SIMPLE: Software for ab initio reconstruction of heterogeneous single-particles.
Elmlund, Dominika; Elmlund, Hans
2012-12-01
The open source software suite SIMPLE: Single-particle IMage Processing Linux Engine provides data analysis methods for single-particle cryo-electron microscopy (cryo-EM). SIMPLE addresses the problem of obtaining 3D reconstructions from 2D projections only, without using an input reference volume for approximating orientations. The SIMPLE reconstruction algorithm is tailored to asymmetrical and structurally heterogeneous single-particles. Its basis is global optimization with the use of Fourier common lines. The advance that enables ab initio reconstruction and heterogeneity analysis is the separation of the tasks of in-plane alignment and projection direction determination via bijective orientation search - a new concept in common lines-based strategies. Bijective orientation search divides the configuration space into two groups of paired parameters that are optimized separately. The first group consists of the rotations and shifts in the plane of the projection; the second group consists of the projection directions and state assignments. In SIMPLE, ab initio reconstruction is feasible because the 3D in-plane alignment is approximated using reference-free 2D rotational alignment. The subsequent common lines-based search hence searches projection directions and states only. Thousands of class averages are analyzed simultaneously in a matter of hours. Novice SIMPLE users get a head start via the well documented front-end. The structured, object-oriented back-end invites advanced users to develop new alignment and reconstruction algorithms. An overview of the package is presented together with benchmarks on simulated data. Executable binaries, source code, and documentation are available at http://simple.stanford.edu. PMID:22902564
Energy Technology Data Exchange (ETDEWEB)
Feller, D.F.
1993-07-01
This collection of benchmark timings represents a snapshot of the hardware and software capabilities available for ab initio quantum chemical calculations at Pacific Northwest Laboratory`s Molecular Science Research Center in late 1992 and early 1993. The ``snapshot`` nature of these results should not be underestimated, because of the speed with which both hardware and software are changing. Even during the brief period of this study, we were presented with newer, faster versions of several of the codes. However, the deadline for completing this edition of the benchmarks precluded updating all the relevant entries in the tables. As will be discussed below, a similar situation occurred with the hardware. The timing data included in this report are subject to all the normal failures, omissions, and errors that accompany any human activity. In an attempt to mimic the manner in which calculations are typically performed, we have run the calculations with the maximum number of defaults provided by each program and a near minimum amount of memory. This approach may not produce the fastest performance that a particular code can deliver. It is not known to what extent improved timings could be obtained for each code by varying the run parameters. If sufficient interest exists, it might be possible to compile a second list of timing data corresponding to the fastest observed performance from each application, using an unrestricted set of input parameters. Improvements in I/O might have been possible by fine tuning the Unix kernel, but we resisted the temptation to make changes to the operating system. Due to the large number of possible variations in levels of operating system, compilers, speed of disks and memory, versions of applications, etc., readers of this report may not be able to exactly reproduce the times indicated. Copies of the output files from individual runs are available if questions arise about a particular set of timings.
AB INITIO AND CALPHAD THERMODYNAMICS OF MATERIALS
Energy Technology Data Exchange (ETDEWEB)
Turchi, P A
2004-04-14
Ab initio electronic structure methods can supplement CALPHAD in two major ways for subsequent applications to stability in complex alloys. The first one is rather immediate and concerns the direct input of ab initio energetics in CALPHAD databases. The other way, more involved, is the assessment of ab initio thermodynamics {acute a} la CALPHAD. It will be shown how these results can be used within CALPHAD to predict the equilibrium properties of multi-component alloys.
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. At the most approximate level, the theory is equivalent to the usual effective-medium theory. At all levels of approximation, every term in the total-energy expression is calculated ab initio, that is, ...
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.
Collective rotation from ab initio theory
Caprio, M A; Vary, J P; Smith, R
2015-01-01
Through ab initio approaches in nuclear theory, we may now seek to quantitatively understand the wealth of nuclear collective phenomena starting from the underlying internucleon interactions. No-core configuration interaction (NCCI) calculations for p-shell nuclei give rise to rotational bands, as evidenced by rotational patterns for excitation energies, electromagnetic moments, and electromagnetic transitions. In this review, NCCI calculations of 7-9Be are used to illustrate and explore ab initio rotational structure, and the resulting predictions for rotational band properties are compared with experiment. We highlight the robustness of ab initio rotational predictions across different choices for the internucleon interaction.
Ab initio nuclear structure theory
Negoita, Gianina Alina
Ab initio no core methods have become major tools for understanding the properties of light nuclei based on realistic nucleon-nucleon (NN) and three-nucleon (NNN) interactions. A brief description is provided for the inter-nucleon interactions that fit two-body scattering and bound state data, as well as NNN interactions. Major new progress, including the goal of applying these interactions to solve for properties of nuclei, is limited by convergence issues. That is, with the goal of obtaining high precision solutions of the nuclear many-body Hamiltonian with no core methods (all nucleons treated on the same footing), one needs to proceed to very large basis spaces to achieve a convergence pattern suitable for extrapolation to the exact result. This thesis investigates (1) the similarity renormalization group (SRG) approach to soften the interaction, while preserving its phase shift properties, and (2) adoption of a realistic basis space using Woods-Saxon (WS) single-particle wavefunctions. Both have their advantages and limitations, discussed here. For (1), SRG was demonstrated by applying it to a realistic NN interaction, JISP16, in a harmonic oscillator (HO) representation. The degree of interaction softening achieved through a regulator parameter is examined. For (2), new results are obtained with the realistic JISP16 NN interaction in ab initio calculations of light nuclei 4He, 6He and 12C, using a WS basis optimized to minimize the ground-state energy within the truncated no core shell model. These are numerically-intensive many-body calculations. Finally, to gain insight into the potential for no core investigations of heavier nuclei, an initial investigation was obtained for the odd mass A = 47 - 49 region nuclei straddling 48Ca. The motivation for selecting these nuclei stems from the aim of preparing for nuclear double beta-decay studies of 48Ca. In these heavier systems, phenomenological additions to the realistic NN interaction determined by previous fits to A = 48 nuclei are needed to fit the data. The modified Hamiltonian produces reasonable spectra for these odd mass nuclei. A look at future pathways opened up with the results presented here concludes this investigation.
Diatomic molecules: results of ab initio calculations
International Nuclear Information System (INIS)
A survey and analysis are given of computational studies of the electronic structure of ground and excited states of diatomic molecules. Ab initio methods in current use are discussed, and many detailed examples and results are provided. 100 references
Ab initio calculation of valley splitting in monolayer ?-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 ?-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
Ab initio model of porous periclase
International Nuclear Information System (INIS)
A two-phase equilibrium equation of state (EOS) for periclase (MgO) was constructed using ab initio quantum mechanics, including a rigorous calculation of quasiharmonic phonon modes. Much of the shock wave data reported for periclase is on porous material. We compared the theoretical EOS with porous data using a simple 'snowplough' treatment and also a model using finite equilibration rates suitable for continuum mechanics simulations. (This model has been applied previously to various heterogeneous explosives as well as other porous materials.) The results were consistent and matched the data well at pressures above the regime affected by strength - and ramp-wave formation - during compaction. Ab initio predictions of the response of porous material have been cited recently as a novel and advanced capability; we feel that this is a fairly routine extension to established ab initio techniques
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
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.
Towards hydrogen metallization: an Ab initio approach
International Nuclear Information System (INIS)
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(H2)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)
AB INITIO FORCE CONSTANTS OF GERMANIUM
Kunc, K.; de Martin, R
1981-01-01
Using the local density-functional formalism, phonon frequencies can be predicted ab initio by comparing the energy of the equilibrium configuration with the one having the atoms displaced in a pattern corresponding to a "frozen" phonon. Here we show that by using less symmetric displacement patterns and by applying the Hellman-Feynman theorem to the self-consistent electronic charge densities, one can obtain al1 force constants determining the entire branches of phonon spectra corresponding ...
Reciprocity Theorems for Ab Initio Force Calculations
Wei, C; Mele, E J; Rappe, A M; Lewis, Steven P.; Rappe, Andrew M.
1996-01-01
We present a method for calculating ab initio interatomic forces which scales quadratically with the size of the system and provides a physically transparent representation of the force in terms of the spatial variation of the electronic charge density. The method is based on a reciprocity theorem for evaluating an effective potential acting on a charged ion in the core of each atom. We illustrate the method with calculations for diatomic molecules.
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...
Thiessen, P. A.; Treder, H.-J.
Jedes initium wird durch experimenta crucis zum eventus. Jedes theoretisch interpretierbare ex-eventu-Resultat fÃ¼hrt auf ein neues Initium. Gerade dies ist die gemeinsame Aussage von Atomistik, Quantenmechanik und RelativitÃ¤tstheorie.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.
Highly scalable Ab initio genomic motif identification
Marchand, BenoÃ®t
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.
Ab initio calculations of thermochemical properties of methanol clusters.
Umer, Muhammad; Leonhard, Kai
2013-02-21
Highly accurate ab initio calculations of binding enthalpies and entropies of gas phase clusters of methanol have been performed, yielding uncertainties smaller than 1 kJ/mol per hydrogen bond in the Gibbs free energy of reaction. This requires quantum chemical RIMP2 and CCSD(T) post-Hartree-Fock methods with basis sets up to aug-cc-pV5Z for energy calculations. An analysis of topological symmetry and hindered rotor effects proves necessary for reliable entropies. This approach goes beyond the rigid rotor plus harmonic oscillator method implemented in standard quantum mechanics software tools. The results demonstrate that (1) thermochemical methanol cluster properties can nowadays be obtained by ab initio methods with an accuracy comparable to or even better than that of the experimental data available, especially for larger species that cannot be studied directly by experiments and (2) cooperativity effects and state-dependent cluster distributions cause a strongly varying average enthalpy and entropy per bond as a function of temperature and density for methanol. PMID:23330733
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.
Ab-initio atomic level stress and role of d-orbitals in CuZr, CuZn and CuY
Ojha, Madhusudan; Nicholson, Don M.; Egami, Takeshi
2015-03-01
Atomic level stress offers a new tool to characterize materials within the local approximation to density functional theory (DFT). Ab-initio atomic level stresses in B2 structures of CuZr, CuZn and CuY are calculated and results are explained on the basis of d-orbital contributions to Density of States (DOS). The overlap of d-orbital DOS plays an important role in the relative magnitude of atomic level stresses in these structures. The trends in atomic level stresses that we observed in these simple B2 structures are also seen in complex structures such as liquids, glasses and solid solutions. The stresses are however modified by the different coordination and relaxed separation distances in these complex structures. We used the Locally Self-Consistent Multiple Scattering (LSMS) code and Vienna Ab-initio Simulation Package (VASP) for ab-initio calculations.
Ab initio energetics of nonsubstituted monocyclic pyrones
Energy Technology Data Exchange (ETDEWEB)
Morais, Victor M.F., E-mail: vmmorais@icbas.up.p [Instituto de Ciencias Biomedicas Abel Salazar, ICBAS, Universidade do Porto, P-4099-003 Porto (Portugal); Centro de Investigacao em Quimica, Departamento de Quimica, Faculdade de Ciencias da Universidade do Porto, Rua do Campo Alegre, 687, P-4169-007 Porto (Portugal)
2011-01-15
The monocyclic nonsubstituted pyrones were studied computationally using state-of-the-art ab initio composite computational techniques. Combination of the accurate energies so obtained with conveniently chosen isodesmic or homodesmotic chemical reactions lead to very confident predictions of their corresponding standard enthalpy of formation. The internal consistency of the results obtained from a vast number of independent chemical schemes serves as a further support of the quality of our results, which are thus proposed to establish the energetics of {alpha}-pyrone and {gamma}-pyrone.
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 ?R IAA1041302; GA ?R 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 energetics of nonsubstituted monocyclic pyrones
International Nuclear Information System (INIS)
The monocyclic nonsubstituted pyrones were studied computationally using state-of-the-art ab initio composite computational techniques. Combination of the accurate energies so obtained with conveniently chosen isodesmic or homodesmotic chemical reactions lead to very confident predictions of their corresponding standard enthalpy of formation. The internal consistency of the results obtained from a vast number of independent chemical schemes serves as a further support of the quality of our results, which are thus proposed to establish the energetics of Î±-pyrone and Î³-pyrone.
Ab initio non-relativistic spin dynamics
Ding, Feizhi; Goings, Joshua J.; Frisch, Michael J.; Li, Xiaosong
2014-12-01
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 Li3 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.
Ab initio non-relativistic spin dynamics
International Nuclear Information System (INIS)
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 Li3 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
Ab initio alpha-alpha scattering
Elhatisari, Serdar; Rupak, Gautam; Epelbaum, Evgeny; Krebs, Hermann; Lähde, Timo A; Luu, Thomas; Meißner, Ulf-G
2015-01-01
Processes involving alpha particles and alpha-like nuclei comprise a major part of stellar nucleosynthesis and hypothesized mechanisms for thermonuclear supernovae. In an effort towards understanding alpha processes from first principles, we describe in this letter the first ab initio calculation of alpha-alpha scattering. We use lattice effective field theory to describe the low-energy interactions of nucleons and apply a technique called the adiabatic projection method to reduce the eight-body system to an effective two-cluster system. We find good agreement between lattice results and experimental phase shifts for S-wave and D-wave scattering. The computational scaling with particle number suggests that alpha processes involving heavier nuclei are also within reach in the near future.
