01
We extend the ab initio no-core shell model/resonating-group method to include three-nucleon (3N) interactions for the description of nucleon-nucleus collisions. We outline the formalism, give algebraic expressions for the 3N-force integration kernels, and discuss computational aspects of two alternative implementations. The extended theoretical framework is then applied to nucleon-4He scattering using similarity-renormalization-group (SRG) evolved nucleon-nucleon plus three-nucleon potentials derived from chiral effective field theory. We analyze the convergence properties of the calculated phase shifts and explore their dependence upon the SRG evolution parameter. We include up to six excited states of the 4He target and find significant effects from the inclusion of the chiral 3N force, e.g., it enhances the spin-orbit splitting between the 3/2- and 1/2- resonances and leads to an improved agreement with the phase shifts obtained from an accurate R-matrix analysis of the five-nucleon experimental data. We ...
An ab initio quantum chemical investigation of the structure and stability of ozone-water complexes
Energy Technology Data Exchange (ETDEWEB)
Kumar, Pradeep [Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016 (India); Sathyamurthy, N., E-mail: nsath@iitk.ac.in [Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016 (India); Indian Institute of Science Education and Research Mohali, Sector 81, SAS Nagar, Manauli 140306 (India)
2013-03-29
Highlights: ► Eclipse geometry most stable for the 1:1 ozone-water complex. ► Cyclic structure most stable for the 1:2 complex. ► Shift in the vertical electronic excitation energy of ozone due to hydration. - Abstract: Ab initio quantum chemical calculations have been carried out to investigate the structure and stability of 1:1 and 1:2 ozone-water complexes. All the geometries have been optimized at the CCSD level of theory using aug-cc-pVDZ and aug-cc-pVTZ basis sets. The importance of correlation-consistent basis sets in deciding the nature of critical points on these complexes is emphasized. An analysis based on the dipole moment of the complexes and the charge distribution on atoms follows. The effect of ozone molecule on the structure and properties of water dimer is also investigated. Values of the vertical electronic excitation energy and the corresponding transition dipole moment have been calculated for the ozone-water complexes using the multi-reference-configuration-interaction method and the aug-cc-pVTZ basis set. The calculated shift in vibrational frequencies due to complex formation is compared with the earlier reported experimental and theoretical values.
Ab initio study of Cr interactions with point defects in bcc Fe
International Nuclear Information System (INIS)
Full text of publication follows. Ferritic martensitic steels are candidate structural materials for fast neutron reactors, and in particular high-Cr reduced-activation steels. In Fe-Cr alloys, Cr plays a major role in the radiation-induced evolution of the mechanical properties. Using ab initio calculations based on density functional theory, the properties of Cr in α-Fe have been investigated. The intrinsic point defect formation energies were found to be larger in model bcc Cr as compared to those in ferromagnetic bcc Fe. The interactions of Cr with point defects (vacancy and self interstitials) have been characterised. Single Cr atoms interact weakly with vacancies but significantly with self-interstitial atoms. Mixed interstitials of any interstitial symmetry are bound. Configurations where two Cr atoms are in nearest neighbour position are generally unfavourable in bcc Fe except when they are a part of a interstitial complex. Mixed interstitials do not have as strong directional stability as pure Fe interstitials have. The effects on the results using the atom description scheme of either the ultrasoft pseudo-potential (USPP) or the projector augmented wave (PAW) formalisms are connected to the differences in local magnetic moments that the two methods predict. As expected for the Fe-Cr system, the results obtained using the PAW method are more reliable than the ones obtained with USPP. (authors)
Oreshko, A P; Ovchinnikova, E N; Beutier, G; Collins, S P; Nisbet, G; Kolchinskaya, A M; Dmitrienko, V E
2012-06-20
Thermal-motion induced (TMI) scattering is caused by the influence of atomic displacements on electronic states in crystals and strongly depends on temperature. It corresponds to dipole-dipole resonant x-ray scattering, but is usually accompanied by dipole-quadrupole scattering. The phenomenological theory supposes the dipole-quadrupole term to be temperature independent (TI). As a result, the transformation of the energy spectra with temperature observed experimentally in ZnO and GaN corresponds to the interference between the TMI and TI terms. In the present paper the direct confirmation of this theoretical prediction is given. Ab initio molecular dynamics was used to simulate the sets of atomic sites at various temperatures followed by quantum mechanical calculation of resonant Bragg reflection energy spectra. The results of simulation are in excellent coincidence with experimental energy spectra of forbidden reflections and confirm the earlier phenomenological conjecture about the interference between the TI dipole-quadrupole and TMI dipole-dipole contributions to the resonant atomic factor. PMID:22627099
Silicon and Germanium Nanostructures for Photovoltaic Applications: Ab-Initio Results
Directory of Open Access Journals (Sweden)
Pulci Olivia
2010-01-01
Full Text Available Abstract Actually, most of the electric energy is being produced by fossil fuels and great is the search for viable alternatives. The most appealing and promising technology is photovoltaics. It will become truly mainstream when its cost will be comparable to other energy sources. One way is to significantly enhance device efficiencies, for example by increasing the number of band gaps in multijunction solar cells or by favoring charge separation in the devices. This can be done by using cells based on nanostructured semiconductors. In this paper, we will present ab-initio results of the structural, electronic and optical properties of (1 silicon and germanium nanoparticles embedded in wide band gap materials and (2 mixed silicon-germanium nanowires. We show that theory can help in understanding the microscopic processes important for devices performances. In particular, we calculated for embedded Si and Ge nanoparticles the dependence of the absorption threshold on size and oxidation, the role of crystallinity and, in some cases, the recombination rates, and we demonstrated that in the case of mixed nanowires, those with a clear interface between Si and Ge show not only a reduced quantum confinement effect but display also a natural geometrical separation between electron and hole.
Direct molecular simulation of nitrogen dissociation based on an ab initio potential energy surface
Energy Technology Data Exchange (ETDEWEB)
Valentini, Paolo, E-mail: vale0142@umn.edu; Schwartzentruber, Thomas E., E-mail: schwart@aem.umn.edu; Bender, Jason D., E-mail: jbender73@gmail.com; Nompelis, Ioannis, E-mail: nompelis@umn.edu; Candler, Graham V., E-mail: candler@umn.edu [Department of Aerospace Engineering and Mechanics, College of Science and Engineering, University of Minnesota, Minneapolis, Minnesota 55455 (United States)
2015-08-15
The direct molecular simulation (DMS) approach is used to predict the internal energy relaxation and dissociation dynamics of high-temperature nitrogen. An ab initio potential energy surface (PES) is used to calculate the dynamics of two interacting nitrogen molecules by providing forces between the four atoms. In the near-equilibrium limit, it is shown that DMS reproduces the results obtained from well-established quasiclassical trajectory (QCT) analysis, verifying the validity of the approach. DMS is used to predict the vibrational relaxation time constant for N{sub 2}–N{sub 2} collisions and its temperature dependence, which are in close agreement with existing experiments and theory. Using both QCT and DMS with the same PES, we find that dissociation significantly depletes the upper vibrational energy levels. As a result, across a wide temperature range, the dissociation rate is found to be approximately 4–5 times lower compared to the rates computed using QCT with Boltzmann energy distributions. DMS calculations predict a quasi-steady-state distribution of rotational and vibrational energies in which the rate of depletion of high-energy states due to dissociation is balanced by their rate of repopulation due to collisional processes. The DMS approach simulates the evolution of internal energy distributions and their coupling to dissociation without the need to precompute rates or cross sections for all possible energy transitions. These benchmark results could be used to develop new computational fluid dynamics models for high-enthalpy flow applications.
Rosenow, Phil; Tonner, Ralf
2016-05-01
The extent of hydrogen coverage of the Si(001) c(4 × 2) surface in the presence of hydrogen gas has been studied with dispersion corrected density functional theory. Electronic energy contributions are well described using a hybrid functional. The temperature dependence of the coverage in thermodynamic equilibrium was studied computing the phonon spectrum in a supercell approach. As an approximation to these demanding computations, an interpolated phonon approach was found to give comparable accuracy. The simpler ab initio thermodynamic approach is not accurate enough for the system studied, even if corrections by the Einstein model for surface vibrations are considered. The on-set of H2 desorption from the fully hydrogenated surface is predicted to occur at temperatures around 750 K. Strong changes in hydrogen coverage are found between 1000 and 1200 K in good agreement with previous reflectance anisotropy spectroscopy experiments. These findings allow a rational choice for the surface state in the computational treatment of chemical reactions under typical metal organic vapor phase epitaxy conditions on Si(001).
Direct molecular simulation of nitrogen dissociation based on an ab initio potential energy surface
International Nuclear Information System (INIS)
The direct molecular simulation (DMS) approach is used to predict the internal energy relaxation and dissociation dynamics of high-temperature nitrogen. An ab initio potential energy surface (PES) is used to calculate the dynamics of two interacting nitrogen molecules by providing forces between the four atoms. In the near-equilibrium limit, it is shown that DMS reproduces the results obtained from well-established quasiclassical trajectory (QCT) analysis, verifying the validity of the approach. DMS is used to predict the vibrational relaxation time constant for N2–N2 collisions and its temperature dependence, which are in close agreement with existing experiments and theory. Using both QCT and DMS with the same PES, we find that dissociation significantly depletes the upper vibrational energy levels. As a result, across a wide temperature range, the dissociation rate is found to be approximately 4–5 times lower compared to the rates computed using QCT with Boltzmann energy distributions. DMS calculations predict a quasi-steady-state distribution of rotational and vibrational energies in which the rate of depletion of high-energy states due to dissociation is balanced by their rate of repopulation due to collisional processes. The DMS approach simulates the evolution of internal energy distributions and their coupling to dissociation without the need to precompute rates or cross sections for all possible energy transitions. These benchmark results could be used to develop new computational fluid dynamics models for high-enthalpy flow applications
Ab initio computational study of vincristine as a biological active compound: NMR and NBO analyses
Directory of Open Access Journals (Sweden)
Shiva Joohari
2015-06-01
Full Text Available Vincristine is a biological active alkaloid that has been used clinically against a variety of neoplasms. In the current study we have theoretically investigated the magnetic properties of titled compound to predict physical and chemical properties of vincristine as a biological inhibitor. Ab initio computation using HF and B3LYP with 3-21G(d and 6-31G(d level of theory have been performed and then magnetic shielding tensor (, ppm, shielding asymmetry (, magnetic shielding anisotropy (aniso, ppm, the skew of a tensor (K, chemical shift anisotropy ( and chemical shift ( were calculated to indicate the details of the interaction mechanism between microtubules and vincristine. Moreover, EHOMO, ELUMO and Ebg were evaluated. The maximum and minimum values of Ebg were found in HF/3-21g and B3LYP/3-21g respectively. It was also uggested that O24, O37, O49 and O55 with minimum values of iso, are active sites of titled compound. Furthermore the calculated chemical shifts were compared with experimental data in DMSO and CDCl3 solvents.
International Nuclear Information System (INIS)
In the present paper we report a theoretical study on mechanistic photodissociation of acetaldehyde (CH3CHO). Stationary structures for H2 and CO eliminations in the ground state (S0) have been optimized with density functional theory method, which is followed by the intrinsic reaction coordinate and ab initio molecular dynamics calculations to confirm the elimination mechanism. Equilibrium geometries, transition states, and intersection structures for the C-C and C-H dissociations in excited states were determined by the complete-active-space self-consistent field (CASSCF) method. Based on the CASSCF optimized structures, the potential energy profiles for the dissociations were refined by performing the single-point calculations using the multireference configuration interaction method. Upon the low-energy irradiation of CH3CHO (265 nm1 C-C bond fission following intersystem crossing from the S1 state is the predominant channel and the minor channel, the ground-state elimination to CH4+CO after internal conversion (IC) from S1 to S0, could not be excluded. With the photon energy increasing, another pathway of IC, achieved via an S1/S0 intersection point resulting from the S1 C-C bond fission, becomes accessible and increases the yield of CH4+CO.