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...
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 path to heavy nuclei
Energy Technology Data Exchange (ETDEWEB)
Binder, Sven, E-mail: sven.binder@physik.tu-darmstadt.de; Langhammer, Joachim, E-mail: joachim.langhammer@physik.tu-darmstadt.de; Calci, Angelo, E-mail: angelo.calci@physik.tu-darmstadt.de; Roth, Robert, E-mail: robert.roth@physik.tu-darmstadt.de
2014-09-07
We present the first ab initio calculations of nuclear ground states up into the domain of heavy nuclei, spanning the range from {sup 16}O to {sup 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 path to heavy nuclei
Binder, Sven; Langhammer, Joachim; Calci, Angelo; Roth, Robert
2014-09-01
We present the first ab initio calculations of nuclear ground states up into the domain of heavy nuclei, spanning the range from 16O to 132Sn, 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.
Time-reversible ab initio molecular dynamics
Niklasson, Anders M. N.; Tymczak, C. J.; Challacombe, Matt
2007-04-01
Time-reversible ab initio molecular dynamics based on a lossless multichannel decomposition for the integration of the electronic degrees of freedom [Phys. Rev. Lett. 97, 123001 (2006)] is explored. The authors present a lossless time-reversible density matrix molecular dynamics scheme. This approach often allows for stable Hartree-Fock simulations using only one single self-consistent field cycle per time step. They also present a generalization, introducing an additional "forcing" term, that in a special case includes a hybrid Lagrangian, i.e., Car-Parrinello-type, method, which can systematically be constrained to the Born-Oppenheimer potential energy surface by using an increasing number of self-consistency cycles in the nuclear force calculations. Furthermore, in analog to the reversible and symplectic leapfrog or velocity Verlet schemes, where not only the position but also the velocity is propagated, the authors propose a Verlet-type density velocity formalism for time-reversible Born-Oppenheimer molecular dynamics.
Discovering chemistry with an ab initio nanoreactor
Wang, Lee-Ping; Titov, Alexey; McGibbon, Robert; Liu, Fang; Pande, Vijay S.; Martínez, Todd J.
2014-12-01
Chemical understanding is driven by the experimental discovery of new compounds and reactivity, and is supported by theory and computation that provide detailed physical insight. Although 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 accelerated first-principles molecular dynamics simulation of chemical reactions that discovers new molecules and mechanisms without preordained reaction coordinates or elementary steps. Using the nanoreactor, we show new pathways for glycine synthesis from primitive compounds proposed to exist on the early Earth, which provide new insight into the classic Urey-Miller experiment. These results highlight the emergence of theoretical and computational chemistry as a tool for discovery, in addition to its traditional role of interpreting experimental findings.
Ab initio alpha-alpha scattering.
Elhatisari, Serdar; Lee, Dean; Rupak, Gautam; Epelbaum, Evgeny; Krebs, Hermann; LÃ¤hde, Timo A; Luu, Thomas; MeiÃŸner, 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 quarks and gluons. PMID:26632590
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 quarks and gluons.
Gross, A; Gross, Axel; Scheffler, Matthias
1997-01-01
The dissociative adsorption of hydrogen on Pd(100) has been studied by ab initio quantum dynamics and ab initio molecular dynamics calculations. Treating all hydrogen degrees of freedom as dynamical coordinates implies a high dimensionality and requires statistical averages over thousands of trajectories. An efficient and accurate treatment of such extensive statistics is achieved in two steps: In a first step we evaluate the ab initio potential energy surface (PES) and determine an analytical representation. Then, in an independent second step dynamical calculations are performed on the analytical representation of the PES. Thus the dissociation dynamics is investigated without any crucial assumption except for the Born-Oppenheimer approximation which is anyhow employed when density-functional theory calculations are performed. The ab initio molecular dynamics is compared to detailed quantum dynamical calculations on exactly the same ab initio PES. The occurence of quantum oscillations in the sticking probab...
Why ferroelectricity? synchrotron radiation and ab initio answers
Scientific Electronic Library Online (English)
R, Olivera; M.E, Fuentes; F, Espinosa; M, García; E, Macías; A, Durán; J, Siqueiros; L, Fuentes.
2007-02-01
Full Text Available Una pregunta histórica de la física del estado sólido está encontrando respuesta en nuestros tiempos: la explicación a nivel atómico del origen de la ferroelectricidad. Las ideas tradicionales sobre fenómenos ferroeléctricos se relacionan con el "ablandamiento" de los fonones en el origen de la zona [...] de Brillouin y con funciones en forma de "W" para la energía libre de Landau. Las contribuciones experimentales (radiación sincrotrónica, neutrones) y teóricas (Cohen, Resta, Spaldin) de la última década han esclarecido aspectos del comportamiento atómico que conducen a la polarización espontánea en estructuras perovskitas y asociadas. Se presenta el trabajo desarrollado por nuestro grupo interdisciplinario. Se obtienen fases ferroeléctricas perovskitas y Aurivillius por diferentes métodos. Se investigan detalles finos de las estructuras cristalinas mediante radiación sincrotrónica en el Laboratorio de Radiación Sincrotrónica de Stanford. Las estructuras electronicas de las fases consideradas se caracterizan por métodos ab initio. Los experimentos de difraccion en alta resolución demuestran ruptura de simetría en un número de sistemas perovskita y Aurivillius. Se discute la relación estructura-simetría- polarización. Se presenta una explicación ab initio de la polarizacion ferroeléctrica en perovskitas. La energía del sistema se calcula mediante el codigo CASTEP bajo un funcional GGA. La optimización de la energía conduce a la ruptura de simetría cubica, con desplazamiento del catión Ti fuera del centro, vía una transformación Jahn-Teller de segundo orden. La estructura electrónica se investiga mediante el software BandLab, bajo un funcional LDA, con el método LMTO. La causa de la deformación de la perovskita es la degeneración de los orbitales Ti 3d z² y Ti 3d (x²+y²). Abstract in english An old question of solid state physics is being answered nowadays: the atomic-level understanding of ferroelectricity. Traditional ideas about ferroelectric phenomena relate with softening of optical phonons at the Brillouin zone origin and with "W-shaped" Landau free energy functions. Last decade e [...] xperimental (synchrotron radiation, neutrons) and quantum-theoretical (Cohen, Resta, Spaldin) contributions have clarified detailed descriptions and explanations for atomic behavior leading to spontaneous polarization in perovskite and perovskite-related crystal structures. Work being performed by our interdisciplinary group on ferroelectricity is presented. Perovskite and Aurivillius ferroelectric phases are obtained by different methods. Fine details on crystal structures are investigated by means of synchrotron radiation at Stanford Synchrotron Radiation Laboratory. Electronic structures of considered phases are theoretically characterized by ab initio methods. High-resolution diffraction experiments demonstrate several symmetry break-downs in perovskite and Aurivillius phases. The structure-symmetry-polarization relationship is discussed for a number of representative cases. Ab initio explanation of ferroelectric polarization in perovskite structures is given. Energy calculation is performed by means of CASTEP code under GGA functional. Energy optimization leads to cubic-tetragonal symmetry break-down with off-centering cation displacements via second-order Jahn-Teller effect. Electronic structure is investigated with BandLab code, under LDA functional with LMTO method. Degeneracy of Ti 3d z² and Ti 3d (x²+y²) orbitals is the cause of cubic-perovskite deformation.
Ab initio study of solution energy and diffusion of caesium in uranium dioxide
Energy Technology Data Exchange (ETDEWEB)
Gupta, F. [IRSN, DPAM, SEMIC, LETR, 13115 Saint Paul Lez Durance (France); Pasturel, A. [CNRS, Laboratoire de Physique et Modelisation des Milieux Condenses, Maison des Magisteres, 25 Avenue des Martyrs BP166, 38042 Grenoble (France); Brillant, G. [IRSN, DPAM, SEMIC, LETR, 13115 Saint Paul Lez Durance (France)], E-mail: guillaume.brillant@irsn.fr
2009-03-31
The behaviour of caesium in nuclear fuels is investigated using density functional theory (DFT). In a first step, the incorporation and solution energies of Cs in pre-existing trap sites of UO{sub 2} (vacancies, interstitials, U-O di-vacancy and Schottky trio defects) are calculated using the projector-augmented-wave (PAW) derived pseudopotentials as implemented in the Vienna ab initio simulation package (VASP). Correlation effects are taken into account within the DFT + U approach. The solubility of caesium is found to be very low, in agreement with experimental data. The migration of Cs is found to be highly anisotropic, it is controlled by uranium diffusion with an Arrhenius activation energy of 4.8 eV in hyperstoichiometric UO{sub 2+x}, in good agreement with experimental values.
Ab Initio Derivation of Model Energy Density Functionals
Dobaczewski, J
2015-01-01
I propose a simple and manageable method that allows for deriving coupling constants of model energy density functionals (EDFs) directly from ab initio calculations performed for finite fermion systems. A proof-of-principle application allows for linking properties of finite nuclei, determined by using the nuclear nonlocal Gogny functional, to the coupling constants of the quasilocal Skyrme functional. The method does not rely on properties of infinite fermion systems but on the ab initio calculations in finite systems. It also allows for quantifying merits of different model EDFs in describing the ab initio results.
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.
On the hierarchical parallelization of ab initio simulations
Ruiz-Barragan, Sergi; Ishimura, Kazuya; Shiga, Motoyuki
2016-02-01
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.
Reconstructions of Ir (110) and (100) an ab initio study
Filippetti, A; Filippetti, Alessio; Fiorentini, Vincenzo
1996-01-01
Prediction criteria for surface reconstructions are discussed, with reference to ab initio calculations of the (110)-$1\\times 2$ missing-row and (100)-$5\\times 1$ quasi-hexagonal reconstructions of Ir and Rh.
Ab initio two-component Ehrenfest dynamics.
Ding, Feizhi; Goings, Joshua J; Liu, Hongbin; Lingerfelt, David B; Li, Xiaosong
2015-09-21
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. PMID:26395685
Ab initio calculation of the Hoyle state
Epelbaum, Evgeny; Lee, Dean; Meißner, Ulf-G
2011-01-01
The Hoyle state plays a crucial role in the hydrogen burning of stars heavier than our sun and in the production of carbon and other elements necessary for life. This excited state of the carbon-12 nucleus was postulated by Hoyle^{1} as a necessary ingredient for the fusion of three alpha particles to produce carbon at stellar temperatures. Although the Hoyle state was seen experimentally more than a half century ago^{2,3}, nuclear theorists have not yet uncovered the nature of this state from first principles. In this letter we report the first ab initio calculation of the low-lying states of carbon-12 using supercomputer lattice simulations and a theoretical framework known as effective field theory. In addition to the ground state and excited spin-2 state, we find a resonance at -85(3) MeV with all of properties of the Hoyle state and in agreement with the experimentally observed energy. These lattice simulations provide insight into the structure of this unique state and new clues as to the amount of fine...
Ab initio calculations of nitramine dimers
Koh-Fallet, Sharon; Schweigert, Igor
2015-06-01
Elevated temperatures and pressures are typically thought to have opposing effects on the reaction channels of nitramine decomposition. These high temperatures promote reactions with loose transition structures (positive activation entropies and volumes), such as N-N bond homolysis. Elevated pressures promote reactions with tight transition structures (negative activation entropies and volumes), such as intramolecular and intermolecular H transfer. However, no quantitative data exists regarding the range of temperatures and pressures at which these effects become pronounced. We are pursuing ab initio calculations of the corresponding unimolecular and bimolecular transition structures with the objective of estimating the relevant thermochemical parameters and quantifying the effects of elevated temperature and pressures on the corresponding rate constants. Here, we present density functional theory and complete active space calculations of gas-phase molecular dimers of nitramines as an intermediate step toward modeling transition structures directly in the condensed phase. This work was supported by the Naval Research Laboratory via the American Society for Engineering and Education and by the Office of Naval Research, both directly and through the Naval Research Laboratory.
Ab initio thermodynamic study of defective strontium titanate
Blokhin, Evgeny
2013-01-01
In the presented thesis the perfect and defective SrTiO3 bulk crystals and their (001) surfaces are considered on ab initio level. Since the experimental study of the complex defective systems is comparatively expensive and difficult, and the computer performance has been greatly increased in the last years, the ab initio modeling became very efficient tool to be applied in this field. Additionally, there is a significant industrial demand for the investigation and improvements of the perform...
Ab-initio Raman spectra of anharmonic and disordered systems
Putrino, Anna
2000-01-01
In this thesis work, a new method for computing Raman spectra of anharmonic and disordered systems through an ab-initio density functional perturbation theory approach is presented. The method combines ab-initio molecular dynamics with the generalization of density functional perturbation theory to non-hamiltonian perturbations. The formalism uses the description of the position operator in periodic systems in terms of Berry phase. The method is implemented in the context of a density functio...
Ab initio atomistic simulation of metals and multicomponent alloys
Tian, Fuyang
2013-01-01
Ab initio theory provides a powerful tool to understand and predict the behavior of materials. This thesis contains both of these aspects. First we use ab initio alloy theory to investigate a new kind of complex alloy (high-entropy alloy). Second we introduce a novel potential (interlayer potential), which can be extracted from ab inito total energy calculations using the Chen-Möbius inversion method. High-entropy alloys (HEAs) are composed of four or more metallic elements with nearly equimo...
Skutterudites under pressure: An ab initio study
Energy Technology Data Exchange (ETDEWEB)
Ram, Swetarekha; Kanchana, V., E-mail: kanchana@iith.ac.in [Department of Physics, Indian Institute of Technology Hyderabad, Ordnance Factory Estate, Yeddumailaram 502 205, Andhra Pradesh (India); Valsakumar, M. C. [School of Engineering Sciences and Technology (SEST), University of Hyderabad, Prof. C. R. Rao Road, Gachibowli, Hyderabad 500 046, Andhra Pradesh (India)
2014-03-07
Ab initio results on the band structure, density of states, and Fermi surface (FS) properties of LaRu{sub 4}X{sub 12} (X?=?P, As, Sb) are presented at ambient pressure as well as under compression. The analysis of density of states reveals the major contribution at the Fermi level to be mainly from the Ru-d and X-p states. We have a complicated Fermi surface with both electron and hole characters for all the three compounds which is derived mainly from the Ru-d and X-p states. There is also a simpler FS with hole character derived from the P-p{sub z} orbital for LaRu{sub 4}P{sub 12} and Ru-d{sub z{sup 2}} orbital in the case of As and Sb containing compounds. More interestingly, Fermi surface nesting feature is observed only in the case of the LaRu{sub 4}P{sub 12}. Under compression, we observe the topology of the complicated FS sheet of LaRu{sub 4}As{sub 12} to change around V/V{sub 0}?=?0.85, leading to a behaviour similar to that of a multiband superconductor, and in addition, we have two more hole pockets centered around ? at V/V{sub 0}?=?0.8 for the same compound. Apart from this, we find the hole pocket to vanish at V/V{sub 0}?=?0.8 in the case of LaRu{sub 4}Sb{sub 12} and the opening of the complicated FS sheet gets reduced. The de Haas van Alphen calculation shows the number of extremal orbits in the complicated sheet to change in As and Sb containing compounds under compression, where we also observe the FS topology to change.
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 illustrations of their unprecedented power in addressing some of the outstanding problems of solid-state chemistry, high-pressure chemistry, or geochemistry. They are the structure and spectra of ice Ih, in particular, the origin of two peaks in the hydrogen-bond-stretching region of its inelastic neutron scattering spectra, a solid-solid phase transition from CO2-I to elusive, metastable CO2-III, pressure tuning of Fermi resonance in solid CO2, and the structure and spectra of solid formic acid, all at the level of second-order Møller-Plesset perturbation theory or higher. PMID:24754304
Why ferroelectricity? synchrotron radiation and ab initio answers
Directory of Open Access Journals (Sweden)
R. Olivera
2007-01-01
Full Text Available Una pregunta hist´orica de la f´?sica del estado s´olido est´a encontrando respuesta en nuestros tiempos: la explicaci´on a nivel at´omico del origen de la ferroelectricidad. Las ideas tradicionales sobre fen´omenos ferroel´ectricos se relacionan con el ?ablandamiento? de los fonones en el origen de la zona de Brillouin y con funciones en forma de ?W? para la energ´?a libre de Landau. Las contribuciones experimentales (radiaci´on sincrotr´onica, neutrones y te´oricas (Cohen, Resta, Spaldin de la ´ultima d´ecada han esclarecido aspectos del comportamiento at´omico que conducen a la polarizaci´on espont´anea en estructuras perovskitas y asociadas. Se presenta el trabajo desarrollado por nuestro grupo interdisciplinario. Se obtienen fases ferroel´ectricas perovskitas y Aurivillius por diferentes m´etodos. Se investigan detalles finos de las estructuras cristalinas mediante radiaci´on sincrotr´onica en el Laboratorio de Radiaci´on Sincrotr´onica de Stanford. Las estructuras electr´onicas de las fases consideradas se caracterizan por m´etodos ab initio. Los experimentos de difracci´on en alta resoluci´on demuestran ruptura de simetr´?a en un n´umero de sistemas perovskita y Aurivillius. Se discute la relaci ´on estructura-simetr´?a- polarizaci´on. Se presenta una explicaci´on ab initio de la polarizaci´on ferroel´ectrica en perovskitas. La energ´?a del sistema se calcula mediante el c´odigo CASTEP bajo un funcional GGA. La optimizaci´on de la energ´?a conduce a la ruptura de simetr´?a c´ubica, con desplazamiento del cati´on Ti fuera del centro, v´?a una transformaci´on Jahn-Teller de segundo orden. La estructura electr´onica se investiga mediante el software BandLab, bajo un funcional LDA, con el m´etodo LMTO. La causa de la deformaci´on de la perovskita es la degeneraci´on de los orbitales Ti 3d z2 y Ti 3d (x2+y2.
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 resulting NN and NNN interactions, characterized by the order of the expansion retained (e.g. 'next-to-next-to leading order' is NNLO), provide a high-quality fit to the NN data and the A = 3 ground-state (g.s.) properties. The derivations of NN, NNN, etc. interactions within meson-exchange and {chi}EFT are well-established but are not subjects of this review. Our focus is solution of the non-relativistic quantum many-body Hamiltonian that includes these interactions using our no core shell model (NCSM) formalism. In the next section we will briefly outline the NCSM formalism and then present applications, results and extensions in later sections.
THERMODYNAMICS OF MATERIALS: FROM AB INITIO TO PHENOMENOLOGY
Energy Technology Data Exchange (ETDEWEB)
Turchi, P A
2004-09-24
Quantum mechanical-based (or ab initio) methods are used to predict the stability properties of materials although their application is limited to relatively simple systems in terms of structures and number of alloy components. However thermodynamics of complex multi-component alloys requires a more versatile approach afforded within the CALPHAD formalism. Despite its success, the lack of experimental data very often prevents the design of robust thermodynamic databases. After a brief survey of ab initio methodologies and CALPHAD, it will be shown how ab initio electronic structure methods can supplement in two ways CALPHAD for subsequent applications. The first one is rather immediate and concerns the direct input of ab initio energetics in CALPHAD databases. The other way, more involved, is the assessment of ab initio thermodynamics '{acute a} la CALPHAD'. It will be shown how these results can be used within CALPHAD to predict the equilibrium properties of multi-component alloys. Finally, comments will be made on challenges and future prospects.
Ab-initio study of electronic structure and thermodynamic properties of aurates BaAu2O4 and SrAu2O4
Jezierski, Andrzej; Kaczkowski, Jakub
2016-03-01
Using ab-initio the Full-Potential Local Orbital minimum basis (FPLO) and Vienna Simulation Package (VASP) methods in the framework of the local density functional theory within the generalized gradient approximation (GGA) the band structure and thermodynamic properties of aurates BaAu2O4, SrAu2O4 and BaSrAu2O4 have been studied. Ab-inito calculations have shown that these systems are semiconductors with the gap between 1.42 and 1.65 eV. The values of gaps decrease with the increase of the volume. The thermodynamic parameters (bulk modulus and Debye temperature) are calculated in the quasi-harmonic Debye-GrÃ¼neisen model. Both ab-initio methods give the similar results.
Ab initio DFT calculations of vibrational properties
Story, S. M.; Vila, F. D.; Kas, J. J.; Rehr, J. J.
2014-03-01
Vibrational properties such as EXAFS and crystallographic Debye-Waller factors, vibrational free energies, phonon self-energies, and phonon contributions to the electron spectral function, are key to understanding many aspects of materials beyond ground state electronic structure. Thus, their simulation using first principles methods is of particular importance. Many of these vibrational properties can be calculated from the dynamical matrix and electron-phonon coupling coefficients obtained from DFT calculations. Here we present a code DMVP that calculates these properties from the output of electronic structure codes such as ABINIT, Gaussian, Quantum Espresso and VASP. Our modular interfacing tool AI2PS allows us to translate the different outputs into a DMVP compatible format and generate vibrational properties in an automated way. Finally, we present some current applications that take advantage of the modular form of AI2PS to extend its capabilities to the calculation of coefficients of thermal expansion and other properties of interest such as infrared spectra. This work was supported by DOE Grant DE-FG02-97ER45623.
Introducing k-point parallelism into VASP
Maniopoulou, A.; Davidson, E. R. M.; Grau-Crespo, R.; Walsh, A.; Bush, I. J.; Catlow, C. R. A.; Woodley, S. M.
2012-01-01
For many years ab initio electronic structure calculations based upon density functional theory have been one of the main application areas in high performance computing (HPC). Typically, the Kohn–Sham equations are solved by minimisation of the total energy functional, using a plane wave basis set for valence electrons and pseudopotentials to obviate the representation of core states. One of the best known and widely used software for performing this type of calculation is the Vienna Ab init...
Ab initio quality neural-network potential for sodium
Eshet, Hagai; Khaliullin, Rustam Z; Kuhne, Thomas D.; Behler, Jorg; Parrinello, Michele
2010-01-01
An interatomic potential for high-pressure high-temperature (HPHT) crystalline and liquid phases of sodium is created using a neural-network (NN) representation of the ab initio potential energy surface. It is demonstrated that the NN potential provides an ab initio quality description of multiple properties of liquid sodium and bcc, fcc, cI16 crystal phases in the P-T region up to 120 GPa and 1200 K. The unique combination of computational efficiency of the NN potential and its ability to re...
Ab Initio Nuclear Structure Theory: From Few to Many
Roth, Robert; Calci, Angelo; Langhammer, Joachim; Binder, Sven
2014-08-01
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.
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.
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.
Transport phenomena in metallic nanostructures: an ab initio approach
Zahn, Peter
2005-01-01
Im Rahmen der vorliegenden Arbeit werden ab initio Berechnungen des Restwiderstandes von metallischen Nanostrukturen vorgestellt. Die elektronische Struktur der idealen Systeme wird mit Hilfe einer Screened KKR Greenschen Funktionsmethode im Rahmen der Vielfachstreutheorie auf der Grundlage der Dichtefunktionaltheorie berechnet. Die Potentiale von Punktdefekten werden selbstkonsistent mit Hilfe einer Dyson-Gleichung für die Greensche Funktion des gestörten Systems berechnet. Unter Nutzung der...
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 found dependent on the type of ionisation process.
Energy Technology Data Exchange (ETDEWEB)
Benrekia, A.R., E-mail: benrekia.ahmed@yahoo.com [Faculty of Science and Technology, University of Medea (Algeria); Benkhettou, N. [Laboratoire des Materiaux Magnetiques, Faculte des Sciences, Universite Djillali Liabes de Sidi Bel Abbes (Algeria); Nassour, A. [Laboratoire de Cristallographie, Resonance Magnetique et Modelisations (CRM2, UMR CNRS 7036) Institut Jean Barriol, Nancy Universite BP 239, Boulevard des Aiguillettes, 54506 Vandoeuvre-les-Nancy (France); Driz, M. [Applied Material Laboratory (AML), Electronics Department, University of Sidi bel Abbes (DZ 22000) (Algeria); Sahnoun, M. [Laboratoire de Physique Quantique de la Matiere et Modelisations Mathematique (LPQ3M), Faculty of Science and Technology,University of Mascara (Algeria); Lebegue, S. [Laboratoire de Cristallographie, Resonance Magnetique et Modelisations (CRM2, UMR CNRS 7036) Institut Jean Barriol, Nancy Universite BP 239, Boulevard des Aiguillettes, 54506 Vandoeuvre-les-Nancy (France)
2012-07-01
We present first-principles VASP calculations of the structural, electronic, vibrational, and optical properties of paraelectric SrTiO{sub 3} and KTaO{sub 3}. The ab initio calculations are performed in the framework of density functional theory with different exchange-correlation potentials. Our calculated lattice parameters, elastic constants, and vibrational frequencies are found to be in good agreement with the available experimental values. Then, the bandstructures are calculated with the GW approximation, and the corresponding band gap is used to obtain the optical properties of SrTiO{sub 3} and KTaO{sub 3}.
Energy Technology Data Exchange (ETDEWEB)
Ventelon, L. [CEA Saclay, Dept. des Materiaux pour le Nucleaire (DEN/DANS/DMN/SRMP), 91 - Gif-sur-Yvette (France)
2008-07-01
In the framework of the multi scale simulation of metals and alloys plasticity, the aim of this study is to develop a methodology of ab initio dislocations study and to apply it to the [111] screw dislocation in the bc iron. (A.L.B.)
Ab initio study of C14 laves phases in Fe-based systems
Directory of Open Access Journals (Sweden)
Pavlu J.
2012-01-01
Full Text Available Structural properties and energetics of Fe-based C14 Laves phases at various compositions (i.e. Fe2Fe, Fe2X, X2Fe, X2X, where X stands for Si, Cr, Mo, W, Ta were investigated using the pseudopotential VASP (Vienna Ab initio Simulation Package code employing the PAW-PBE (Projector Augmented Wave - Perdew Burke-Ernzerhof pseudopotentials. Full relaxation was performed for all structures studied including the reference states of elemental constituents and the equilibrium structure parameters as well as bulk moduli were found. The structure parameters of experimentally found structures were very well reproduced by our calculations. It was also found that the lattice parameters and volumes of the unit cell decrease with increasing molar fraction of iron. Thermodynamic analysis shows that the Fe2X configurations of Laves phases are more stable than the X2Fe ones. Some of the X2Fe configurations are even unstable with respect to the weighted average of the Laves phases of elemental constituents. Our calculations predict the stability of Fe2Ta. On the other hand, Fe2Mo and Fe2W are slightly unstable (3.19 and 0.68 kJ.mol-1, respectively and hypothetical structures Fe2Cr and Fe2Si are found unstable as well.