Energy Technology Data Exchange (ETDEWEB)
Monteseguro, V. [Departamento de Física and MALTA Consolider Team, Universidad de La Laguna, 38200 San Cristóbal de La Laguna, Santa Cruz de Tenerife (Spain); Rodríguez-Hernández, P.; Muñoz, A., E-mail: amunoz@ull.es [Departamento de Física and MALTA Consolider Team, Universidad de La Laguna, 38200 San Cristóbal de La Laguna, Santa Cruz de Tenerife (Spain); Instituto de Materiales y Nanotecnología. Universidad de La Laguna, 38200 San Cristóbal de La Laguna, Santa Cruz de Tenerife (Spain)
2015-12-28
The structural, elastic, and vibrational properties of yttrium aluminum garnet Y{sub 3}Al{sub 5}O{sub 12} are studied under high pressure by ab initio calculations in the framework of the density functional theory. The calculated ground state properties are in good agreement with the available experimental data. Pressure dependences of bond length and bulk moduli of the constituent polyhedra are reported. The evolution of the elastic constants and the major elastic properties, Young and shear modulus, Poisson's ratios, and Zener anisotropy ratio, are described. The mechanical stability is analyzed, on the light of “Born generalized stability criteria,” showing that the garnet is mechanically unstable above 116 GPa. Symmetries, frequencies, and pressure coefficients of the Raman-active modes are discussed on the basis of the calculated total and partial phonon density of states, which reflect the dynamical contribution of each atom. The relations between the phonon modes of Y{sub 3}Al{sub 5}O{sub 12} and the internal and external molecular modes of the different polyhedra are discussed. Infrared-active modes, as well as the silent modes, and their pressure dependence are also investigated. No dynamical instabilities were found below 116 GPa.
Ab initio molecular dynamics of the reaction of quercetin with superoxide radical
Lespade, Laure
2016-08-01
Superoxide plays an important role in biology but in unregulated concentrations it is implicated in a lot of diseases such as cancer or atherosclerosis. Antioxidants like flavonoids are abundant in plant and are good scavengers of superoxide radical. The modeling of superoxide scavenging by flavonoids from the diet still remains a challenge. In this study, ab initio molecular dynamics of the reaction of the flavonoid quercetin toward superoxide radical has been carried out using Car-Parrinello density functional theory. The study has proven different reactant solvation by modifying the number of water molecules surrounding superoxide. The reaction consists in the gift of a hydrogen atom of one of the hydroxyl groups of quercetin to the radical. When it occurs, it is relatively fast, lower than 100 fs. Calculations show that it depends largely on the environment of the hydroxyl group giving its hydrogen atom, the geometry of the first water layer and the presence of a certain number of water molecules in the second layer, indicating a great influence of the solvent on the reactivity.
Ab initio quasiparticle bandstructure of ABA and ABC-stacked graphene trilayers
Menezes, Marcos; Capaz, Rodrigo; Louie, Steven
2013-03-01
We obtain the quasiparticle band structure of ABA and ABC-stacked graphene trilayers through ab initio density functional theory (DFT) and many-body quasiparticle calculations within the GW approximation. To interpret our results, we fit the DFT and GW π bands to a low energy tight-binding model, which is found to reproduce very well the observed features near the K point. The values of the extracted hopping parameters are reported and compared with available theoretical and experimental data. For both stackings, the quasiparticle corrections lead to a renormalization of the Fermi velocity, an effect also observed in previous calculations on monolayer graphene. They also increase the separation between the higher energy bands, which is proportional to the nearest neighbor interlayer hopping parameter γ1. Both features are brought to closer agreement with experiment through the quasiparticle corrections. Finally, other effects, such as trigonal warping, electron-hole assymetry and energy gaps are discussed in terms of the associated parameters. This work was supported by the Brazilian funding agencies: CAPES, CNPq, FAPERJ and INCT-Nanomateriais de Carbono. It was also supported by NSF grant No. DMR10-1006184 and U.S. DOE under Contract No. DE-AC02-05CH11231.
A Simple ab Initio Model for the Hydrated Electron That Matches Experiment.
Kumar, Anil; Walker, Jonathan A; Bartels, David M; Sevilla, Michael D
2015-08-27
Since its discovery over 50 years ago, the "structure" and properties of the hydrated electron have been a subject for wonderment and also fierce debate. In the present work we seriously explore a minimal model for the aqueous electron, consisting of a small water anion cluster embedded in a polarized continuum, using several levels of ab initio calculation and basis set. The minimum energy "zero Kelvin" structure found for any 4-water (or larger) anion cluster, at any post-Hartree–Fock theory level, is very similar to a recently reported embedded-DFT-in-classical-water-MD simulation (Uhlig, Marsalek, and Jungwirth, J. Phys. Chem. Lett. 2012, 3, 3071−3075), with four OH bonds oriented toward the maximum charge density in a small central "void". The minimum calculation with just four water molecules does a remarkably good job of reproducing the resonance Raman properties, the radius of gyration derived from the optical spectrum, the vertical detachment energy, and the hydration free energy. For the first time we also successfully calculate the EPR g-factor and (low temperature ice) hyperfine couplings. The simple tetrahedral anion cluster model conforms very well to experiment, suggesting it does in fact represent the dominant structural motif of the hydrated electron. PMID:26275103
Ab initio Mechanism Study on the Reaction of Chlorine Atom with Formic Acid
Institute of Scientific and Technical Information of China (English)
于海涛; 付宏刚; 等
2003-01-01
The potential energy surface(PES) for the reaction of Cl atom with HCOOH is predicted using ab initio molecular orbital calculation methods at UQCIDS(T,full)6-311++G(3df,2p)//UMP2(full)/6-311+G(d,P) level of theory with zero-point vibrational energy (ZPVE) correction.The calculated results show that the reaction mechanism of Cl atom with formic acid is a C-site hydrogen abstraction reaction from cis-HOC(H)O molecule by Cl atom with a 3.73kJ/mol reaction barrier height,leading to the formation of cis-HOCO radical which will reacts with Cl atom or other molecules in such a reaction system.Because the reaction barrier height of O-site hydrogen abstraction reaction from cis-HOC(H)O molecule by Cl atom which leads to the formation of HCO2 radical is 67.95kJ/mol,it is a secondary reaction channel in experiment,This is in good agreement with the prediction based on the previous experiments.
Singh, Ram Sevak; Solanki, Ankit
2016-03-01
Silicon carbide nanotubes (SiCNTs) have received a great deal of scientific and commercial interest due to their intriguing properties that include high temperature stability and electronic properties. For their efficient and widespread applications, tuning of electronic properties of SiCNTs is an attractive study. In this article, electronic properties of sulphur doped (S-doped) zigzag (9 , 0) SiCNT is investigated by ab initio calculations based on density functional theory (DFT). Energy band structures and density of states of fully optimized undoped and doped structures with varying dopant concentration are calculated. S-doped on C-site of the nanotube exhibits a monotonic reduction of energy gap with increase in dopant concentration, and the nanotube transforms from semiconductor to metal at high dopant concentration. In case of S-doped on Si-site doping has less influence on modulating electronic structures, which results in reduction of energy gap up to a moderate doping concentration. Importantly, S preferential substitutes of Si-sites and the nanotube with S-doped on Si-site are energetically more stable as compared to the nanotube with S-doped on C-site. The study of tunable electronic properties in S-doped SiCNT may have potential in fabricating nanoelectronic devices, hydrogen storage and gas sensing applications.
Ab initio Stellar Astrophysics: Reliable Modeling of Cool White Dwarf Atmospheres
Kowalski, Piotr M
2010-01-01
Over the last decade {\\it ab initio} modeling of material properties has become widespread in diverse fields of research. It has proved to be a powerful tool for predicting various properties of matter under extreme conditions. We apply modern computational chemistry and materials science methods, including density functional theory (DFT), to solve lingering problems in the modeling of the dense atmospheres of cool white dwarfs ($T_{\\rm eff}\\rm <7000 \\, K$). Our work on the revision and improvements of the absorption mechanisms in the hydrogen and helium dominated atmospheres resulted in a new set of atmosphere models. By inclusion of the Ly-$\\rm \\alpha$ red wing opacity we successfully fitted the entire spectral energy distributions of known cool DA stars. In the subsequent work we fitted the majority of the coolest stars with hydrogen-rich models. This finding challenges our understanding of the spectral evolution of cool white dwarfs. We discuss a few examples, including the cool companion to the pulsar...
An ab initio study of magneto-electric coupling of YMnO3
International Nuclear Information System (INIS)
This paper proposes the direct calculation of the microscopic contributions to the magneto-electric coupling, using ab initio methods. The electrostrictive and the Dzyaloshinskii–Moriya contributions were evaluated individually. For this purpose a specific method was designed, combining density functional theory calculations and embedded fragment, explicitly correlated, quantum chemical calculations. This method allowed us to calculate the evolution of the magnetic couplings as a function of an applied electric field. We found that in YMnO3 the Dzyaloshinskii–Moriya contribution to the magneto-electric effect is three orders of magnitude weaker than the electrostrictive contribution. Strictive effects are thus dominant in the magnetic exchange evolution under an applied electric field, and by extension in the magneto-electric effect. These effects however, remain quite small, and the modifications of the magnetic excitations under an applied electric field will be difficult to observe experimentally. Another important conclusion is that it can be shown that the linear magneto-electric tensor is null due to the inter-layer symmetry operations. (paper)
Amorphous Ge quantum dots embedded in crystalline Si: ab initio results
International Nuclear Information System (INIS)
We study amorphous Ge quantum dots embedded in a crystalline Si matrix through structure modeling and simulation using ab initio density functional theory including spin–orbit interaction and quasiparticle effects. Three models are generated by replacing a spherical region within diamond Si by Ge atoms and creating a disordered bond network with appropriate density inside the Ge quantum dot. After total-energy optimisations of the atomic geometry we compute the electronic and optical properties. We find three major effects: (i) the resulting nanostructures adopt a type-I heterostructure character; (ii) the lowest optical transitions occur only within the Ge quantum dots, and do not involve or cross the Ge–Si interface. (iii) for larger amorphous Ge quantum dots, with diameters of about 2.0 and 2.7 nm, absorption peaks appear in the mid-infrared spectral region. These are promising candidates for intense luminescence at photon energies below the gap energy of bulk Ge. (paper)
Ab initio energetic study of oxide ceramics with rare-earth elements
Institute of Scientific and Technical Information of China (English)
WU Bo; Matvei Zinkevich; WANG Chong; Fritz Aldinger
2006-01-01
Ab initio energetic calculations based on the density functional theory (DFT) and the projector augmented wave method (PAW) for determining the polymorphisms of lanthanide sesquioxides Ln2O3 (where Ln = rare-earth element. Y,and Sc), LnMO3 perovskites (where M = Al and Ga), and Ln2B2O7 pyrochlores (where B = Ti, Zr, and Hf) were reported. The relative lattice stabilities agreed well with the critically assessed results or the experimental results except the C-type Ln2O3 with a cubic structure, for which the calculated total energies were considerably more negative. With the increase of the Ln3+-cation radius, the polymorphic structures showed a degenerative tendency. The tendencies and quantities of the enthalpies of formation of the ternary oxide ceramics synthesized from their constituent binary oxides reasonably agreed with the available experimental results, and valuable thermodynamic properties were afforded to the compound, for which no experimental data is available. The enthalpies of formation of both perovskites and pyrochlores tend to become more negative with the increase of the Ln3+-cation radius.
Ab initio investigation of the adsorption of atomic and molecular hydrogen on AlN nanotubes
International Nuclear Information System (INIS)
Highlights: • The adsorption characteristics of hydrogen inside the zigzag and armchair AlN nanotubes are explored. • The AlN nanotubes can store hydrogen up to 8.89 wt% with the average binding energies of 0.2–0.4 eV/H-2. - Abstract: The adsorption of atomic and molecular hydrogen on zigzag and armchair AlN nanotubes is investigated within the ab initio density functional theory. The adsorption configurations are magnetic when the H atom is adsorbed on the Al atom and the center of a hexagon. The total magnetic moment is 1.00 μB which comes from the H atom and the nearest neighbor N atoms. The barrier height of various adsorption configurations is very low, indicating that the adsorbed H atom can easily transform into other forms. The adsorption energies of hydrogen atoms to the zigzag and armchair AlN nanotubes are calculated at 25%, 50%, 75%, 100%, 133%, and 200% coverages, the most favorable adsorption configurations are 100% hydrogen coverages. The adsorption configuration of hydrogen molecule adsorbed on the Al atom is the most energetically favorable. Each Al atom is capable of binding one hydrogen molecule, corresponding to the hydrogen gravimetric density to 8.89 wt%. Our theoretical study demonstrates that AlN nanotube can be a potential candidate for the hydrogen storage materials
Ab initio thermodynamic evaluation of Pd atom interaction with CeO(2) surfaces.
Mayernick, Adam D; Janik, Michael J
2009-08-28
Palladium supported on ceria is an effective catalytic material for three-way automotive catalysis, catalytic combustion, and solid-oxide fuel cell (SOFC) anodes. The morphology, oxidation state, and particle size of Pd on ceria affect catalytic activity and are a function of experimental conditions. This work utilizes ab initio thermodynamics using density functional theory (DFT) (DFT+U) methods to evaluate the stability of Pd atoms, PdO(x) species, and small Pd particles in varying configurations on CeO(2) (111), (110), and (100) single crystal surfaces. Over specific oxygen partial pressure and temperature ranges, palladium incorporation to form a mixed surface oxide is thermodynamically favorable versus other single Pd atom states on each ceria surface. For example, Pd atoms may incorporate into Ce fluorite lattice positions in a Pd(4+) oxidation state on the CeO(2) (111) surface. The ceria support shifts the transition between formal Pd oxidation states (Pd(0), Pd(2+), Pd(4+)) relative to bulk palladium and stabilizes certain oxidized palladium species on each surface. We show that temperature, oxygen pressure, and cell potential in a SOFC can influence the stable states of palladium supported on ceria surfaces, providing insight into structural stability during catalytic operation. PMID:19725615
Ab-initio calculation of the photonuclear cross section of $^{10}$B
Kruse, M K G; Johnson, C W
2015-01-01
We present for the first-time the photonuclear cross section of $^{10}$B calculated within the ab-initio No Core Shell Model framework. Realistic two-nucleon (NN) chiral forces up to next-to-next-to-next-order (N3LO), which have been softened by the similarity renormalization group method (SRG) to $\\lambda=2.02$ fm$^{-1}$, were utilized. The electric-dipole response function is calculated using the Lanczos method. The effects of the continuum were accounted for by including neutron escape widths derived from R-matrix theory. The calculated cross section agrees well with experimental data in terms of structure as well as in absolute peak height, $\\sigma_{\\rm max}=4.85~{\\rm mb}$ at photon energy $\\omega=23.61~{\\rm MeV}$, and integrated cross section $85.36\\, {\\rm MeV \\cdotp mb}$. We test the Brink hypothesis by calculating the electric-dipole response for the first five positive-parity states in $^{10}$B and verify that dipole excitations built upon the ground- and excited states have similar characteristics.
Ab Initio Enhanced calphad Modeling of Actinide-Rich Nuclear Fuels
International Nuclear Information System (INIS)
The process of fuel recycling is central to the Advanced Fuel Cycle Initiative (AFCI), where plutonium and the minor actinides (MA) Am, Np, and Cm are extracted from spent fuel and fabricated into new fuel for a fast reactor. Metallic alloys of U-Pu-Zr-MA are leading candidates for fast reactor fuels and are the current basis for fast spectrum metal fuels in a fully recycled closed fuel cycle. Safe and optimal use of these fuels will require knowledge of their multicomponent phase stability and thermodynamics (Gibbs free energies). In additional to their use as nuclear fuels, U-Pu-Zr-MA contain elements and alloy phases that pose fundamental questions about electronic structure and energetics at the forefront of modern many-body electron theory. This project will validate state-of-the-art electronic structure approaches for these alloys and use the resulting energetics to model U-Pu-Zr-MA phase stability. In order to keep the work scope practical, researchers will focus on only U-Pu-Zr-m), leaving Cm for later study. The overall objectives of this project are to: Provide a thermodynamic model for U-Pu-Zr-MA for improving and controlling reactor fuels; and, Develop and validate an ab initio approach for predicting actinide alloy energetics for thermodynamic modeling
Ab initio parameterization of YFF1, a universal force field for drug-design applications.
Yakovenko, Olexandr Ya; Li, Yvonne Y; Oliferenko, Alexander A; Vashchenko, Ganna M; Bdzhola, Volodymyr G; Jones, Steven J M
2012-02-01
The YFF1 is a new universal molecular mechanic force field designed for drug discovery purposes. The electrostatic part of YFF1 has already been parameterized to reproduce ab initio calculated dipole and quadrupole moments. Now we report a parameterization of the van der Waals interactions (vdW) for the same atom types that were previously defined. The 6-12 Lennard-Jones potential terms were parameterized against homodimerization energies calculated at the MP2/6-31 G level of theory. The Boys-Bernardi counterpoise correction was employed to account for the basis-set superposition error. As a source of structural information we used about 2,400 neutral compounds from the ZINC2007 database. About 6,600 homodimeric configurations were generated from this dataset. A special "closure" procedure was designed to accelerate the parameters fitting. As a result, dimerization energies of small organic compounds are reproduced with an average unsigned error of 1.1 kcal mol(-1). Although the primary goal of this work was to parameterize nonbonded interactions, bonded parameters were also derived, by fitting to PM6 semiempirically optimized geometries of approximately 20,000 compounds. PMID:21562826
Energy Technology Data Exchange (ETDEWEB)
Liu, Shi-Yu, E-mail: buaasyliu@gmail.com [College of Physics and Electronic Information Science, Tianjin Normal University, Tianjin 300387 (China); Department of Mechanical and Biomedical Engineering, City University of Hong Kong, Kowloon, Hong Kong (China); Liu, Shiyang [Institute of Information Optics, Zhejiang Normal University, Jinhua, Zhejiang 321004 (China); Li, De-Jun [College of Physics and Electronic Information Science, Tianjin Normal University, Tianjin 300387 (China); Wang, Sanwu, E-mail: sanwu-wang@utulsa.edu [Department of Physics and Engineering Physics, The University of Tulsa, Tulsa, Oklahoma 74104 (United States); Guo, Jing; Shen, Yaogen, E-mail: meshen@cityu.edu.hk [Department of Mechanical and Biomedical Engineering, City University of Hong Kong, Kowloon, Hong Kong (China)
2015-02-14
Utilizing a combination of ab initio density-functional theory and thermodynamics formalism, we have established the microscopic mechanisms for oxidation of the binary and ternary alloy surfaces and provided a clear explanation for the experimental results of the oxidation. We construct three-dimensional surface phase diagrams (SPDs) for oxygen adsorption on three different Nb-X(110) (X = Ti, Al or Si) binary alloy surfaces. On the basis of the obtained SPDs, we conclude a general microscopic mechanism for the thermodynamic oxidation, that is, under O-rich conditions, a uniform single-phase SPD (type I) and a nonuniform double-phase SPD (type II) correspond to the sustained complete selective oxidation and the non-sustained partial selective oxidation by adding the X element, respectively. Furthermore, by revealing the framework of thermodynamics for the oxidation mechanism of ternary alloys through the comparison of the surface energies of two separated binary alloys, we provide an understanding for the selective oxidation behavior of the Nb ternary alloy surfaces. Using these general microscopic mechanisms, one could predict the oxidation behavior of any binary and multi-component alloy surfaces based on thermodynamics considerations.
Using ab initio calculations in designing bcc Mg-Li alloys for ultra-lightweight applications
International Nuclear Information System (INIS)
Ab initio calculations are becoming increasingly useful to engineers interested in designing new alloys, because these calculations are able to accurately predict basic material properties only knowing the atomic composition of the material. In this paper, single crystal elastic constants of 11 bcc Mg-Li alloys are calculated using density functional theory (DFT) and compared with available experimental data. Based on DFT determined properties, engineering parameters such as the ratio of bulk modulus over shear modulus (B/G) and the ratio of Young's modulus over mass density (Y/ρ) are calculated. Analysis of B/G and Y/ρ shows that bcc Mg-Li alloys with 30-50 at.% Li offer the most potential as lightweight structural material. Compared with fcc Al-Li alloys, bcc Mg-Li alloys have a lower B/G ratio, but a comparable Y/ρ ratio. An Ashby map containing Y/ρ vs B/G shows that it is not possible to increase both Y/ρ and B/G by changing only the composition of a binary alloy
Ab initio computational studies on molecular conformation of N-methyl-glyphosate
Kaliannan, P.; Naseer Ali, M. Mohamed; Venuvanalingam, P.
Conformational analysis of N-methyl-glyphosate has been carried out using an ab initio molecular orbital (MO) method at the HF/3-21G* levels of theory and the results are compared with the results of a previously studied compound, namely glyphosate. The potential energy surface of the molecule obtained by varying the central torsion angles (Φ, Ψ) was investigated in detail. Fourteen conformers with 5 kcal mol-1 energy cut-off have been selected from the potential energy surface for geometry optimization to locate the true minimum on the conformational space. The minimum has been found to be at (-62°, 110°) for the central torsion angles. This conformation is stabilized by hydrogen bond interactions (O-H···O and C-H···O) and the interactions due to protons nearer to each other. This cationic field leads to the formation of a hydrophobic patch in this structure, as well as in the structures closer to the global minimum. This patch may destabilize the favourable interaction of N-methyl-glyphosate with the surrounding amino acid residues in the binding cavity as they form the cationic field throughout the glyphosate binding region.
Ab initio Calculations of Charge Symmetry Breaking in the A =4 Hypernuclei
Gazda, Daniel; Gal, Avraham
2016-03-01
We report on ab initio no-core shell model calculations of the mirror Λ hypernuclei H4Λ and He4Λ , using the Bonn-Jülich leading-order chiral effective field theory hyperon-nucleon potentials plus a charge symmetry breaking Λ -Σ0 mixing vertex. In addition to reproducing rather well the 0g.s . + and 1exc+ binding energies, these four-body calculations demonstrate for the first time that the observed charge symmetry breaking splitting of mirror levels, reaching hundreds of keV for 0g.s . +, can be reproduced using realistic theoretical interaction models, although with a non-negligible momentum cutoff dependence. Our results are discussed in relation to recent measurements of the H4Λ(0g.s . +) binding energy at the Mainz Microtron [A. Esser et al. (A1 Collaboration), Phys. Rev. Lett. 114, 232501 (2015)] and the He4Λ(1exc+) excitation energy [T.O. Yamamoto et al. (J-PARC E13 Collaboration), Phys. Rev. Lett. 115, 222501 (2015)].
Ab Initio Enhanced calphad Modeling of Actinide-Rich Nuclear Fuels
Energy Technology Data Exchange (ETDEWEB)
Morgan, Dane [Univ. of Wisconsin, Madison, WI (United States); Yang, Yong Austin [Univ. of Wisconsin, Madison, WI (United States)
2013-10-28
The process of fuel recycling is central to the Advanced Fuel Cycle Initiative (AFCI), where plutonium and the minor actinides (MA) Am, Np, and Cm are extracted from spent fuel and fabricated into new fuel for a fast reactor. Metallic alloys of U-Pu-Zr-MA are leading candidates for fast reactor fuels and are the current basis for fast spectrum metal fuels in a fully recycled closed fuel cycle. Safe and optimal use of these fuels will require knowledge of their multicomponent phase stability and thermodynamics (Gibbs free energies). In additional to their use as nuclear fuels, U-Pu-Zr-MA contain elements and alloy phases that pose fundamental questions about electronic structure and energetics at the forefront of modern many-body electron theory. This project will validate state-of-the-art electronic structure approaches for these alloys and use the resulting energetics to model U-Pu-Zr-MA phase stability. In order to keep the work scope practical, researchers will focus on only U-Pu-Zr-{Np,Am}, leaving Cm for later study. The overall objectives of this project are to: Provide a thermodynamic model for U-Pu-Zr-MA for improving and controlling reactor fuels; and, Develop and validate an ab initio approach for predicting actinide alloy energetics for thermodynamic modeling.