Ab-initio modelling of defect properties in B2 NiAl
Energy Technology Data Exchange (ETDEWEB)
Lerch, Daniel; Doessel, Kerrin; Mueller, Stefan; Heinz, Klaus [Festkoerperphysik, Universitaet Erlangen-Nuernberg, Staudtstr. 7, D- 91058 Erlangen (Germany)
2007-07-01
The strongly ordering B2 phase of the intermetallic compound NiAl is known to be stable for a broad concentration range off its stoichiometry. Evidently this is due to the existence of constitutional defects. Experimental studies hav e shown that the dominant defect types in Al rich and Al poor parts of the B2 ph ase are Ni vacancies and Ni antisites (Ni atoms on the Al sublattice) respectively. We elucidate the ordering and ground state properties of defects in B2 NiAl on the basis of ab-initio calculations. We have applied the Cluster-Expansion (CE) formalism with Density Functional Theory (DFT) and the nudged elastic band method (as implemented within the computer code VASP) supplying the required input energies, as well as activation barriers for defect diffusion. Consistent with the existence of vacancies a full description of defects in NiAl bulk is only possible if vacancies are included by treating them as a third independent component within the CE formalism. One possible choice for a complete orthonormal basis set of point functions, required in such a ternary CE, are the first two Chebychev polynomials which were chosen in this case.
Ab Initio Calculations Of Light-Ion Reactions
Energy Technology Data Exchange (ETDEWEB)
Navratil, P; Quaglioni, S; Roth, R; Horiuchi, W
2012-03-12
The exact treatment of nuclei starting from the constituent nucleons and the fundamental interactions among them has been a long-standing goal in nuclear physics. In addition to the complex nature of nuclear forces, 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. In this contribution, we present one of such promising techniques capable of describing simultaneously both bound and scattering states in light nuclei. By combining the resonating-group method (RGM) with the ab initio no-core shell model (NCSM), we complement a microscopic cluster approach with the use of realistic interactions and a microscopic and consistent description of the clusters. We discuss applications to light nuclei scattering, radiative capture and fusion reactions.
Spin-orbit decomposition of ab initio wavefunctions
Johnson, Calvin W
2014-01-01
Although the modern shell-model picture of atomic nuclei is built from single-particle orbits with good total angular momentum $j$, leading to $j$-$j$ coupling, phenomenological models suggested decades ago that for $0p$-shell nuclides a simpler picture can be realized via coupling of total spin $S$ and total orbital angular momentum $L$. I revisit this idea with large-basis, no-core shell model (NCSM) calculations using modern \\textit{ab initio} two-body interactions, and dissect the resulting wavefunctions into their component $L$- and $S$-components. Remarkably, there is broad agreement with calculations using the phenomenological Cohen-Kurath forces, despite a gap of nearly fifty years and six orders of magnitude in basis dimensions. I suggest $L$-$S$ may be a useful tool for analyzing \\textit{ab initio} wavefunctions of light nuclei, for example in the case of rotational bands.
Ab initio Monte Carlo investigation of small lithium clusters.
Energy Technology Data Exchange (ETDEWEB)
Srinivas, S.
1999-06-16
Structural and thermal properties of small lithium clusters are studied using ab initio-based Monte Carlo simulations. The ab initio scheme uses a Hartree-Fock/density functional treatment of the electronic structure combined with a jump-walking Monte Carlo sampling of nuclear configurations. Structural forms of Li{sub 8} and Li{sub 9}{sup +} clusters are obtained and their thermal properties analyzed in terms of probability distributions of the cluster potential energy, average potential energy and configurational heat capacity all considered as a function of the cluster temperature. Details of the gradual evolution with temperature of the structural forms sampled are examined. Temperatures characterizing the onset of structural changes and isomer coexistence are identified for both clusters.
Multiple Time Step Integrators in Ab Initio Molecular Dynamics
Luehr, Nathan; Martinez, Todd J
2013-01-01
Multiple time-scale algorithms exploit the natural separation of time-scales in chemical systems to greatly accelerate the efficiency of molecular dynamics simulations. Although the utility of these methods in systems where the interactions are described by empirical potentials is now well established, their application to ab initio molecular dynamics calculations has been limited by difficulties associated with splitting the ab initio potential into fast and slowly varying components. Here we show that such a timescale separation is possible using two different schemes: one based on fragment decomposition and the other on range separation of the Coulomb operator in the electronic Hamiltonian. We demonstrate for both water clusters and a solvated hydroxide ion that multiple time-scale molecular dynamics allows for outer time steps of 2.5 fs, which are as large as those obtained when such schemes are applied to empirical potentials, while still allowing for bonds to be broken and reformed throughout the dynami...
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.
Four-body forces in ab initio nuclear structure
International Nuclear Information System (INIS)
Nucleon-nucleon (NN) and three-nucleon (3N) interactions derived from chiral effective field theory, transformed using the similarity renormalization group (SRG), have been successful in the ab initio description of nuclear-structure. Previous investigations have shown strong indications for sizable effects originating by SRG-induced four-nucleon (4N) contributions. To obtain reliable results, the 4N contributions can either be suppressed or included in nuclear-structure calculations. We present an efficient scheme to take induced as well as initial 4N forces into account by performing the SRG transformation in four-body space and extending the importance-truncated no-core shell model (IT-NCSM) for their explicit inclusion. We investigate the effect of induced forces beyond the three-body level in ab initio nuclear-structure calculations and present results for p-shell nuclei up to 16O.
Four-body forces in ab initio nuclear structure
Energy Technology Data Exchange (ETDEWEB)
Schulz, Stefan; Calci, Angelo; Roth, Robert [Institut fuer Kernphysik, Technische Universitaet Darmstadt (Germany)
2014-07-01
Nucleon-nucleon (NN) and three-nucleon (3N) interactions derived from chiral effective field theory, transformed using the similarity renormalization group (SRG), have been successful in the ab initio description of nuclear-structure. Previous investigations have shown strong indications for sizable effects originating by SRG-induced four-nucleon (4N) contributions. To obtain reliable results, the 4N contributions can either be suppressed or included in nuclear-structure calculations. We present an efficient scheme to take induced as well as initial 4N forces into account by performing the SRG transformation in four-body space and extending the importance-truncated no-core shell model (IT-NCSM) for their explicit inclusion. We investigate the effect of induced forces beyond the three-body level in ab initio nuclear-structure calculations and present results for p-shell nuclei up to {sup 16}O.
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
Relating Ab Initio Mechanical Behavior of Intergranular Glassy Films in ?-Si3N4 to Continuum Scales
Ouyang, L.; Chen, J.; Ching, W.; Misra, A.
2006-05-01
Nanometer thin intergranular glassy films (IGFs) form in polycrystalline ceramics during sintering at high temperatures. The structure and properties of these IGFs are significantly changed by doping with rare earth elements. We have performed highly accurate large-scale ab initio calculations of the mechanical properties of both undoped and Yittria doped (Y-IGF) model by theoretical uniaxial tensile experiments. Uniaxial strain was applied by incrementally stretching the super cell in one direction, while the other two dimensions were kept constant. At each strain, all atoms in the model were fully relaxed using Vienna Ab initio Simulation Package VASP. The relaxed model at a given strain serves as the starting position for the next increment of strain. This process is carried on until the total energy (TE) and stress data show that the "sample" is fully fractured. Interesting differences are seen between the stress-strain response of undoped and Y-doped models. For the undoped model, the stress-strain behavior indicates that the initial atomic structure of the IGF is such that there is negligible coupling between the x- and the y-z directions. However, once the behavior becomes non- linear the lateral stresses increase, indicating that the atomic structure evolves with loading [1]. To relate the ab initio calculations to the continuum scales we analyze the atomic-scale deformation field under this uniaxial loading [1]. The applied strain in the x-direction is mostly accommodated by the IGF part of the model and the crystalline part experiences almost negligible strain. As the overall strain on the sample is incrementally increased, the local strain field evolves such that locations proximal to the softer spots attract higher strains. As the load progresses, the strain concentration spots coalesce and eventually form persistent strain localization zone across the IGF. The deformation pattern obtained through ab initio calculations indicates that it is possible to construct discrete grain-scale models that may be used to bridge these calculations to the continuum scale for finite element analysis. Reference: 1. J. Chen, L. Ouyang, P. Rulis, A. Misra, W. Y. Ching, Phys. Rev. Lett. 95, 256103 (2005)
TiC lattice dynamics from ab initio calculations
Jochym, Pawe? T; Sternik, Ma?gorzata; Parlinski, Krzysztof
2013-01-01
Ab initio calculations and a direct method have been applied to derive the phonon dispersion curves and phonon density of states for the TiC crystal. The results are compared and found to be in a good agreement with the experimental neutron scattering data. The force constants have been determined from the Hellmann-Feynman forces induced by atomic displacements in the 2x2x2 supercell. The calculated phonon density of states suggests that vibrations of Ti atoms form acoustic ...
Ab Initio study of neutron drops with chiral Hamiltonians
Potter, H. D.; Fischer, S.; Maris, P.; vary, J. P.; Binder, S.; Calci, A.; Langhammer, J.; Roth, R.
2014-01-01
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. Comparison...
Spin caloritronics in magnetic tunnel junctions: Ab initio studies
Czerner, Michael; Bachmann, Michael; Heiliger, Christian
2011-01-01
This Letter presents ab initio calculations of the magneto-thermoelectric power (MTEP) and of the spin-Seebeck coefficient in MgO based tunnel junctions with Fe and Co leads. In addition, the normal thermopower is calculated and gives for pure Fe and Co an quantitative agreement with experiments. Consequently, the calculated values in tunnel junctions are a good estimation of upper limits. In particular, spin-Seebeck coefficients of more than 100 \\mu V/K are possible. The MT...
Ab initio calculations of mechanical properties: Methods and applications.
Czech Academy of Sciences Publication Activity Database
Pokluda, J.; ?erný, Miroslav; Šob, Mojmír; Umeno, Y.
2015-01-01
Ro?. 73, AUG (2015), s. 127-158. ISSN 0079-6425 R&D Projects: GA MŠk(CZ) ED1.1.00/02.0068; GA ?R GAP108/12/0144; GA ?R(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
The study of molecular spectroscopy by ab initio methods
Bauschlicher, Charles W., Jr.; Langhoff, Stephen R.
1991-01-01
This review illustrates the potential of theory for solving spectroscopic problems. The accuracy of approximate techniques for including electron correlation have been calibrated by comparison with full configuration-interaction calculations. Examples of the application of ab initio calculations to vibrational, rotational, and electronic spectroscopy are given. It is shown that the state-averaged, complete active space self-consistent field, multireference configuration-interaction procedure provides a good approach for treating several electronic states accurately in a common molecular orbital basis.
Ab-initio calculations for dilute magnetic semiconductors
Belhadji, Brahim
2009-01-01
Der Schwerpunkt dieser Arbeit sind ab-initio Rechnungen zu der elektronischen Struktur und den magnetischen Eigenschaften von verdünnten magnetischen Halbleitern (DMS). Ein besonderes Ziel ist es, die komplexen Austausch-Wechselwirkungen, die in diesen Systemen auftreten, zu verstehen. Unsere Rechnungen basieren auf der Dichte-Funktional-Theorie, die sehr gut für die Beschreibung der material-spezifischen Eigenschaften von DMS geeignet ist. Wir verwenden die Koringa-Kohn-Rostoker (KKR) Green...
Ab initio analysis of plasmon dispersion in sodium under pressure
Ibañez-Azpiroz, Julen; Rousseau, Bruno; Eiguren, Asier; Bergara, Aitor
2014-01-01
We present an ab initio study of the electronic response function of sodium in its five known metallic phases from 0 to 180 GPa at room temperature. The considered formalism is based on a interpolation scheme within time-dependent density functional theory that uses maximally localized Wannier functions, providing an accurate sampling of the reciprocal space. Besides showing an excellent agreement with inelastic x-ray scattering experiments [Phys. Rev. Lett. 107, 086402 (2011), Proc. Natl....
Ab initio electronic properties of dual phosphorus monolayers in silicon
Drumm, Daniel W.; Per, Manolo C.; Budi, Akin; Hollenberg, Lloyd CL; Russo, Salvy P.
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, investigating the fundamental electronic properties of monolayer pairs. Quantitative band splittings and the electronic density are presented, along with effects of the layers’ relative alignment and ...
Quantifying transition voltage spectroscopy of molecular junctions: Ab initio calculations
DEFF Research Database (Denmark)
Chen, Jingzhe; Markussen, Troels; Thygesen, Kristian Sommer
2010-01-01
Transition voltage spectroscopy (TVS) has recently been introduced as a spectroscopic tool for molecular junctions where it offers the possibility to probe molecular level energies at relatively low bias voltages. In this work we perform extensive ab initio calculations of the nonlinear current-voltage relations for a broad class of single-molecule transport junctions in order to assess the applicability and limitations of TVS. We find, that in order to fully utilize TVS as a quantitative spectr...
Ab initio study of alanine polypeptide chain twisting
DEFF Research Database (Denmark)
Solov'yov, Ilia; Yakubovich, Alexander V.; Solov'yov, Andrey V.; Greiner, Walter
2006-01-01
We have investigated the potential energy surfaces for alanine chains consisting of three and six amino acids. For these molecules we have calculated potential energy surfaces as a function of the Ramachandran angles ph$ and psi, which are widely used for the characterization of the polypeptide chains. These particular degrees of freedom are essential for the characterization of the proteins folding process. Calculations have been carried out within the ab initio theoretical framework based on t...
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 have been compared to the experimental results of de Wilde and van Tiggelen (1968) who measured the laminar burning velocities for HOCHO flames over a range of stoichiometries and dilution ratios. The mo...
DNA base trimers: empirical and quantum chemical ab initio calculations.
Czech Academy of Sciences Publication Activity Database
Kabelá?, Martin; Sherer, E. C.; Cramer, Ch. J.; Hobza, Pavel
2007-01-01
Ro?. 13, ?. 7 (2007), s. 2067-2077. ISSN 0947-6539 R&D Projects: GA MŠk LC512; GA ?R GA203/05/0009 Institutional research plan: CEZ:AV0Z40550506 Keywords : ab initio calculations * nucleic acid bases trimer * empirical force field * Amber * potential energy surface Subject RIV: CF - Physical ; Theoretical Chemistry Impact factor: 5.330, year: 2007
Current rectification by asymmetric molecules: An ab initio study
Zhou, Yan-hong; Zheng, Xiao-hong; Ying XU; Zeng, Zhao Yang
2006-01-01
We study current rectification effect in an asymmetric molecule HOOC-C$_6$H$_4$-(CH$_2$)$_n$ sandwiched between two Aluminum electrodes using an {\\sl ab initio} nonequilibrium Green function method. The conductance of the system decreases exponentially with the increasing number $n$ of CH$_2$. The phenomenon of current rectification is observed such that a very small current appears at negative bias and a sharp negative differential resistance at a critical positive bias when $n\\ge 2$. The re...
Ab initio studies of electromechanical effects in carbon nanotubes
Scientific Electronic Library Online (English)
M. Verissimo, Alves; R.B., Capaz; Belita, Koiller; Emilio, Artacho; H., Chacham.
2002-06-01
Full Text Available Carbon nanotubes have recently attracted interest for their possible applications as nanoactuators and nanoswitches, as well as possible building blocks for nanoelectronics. We present ab initio calculations for uniformly charged graphene and (11,0),(9,0) and (5,5) carbon nanotubes. We also consider [...] the effects of polaron formation in these systems. The strain-charge coeÆcient is calculated and compared for both graphene and the nanotubes under study.
Local Structure Analysis in $Ab$ $Initio$ Liquid Water
Santra, Biswajit; Robert A DiStasio Jr; Martelli, Fausto; Car, Roberto
2015-01-01
Within the framework of density functional theory, the inclusion of exact exchange and non-local van der Waals/dispersion (vdW) interactions is crucial for predicting a microscopic structure of ambient liquid water that quantitatively agrees with experiment. In this work, we have used the local structure index (LSI) order parameter to analyze the local structure in such highly accurate $ab$ $initio$ liquid water. At ambient conditions, the LSI probability distribution, P($I$...
Nuclear forces and ab initio calculations of atomic nuclei
Energy Technology Data Exchange (ETDEWEB)
Meißner, Ulf-G. [Universität Bonn, Helmholtz-Institut für Strahlen- und Kernphysik and Bethe Center for Theoretical Physics, D-53115 Bonn (Germany); Forschungszentrum Jülich, Institut für Kernphysik, Institute for Advanced Simulation, and Jülich Center for Hadron Physics, D-52425 Jülich (Germany)
2014-08-15
Nuclear forces and the nuclear many-body problem have been some of Gerry Brown's main topics in his so productive life as a theoretical physicist. In this talk, I outline how Gerry's work laid the foundations of the modern theory of nuclear forces and ab initio calculations of atomic nuclei. I also present some recent developments obtained in the framework of nuclear lattice simulations.
Clustering in nuclei from ab initio nuclear lattice simulations
Meißner, Ulf-G
2015-01-01
Nuclear Lattice Effective Field Theory is a new many-body approach that is firmly rooted in the symmetries of QCD. In particular, it allows for truly ab initio calculations of nuclear structure and reactions. In this talk, I focus on the emergence of alpha-clustering in nuclei based on this approach. I also discuss various recent achievements, the deficiencies of the chiral forces used at present and the prospects to improve upon these and the calculations of nuclear properties and dynamics.
An AB initio model for cosmic-ray modulation
Engelbrecht, N.E.; Burger, R A
2013-01-01
A proper understanding of the effects of turbulence on the diffusion and drift of cosmic rays (CRs) is of vital importance for a better understanding of CR modulation in the heliosphere. This study presents an ab initio model for CR modulation, incorporating for the first time the results yielded by a two-component turbulence transport model. This model is solved for solar minimum heliospheric conditions, utilizing boundary values chosen so that model results are in reasonable agr...
Cosmic-ray modulation: an ab initio approach
Engelbrecht, N.E.; Burger, R A
2014-01-01
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 ...
Ab initio calculation of tight-binding parameters
Energy Technology Data Exchange (ETDEWEB)
McMahan, A.K.; Klepeis, J.E.
1997-12-01
We calculate ab initio values of tight-binding parameters for the f- electron metal Ce and various phases of Si, from local-density functional one-electron Hamiltonian and overlap matrix elements. Our approach allows us to unambiguously test the validity of the common minimal basis and two-center approximations as well as to determine the degree of transferability of both nonorthogonal and orthogonal hopping parameters in the cases considered.
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 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 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 â€œcloning,â€ in analogy to the â€œspawningâ€ 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, â€œtrains,â€ 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 â€œcloning,â€ in analogy to the â€œspawningâ€ 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, â€œtrains,â€ 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 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.
Ab initio calculations of reactions with light nuclei
Quaglioni, S; Calci, A; Navratil, P; Roth, R
2015-01-01
An {\\em 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 {\\em 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 {\\em 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...
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. PMID:25106573
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.
Ab Initio Interactive Molecular Dynamics on Graphical Processing Units (GPUs).
Luehr, Nathan; Jin, Alex G B; Martínez, Todd J
2015-10-13
A virtual molecular modeling kit is developed based on GPU-enabled interactive ab initio molecular dynamics (MD). The code uses the TeraChem and VMD programs with a modified IMD interface. Optimization of the GPU accelerated TeraChem program specifically for small molecular systems is discussed, and a robust multiple time step integrator is employed to accurately integrate strong user-supplied pulling forces. Smooth and responsive visualization techniques are developed to allow interactive manipulation at minimum simulation rates below five MD steps per second. Representative calculations at the Hartree-Fock level of theory are demonstrated for molecular systems containing up to a few dozen atoms. PMID:26574246
Ozone adsorption on graphene: ab initio study and experimental validation
Lee, Geunsik; Lee, Bongki; Kim, Jiyoung; Cho, Kyeongjae
2009-01-01
We have investigated ozone adsorption on graphene using the ab initio density functional theory method. Ozone molecules adsorb on graphene basal plane with binding energy of 0.25 eV, and the physisorbed molecule can chemically react with graphene to form an epoxide group and an oxygen molecule. The activation energy barrier from physisorption to chemisorption is 0.72 eV, and the chemisorbed state has the binding energy of 0.33 eV. These binding energies and energy barrier in...
Ab-Initio Shell Model with a Core
International Nuclear Information System (INIS)
We construct effective 2- and 3-body Hamiltonians for the p-shell by performing 12(hbar)(Omega) ab initio no-core shell model (NCSM) calculations for A=6 and 7 nuclei and explicitly projecting the many-body Hamiltonians onto the 0(hbar)(Omega) space. We then separate these effective Hamiltonians into 0-, 1- and 2-body contributions (also 3-body for A=7) and analyze the systematic behavior of these different parts as a function of the mass number A and size of the NCSM basis space. The role of effective 3- and higher-body interactions for A > 6 is investigated and discussed
Ab-Initio Shell Model with a Core
Energy Technology Data Exchange (ETDEWEB)
Lisetskiy, A F; Barrett, B R; Kruse, M; Navratil, P; Stetcu, I; Vary, J P
2008-06-04
We construct effective 2- and 3-body Hamiltonians for the p-shell by performing 12{h_bar}{Omega} ab initio no-core shell model (NCSM) calculations for A=6 and 7 nuclei and explicitly projecting the many-body Hamiltonians onto the 0{h_bar}{Omega} space. We then separate these effective Hamiltonians into 0-, 1- and 2-body contributions (also 3-body for A=7) and analyze the systematic behavior of these different parts as a function of the mass number A and size of the NCSM basis space. The role of effective 3- and higher-body interactions for A > 6 is investigated and discussed.
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. Â© 2012 American Physical Society.
Fast ab initio calculation of solvent envelopes for protein structures.
Harris, G W
1995-09-01
A fast and simple method has been developed for the ab initio calculation of low-resolution solvent envelopes for macromolecular structures. In essence, a sphere of point scatterers is moved through the asymmetric unit cell in a part random, part systematic search for the configuration which corresponds to the lowest R value. The spheres correspond to the solvent regions in the cell. The program has been shown to work successfully for a number of test structures in a variety of space groups. No prior knowledge of the structures is needed, and c.p.u. requirements are extremely modest. PMID:15299798
Ab initio investigation of intermolecular interactions in solid benzene
Bludsky, O; Soldan, P; 10.1103/PhysRevB.77.092103
2009-01-01
A computational strategy for the evaluation of the crystal lattice constants and cohesive energy of the weakly bound molecular solids is proposed. The strategy is based on the high level ab initio coupled-cluster determination of the pairwise additive contribution to the interaction energy. The zero-point-energy correction and non-additive contributions to the interaction energy are treated using density functional methods. The experimental crystal lattice constants of the solid benzene are reproduced, and the value of 480 meV/molecule is calculated for its cohesive energy.
Symmetry-Adapted Ab Initio Open Core Shell Model Theory
International Nuclear Information System (INIS)
By using only a fraction of the model space, we gain further insight – within a symmetry-guided no-core shell model framework – into the many-body nuclear dynamics that gives rise to important single-particle configurations together with correlated highly-deformed and alpha-cluster structures. We show results of the novel ab initio symmetry-adapted no-core shell model for large-scale nuclear structure computations. In addition, we use the symmetry patterns unveiled in these results to explore ultra-large model spaces.
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.
Hydrogen Desorption from Mg Hydride: An Ab Initio Study
Directory of Open Access Journals (Sweden)
Simone Giusepponi
2012-07-01
Full Text Available Hydrogen desorption from hydride matrix is still an open field of research. By means of accurate first-principle molecular dynamics (MD simulations an Mg–MgH2 interface is selected, studied and characterized. Electronic structure calculations are used to determine the equilibrium properties and the behavior of the surfaces in terms of structural deformations and total energy considerations. Furthermore, extensive ab-initio molecular dynamics simulations are performed at several temperatures to characterize the desorption process at the interface. The numerical model successfully reproduces the experimental desorption temperature for the hydride.
Ab initio and model calculations on different phases of zirconia
International Nuclear Information System (INIS)
In order to get a better understanding of the energetics of ZrO2 (zirconia) ab initio calculations with the full potential linear muffin tin orbital method (fp LMTO) have been performed on the tetragonal structure over a range of c/a and sublattice displacement. A new semi-empirical shell model is developed which makes use of Hartree-Fock calculations and includes compressible anions and quadrupolar distortions. The empirical model predicts energies for tetragonal distortion in agreement with the fp LMTO calculations. Furthermore, it enables us to understand why the seven-fold coordinated monoclinic phase is the low temperature equilibrium structure
Ab initio electronic properties of dual phosphorus monolayers in silicon.
Drumm, Daniel W; Per, Manolo C; Budi, Akin; Hollenberg, Lloyd Cl; Russo, Salvy P
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, investigating the fundamental electronic properties of monolayer pairs. Quantitative band splittings and the electronic density are presented, along with effects of the layers' relative alignment and comments on disordered systems, and for the first time, the effective electronic widths of such device components are calculated. PMID:25246862
Ab-initio study of transition metal hydrides
Energy Technology Data Exchange (ETDEWEB)
Sharma, Ramesh [Dept. of Physics, Feroze Gandhi Insititute of Engineering and Technology, Raebareli-229001 (India); Shukla, Seema, E-mail: sharma.yamini62@gmail.com; Dwivedi, Shalini, E-mail: sharma.yamini62@gmail.com; Sharma, Yamini, E-mail: sharma.yamini62@gmail.com [Theoretical Condensed Matter Physics Laboratory, Dept. of Physics Feroze Gandhi College, Raebareli-229001 (India)
2014-04-24
We have performed ab initio self consistent calculations based on Full potential linearized augmented plane wave (FP-LAPW) method to investigate the optical and thermal properties of yttrium hydrides. From the band structure and density of states, the optical absorption spectra and specific heats have been calculated. The band structure of Yttrium metal changes dramatically due to hybridization of Y sp orbitals with H s orbitals and there is a net charge transfer from metal to hydrogen site. The electrical resistivity and specific heats of yttrium hydrides are lowered but the thermal conductivity is slightly enhanced due to increase in scattering from hydrogen sites.
Ab initio study of phase equilibria in TiCx
DEFF Research Database (Denmark)
Korzhavyi, P.A.; Pourovskii, L.V.; Hugosson, H.W.; Ruban, Andrei; Johansson, B.
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, Ti3C2, and Ti6C5) are found to be ground state configurations. Their stability has been verified by full-potential total energy calculations of the fully relaxed structures.