Ab-initio Electronic, Transport and Related Properties of Zinc Blende Boron Arsenide (zb-BAs)
Nwigboji, Ifeanyi H.; Malozovsky, Yuriy; Bagayoko, Diola
We present results from ab-initio, self-consistent density functional theory (DFT) calculations of electronic, transport, and bulk properties of zinc blende boron arsenide (zb-BAs). We utilized a local density approximation (LDA) potential and the linear combination of atomic orbital (LCAO) formalism. Our computational technique follows the Bagayoko, Zhao, and Williams method, as enhanced by Ekuma and Franklin. Our results include electronic energy bands, densities of states, and effective masses. We explain the agreement between these findings, including the indirect band gap, and available, corresponding, experimental ones. This work confirms the capability of DFT to describe accurately properties of materials, provided the computations adhere to the conditions of validity of DFT [AIP Advances, 4, 127104 (2014)]. Acknowledgments: This work was funded in part by the National Science Foundation (NSF) and the Louisiana Board of Regents, through LASiGMA [Award Nos. EPS- 1003897, NSF (2010-15)-RII-SUBR] and NSF HRD-1002541, the US Department of Energy - National, Nuclear Security Administration (NNSA) (Award No. DE- NA0002630), LaSPACE, and LONI-SUBR.
Ab-initio Calculation of Optoelectronic and Structural Properties of Cubic Lithium Oxide (Li2O)
Ziegler, Joshua; Polin, Daniel; Malozovsky, Yuriy; Bagayoko, Diola
Using the Bagayoko, Zhao, and Williams (BZW) method, as enhanced by Ekuma and Franklin (BZW-EF), we performed ab-initio, density functional theory (DFT) calculations of optoelectronic, transport, and bulk properties of Li2S. In so doing, we avoid ``band gap'' and problems plaguing many DET calculations [AIP Advances 4, 127104 (2014)]. We employed a local density approximation (LDA) potential and the linear combination of atomic orbitals (LCAO). With the BZW-EF method, our results possess the full, physical content of DFT and agree with available, corresponding experimental ones. In particular, we found a room temperature indirect band gap of 6.659 eV that compares favorably with experimental values ranging from 5 to 7.99 eV. We also calculated total and partial density of states (DOS and PDOS), effective masses of charge carriers, the equilibrium lattice constant, and the bulk modulus. Acknowledgments: This work was funded in part by the National Science Foundation (NSF) and the Louisiana Board of Regents, through LASiGMA [Award Nos. EPS- 1003897, NSF (2010-15)-RII-SUBR] and NSF HRD-1002541, the US Department of Energy - National, Nuclear Security Administration (NNSA) (Award Nos. DE-NA0001861 and DE- NA0002630), LaSPACE, and LONI-SUBR.
Ab-initio Calculations of Accurate Electronic Properties of Wurzite AlN
Nwigboji, Ifeanyi; Malozovsky, Yuriy; Bagayoko, Diola; Bagayoko Research Group Team
2014-03-01
We present results from ab-initio, self consistent local density approximation (LDA) calculations of electronic and related properties of wurtzite Aluminum Nitride (w-AlN). Our non-relativistic computations employed the Ceperley and Alder LDA potential and the linear combination of atomic orbital (LCAO) formalism. The implementation of the LCAO formalism followed the Bagayoko, Zhao, and Williams' method as enhanced by Ekuma and Franklin (BZW-EF). The BZW-EF method verifiably obtains the minima of the occupied energies; these minima provide the most variationally and physically valid density functional theory (DFT) description of the ground states of materials under study. Our preliminary results for w-AlN show that w-AlN has a direct band gap of 5.82 eV at the Γ point. The preliminary energy bands were obtained with a basis set comprising 48 functions. None of the several, larger basis sets tested to date led to occupied energies lower than those obtained with the above 48. While most previous LDA calculations are 2 eV smaller or more than the experimental value of 5.9 eV that is in excellent agreement with our finding, considering the typical experimental uncertainty of 0.2 eV for absorption measurements on AlN. We also discuss our calculated density of states (DOS) and partial densities of states (pDOS).
Ottonello, G; Zuccolini, M Vetuschi; Belmonte, D
2010-09-14
We present the results of a computational investigation with ab initio procedures of the structure-energy and vibrational properties of silica clusters in a dielectric continuum with dielectric constant ε=3.8, through density functional theory/B3LYP gas phase calculations coupled with a polarized continuum model approach [integral equation formalism applied to a polarized continuum (IEFPCM)] and those of the periodical structure D(6h) which leads to the α-cristobalite polymorph of silica when subjected to symmetry operations with the same functional within the linear combination of atomic orbitals (LCAO) approximation and in the framework of Bloch's theorem. Based on the computed energies and vibrational features, an aggregate of the D(6h) network and the monomer locally ordered in the short-medium range and both present in the glass in a mutual arrangement lacking of spatial continuity reproduces satisfactorily the experimentally observed low T heat capacity and the deviation from the Debye T(3) law. Above T(g), the experimental heat capacity of the liquid is perfectly reproduced summing to the internal modes the translational and rotational contributions to the bulk heat capacity and subtracting the (acoustic) terms arising from coherent motion (no longer existent). PMID:20849179
Ab initio prediction of electronic, transport and bulk properties of Li2S
Malozovsky, Yuriy; Franklin, Lashounda; Ekuma, Chinedu; Bagayoko, Diola
2015-08-01
In this paper, we present results from ab initio, self-consistent, local density approximation (LDA) calculations of electronic and related properties of cubic antifluorite (anti-CaF2) lithium sulfide (Li2S). Our nonrelativistic computations implemented the linear combination of atomic orbital (LCAO) formalism following the Bagayoko, Zhao and Williams method, as enhanced by Ekuma and Franklin (BZW-EF). Consequently, using several self-consistent calculations with increasing basis sets, we searched for the smallest basis set that yields the absolute minima of the occupied energies. The outcomes of the calculation with this basis set, called the optimal basis set, have the full physical content of density functional theory (DFT). Our calculated indirect band gap, from Γ to X, is 3.723 eV, for the low temperature experimental lattice constant of 5.689 Å. The predicted indirect band gap of 3.702 eV is obtained for the computationally determined equilibrium lattice constant of 5.651 Å. We have also calculated the total density of states (DOS) and partial densities of states (pDOS), electron and hole effective masses and the bulk modulus of Li2S. Due to a lack of experimental results, most of the calculated ones reported here are predictions for this material suspected of exhibiting a high temperature superconductivity similar to that of MgB2.
Mullaney, John C; Zaleski, Daniel P; Tew, David P; Walker, Nicholas R; Legon, Anthony C
2016-04-18
An isolated, gas-phase dimer of imidazole is generated through laser vaporisation of a solid rod containing a 1:1 mixture of imidazole and copper in the presence of an argon buffer gas undergoing supersonic expansion. The complex is characterised through broadband rotational spectroscopy and is shown to have a twisted, hydrogen-bonded geometry. Calculations at the CCSD(T)(F12*)/cc-pVDZ-F12 level of theory confirm this to be the lowest-energy conformer of the imidazole dimer. The distance between the respective centres of mass of the imidazole monomer subunits is determined to be 5.2751(1) Å, and the twist angle γ describing rotation of one monomer with respect to the other about a line connecting the centres of mass of the monomers is determined to be 87.9(4)°. Four out of six intermolecular parameters in the model geometry are precisely determined from the experimental rotational constants and are consistent with results calculated ab initio. PMID:26812549
Potential energy surface and second virial coefficient of methane-water from ab initio calculations
Akin-Ojo, Omololu; Szalewicz, Krzysztof
2005-10-01
Six-dimensional intermolecular potential energy surfaces (PESs) for the interaction of CH4 with H2O are presented, obtained from ab initio calculations using symmetry-adapted perturbation theory (SAPT) at two different levels of intramonomer correlation and the supermolecular approach at three different levels of electron correlation. Both CH4 and H2O are assumed to be rigid molecules with interatomic distances and angles fixed at the average values in the ground-state vibration. A physically motivated analytical expression for each PES has been developed as a sum of site-site functions. The PES of the CH4-H2O dimer has only two symmetry-distinct minima. From the SAPT calculations, the global minimum has an energy of -1.03kcal /mol at a geometry where H2O is the proton donor, HO -H⋯CH4, with the O-H-C angle of 165°, while the secondary minimum, with an energy of -0.72kcal/mol, has CH4 in the role of the proton donor (H3C -H⋯OH2). We estimated the complete basis set limit of the SAPT interaction energy at the global minimum to be -1.06kcal/mol. The classical cross second virial coefficient B12(T) has been calculated for the temperature range 298-653K. Our best results agree well with some experiments, allowing an evaluation of the quality of experimental results.
Ab initio study of intrinsic defects and diffusion behaviors in solid molecular hydrogens
Sun, Qingqiang; Ye, Yingting; Yang, Tianle; Yang, Li; Peng, Shuming; Long, Xinggui; Zhou, Xiaosong; Zu, Xiaotao; Du, Jincheng
2015-12-01
The behaviors of intrinsic defects in solid molecular hydrogens (H2) were investigated using ab initio calculations based on density functional theory. The results show that the formation energy of a vacancy is dependent on molecule orientation in disordered hexagonal close-packed (hcp) H2 crystals, but independent of molecular orientation in face-centered cubic-Pa3 H2. Furthermore, H2 molecules generally prefer to occupy the basal octahedral sites in hcp and octahedral sites in Pa3 structures. The formation energies of an interstitial H2 depend sensitively on the volume of interstitial sites, and also on near spatial distributions of molecular axes of a H2 in hcp H2 crystals. The strong force field introduced by an interstitial H2 might induce the rotation of molecular axes and reduce the formation energy of the interstitial H2. The migration barrier energy of a vacancy in Pa3 is larger than the average in hcp structures. However, the H2 molecules prefer to jump though vacancies in the two structures of solid H2 than the interstitial migrations.
Acidity of HOCH, HSCN, HNCO, HNCS: a treatment from the viewpoint of ab initio approach
Directory of Open Access Journals (Sweden)
ALEXEI N. PANKRATOV
2005-10-01
Full Text Available The electronic structures of the molecules HOCN, HSCN, HNCO, HNCS and the anions [OCN]-, [SCN]- have been investigated ab initio at the RHF/6-31G(d, RHF/6-31G(d,p, MP2/6-31G(d//RHF/6-31G(d and MP2/6-31G(d,p//RHF/6-31G(d,p theory levels. The thermodynamic stability of the HNCO and HNCS molecules was shown to be higher than that of the HOCH and HSCN molecules, respectively.The following series of the alteration of the protolytes strength HSCN > HOCH, HNCS > HNCO, HOCN > HNCO, HSCN > HNCS was substantiated. The computations taking into account the electronic correlation (MP2/6-31G(d//RHF/6-31G(d and MP2/6-31G(d,p//RHF/6-31G(d,p reflect the general sequence of change in the proton-donor properties: HSCN > HOCN > HNCS > HNCO, coinciding with the order of descending hydrophobicity of the compounds. The comparative proton-donor ability of the above acids in aqueous solutions is determined basically from the electronic structure and size of their molecules and anions [OCN]-, [SCN]-, but not on medium effects.
Ab initio Study on Structures and Isomerization of Magnesium Fluorosilylenoid H2SiFMgF
Institute of Scientific and Technical Information of China (English)
Yi-jian Zhang; Mei-jiang Li; Guo-qiao Lai; Da-cheng Feng; Sheng-yu Feng
2008-01-01
The structures and isomerization of magnesium fluorosilylenoid H2SiFMgF were investigated by ab initio molecular orbital theory. Four equilibrium structures and three isomeric transition states were located and fully optimized at the B3LYP/6-31G(d,p) and G3MP2B3 levels, respectively. Based on the B3LYP/6-31G(d,p) optimized geometries, harmonic frequencies of various structures were obtained and 29Si chemical shifts were calculated. The solvent effects were investigated by means of the polarizable continuum model using THF as a solvent at B3LYP/6-31G(d,p) level. Isomerization paths for isomers were confirmed by in-trinsic reaction coordinate calculations. The calculated results show that tetrahedral structure has the lowest energy and is the most stable; tetrahedral, three-membered ring, and p-complex structures are suggested to be the experimentally detectable ones; and σ-complex structure has the highest energy and will not exist.