Ab initio study of neutron drops with chiral Hamiltonians
Energy Technology Data Exchange (ETDEWEB)
Potter, H.D., E-mail: hdpotter@iastate.edu [Department of Physics and Astronomy, Iowa State University, Ames, IA 50011 (United States); Fischer, S., E-mail: s.fischer@stud.tu-darmstadt.de [Institut für Kernphysik, Technische Universität Darmstadt, 64289 Darmstadt (Germany); Maris, P., E-mail: pmaris@iastate.edu [Department of Physics and Astronomy, Iowa State University, Ames, IA 50011 (United States); Vary, J.P., E-mail: jvary@iastate.edu [Department of Physics and Astronomy, Iowa State University, Ames, IA 50011 (United States); Binder, S., E-mail: sven.binder@physik.tu-darmstadt.de [Institut für Kernphysik, Technische Universität Darmstadt, 64289 Darmstadt (Germany); Calci, A., E-mail: angelo.calci@physik.tu-darmstadt.de [Institut für Kernphysik, Technische Universität Darmstadt, 64289 Darmstadt (Germany); Langhammer, J., E-mail: joachim.langhammer@physik.tu-darmstadt.de [Institut für Kernphysik, Technische Universität Darmstadt, 64289 Darmstadt (Germany); Roth, R., E-mail: robert.roth@physik.tu-darmstadt.de [Institut für Kernphysik, Technische Universität Darmstadt, 64289 Darmstadt (Germany)
2014-12-12
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 v{sub 8}{sup ?} with three-nucleon interactions reveal important dependences on the chosen Hamiltonian.
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.
Ab Initio NMR Chemical Shift Calculations Using Fragment Molecular Orbitals
Yokojima, Satoshi; Gao, Qi; Nakamura, Shinichiro
2009-03-01
The recently proposed method to calculate NMR chemical shift of large biomolecular systems using fragment molecular orbitals (FMO) is evaluated in comparison with the conventional ab initio results using a ?-sheet (32 residues). The errors in isotropic shielding constants are slightly larger than the previous errors. [Q. Gao, S. Yokojima, T. Kohno, T. Ishida, D. G. Fedorov, K. Kitaura, M. Fujihira, and S. Nakamura, Chem. Phys. Lett. 445, 331-339 (2007).] The increase of the errors in anisotropic shielding constants of 15N is attributed to the neglect of the shielding of the neighboring hydrogen bonded residue, which suggests the crucial importance of hydrogen bonds in biomolecular systems.
Thermochemical study of uranium by Ab initio quantum chemical calculations
International Nuclear Information System (INIS)
Ab initio Hartree Fock (HF) theory and Density Functional theory (DFT) have been used to predict the Gibbs free energy of formation (?Gf0) for uranium (VI) hydrolysis complexes. The calculations were performed in the presence of solvent (water) by using the continuum dielectric solvation modeling PCM method as well as the revised Born model. The pressure parameter p=1354 atm was used to reproduce the entropy of ions in solution. The calculated energy shows that PCM model is better than Born model in calculating the hydration energy. The primary hydration shell has been included in the calculation and it has improved the calculated energy
TiAl doping by vanadium: ab initio study
International Nuclear Information System (INIS)
Tetragonality degree in TiAl and vanadium doping effect on it were studied using the methods of calculation based on approximation of coherent potential and ab initio pseudopotentials. It is shown that vanadium substitution for Ti sublattice atoms entails increase in tetragonality degree but with substitution of the atoms in aluminium sublattice the tetragonality of the TiAl:V alloy decreases and at the content of vanadium about 8 at. % the lattice becomes actually cubical. In its turn, it may result in increase in TiAl ductility, the alloy being brittle at low temperatures
Ab Initio study of neutron drops with chiral Hamiltonians
Potter, H.D.; Fischer, S.; Maris, P.; Vary, J.P.; Binder, S.; Calci, A.; Langhammer, J.; Roth, R.
2014-01-01
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 Green's Fun...
Ab Initio study of neutron drops with chiral Hamiltonians
Potter, H D; Maris, P; Vary, J P; Binder, S; Calci, A; Langhammer, J; Roth, R
2014-01-01
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 Green's Function Monte Carlo results, where available, using Argonne v8' with three-nucleon interactions reveal important dependences on the chosen Hamiltonian.
Ab initio study of hydrogen on beryllium surfaces
Bachurin, D. V.; Vladimirov, P. V.
2015-11-01
Static ab initio calculations were performed for five principal hexagonal close-packed beryllium surfaces: basal, prismatic (type I and II) and pyramidal (type I and II). The basal plane was found to be the most energetically favorable, while the energies of the prismatic (type I) and pyramidal (type I) planes were slightly higher followed by the type II planes. Beryllium is known to show extreme interlayer distance relaxation near the surface. Up to five outermost atomic layers were involved in surface relaxation. The presence of hydrogen on the beryllium surfaces led to a noticeable reduction of the surface energy.
Accelerating VASP electronic structure calculations using graphic processing units
Hacene, Mohamed
2012-08-20
We present a way to improve the performance of the electronic structure Vienna Ab initio Simulation Package (VASP) program. We show that high-performance computers equipped with graphics processing units (GPUs) as accelerators may reduce drastically the computation time when offloading these sections to the graphic chips. The procedure consists of (i) profiling the performance of the code to isolate the time-consuming parts, (ii) rewriting these so that the algorithms become better-suited for the chosen graphic accelerator, and (iii) optimizing memory traffic between the host computer and the GPU accelerator. We chose to accelerate VASP with NVIDIA GPU using CUDA. We compare the GPU and original versions of VASP by evaluating the Davidson and RMM-DIIS algorithms on chemical systems of up to 1100 atoms. In these tests, the total time is reduced by a factor between 3 and 8 when running on n (CPU core + GPU) compared to n CPU cores only, without any accuracy loss. Â© 2012 Wiley Periodicals, Inc.
Ab initio description of p-shell hypernuclei
International Nuclear Information System (INIS)
Tremendous progress is being made on the experimental study of hypernuclei, especially on the spectroscopy of p-shell hypernuclei. Their theoretical description, however, is limited to phenomenological models or to very light (i.e. s-shell) systems. We present ab initio calculations of p-shell hypernuclei using chiral Hamiltonians including leading-order (LO) hyperon-nucleon as well as two- and three-nucleon interactions at N3LO and N2LO, respectively. To improve convergence with respect to model space size, the Hamiltonians are evolved using a Similarity Renormalization Group (SRG) transformation. The many-body calculations are carried out in the framework of the importance-truncated no-core shell model. We present the first ab initio results for the spectroscopy of ?7Li, ?9Be and ?13C obtained using chiral and phenomenological hyperon-nucleon interactions. We also discuss the role of SRG-induced hyperon-nucleon-nucleon (YNN) terms which hint at the impact of chiral YNN interactions.
Why ferro electricity? synchrotron radiation and ab initio answers
International Nuclear Information System (INIS)
An old question of solid state physics is being answered nowadays: the atomic-level understanding of ferro electricity. Traditional ideas about ferroelectric phenomena relate with softening of optical phonons at the Brillouin zone origin and with 'W-shaped' Landau free energy functions. Last decade experimental (synchrotron radiation, neutrons) and quantum-theoretical (Cohen, Resta, Spaldin) contributions have clarified detailed descriptions and explanations for atomic behavior leading to spontaneous polarization in perovskite and perovskite-related crystal structures. Work being performed by our interdisciplinary group on ferro electricity is presented. Perovskite and Aurivillius ferroelectric phases are obtained by different methods. Fine details on crystal structures are investigated by means of synchrotron radiation at Stanford Synchrotron Radiation Laboratory. Electronic structures of considered phases are theoretically characterized by ab initio methods. High-resolution diffraction experiments demonstrate several symmetry break-downs in perovskite and Aurivillius phases. The structure- symmetry-polarization relationship is discussed for a number of representative cases. Ab initio explanation of ferroelectric polarization in perovskite structures is given. Energy calculation is performed by means of CASTEP code under GGA functional. Energy optimization leads to cubic-tetragonal symmetry break-down with off-centering cation displacements via second-order Jahn-Teller effect. Electronic structure is investigated with Band Lab code, under LDA functional with LMTO method. Degeneracy of Ti 3d z2 and Ti 3d (x2 + y2) orbitals is the cause of cubic-perovskite deformation. (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.
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 ?-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.
A fragmentation and reassembly method for ab initio phasing.
Shrestha, Rojan; Zhang, Kam Y J
2015-02-01
Ab initio phasing with de novo models has become a viable approach for structural solution from protein crystallographic diffraction data. This approach takes advantage of the known protein sequence information, predicts de novo models and uses them for structure determination by molecular replacement. However, even the current state-of-the-art de novo modelling method has a limit as to the accuracy of the model predicted, which is sometimes insufficient to be used as a template for successful molecular replacement. A fragment-assembly phasing method has been developed that starts from an ensemble of low-accuracy de novo models, disassembles them into fragments, places them independently in the crystallographic unit cell by molecular replacement and then reassembles them into a whole structure that can provide sufficient phase information to enable complete structure determination by automated model building. Tests on ten protein targets showed that the method could solve structures for eight of these targets, although the predicted de novo models cannot be used as templates for successful molecular replacement since the best model for each target is on average more than 4.0?Å away from the native structure. The method has extended the applicability of the ab initio phasing by de novo models approach. The method can be used to solve structures when the best de novo models are still of low accuracy. PMID:25664740
Why ferro electricity? synchrotron radiation and ab initio answers
Energy Technology Data Exchange (ETDEWEB)
Olivera, R.; Espinosa, F.; Garcia, M.; Macias, E. [Centro de Investigacion en Materiales Avanzados (CIMAV), Chihuahua (Mexico); Fuentes, M.E. [Universidad Autonoma de Chihuahua (UACh), Chihuahua (Mexico); Duran, A.; Siqueiros, J. [Centro de Ciencias de la Materia Condensada (CCMC), Ensenada (Mexico)]. e-mail: luis.fuentes@cimav.edu.mx
2007-07-01
An old question of solid state physics is being answered nowadays: the atomic-level understanding of ferro electricity. Traditional ideas about ferroelectric phenomena relate with softening of optical phonons at the Brillouin zone origin and with 'W-shaped' Landau free energy functions. Last decade experimental (synchrotron radiation, neutrons) and quantum-theoretical (Cohen, Resta, Spaldin) contributions have clarified detailed descriptions and explanations for atomic behavior leading to spontaneous polarization in perovskite and perovskite-related crystal structures. Work being performed by our interdisciplinary group on ferro electricity is presented. Perovskite and Aurivillius ferroelectric phases are obtained by different methods. Fine details on crystal structures are investigated by means of synchrotron radiation at Stanford Synchrotron Radiation Laboratory. Electronic structures of considered phases are theoretically characterized by ab initio methods. High-resolution diffraction experiments demonstrate several symmetry break-downs in perovskite and Aurivillius phases. The structure- symmetry-polarization relationship is discussed for a number of representative cases. Ab initio explanation of ferroelectric polarization in perovskite structures is given. Energy calculation is performed by means of CASTEP code under GGA functional. Energy optimization leads to cubic-tetragonal symmetry break-down with off-centering cation displacements via second-order Jahn-Teller effect. Electronic structure is investigated with Band Lab code, under LDA functional with LMTO method. Degeneracy of Ti 3d z{sup 2} and Ti 3d (x{sup 2} + y{sup 2}) orbitals is the cause of cubic-perovskite deformation. (Author)
Quantum Fragment Based ab Initio Molecular Dynamics for Proteins.
Liu, Jinfeng; Zhu, Tong; Wang, Xianwei; He, Xiao; Zhang, John Z H
2015-12-01
Developing ab initio molecular dynamics (AIMD) methods for practical application in protein dynamics is of significant interest. Due to the large size of biomolecules, applying standard quantum chemical methods to compute energies for dynamic simulation is computationally prohibitive. In this work, a fragment based ab initio molecular dynamics approach is presented for practical application in protein dynamics study. In this approach, the energy and forces of the protein are calculated by a recently developed electrostatically embedded generalized molecular fractionation with conjugate caps (EE-GMFCC) method. For simulation in explicit solvent, mechanical embedding is introduced to treat protein interaction with explicit water molecules. This AIMD approach has been applied to MD simulations of a small benchmark protein Trpcage (with 20 residues and 304 atoms) in both the gas phase and in solution. Comparison to the simulation result using the AMBER force field shows that the AIMD gives a more stable protein structure in the simulation, indicating that quantum chemical energy is more reliable. Importantly, the present fragment-based AIMD simulation captures quantum effects including electrostatic polarization and charge transfer that are missing in standard classical MD simulations. The current approach is linear-scaling, trivially parallel, and applicable to performing the AIMD simulation of proteins with a large size. PMID:26642993
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.
Ab initio thermodynamic model for magnesium carbonates and hydrates.
Chaka, Anne M; Felmy, Andrew R
2014-09-01
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 analogues 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. PMID:24679248
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 tight-binding Hamiltonian for transition metal dichalcogenides
Fang, Shiang; Kuate Defo, Rodrick; Shirodkar, Sharmila N.; Lieu, Simon; Tritsaris, Georgios A.; Kaxiras, Efthimios
2015-11-01
We present an accurate ab initio tight-binding Hamiltonian for the transition metal dichalcogenides, MoS2, MoSe2, WS2, WSe2, with a minimal basis (the d orbitals for the metal atoms and p orbitals for the chalcogen atoms) based on a transformation of the Kohn-Sham density functional theory Hamiltonian to a basis of maximally localized Wannier functions. The truncated tight-binding Hamiltonian, with only on-site, first, and partial second neighbor interactions, including spin-orbit coupling, provides a simple physical picture and the symmetry of the main band-structure features. Interlayer interactions between adjacent layers are modeled by transferable hopping terms between the chalcogen p orbitals. The full-range tight-binding Hamiltonian can be reduced to hybrid-orbital k .p effective Hamiltonians near the band extrema that capture important low-energy excitations. These ab initio Hamiltonians can serve as the starting point for applications to interacting many-body physics including optical transitions and Berry curvature of bands, of which we give some examples.
Ab initio methods to oxides, and atomic orbitals in te PAW framework
International Nuclear Information System (INIS)
The accuracy of ab initio density functional theory calculations depends crucially on the applied exchange-correlation functional. In recent years, a true zoo of functionals has emerged in particular in the quantum chemistry community. All these developments were aimed on an improved description of different properties of molecules, solids and surfaces. In the first part of this thesis we apply four different exchange-correlation functionals (LDA, PBE, PBEsol and HSE03) to two model perovskites, namely SrTiO3 and BaTiO3. We evaluate structural, electronic and phonon properties in the high temperature cubic as well as the tetragonal distorted phase with special attention on the zone-centred phonon mode. Our results show that the ferroelectric instability strongly depends on the volume and to a lesser extent on the applied functional. The second part of the thesis concentrates on the description of the polar oxygen terminated surface of ZnO. In the simple ionic picture the surface is electrostatically unstable due to a diverging electrostatic energy and has to adopt a modified surface structure to compensate for the polarity. In a close collaboration with experiment, a novel reconstruction is suggested. The remarkable observation is that the previously investigated oxygen terminated surface behaves very different than the zinc terminated surface. The difference is explained by the different bond preferences of Zn and O atoms: as a d-element Zn atoms are more flexible than O atoms. The final part of the thesis concentrates on the implementation of a local basis set code in VASP. We derive all required formulas for an atomic orbital basis set expanded in spherical Bessel functions within the projector augmented wave framework. Particular emphasis was placed on an implementation that maintains to a very high accuracy the translational invariance of the Hamiltonian. The basis set was optimized to achieve this aim. This is demonstrated for few simple test cases. (author)
Ab initio and semiempirical studies of molecules adsorbed on surfaces
Yang, Shizhong
By the use of semiempirical method SAM1 and ab initio Density Functional Theory (DFT) with gaussian basis sets, the following systems: HCOO-/Cu(110), H3CCOO-/Cu(110), CO/Cu(110) and O/diamond(100) have been studied. Ab initio DFT method with plane wave basis set has been used in studying the following systems: CH4/graphite(0001), commensurate Ag(111) - ( 7x7 )R19.1° - 4Ar monolayer and CO/Au/TiO3/Mo(112). The preferred adsorption sites, molecular structure, infrared spectra, and energies relative to stable gas phase molecules for carbon monoxide, the formate ion, and the a cetate ion when adsorbed on the (110) face of copper were calculated. The semiempirical calculations compare well with those from ab initio, density functional theory, and with experiments. A method of estimating the adsorption energy is offered. Quantum chemical SAM I calculations for coupled oscillations examining CO adsorbed on Cu(110) and atomic oxygen on the (100) diamond face have been performed. For CO, the calculations show coupling consistent with a dynamic dipole-dipole interaction. In the case of the adsorbed oxygen atoms, the dominant coupling seems to be mechanical. In CH4/graphite(0001), the CH4 molecules favored atop sites on the graphite surface with the hydrogen tripod down. The methane carbon was found 3.21 A above the graphite carbon and the adsorption energy to be 118 meV for the lower coverage. The independent harmonic oscillator vibrational frequency perpendicular to the surface for the CH4 molecule was computed to be 87 cm-1. The commensurate Ag(111) - ( 7x7 )R19.1° - 4Ar monolayer results show that both the hcp and fcc hollow sites are possible sites and slightly more stable than an atop site, with the total energy 1.5 meV and 4.5 meV lower than atop site per four Ar atoms respectively. The surface structure of (1x3) Au/TiO3/Mo(112) forms a zigzag surface profile. Three kinds of Au charge states were found in the (1x3) structure: Au0, Audelta+ and Au delta-. A red shifted CO vibrational frequency was proposed for experimental verification. The DOS of CO/(Audelta+/Au 0 bridge) shows a continuous 2 pi* band extension to 1 pi band.
A hydronitrogen solid: high pressure ab initio evolutionary structure searches
Energy Technology Data Exchange (ETDEWEB)
Hu Anguang; Zhang Fan, E-mail: Anguang.Hu@drdc-rddc.gc.ca [Defence Research and Development Canada-Suffield, PO Box 4000 Stn Main, Medicine Hat, AB, T1A 8K6 (Canada)
2011-01-19
High pressure ab initio evolutionary structure searches resulted in a hydronitrogen solid with a composition of (NH){sub 4}. The structure searches also provided two molecular isomers, ammonium azide (AA) and trans-tetrazene (TTZ) which were previously discovered experimentally and can be taken as molecular precursors for high pressure synthesis of the hydronitrogen solid. The computed pressure versus enthalpy diagram showed that the transformation pressure to the hydronitrogen solid is 36 GPa from AA and 75 GPa from TTZ. Its metastability was analyzed by the phonon dispersion spectrum and room-temperature vibrational density of state together with the transformation energy barrier back to molecular phases at 298 K. The predicted energy barrier of 0.21 eV/atom means that the proposed hydronitrogen solid should be very stable at ambient conditions. (fast track communication)
A hydronitrogen solid: high pressure ab initio evolutionary structure searches
International Nuclear Information System (INIS)
High pressure ab initio evolutionary structure searches resulted in a hydronitrogen solid with a composition of (NH)4. The structure searches also provided two molecular isomers, ammonium azide (AA) and trans-tetrazene (TTZ) which were previously discovered experimentally and can be taken as molecular precursors for high pressure synthesis of the hydronitrogen solid. The computed pressure versus enthalpy diagram showed that the transformation pressure to the hydronitrogen solid is 36 GPa from AA and 75 GPa from TTZ. Its metastability was analyzed by the phonon dispersion spectrum and room-temperature vibrational density of state together with the transformation energy barrier back to molecular phases at 298 K. The predicted energy barrier of 0.21 eV/atom means that the proposed hydronitrogen solid should be very stable at ambient conditions. (fast track communication)
Ab Initio Study of KCl and NaCl Clusters
Brownrigg, Clifton; Hira, Ajit; Pacheco, Jose; Salazar, Justin
2013-03-01
We continue our interest in the theoretical study of molecular clusters to examine the chemical properties of small KnCln and NanCln clusters (n = 2 - 15). The potentially important role of these molecular species in biochemical and medicinal processes is well known. This work applies the hybrid ab initio methods of quantum chemistry to derive the different alkali-halide (MnHn) geometries. Of particular interest is the competition between hexagonal ring geometries and rock salt structures. Electronic energies, rotational constants, dipole moments, and vibrational frequencies for these geometries are calculated. Magic numbers for cluster stability are identified and are related to the property of cluster compactness. Mapping of the singlet, triplet, and quintet, potential energy surfaces is performed. Calculations have been performed to examine the interactions of these clusters with some atoms and molecules of biological interest, including O, O2, and Fe. The potential for design of new medicinal drugs is explored.
The ab-initio density matrix renormalization group in practice
Energy Technology Data Exchange (ETDEWEB)
Olivares-Amaya, Roberto; Hu, Weifeng; Sharma, Sandeep; Yang, Jun; Chan, Garnet Kin-Lic [Department of Chemistry, Princeton University, Princeton, New Jersey 08544 (United States); Nakatani, Naoki [Department of Chemistry, Princeton University, Princeton, New Jersey 08544 (United States); Catalysis Research Center, Hokkaido University, Kita 21 Nishi 10, Sapporo, Hokkaido 001-0021 (Japan)
2015-01-21
The ab-initio density matrix renormalization group (DMRG) is a tool that can be applied to a wide variety of interesting problems in quantum chemistry. Here, we examine the density matrix renormalization group from the vantage point of the quantum chemistry user. What kinds of problems is the DMRG well-suited to? What are the largest systems that can be treated at practical cost? What sort of accuracies can be obtained, and how do we reason about the computational difficulty in different molecules? By examining a diverse benchmark set of molecules: ?-electron systems, benchmark main-group and transition metal dimers, and the Mn-oxo-salen and Fe-porphine organometallic compounds, we provide some answers to these questions, and show how the density matrix renormalization group is used in practice.
Ab initio investigation of the mechanical properties of copper
International Nuclear Information System (INIS)
Employing the ab initio total energy method based on the density functional theory with the generalized gradient approximation, we have systematically investigated the theoretical mechanical properties of copper (Cu). The theoretical tensile strengths are calculated to be 25.3 GPa, 5.9 GPa, and 37.6 GPa for the fcc Cu single crystal in the [001], [110], and [111] directions, respectively. Among the three directions, the [110] direction is the weakest one due to the occurrence of structure transition at the lower strain and the weakest interaction of atoms between the (110) planes, while the [111] direction is the strongest direction because of the strongest interaction of atoms between the (111) planes. In terms of the elastic constants of Cu single crystal, we also estimate some mechanical quantities of polycrystalline Cu, including bulk modulus B, shear modulus G, Young's modulus Ep, and Poisson's ratio ?
Local Structure Analysis in $Ab$ $Initio$ Liquid Water
Santra, Biswajit; Martelli, Fausto; Car, Roberto
2015-01-01
Within the framework of density functional theory, the inclusion of exact exchange and non-local van der Waals/dispersion (vdW) interactions is crucial for predicting a microscopic structure of ambient liquid water that quantitatively agrees with experiment. In this work, we have used the local structure index (LSI) order parameter to analyze the local structure in such highly accurate $ab$ $initio$ liquid water. At ambient conditions, the LSI probability distribution, P($I$), was unimodal with most water molecules characterized by more disordered high-density-like local environments. With thermal excitations removed, the resultant bimodal P($I$) in the inherent potential energy surface (IPES) exhibited a 3:1 ratio between high- and low-density-like molecules, with the latter forming small connected clusters amid the predominant population. By considering the spatial correlations and hydrogen bond network topologies $among$ water molecules with the same LSI identities, we demonstrate that the signatures of th...
Ab-initio study of diluted magnetic semiconductors.
Czech Academy of Sciences Publication Activity Database
Kudrnovský, Josef; Drchal, Václav; Máca, František; Turek, Ilja; Bouzerar, G.; Bruno, P.
New York : Springer Science+ Business Media , Inc, 2005 - (Turchi, P.; Gonis, A.; Rajan, K.; Meike, A.), s. 277-293 ISBN 0-387-24811-0. [Third International Alloy Conference. Estoril Sol (PT), 30.06.2002-05.07.2002] R&D Projects: GA ?R(CZ) GA202/00/0122; GA AV ?R(CZ) IAA1010203; GA AV ?R IAA1010214; GA MŠk(CZ) OC P5.30 Grant ostatní: RTN(XE) HPRN-CT-2000-00143 Institutional research plan: CEZ:AV0Z10100520 Keywords : ab-initio * diluted magnetic semiconductors * Curie temperatures * III-V semiconductors Subject RIV: BM - Solid Matter Physics ; Magnetism
Ab initio relativistic calculations on heavy elements and their molecules
International Nuclear Information System (INIS)
Methods of ab initio electronic structure theory have been used in conjunction with effective core potentials, derived from all-electron numerical solutions of the atomic Dirac-Fock equations, in calculations of a wide variety of properties of atomic and molecular systems containing heavy elements. The presentation includes discussions of such phenomena as intermediate angular momentum coupling, relativistic effects on orbital sizes, the subtle interplay between electron correlation and spin-orbit coupling, and changes in molecular geometries attributable to relativity. This first-principles approach is used for obtaining both qualitative and quantitative information as shown in a number of specific studies. Included are calculations of electronic spectra and properties of U, Np, Pu and selected transplutoniuum elements, bonding and spectra of molecules containing the elements Pt-Rn, and halogen group spin-orbit splitting
Isofulminic acid, HONC: Ab initio theory and microwave spectroscopy.