Rayne, Sierra; Forest, Kaya
2016-06-01
The gas phase standard state (298.15 K, 1 atm) isomerization enthalpy (ΔisomH°(g)) prediction performance of the major semiempirical, ab initio, and density functional levels of theory for environmentally relevant transformations was investigated using the linear to branched heptanes as a representative case study. The M062X density functional, MP2 (and higher) levels of Moller-Plesset perturbation theory, and the CBS and Gaussian-n composite methods are well suited for investigating the thermodynamic properties of environmentally interesting isomerizations, although the M062X functional may not be appropriate for all systems. Where large molecular systems prohibit the use of higher levels of theory, the PM6 and PDDG semiempirical methods may offer an appropriate computational cost-accuracy compromise. PMID:26979512
Energy Technology Data Exchange (ETDEWEB)
Matsushita, Y., E-mail: kurita@cs.tut.ac.jp; Murakawa, T., E-mail: kurita@cs.tut.ac.jp; Shimamura, K., E-mail: kurita@cs.tut.ac.jp; Oishi, M., E-mail: kurita@cs.tut.ac.jp; Ohyama, T., E-mail: kurita@cs.tut.ac.jp; Kurita, N., E-mail: kurita@cs.tut.ac.jp [Department of Computer Science and Engineering, Toyohashi University of Technology, Tempaku-cho, Toyohashi, Aichi, 441-8580 (Japan)
2015-02-27
The catabolite activator protein (CAP) is one of the regulatory proteins controlling the transcription mechanism of gene. Biochemical experiments elucidated that the complex of CAP with cyclic AMP (cAMP) is indispensable for controlling the mechanism, while previous molecular simulations for the monomer of CAP+cAMP complex revealed the specific interactions between CAP and cAMP. However, the effect of cAMP-binding to CAP on the specific interactions between CAP and DNA is not elucidated at atomic and electronic levels. We here considered the ternary complex of CAP, cAMP and DNA in solvating water molecules and investigated the specific interactions between them at atomic and electronic levels using ab initio molecular simulations based on classical molecular dynamics and ab initio fragment molecular orbital methods. The results highlight the important amino acid residues of CAP for the interactions between CAP and cAMP and between CAP and DNA.
International Nuclear Information System (INIS)
The catabolite activator protein (CAP) is one of the regulatory proteins controlling the transcription mechanism of gene. Biochemical experiments elucidated that the complex of CAP with cyclic AMP (cAMP) is indispensable for controlling the mechanism, while previous molecular simulations for the monomer of CAP+cAMP complex revealed the specific interactions between CAP and cAMP. However, the effect of cAMP-binding to CAP on the specific interactions between CAP and DNA is not elucidated at atomic and electronic levels. We here considered the ternary complex of CAP, cAMP and DNA in solvating water molecules and investigated the specific interactions between them at atomic and electronic levels using ab initio molecular simulations based on classical molecular dynamics and ab initio fragment molecular orbital methods. The results highlight the important amino acid residues of CAP for the interactions between CAP and cAMP and between CAP and DNA
Bridging a gap between continuum-QCD and ab initio predictions of hadron observables
Binosi, Daniele; Papavassiliou, Joannis; Roberts, Craig D
2014-01-01
Within contemporary hadron physics there are two common methods for determining the momentum-dependence of the interaction between quarks: the top-down approach, which works toward an ab initio computation of the interaction via direct analysis of the gauge-sector gap equations; and the bottom-up scheme, which aims to infer the interaction by fitting data within a well-defined truncation of those equations in the matter sector that are relevant to bound-state properties. We unite these two approaches by demonstrating that the renormalisation-group-invariant running-interaction predicted by contemporary analyses of QCD's gauge sector coincides with that required in order to describe ground-state hadron observables using a nonperturbative truncation of QCD's Dyson-Schwinger equations in the matter sector. This bridges a gap that had lain between nonperturbative continuum-QCD and the ab initio prediction of bound-state properties.
Liquid Be, Ca and Ba. An orbital-free ab-initio molecular dynamics study
International Nuclear Information System (INIS)
Several static and dynamic properties of liquid beryllium (l-Be), liquid calcium (l-Ca) and liquid barium (l-Ba) near their triple point have been evaluated by the orbital-free ab initio molecular dynamics method (OF-AIMD), where the interaction between valence electrons and ions is described by means of local pseudopotentials. These local pseudopotentials used were constructed through a force-matching process with those obtained from a Kohn-Sham ab initio molecular dynamics study (KS-AIMD) of a reduced system with non-local pseudopotentials. The calculated static structures show good agreement with the available experimental data, including an asymmetric second peak in the structure factor which has been linked to the existence of a marked icosahedral short-range order in the liquid. As for the dynamic properties, we obtain collective density excitations whose associated dispersion relations exhibit a positive dispersion
Knoop, S.; Żuchowski, P. S.; KÈ©dziera, D.; Mentel, Ł.; Puchalski, M.; Mishra, H. P.; Flores, A. S.; Vassen, W.
2014-08-01
We have investigated the ultracold interspecies scattering properties of metastable triplet He and Rb. We performed state-of-the-art ab initio calculations of the relevant interaction potential, and measured the interspecies elastic cross section for an ultracold mixture of metastable triplet He4 and Rb87 in a quadrupole magnetic trap at a temperature of 0.5 mK. Our combined theoretical and experimental study gives an interspecies scattering length a4+87=+17-4+1a0, which prior to this work was unknown. More general, our work shows the possibility of obtaining accurate scattering lengths using ab initio calculations for a system containing a heavy, many-electron atom, such as Rb.
Ab initio studies of equations of state and chemical reactions of reactive structural materials
Zaharieva, Roussislava
subject of studies of the shock or thermally induced chemical reactions of the two solids comprising these reactive materials, from first principles, is a relatively new field of study. The published literature on ab initio techniques or quantum mechanics based approaches consists of the ab initio or ab initio-molecular dynamics studies in related fields that contain a solid and a gas. One such study in the literature involves a gas and a solid. This is an investigation of the adsorption of gasses such as carbon monoxide (CO) on Tungsten. The motivation for these studies is to synthesize alternate or synthetic fuel technology by Fischer-Tropsch process. In this thesis these studies are first to establish the procedure for solid-solid reaction and then to extend that to consider the effects of mechanical strain and temperature on the binding energy and chemisorptions of CO on tungsten. Then in this thesis, similar studies are also conducted on the effect of mechanical strain and temperature on the binding energies of Titanium and hydrogen. The motivations are again to understand the method and extend the method to such solid-solid reactions. A second motivation is to seek strained conditions that favor hydrogen storage and strain conditions that release hydrogen easily when needed. Following the establishment of ab initio and ab initio studies of chemical reactions between a solid and a gas, the next step of research is to study thermally induced chemical reaction between two solids (Ni+Al). Thus, specific new studies of the thesis are as follows: (1) Ab initio Studies of Binding energies associated with chemisorption of (a) CO on W surfaces (111, and 100) at elevated temperatures and strains and (b) adsorption of hydrogen in titanium base. (2) Equations of state of mixtures of reactive material structures from ab initio methods. (3) Ab initio studies of the reaction initiation, transition states and reaction products of intermetallic mixtures of (Ni+Al) at elevated
Conformational space of clindamycin studied by ab initio and full-atom molecular dynamics.
Kulczycka-Mierzejewska, Katarzyna; Trylska, Joanna; Sadlej, Joanna
2016-01-01
Molecular dynamics (MD) simulations allow determining internal flexibility of molecules at atomic level. Using ab initio Born-Oppenheimer molecular dynamics (BOMD), one can simulate in a reasonable time frame small systems with hundreds of atoms, usually in vacuum. With quantum mechanics/molecular mechanics (QM/MM) or full-atom molecular dynamics (FAMD), the influence of the environment can also be simulated. Here, we compare three types of MD calculations: ab initio BOMD, hybrid QM/MM, and classical FAMD. As a model system, we use a small antibiotic molecule, clindamycin, which is one of the lincosamide antibiotics. Clindamycin acquires two energetically stable forms and we investigated the transition between these two experimentally known conformers. We performed 60-ps BOMD simulations in vacuum, 50-ps QM/MM, and 100-ns FAMD in explicit water. The transition between two antibiotic conformers was observed using both BOMD and FAMD methods but was not noted in the QM/MM simulations. PMID:26733483
Ab initio calculations on the magnetic properties of transition metal complexes
International Nuclear Information System (INIS)
We present a protocol for the ab initio determination of the magnetic properties of mono- and polynuclear transition metal compounds. First, we obtain the low lying electronic states by multireference methods. Then, we include spin-orbit coupling and an external magnetic field for the determination of zero-field splitting and g-tensors. For the polynuclear complexes the magnetic exchange coupling constants are determined by a modified complete active space self consistent field method. Based on the results of the ab initio calculations, magnetic data such as magnetic susceptibility or magnetization are simulated and compared to experimental data. The results obtained for the polynuclear complexes are further analysed by calculations on model complexes where part of the magnetic centers are substituted by diamagnetic ions. The methods are applied to different Co and Ni containing transition metal complexes
Study of atomic structure of liquid Hg-In alloys using ab-initio molecular dynamics
Energy Technology Data Exchange (ETDEWEB)
Sharma, Nalini; Ahluwalia, P. K. [Department of Physics, Himachal Pradesh University, Shimla(HP)-171005 (India); Thakur, Anil [Department of Physics, Govt. P. G. College Solan (HP)-173212 (India)
2015-05-15
Ab-initio molecular dynamics simulations are performed to study the structural properties of liquid Hg-In alloys. The interatomic interactions are described by ab-initio pseudopotentials given by Troullier and Martins. Five liquid Hg-In mixtures (Hg{sub 10}In{sub 90}, Hg{sub 30}In{sub 70}, Hg{sub 50}In{sub 50}, Hg{sub 70}In{sub 30} and Hg{sub 90}In{sub 10}) at 299K are considered. The radial distribution function g(r) and structure factor S(q) of considered alloys are compared with respective experimental results for liquid Hg (l-Hg) and (l-In). The radial distribution function g(r) shows the presence of short range order in the systems considered. Smooth curves of Bhatia-Thornton partial structure factors factor shows the presence of liquid state in the considered alloys.
Study of atomic structure of liquid Hg-In alloys using ab-initio molecular dynamics
International Nuclear Information System (INIS)
Ab-initio molecular dynamics simulations are performed to study the structural properties of liquid Hg-In alloys. The interatomic interactions are described by ab-initio pseudopotentials given by Troullier and Martins. Five liquid Hg-In mixtures (Hg10In90, Hg30In70, Hg50In50, Hg70In30 and Hg90In10) at 299K are considered. The radial distribution function g(r) and structure factor S(q) of considered alloys are compared with respective experimental results for liquid Hg (l-Hg) and (l-In). The radial distribution function g(r) shows the presence of short range order in the systems considered. Smooth curves of Bhatia-Thornton partial structure factors factor shows the presence of liquid state in the considered alloys
Ab Initio Calculations and Raman and SERS Spectral Analyses of Amphetamine Species
DEFF Research Database (Denmark)
Berg, Rolf W.; Nørbygaard, Thomas; White, Peter C.;
2011-01-01
For the first time, the differences between the spectra of amphetamine and amphetamine-H+ and between different conformers are thoroughly studied by ab initio model calculations, and Raman and surface-enhanced Raman spectroscopy (SERS) spectra are measured for different species of amphetamine....... The spectra of amphetamine and amphetamine-H+ sampleswere obtained and assigned according to a comparison of the experimental spectra and the ab initio MO calculations, performed using the Gaussian 03W program (Gaussian, Inc., Pittsburgh, PA). The analyses were based on complete geometry minimization...... with internal bonds (sulfates, hydrogen phosphates, etc.) need to be taken into account when employing these spectra for identification purposes. These results also show how Raman spectroscopy can assist the forensic community in drug profiling studies. Furthermore, because their spectra are different...
A fully ab initio potential energy surface for ClH2 reactive system
Institute of Scientific and Technical Information of China (English)
无
2000-01-01
An ab initio analytical potential energy surface called BW3 for the ClH2 reactive system is presented. The fit of this surface is based on about 1 200 ab initio energy points, computed with multi-reference configuration interaction(MRCI) and scaling external correlation (SEC) method and a very large basis set. The precision in the fit is very high. The BW3 surface could reproduce correctly the dissociation energy of H2 and HCl, and the endothermicity of the Cl + H2 abstraction reaction. For the Cl + H2 abstraction reaction, the saddle point of BW3 lies in collinear geometries, and the barrier height is 32.84 kJ/mol; for the H + ClH exchange reaction, the barrier of BW3 is also linear, with a height of 77.40 kJ/mol.
A fully ab initio potential energy surface for C1H2 reactive system
Institute of Scientific and Technical Information of China (English)
边文生; 刘成卜; H.J.Werner
2000-01-01
An ab initio analytical potential energy surface called BW3 for the CIH2 reactive system is presented. The fit of this surface is based on about 1 200 ab initio energy points, computed with multi-reference configuration interaction(MRCI) and scaling external correlation (SEC) method and a very large basis set. The precision in the fit is very high. The BW3 surface could reproduce correctly the dissociation energy of H2 and HCI, and the endothermicity of the Cl + H2 abstraction reaction. For the Cl + H2 abstraction reaction, the saddle point of BW3 lies in collinear geometries, and the barrier height is 32.84 kJ/mol; for the H + CIH exchange reaction, the barrier of BW3 is also linear, with a height of 77.40 kJ/mol.
Ab Initio No-Core Shell Model Calculations Using Realistic Two- and Three-Body Interactions
Energy Technology Data Exchange (ETDEWEB)
Navratil, P; Ormand, W E; Forssen, C; Caurier, E
2004-11-30
There has been significant progress in the ab initio approaches to the structure of light nuclei. One such method is the ab initio no-core shell model (NCSM). Starting from realistic two- and three-nucleon interactions this method can predict low-lying levels in p-shell nuclei. In this contribution, we present a brief overview of the NCSM with examples of recent applications. We highlight our study of the parity inversion in {sup 11}Be, for which calculations were performed in basis spaces up to 9{Dirac_h}{Omega} (dimensions reaching 7 x 10{sup 8}). We also present our latest results for the p-shell nuclei using the Tucson-Melbourne TM three-nucleon interaction with several proposed parameter sets.
Electronic properties of liquid Hg-In alloys : Ab-initio molecular dynamics study
Sharma, Nalini; Thakur, Anil; Ahluwalia, P. K.
2016-05-01
Ab-initio molecular dynamics simulations are performed to study the structural properties of liquid Hg-In alloys. The interatomic interactions are described by ab-initio pseudopotentials given by Troullier and Martins. Three liquid Hg-In alloys (Hg10In90, Hg30In70,. Hg50In50, Hg70In30, and Hg90Pb10) at 299 K are considered. The calculated results for liquid Hg (l-Hg) and lead (l-In) are also drawn. Along with the calculated results of considered five liquid alloys of Hg-In alloy. The results obtained from electronic properties namely total density of state and partial density of states help to find the local arrangement of Hg and In atoms and the presence of liquid state in the considered five alloys.
Melting of sodium under high pressure. An ab-initio study
Energy Technology Data Exchange (ETDEWEB)
González, D. J.; González, L. E. [Departamento de Física Teórica, Atómica y Optica, Universidad de Valladolid, 47011 Valladolid (Spain)
2015-08-17
We report ab-initio molecular dynamics simulations of dense liquid/solid sodium for a pressure range from 0 to 100 GPa. The simulations have been performed with the orbital free ab-initio molecular dynamics method which, by using the electron density as the basic variable, allows to perform simulations with large samples and for long runs. The calculated melting curve shows a maximum at a pressure ≈ 30 GPa and it is followed by a long, steep decrease. These features are in good agreement with the experimental data. For various pressures along the melting curve, we have calculated several liquid static properties (pair distribution functions, static structure factors and short-range order parameters) in order to analyze the structural effects of pressure.
Ab Initio Calculations for the BaTiO3 (001) Surface Structure
Institute of Scientific and Technical Information of China (English)
XUE Xu-Yan; WANG Chun-Lei; ZHONG Wei-Lie
2004-01-01
@@ The ab initio method within the local density approximation is applied to calculate cubic BaTiO3 (001) surface relaxation and rumpling for two different terminations (BaO and TiO2). Our calculations demonstrate that cubic perovskite BaTiO3 crystals possess surface polarization, accompanied by the presence of the relevant electric field.We analyse their electronic structures (band structure, density of states and the electronic density redistribution with emphasis on the covalency effects). The results are also compared with that of the previous ab initio calculations. Considerable increases of Ti-O chemical bond covalency nearby the surface have been observed.The band gap reduces especially for the TiO2 termination.
Raman spectroscopy, ab-initio model calculations, and conformational, equilibria in ionic liquids
DEFF Research Database (Denmark)
Berg, Rolf W.
hoped that the structural resolving power of Raman spectroscopy will be appreciated by the reader, when used on crystals of known conformation and on the corresponding liquids, especially in combination with modern quantum mechanics calculations. It is hoped that these inetrdisciplinary methods will be...... spectroscopy and ab-initio molecular orbital calculations. A discussion is given, based mainly on some recent FT- Raman spectroscopic results on the model ionic liquid system of 1-butyl-3-methyl-imidazolium ([C4C1Im]+X-) salts. The rotational isomerism of the [C4C1Im]+ cation is described: the presence of anti...... instrumentation ...... 311 12.3 Brief introduction to ab-initio model calculations .... 312 12.4 Case study on Raman spectroscopy and structure of imidazolium-based ionic liquids ..... 312 12.5 Raman spectra and structure of [C4C1Im]+ liquids ..... 315 12.6 Normal mode analysis and rotational isomerism of the [C4...
Implementation of a vector potential method in an ab initio Hartree-Fock code
Tevekeliyska, Violina; Springborg, Michael; Champagne, Benoît; Kirtman, Bernard
2012-12-01
For extended systems exposed to an external, electrostatic field, the presence of the field leads to an extra term (E⃗. P⃗) to the Hamiltonian, where E⃗ is the field vector and P⃗ is the polarization of the system of interest. In order to find out how a polymer chain responds to an external electric perturbation, a field with a charge and a current term for the polarization is added to an ab initio Hartree-Fock Hamiltonian. The polarization expression is taken from an efficient vector potential approach (VPA) [1] for calculating electronic and nuclear responses of infinite periodic systems to finite electric fields and is implemented in the ab initio LCAO-SCF algorithm [3], which computes band structure of regular or helical polymers, taking into account the one-dimensional translational symmetry. A smoothing procedure for numerical differentiation of the orbital coefficients is used in order to calculate self-consistently the charge flow contribution to the polarization.
Large-scale ab initio configuration interaction calculations for light nuclei
International Nuclear Information System (INIS)
In ab-initio Configuration Interaction calculations, the nuclear wavefunction is expanded in Slater determinants of single-nucleon wavefunctions and the many-body Schrodinger equation becomes a large sparse matrix problem. The challenge is to reach numerical convergence to within quantified numerical uncertainties for physical observables using finite truncations of the infinite-dimensional basis space. We discuss strategies for constructing and solving the resulting large sparse matrix eigenvalue problems on current multicore computer architectures. Several of these strategies have been implemented in the code MFDn, a hybrid MPI/OpenMP Fortran code for ab-initio nuclear structure calculations that can scale to 100,000 cores and more. Finally, we will conclude with some recent results for 12C including emerging collective phenomena such as rotational band structures using SRG evolved chiral N3LO interactions.
Ab-initio approach to study hydrogen diffusion in 9Cr steels
International Nuclear Information System (INIS)
We calculate the equilibrium energies and migration barriers of Fe, Cr and H interstitial defects in α-FeX(X=Cr). We use the ab-initio electronic structure code, SIESTA, coupled to the monomer method to find activated states (or migration barriers), in order to study atomic defects diffusion. Ab-initio calculations reveal that in the presence of Cr the H migration barriers are higher than in pure α-Fe. On the other hand, our permeation tests on 9%Cr-91%Fe alloys reveal a permeation coefficient 10 times lower and a diffusion coefficient 200 times lower than in pure, annealed iron. Focusing on our experimental results, we explore very simple model of new H trapping sites and possible migration paths that can explain the experimental observations.
Experimental studies and ab initio calculations on characteristics of the C state of SF2 radical
International Nuclear Information System (INIS)
SF2 radicals were generated by a pulsed dc discharge in the mixture gas beam of SF2 and Ar. The (2+1) resonance-enhanced multiphoton ionization (REMPI) excitation spectroscopy of SF2 radical was obtained between 325 and 365 nm. The SF+ ion signals were also observed in the same wavelength range. The analysis shows that the spectrum can be assigned as the two-photon resonant excitation of SF2 radical (B-tilde1 B1 and (C-tilde1 A1 states). And also, ab initio calculations suggest that the C-tilde state is a bonding state with Rydberg characteristic. The potential energy surfaces (PESs) of SF2 and SF2+ by ab initio calculations suggest that SF+ ions originate from dissociation processes of excited SF2+ ions. (author)
An efficient time-stepping scheme for ab initio molecular dynamics simulations
Tsuchida, Eiji
2015-01-01
In ab initio molecular dynamics simulations of real-world problems, the simple Verlet method is still widely used for integrating the equations of motion, while more efficient algorithms are routinely used in classical molecular dynamics. We show that if the Verlet method is used in conjunction with pre- and postprocessing, the accuracy of the time integration is significantly improved with only a small computational overhead. The validity of the processed Verlet method is demonstrated in several examples including ab initio molecular dynamics simulations of liquid water. The structural properties obtained from the processed Verlet method are found to be sufficiently accurate even for large time steps close to the stability limit. This approach results in a 2x performance gain over the standard Verlet method for a given accuracy.
Ab Initio Studies on Hematite Surface and the Adsorption of Phosphate
Swati Chaudhury; Chandrika Varadachari; Kunal Ghosh
2014-01-01
This investigation explores the ab initio DFT method for understanding surface structure of hematite and the nature and energetics of phosphate adsorption. Using the full potential linearized plane wave method (FP-LAPW), we derived the structure and energies of various magnetic forms of hematite. The antiferromagnetic (AFM) form was observed to be the most stable. Hematite surfaces with Fe-termination, O-termination, or OH-termination were studied. The OH-terminated surface was the most stabl...
Modelling of carbohydrate–aromatic interactions: ab initio energetics and force field performance
Czech Academy of Sciences Publication Activity Database
Spiwok, V.; Lipovová, P.; Skálová, Tereza; Vondráčková, Eva; Dohnálek, Jan; Hašek, Jindřich; Králová, B.
2005-01-01
Roč. 19, č. 12 (2005), s. 887-901. ISSN 0920-654X R&D Projects: GA ČR GA204/02/0843; GA AV ČR KJB500500512 Institutional research plan: CEZ:AV0Z40500505 Keywords : ab initio * carbohydrate recognition * C-H/.pi. interactions Subject RIV: CD - Macromolecular Chemistry Impact factor: 2.082, year: 2005
Ab initio excited states calculations of Kr3+, probing semi-empirical modelling
Czech Academy of Sciences Publication Activity Database
Milko, Petr; Kalus, R.; Paidarová, Ivana; Hrušák, Jan; Gadéa, F. X.
-, 23 June (2009), s. 25. ISSN 1432-2234 R&D Projects: GA AV ČR IAA100400501 Institutional research plan: CEZ:AV0Z40400503 Keywords : cluster modelling * rare gas ions * ab initio potential energie * evaporation energies Subject RIV: CF - Physical ; Theoretical Chemistry http://www.springerlink.com/content/100493/?Content+Status=Accepted&sort=p_OnlineDate&sortorder=desc&v=condensed&o=20
Ab initio calculation of positron distribution, ACAR and lifetime in TTF-TCNQ
International Nuclear Information System (INIS)
We have performed ab initio calculations of positron distribution, ACAR and lifetime in the quasi-one-dimensional organic conductor TTF-TCNQ. The electronic structure is obtained within the LDA, while the positron state is calculated either with the LDA or with the GGA. Except the positron lifetime, differences between the LDA and GGA results are rather small. The obtained results are compared with our previous experiments and calculations.
Ab initio calculation of positron distribution, ACAR and lifetime in TTF-TCNQ
Ishibashi, Shoji; Kohyama, Masanori
2000-06-01
We have performed ab initio calculations of positron distribution, ACAR and lifetime in the quasi-one-dimensional organic conductor TTF-TCNQ. The electronic structure is obtained within the LDA, while the positron state is calculated either with the LDA or with the GGA. Except the positron lifetime, differences between the LDA and GGA results are rather small. The obtained results are compared with our previous experiments and calculations.
Projector augmented wave method: ab initio molecular dynamics with full wave functions
Indian Academy of Sciences (India)
Peter E Blöchl; Clemens J Först; Johannes Schimpl
2003-01-01
A brief introduction to the projector augmented wave method is given and recent developments are reviewed. The projector augmented wave method is an all-electron method for efficient ab initio molecular dynamics simulations with full wave functions. It extends and combines the traditions of existing augmented wave methods and the pseudopotential approach. Without sacrificing efficiency, the PAW method avoids transferability problems of the pseudopotential approach and it has been valuable to predict properties that depend on the full wave functions.
How Is Acetylcholinesterase Phosphonylated by Soman? An Ab Initio QM/MM Molecular Dynamics Study
Sirin, Gulseher Sarah; Zhang, Yingkai
2014-01-01
Acetylcholinesterase (AChE) is a crucial enzyme in the cholinergic nerve system that hydrolyzes acetylcholine (ACh) and terminates synaptic signals by reducing the effective concentration of ACh in the synaptic clefts. Organophosphate compounds irreversibly inhibit AChEs, leading to irreparable damage to nerve cells. By employing Born–Oppenheimer ab initio QM/MM molecular dynamics simulations with umbrella sampling, a state-of-the-art approach to simulate enzyme reactions, we have characteriz...
Ab Initio Study on the Anti-HIV Activities of Hydroxyflavones
Institute of Scientific and Technical Information of China (English)
ZHANG Yu
2005-01-01
Flavone and 95 hydroxyflavones have been studied with ab initio method, and their total energies, atomic charges, dipole moments, multipole moments, molecular orbital compositions, orbital energies etc. were obtained. Among them the relationship between total atomic charges and activities against HIV is basically in accordance with the experimental results. The beneficial references are provided for the extraction and synthesis of strong active anti-HIV medicines.
Large scale ab initio calculations based on three levels of parallelization
Bottin, François; Leroux, Stéphane; Knyazev, Andrew; Zérah, Gilles
2007-01-01
We suggest and implement a parallelization scheme based on an efficient multiband eigenvalue solver, called the locally optimal block preconditioned conjugate gradient LOBPCG method, and using an optimized three-dimensional (3D) fast Fourier transform (FFT) in the ab initio}plane-wave code ABINIT. In addition to the standard data partitioning over processors corresponding to different k-points, we introduce data partitioning with respect to blocks of bands as well as spatial partitioning in t...
Ab initio study of one-dimensional disorder on III-V semiconductor surfaces
Czech Academy of Sciences Publication Activity Database
Romanyuk, Olexandr; Grosse, F.; Braun, W.
2010-01-01
Roč. 7, č. 2 (2010), s. 330-333. ISSN 1862-6351 R&D Projects: GA AV ČR KAN300100802 Grant ostatní: German Resarch Fondation(DE) GZ:436 TSE 113/62/0-1 Institutional research plan: CEZ:AV0Z10100521 Keywords : atomic disorder * ab initio * semiconductor * reconstruction Subject RIV: BM - Solid Matter Physics ; Magnetism
Ab initio study of antiphase boundaries and stacking faults in L12 and DO22 compounds
DEFF Research Database (Denmark)
Rosengaard, N. M.; Skriver, Hans Lomholt
1994-01-01
We have performed ab initio calculations of the energies of antiphase boundaries as well as complex and superlattice intrinsic stacking faults in nine intermetallic compounds observed in the face-centered-cubic L1(2) and DO22 structures. The calculations were performed by means of a Green's funct......'s function technique based on the linear-muffin-tin-orbitals method within the tight-binding and atomic-sphere approximations....
Ab initio study of phase transformations in transition-metal disilicides
Czech Academy of Sciences Publication Activity Database
Káňa, Tomáš; Šob, Mojmír; Vitek, V.
2011-01-01
Roč. 19, č. 7 (2011), s. 919-926. ISSN 0966-9795 R&D Projects: GA AV ČR IAA100100920; GA MŠk(CZ) OC10008 Institutional research plan: CEZ:AV0Z20410507 Keywords : silicides various * phase transformation * plastic deformation mechanisms * defects * dislocation geometry and arrangement * ab-initio calculations * aero- engine components Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 1.649, year: 2011
Ab initio study on the magnetostructural properties of MnAs
Sanvito, Stefano; RUNGGER, IVAN
2006-01-01
The magnetic and structural properties of MnAs are studied with ab initio methods, and by mapping total energies onto a Heisenberg model. The stability of the different phases is found to depend mainly on the volume and on the amount of magnetic order, confirming previous experimental findings and phenomenological models. It is generally found that for large lattice constants the ferromagnetic state is favored, whereas for small lattice constants different antiferromagnetic states can be stab...
Ab initio study of ideal tensile strength and mechanical stability of transition-metal disilicides
Czech Academy of Sciences Publication Activity Database
Friák, Martin; Šob, Mojmír; Vitek, V.
2003-01-01
Roč. 68, č. 18 (2003), s. 184101-1 - 181101-10. ISSN 0163-1829 R&D Projects: GA AV ČR IAA1041302; GA ČR GA202/03/1351; GA MŠk OC 523.90 Institutional research plan: CEZ:AV0Z2041904 Keywords : ab initio calculations * electronic structure * theoretical tensile strength Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 2.962, year: 2003
Czech Academy of Sciences Publication Activity Database
Ehala, Sille; Dybal, Jiří; Makrlík, E.; Kašička, Václav
2009-01-01
Roč. 32, č. 4 (2009), s. 597-604. ISSN 1615-9306 R&D Projects: GA ČR(CZ) GA203/06/1044; GA ČR(CZ) GA203/08/1428; GA AV ČR 1ET400500402 Institutional research plan: CEZ:AV0Z40550506; CEZ:AV0Z40500505 Keywords : capillary affinity electrophoresis * valinomycin * ab initio calculation Subject RIV: CB - Analytical Chemistry, Separation Impact factor: 2.551, year: 2009
Czech Academy of Sciences Publication Activity Database
Hemzalová, P.; Friák, Martin; Šob, Mojmír; Ma, D.; Udyansky, A.; Raabe, D.; Neugebauer, J.
2013-01-01
Roč. 88, č. 17 (2013), Art. no. 174103. ISSN 1098-0121 R&D Projects: GA ČR(CZ) GAP108/12/0311; GA ČR GD106/09/H035; GA AV ČR IAA100100920 Institutional support: RVO:68081723 Keywords : nitrides * ab initio * thermodynamics * elasticity Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 3.664, year: 2013
On limits of ab initio calculations of pairing gap in nuclei
Saperstein, E. E.; Baldo, M.; Lombardo, U.; Pankratov, S. S.; Zverev, M. V.
2010-01-01
A brief review of recent microscopic calculations of nuclear pairing gap is given. A semi-microscopic model is suggested in which the ab-initio effective pairing interaction is supplemented with a small phenomenological addendum. It involves a parameter which is universal for all medium and heavy nuclei. Calculations for several isotopic and isotonic chains of semi-magic nuclei confirm the relevance of the model.
Ab initio simulations of liquid NaSn alloys: Zintl anions and network formation
Schoene, M.; Kaschner, R.; Seifert, G
1994-01-01
Using the Car-Parrinello technique, ab initio molecular dynamics simulations are performed for liquid NaSn alloys in five different compositions (20, 40, 50, 57 and 80 % sodium). The obtained structure factors agree well with the data from neutron scattering experiments. The measured prepeak in the structure factor is reproduced qualitatively for most compositions. The calculated and measured positions of all peaks show the same trend as function of the composition.\\\\ The dynamic simulations ...
Ab initio molecular dynamics study of liquid sodium and cesium up to critical point
Energy Technology Data Exchange (ETDEWEB)
Yuryev, Anatoly A. [Institute of Metallurgy of Ural Branch of the Russian Academy of Sciences, Amundsen st. 101,620016, Yekaterinburg (Russian Federation); Ural Federal University, Vira st. 19, 620002, Yekaterinburg (Russian Federation); Gelchinski, Boris R. [Institute of Metallurgy of Ural Branch of the Russian Academy of Sciences, Amundsen st. 101,620016, Yekaterinburg (Russian Federation)
2015-08-17
Ab initio modeling of liquid metals Na and K is carried out using the program SIESTA. We have determined the parameters of the model (the optimal step, the number of particles, the initial state etc) and calculated a wide range of properties: the total energy, pair correlation function, coefficient of self-diffusion, heat capacity, statistics of Voronoi polyhedra, the density of electronic states up to the critical temperature.
Fürst, Joachim Alexander; J Hashemi; Markussen, Troels; Brandbyge, Mads; Jauho, Antti-Pekka; Nieminen, R. M.
2009-01-01
Fullerene functionalized carbon nanotubes-NanoBuds-form a novel class of hybrid carbon materials, which possesses many advantageous properties as compared to the pristine components. Here, we report a theoretical study of the electronic transport properties of these compounds. We use both ab initio techniques and tight-binding calculations to illustrate these materials' transmission properties and give physical arguments to interpret the numerical results. Specifically, above the Fermi energy...
Fürst, J. A.; J Hashemi; Markussen, T.; Brandbyge, M.; Jauho, A.P.; Nieminen, Risto M.
2009-01-01
Fullerene functionalized carbon nanotubes—NanoBuds—form a novel class of hybrid carbon materials, which possesses many advantageous properties as compared to the pristine components. Here, we report a theoretical study of the electronic transport properties of these compounds. We use both ab initio techniques and tight-binding calculations to illustrate these materials’ transmission properties and give physical arguments to interpret the numerical results. Specifically, above the Fermi energy...
Ab initio studies on the lattice thermal conductivity of silicon clathrate frameworks II and VIII
Härkönen, Ville J.; Karttunen, Antti J.
2015-01-01
The lattice thermal conductivities of silicon clathrate frameworks II and VIII are investigated by using ab initio lattice dynamics and iterative solution of the linearized Boltzmann transport equation(BTE) for phonons. Within the temperature range 100-350 K, the clathrate structures II and VIII were found to have lower lattice thermal conductivity values than silicon diamond structure (d-Si) by factors of 1/2 and 1/5, respectively. The main reason for the lower lattice thermal conductivity o...
Surface Tension of Ab Initio Liquid Water at the Water-Air Interface
Nagata, Yuki; Ohto, Tatsuhiko; Bonn, Mischa; Kühne, Thomas D.
2016-01-01
We report calculations of the surface tension of the water-air interface using ab initio molecular dynamics (AIMD) simulations. We investigate the simulation cell size dependence of the surface tension of water from force field molecular dynamics (MD) simulations, which show that the calculated surface tension increases with increasing simulation cell size, thereby illustrating that a correction for finite size effects is required for the small system used in the AIMD simulation. The AIMD sim...
Czech Academy of Sciences Publication Activity Database
Slavíček, Petr; Fárník, Michal
2011-01-01
Roč. 13, č. 26 (2011), s. 12123-12137. ISSN 1463-9076 R&D Projects: GA ČR GA203/09/0422; GA ČR GAP208/11/0161 Institutional research plan: CEZ:AV0Z40400503 Keywords : photochemistry * hydrogen bonded heterocycles * ab initio methods Subject RIV: CF - Physical ; Theoretical Chemistry Impact factor: 3.573, year: 2011
A Deep Learning Network Approach to ab initio Protein Secondary Structure Prediction
Spencer, Matt; Eickholt, Jesse; Cheng, Jianlin
2014-01-01
Ab initio protein secondary structure (SS) predictions are utilized to generate tertiary structure predictions, which are increasingly demanded due to the rapid discovery of proteins. Although recent developments have slightly exceeded previous methods of SS prediction, accuracy has stagnated around 80% and many wonder if prediction cannot be advanced beyond this ceiling. Disciplines that have traditionally employed neural networks are experimenting with novel deep learning techniques in atte...
Ab initio studies of the kinetic isotope effect of the CH4 + OH atmospheric reaction
Lasaga, Antonio C.; Gibbs, G. V.
1991-07-01
High level ab initio calculations have been carried out on the C-13 - C-12 kinetic isotope effect of the CH4 + OH reaction in the atmosphere. The results agree quite well with both the absolute value of the isotope effect and the temperature dependence of the effect, based on new experimental data. The calculated kinetic isotope effect supports a bigger effect of biomass burning on the methane global budget.
The onset of ion solvation by ab initio calculations: Comparison of water and methanol
Czech Academy of Sciences Publication Activity Database
Pluhařová, Eva; Jungwirth, Pavel
2008-01-01
Roč. 73, 6/7 (2008), s. 733-744. ISSN 0010-0765 R&D Projects: GA MŠk LC512; GA ČR GA203/07/1006 Institutional research plan: CEZ:AV0Z40550506 Keywords : ions * water cluster * methanol * ab initio calculations Subject RIV: CF - Physical ; Theoretical Chemistry Impact factor: 0.784, year: 2008
Hyperfine tensors of nitrogen-vacancy center in diamond from \\emph{ab initio} calculations
Gali, Adam
2009-01-01
We determine and analyze the charge and spin density distributions of nitrogen-vacancy (N-V) center in diamond for both the ground and excited states by \\emph{ab initio} supercell calculations. We show that the hyperfine tensor of $^{15}$N nuclear spin is negative and strongly anisotropic in the excited state, in contrast to previous models used extensively to explain electron spin resonance measurements. In addition, we detect a significant redistribution of the spin density due to excitatio...
DNA oligonucleotide-cis-platin Binding: Ab initio interpretation of the vibrational spectra
Czech Academy of Sciences Publication Activity Database
Andrushchenko, Valery; Wieser, H.; Bouř, Petr
2007-01-01
Roč. 111, č. 39 (2007), s. 9714-9723. ISSN 1089-5639 R&D Projects: GA AV ČR IAA400550702; GA ČR GA202/07/0732 Institutional research plan: CEZ:AV0Z40550506 Keywords : cis-platin * DNA * vibrational spektra * ab initio Subject RIV: CF - Physical ; Theoretical Chemistry Impact factor: 2.918, year: 2007
Ab initio molecular dynamics simulations with linear scaling: application to liquid ethanol
International Nuclear Information System (INIS)
The structural and dynamical properties of liquid ethanol (C2H5OH) at ambient conditions have been studied by ab initio molecular dynamics simulations using a large supercell containing 125 molecules (1125 atoms). The results obtained from a trajectory of 10 ps are found to be in good agreement with available experimental data. Without sacrificing accuracy, the computational cost of simulations is reduced by more than a factor of four by the linear scaling algorithm based on the augmented orbital minimization method
International Nuclear Information System (INIS)
Full text: To understand the plasma-wall interaction on divertor plates, we investigate the interaction of hydrogen atoms and carbon materials used in the high heat flux components by the use of the following simulations. Monte-Carlo (MC) method based on binary collision approximation can calculate the sputtering process of hydrogen atoms on the carbon material quickly. Classical molecular dynamics (MD) method employs multi-body potential models and can treat realistic structures of crystal and molecule. The ab-initio method can calculate electron energy in quantum mechanics, which is regarded as realistic potential for atoms. In the present paper, the interaction of the hydrogen and the carbon material is investigated using the multi-scale (MC, MD and ab-initio) methods. The bombardment of hydrogen atoms onto the carbon material is simulated by the ACAT-code of the MC method, which cannot represent the structure of crystal, and the MD method using modified reactive empirical bond order (REBO) potential, which treats single crystal graphite and amorphous carbon. Consequently, we clarify that the sputtering yield and the reflection rate calculated by the ACAT-code agree with those on the amorphous carbon calculated by the MD. Moreover, there are many kinds of REBO potential for the MD. Adsorption, reflection and penetration rates between a hydrogen atom and a graphene surface are calculated by the MD simulations using the two kinds of potential model. For the incident energy of less than 1 eV, the MD simulation using the modified REBO potential, which is based on Brenner's REBO potential in 2002, shows that reflection is dominant, while the most popular Brenner's REBO potential in 1990 shows that adsorption is dominant. This reflection of the low energy injection is caused by a small potential barrier for the hydrogen atom in the modified REBO potential. The small potential barrier is confirmed by the ab-initio calculations, which are hybrid DFT (B3LYP/cc-pVDZ), ab-initio
International Nuclear Information System (INIS)
The behaviour of silicon carbide under irradiation has been studied using classical and ab initio simulations, focusing on the nano scale elementary processes. First, we have been interested in the calculation of threshold displacement energies, which are difficult to determine both experimentally and theoretically, and also the associated Frenkel pairs. In the framework of this thesis, we have carried out simulations in classical and ab initio molecular dynamics. For the classical approach, two types of potentials have been used: the Tersoff potential, which led to non satisfactory results, and a new one which has been developed during this thesis. This potential allows a better modelling of SiC under irradiation than most of the empirical potentials available for SiC. It is based on the EDIP potential, initially developed to describe defects in silicon, that we have generalized to SiC. For the ab initio approach, the feasibility of the calculations has been validated and average energies of 19 eV for the C and 38 eV for the Si sublattices have been determined, close to the values empirically used in the fusion community. The results obtained with the new potential EDIP are globally in agreement with those values. Finally, the elementary processes involved in the crystal recovery have been studied by calculating the stability of the created Frenkel pairs and determining possible recombination mechanisms with the nudged elastic band method. (author)
Geng, Hua Y
2014-01-01
A multilevel approach to sample the potential energy surface in a path integral formalism is proposed. The purpose is to reduce the required number of ab initio evaluations of energy and forces in ab initio path integral molecular dynamics (AI-PIMD) simulation, without compromising the overall accuracy. To validate the method, the internal energy and free energy of an Einstein crystal are calculated and compared with the analytical solutions. As a preliminary application, we assess the performance of the method in a realistic model, the FCC phase of dense atomic hydrogen, in which the calculated result shows that the acceleration rate is about 3 to 4 fold for a two-level implementation, and can be increased to 10 times if extrapolation is used. With only 16 beads used for the ab initio potential sampling, this method gives a well converged internal energy. The residual error in pressure is just about 3 GPa, whereas it is about 20 GPa for a plain AI-PIMD calculation with the same number of beads. The vibration...
Dominant Modes in Light Nuclei - Ab Initio View of Emergent Symmetries
Draayer, J. P.; Dytrych, T.; Launey, K. D.; Dreyfuss, A. C.; Langr, D.
2015-01-01
An innovative symmetry-guided concept is discussed with a focus on emergent symmetry patterns in complex nuclei. In particular, the ab initio symmetry-adapted no-core shell model (SA-NCSM), which capitalizes on exact as well as partial symmetries that underpin the structure of nuclei, provides remarkable insight into how simple symmetry patterns emerge in the many-body nuclear dynamics from first principles. This ab initio view is complemented by a fully microscopic no-core symplectic shell-model framework (NCSpM), which, in turn, informs key features of the primary physics responsible for the emergent phenomena of large deformation and alpha-cluster substructures in studies of the challenging Hoyle state in Carbon-12 and enhanced collectivity in intermediate-mass nuclei. Furthermore, by recognizing that deformed configurations often dominate the low-energy regime, the SA-NCSM provides a strategy for determining the nature of bound states of nuclei in terms of a relatively small subspace of the symmetry-reorganized complete model space, which opens new domains of nuclei for ab initio investigations, namely, the intermediate-mass region, including isotopes of Ne, Mg, and Si.
Wang, Zi; Bevan, Kirk H.
2016-01-01
In the present work, we study the effects of the electronic relaxation of semicore levels on polaron activation energies and dynamics. Within the framework of adiabatic ab initio theory, we utilize both static transition state theory and molecular dynamics methods for an in-depth study of polaronic hopping in delithiated LiFePO4 (FePO4). Our results show that electronic relaxation of semicore states is significant in FePO4, resulting in a lower activation barrier and kinetics that is one to two orders faster compared to the result of calculations that do not incorporate semicore states. In general, the results suggest that the relaxation of states far below the Fermi energy could dramatically impact the ab initio polaronic barrier estimates for many transition metal oxides and phosphates.
A theoretical-spectroscopy, ab initio-based study of the electronic ground state of 121SbH3
International Nuclear Information System (INIS)
For the stibine isotopologue 121SbH3, we report improved theoretical calculations of the vibrational energies below 8000 cm-1 and simulations of the rovibrational spectrum in the 0-8000 cm-1 region. The calculations are based on a refined ab initio potential energy surface and on a new dipole moment surface obtained at the coupled cluster CCSD(T) level. The theoretical results are compared with the available experimental data in order to validate the ab initio surfaces and the TROVE computational method [Yurchenko SN, Thiel W, Jensen P. J Mol Spectrosc 2007;245:126-40] for calculating rovibrational energies and simulating rovibrational spectra of arbitrary molecules in isolated electronic states. A number of predicted vibrational energies of 121SbH3 are provided in order to stimulate new experimental investigations of stibine. The local-mode character of the vibrations in stibine is demonstrated through an analysis of the results in terms of local-mode theory.
Ab initio design of elastically isotropic TiZrNbMoV{sub x} high-entropy alloys
Energy Technology Data Exchange (ETDEWEB)
Tian, Fuyang, E-mail: fuytian@gmail.com [Applied Materials Physics, Department of Materials Science and Engineering, Royal Institute of Technology, Stockholm SE-100 44 (Sweden); Institute for Applied Physics, University of Science and Technology Beijing, Beijing 100083 (China); Varga, Lajos Karoly [Institute for Solid State Physics and Optics, Wigner Research Centre for Physics, Hungarian Academy of Sciences, P.O. Box 49, H-1525 Budapest (Hungary); Chen, Nanxian [Institute for Applied Physics, University of Science and Technology Beijing, Beijing 100083 (China); Department of Physics, Tsinghua University, Beijing 100084 (China); Shen, Jiang [Institute for Applied Physics, University of Science and Technology Beijing, Beijing 100083 (China); Vitos, Levente [Applied Materials Physics, Department of Materials Science and Engineering, Royal Institute of Technology, Stockholm SE-100 44 (Sweden); Institute for Solid State Physics and Optics, Wigner Research Centre for Physics, Hungarian Academy of Sciences, P.O. Box 49, H-1525 Budapest (Hungary); Department of Physics and Astronomy, Division of Materials Theory, Uppsala University, Box 516, SE-751210 Uppsala (Sweden)
2014-06-25
Highlights: • The refractory high-entropy alloys are studied with ab initio theory. • We study the effect of alloying elements on the elastic parameters. • We propose an criterion of elastically isotropic refractory high-entropy alloys. - Abstract: The TiZrVNb and TiZrNbMoV{sub x} (x = 0–1.5) high-entropy alloys (HEAs) are single-phase solid solutions having the body centered cubic crystallographic structure. Here we use the ab initio exact muffin-tin orbitals method in combination with the coherent potential approximation to study the equilibrium bulk properties of the above refractory HEAs. We provide a detailed investigation of the effect of alloying elements on the electronic structure and elastic parameters. Our results indicate that vanadium enhances the anisotropy of TiZrNbMoV{sub x}. As an application of the present theoretical database, we verify the often quoted correlation between the valence electron concentration (VEC) and the micro-mechanical properties in the case of multi-component alloys. Furthermore, we predict that the present HEAs become elastically isotropic for VEC∼4.72.