Mladenovi?, Mirjana; Lewerenz, Marius; McCarthy, Michael C; Thaddeus, Patrick
2009-11-01
Isofulminic acid, HONC, the most energetic stable isomer of isocyanic acid HNCO, higher in energy by 84 kcal/mol, has been detected spectroscopically by rotational spectroscopy supported by coupled cluster electronic structure calculations. The fundamental rotational transitions of the normal, carbon-13, oxygen-18, and deuterium isotopic species have been detected in the centimeter band in a molecular beam by Fourier transform microwave spectroscopy, and rotational constants and nitrogen and deuterium quadrupole coupling constants have been derived. The measured constants agree well with those predicted by ab initio calculations. A number of other electronic and spectroscopic parameters of isofulminic acid, including the dipole moment, vibrational frequencies, infrared intensities, and centrifugal distortion constants have been calculated at a high level of theory. Isofulminic acid is a good candidate for astronomical detection with radio telescopes because it is highly polar and its more stable isomers (HNCO, HOCN, and HCNO) have all been identified in space. PMID:19895013
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
Symplectic ab initio no-core shell model
International Nuclear Information System (INIS)
The present study confirms the significance of the symplectic Sp(3,R) symmetry in nuclear dynamics as unveiled, for the first time, by examinations of realistic nucleon-nucleon interactions as well as of eigenstates calculated in the framework of the ab initio No-Core Shell Model (NCSM). The results reveal that the NCSM wave functions for light nuclei highly overlap (at the ? 90% level) with only a few of the most deformed Sp(3,R)-symmetric basis states. This points to the possibility of achieving convergence of higher-lying collective modes and reaching heavier nuclei by expanding the NCSM basis space beyond its current limits through Sp(3,R) basis states. Furthermore the symplectic symmetry is found to be favored by the JISP 16 and CD-Bonn realistic nucleon-nucleon interactions, which points to a more fundamental origin of the symplectic symmetry. (Author)
Symplectic ab initio no-core shell model
Energy Technology Data Exchange (ETDEWEB)
Draayer, J. P.; Dytrych, T.; Sviratcheva, K. D.; Bahri, C. [Department of Physics and Astronomy, Lousiana State University, Baton Rouge, 70803 Lousiana (United States); Vary, J. P. [Department of Physics and Astronomy, Iowa State University, Ames, 50011 Iowa (United States)
2008-12-15
The present study confirms the significance of the symplectic Sp(3,R) symmetry in nuclear dynamics as unveiled, for the first time, by examinations of realistic nucleon-nucleon interactions as well as of eigenstates calculated in the framework of the ab initio No-Core Shell Model (NCSM). The results reveal that the NCSM wave functions for light nuclei highly overlap (at the {approx} 90% level) with only a few of the most deformed Sp(3,R)-symmetric basis states. This points to the possibility of achieving convergence of higher-lying collective modes and reaching heavier nuclei by expanding the NCSM basis space beyond its current limits through Sp(3,R) basis states. Furthermore the symplectic symmetry is found to be favored by the JISP 16 and CD-Bonn realistic nucleon-nucleon interactions, which points to a more fundamental origin of the symplectic symmetry. (Author)
Simple calculation of ab initio melting curves: Application to aluminum
Robert, Grégory; Legrand, Philippe; Arnault, Philippe; Desbiens, Nicolas; Clérouin, Jean
2015-03-01
We present a simple, fast, and promising method to compute the melting curves of materials with ab initio molecular dynamics. It is based on the two-phase thermodynamic model of Lin et al [J. Chem. Phys. 119, 11792 (2003), 10.1063/1.1624057] and its improved version given by Desjarlais [Phys. Rev. E 88, 062145 (2013), 10.1103/PhysRevE.88.062145]. In this model, the velocity autocorrelation function is utilized to calculate the contribution of the nuclei motion to the entropy of the solid and liquid phases. It is then possible to find the thermodynamic conditions of equal Gibbs free energy between these phases, defining the melting curve. The first benchmark on the face-centered cubic melting curve of aluminum from 0 to 300 GPa demonstrates how to obtain an accuracy of 5%-10%, comparable to the most sophisticated methods, for a much lower computational cost.
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 electronic properties of monolayer phosphorus nanowires in silicon.
Drumm, D W; Smith, J S; Per, M C; Budi, A; Hollenberg, L C L; Russo, S P
2013-03-22
Epitaxial circuitry offers a revolution in silicon technology, with components that can be fabricated on atomic scales. We perform the first ab initio calculation of atomically thin epitaxial nanowires in silicon, investigating the fundamental electronic properties of wires two P atoms thick, similar to those produced this year by Weber et al. For the first time, we catch a glimpse of disorder-related effects in the wires--a prerequisite for understanding real fabricated systems. Interwire interactions are made negligible by including 40 ML of silicon in the vertical direction (and the equivalent horizontally). Accurate pictures of band splittings and the electronic density are presented, and for the first time the effective masses of electrons in such device components are calculated. PMID:25166832
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...... have been compared to the experimental results of de Wilde and van Tiggelen (1968) who measured the laminar burning velocities for HOCHO flames over a range of stoichiometries and dilution ratios. The modeling predictions are generally satisfactory. The governing reaction mechanisms are outlined based...
Ab initio methods for electron-molecule collisions
Energy Technology Data Exchange (ETDEWEB)
Collins, L.A.; Schneider, B.I.
1987-01-01
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 (approx. = 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.
Ab initio structural, elastic, and vibrational properties of carbon nanotubes
Sánchez-Portál, D; Soler, J M; Rubio, A; Ordejón, P
1999-01-01
A study based on ab initio calculations is presented on the estructural, elastic, and vibrational properties of single-wall carbon nanotubes with different radii and chiralities. We use SIESTA, an implementation of pseudopotential-density-functional theory which allows calculations on systems with a large number of atoms per cell. Different quantities like bond distances, Young moduli, Poisson ratio and the frequencies of different phonon branches are monitored versus tube radius. The validity of expectations based on graphite is explored down to small radii, where some deviations appear related to the curvature effects. For the phonon spectra, the results are compared with the predictions of the simple zone-folding approximation. Except for the known defficiencies of this approximation in the low-frequency vibrational regions, it offers quite accurate results, even for relatively small radii.
Structural phase transformations via ab initio molecular dynamics
Focher, P; Bernasconi, M; Tosatti, E; Parrinello, M
1993-01-01
Available simulation methods, suitable to describe solid-solid phase transitions occurring upon increasing of presssure and/or temperature, are based on empirical interatomic potentials: this restriction reduces the predictive power, and thus the general usefulness of numeric simulations in this very relevant field. We present a new simulation scheme which allows, for the first time, the simulation of these phenomena with the correct quantum-mechanical description of interatomic forces and internal stress, along with the correct statistical mechanics of ionic degrees of freedom. The method is obtained by efficiently combining the Car-Parrinello method for ab- initio molecular dynamics with the Parrinello Rahman method to account for a variable cell shape. Within this scheme phase transformations may spontaneously take place during the simulation with variation of external pressure and/or temperature. The validity of the method is demonstrated by simulating the metal-insulator transition in Silicon (from diamo...
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, S2?, in the range 0.004-0.16 ?W /cm K2.
Interatomic Coulombic decay widths of helium trimer: Ab initio calculations
Koloren?, P?emysl; Sisourat, Nicolas
2015-12-01
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.
The ab-initio density matrix renormalization group in practice
International Nuclear Information System (INIS)
The ab-initio density matrix renormalization group (DMRG) is a tool that can be applied to a wide variety of interesting problems in quantum chemistry. Here, we examine the density matrix renormalization group from the vantage point of the quantum chemistry user. What kinds of problems is the DMRG well-suited to? What are the largest systems that can be treated at practical cost? What sort of accuracies can be obtained, and how do we reason about the computational difficulty in different molecules? By examining a diverse benchmark set of molecules: ?-electron systems, benchmark main-group and transition metal dimers, and the Mn-oxo-salen and Fe-porphine organometallic compounds, we provide some answers to these questions, and show how the density matrix renormalization group is used in practice
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 (â‰… 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
Large-scale ab initio calculations for C3
Mladenovi?, M.; Schmatz, S.; Botschwina, P.
1994-10-01
Large-scale calculations by the single, double, and perturbative triple excitation coupled cluster [CCSD(T)] method have been carried out for C3. A linear equilibrium geometry with Re=1.2945 Å was obtained. The use of large basis sets and the inclusion of connected triple substitutions is crucial for an accurate representation of the extremely shallow bending potential. Rovibrational term energies were calculated from a three-dimensional CCSD(T) potential (basis: 177 contracted Gaussian-type orbitals) by the discrete variable representation (DVR). Compared with the best previous ab initio calculations [P. Jensen et al., J. Chem. Phys. 97, 3399 (1992)] agreement with experiment is improved by almost an order of magnitude. The errors in the wave numbers of the fundamentals are -7.7 (?1), 0.9 (?2), and 0.6 (?3) cm-1, respectively.
Ab Initio Symmetry-Adapted No-Core Shell Model
International Nuclear Information System (INIS)
A multi-shell extension of the Elliott SU(3) model, the SU(3) symmetry-adapted version of the no-core shell model (SA-NCSM), is described. The significance of this SA-NCSM emerges from the physical relevance of its SU(3)-coupled basis, which – while it naturally manages center-of-mass spuriosity – provides a microscopic description of nuclei in terms of mixed shape configurations. Since typically configurations of maximum spatial deformation dominate, only a small part of the model space suffices to reproduce the low-energy nuclear dynamics and hence, offers an effective symmetry-guided framework for winnowing of model space. This is based on our recent findings of low-spin and high-deformation dominance in realistic NCSM results and, in turn, holds promise to significantly enhance the reach of ab initio shell models.
Ab Initio Symmetry-Adapted No-Core Shell Model
Draayer, J. P.; Dytrych, T.; Launey, K. D.
2011-10-01
A multi-shell extension of the Elliott SU(3) model, the SU(3) symmetry-adapted version of the no-core shell model (SA-NCSM), is described. The significance of this SA-NCSM emerges from the physical relevance of its SU(3)-coupled basis, which - while it naturally manages center-of-mass spuriosity - provides a microscopic description of nuclei in terms of mixed shape configurations. Since typically configurations of maximum spatial deformation dominate, only a small part of the model space suffices to reproduce the low-energy nuclear dynamics and hence, offers an effective symmetry-guided framework for winnowing of model space. This is based on our recent findings of low-spin and high-deformation dominance in realistic NCSM results and, in turn, holds promise to significantly enhance the reach of ab initio shell models.
Ab initio MCDHF calculations of electron-nucleus interactions
Biero?, Jacek; Froese Fischer, Charlotte; Fritzsche, Stephan; Gaigalas, Gediminas; Grant, Ian P.; Indelicato, Paul; Jönsson, Per; Pyykkö, Pekka
2015-05-01
We present recent advances in the development of atomic ab initio multiconfiguration Dirac-Hartree-Fock theory, implemented in the GRASP relativistic atomic structure code. For neutral atoms, the deviations of properties calculated within the Dirac-Hartree-Fock (DHF) method (based on independent particle model of an atomic cloud) are usually dominated by electron correlation effects, i.e. the non-central interactions of individual electrons. We present the recent advances in accurate calculations of electron correlation effects in small, medium, and heavy neutral atoms. We describe methods of systematic development of multiconfiguration expansions leading to systematic, controlled improvement of the accuracy of the ab initio calculations. These methods originate from the concept of the complete active space (CAS) model within the DHF theory, which, at least in principle, permits fully relativistic calculations with full account of electron correlation effects. The calculations within the CAS model on currently available computer systems are feasible only for very light systems. For heavier atoms or ions with more than a few electrons, restrictions have to be imposed on the multiconfiguration expansions. We present methods and tools, which are designed to extend the numerical calculations in a controlled manner, where multiconfiguration expansions account for all leading electron correlation effects. We show examples of applications of the GRASP code to calculations of hyperfine structure constants, but the code may be used for calculations of arbitrary bound-state atomic properties. In recent years it has been applied to calculations of atomic and ionic spectra (transition energies and rates), to determinations of nuclear electromagnetic moments, as well as to calculations related to interactions of bound electrons with nuclear electromagnetic moments leading to violations of discrete symmetries.
NestedMICA as an ab initio protein motif discovery tool
Directory of Open Access Journals (Sweden)
Down Thomas A
2008-01-01
Full Text Available Abstract Background Discovering overrepresented patterns in amino acid sequences is an important step in protein functional element identification. We adapted and extended NestedMICA, an ab initio motif finder originally developed for finding transcription binding site motifs, to find short protein signals, and compared its performance with another popular protein motif finder, MEME. NestedMICA, an open source protein motif discovery tool written in Java, is driven by a Monte Carlo technique called Nested Sampling. It uses multi-class sequence background models to represent different "uninteresting" parts of sequences that do not contain motifs of interest. In order to assess NestedMICA as a protein motif finder, we have tested it on synthetic datasets produced by spiking instances of known motifs into a randomly selected set of protein sequences. NestedMICA was also tested using a biologically-authentic test set, where we evaluated its performance with respect to varying sequence length. Results Generally NestedMICA recovered most of the short (3–9 amino acid long test protein motifs spiked into a test set of sequences at different frequencies. We showed that it can be used to find multiple motifs at the same time, too. In all the assessment experiments we carried out, its overall motif discovery performance was better than that of MEME. Conclusion NestedMICA proved itself to be a robust and sensitive ab initio protein motif finder, even for relatively short motifs that exist in only a small fraction of sequences. Availability NestedMICA is available under the Lesser GPL open-source license from: http://www.sanger.ac.uk/Software/analysis/nmica/
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.
Hayes, J. M.; Greer, J. C.
2002-09-01
The program PAROPT has been written to extract forcefield parameters from ab initio calculations of potential energy surfaces (PES) and ab initio analytical forces. The ability to use either energies or forces, or a combination of the two to determine forcefield parameters is a novel feature of the program. Simulated annealing is used within the program to minimise the difference between a set of forces and energies calculated using ab initio techniques and the same quantities calculated using an empirical forcefield. Details of the program and methods used to extract forcefield parametrizations are presented.
DEFF Research Database (Denmark)
Wolf, T. J. A.; Kuhlman, Thomas Scheby; Schalk, O.; Martínez, T. J.; Møller, Klaus Braagaard; Stolow, A.; Unterreiner, A.-N.
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....
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.
International Nuclear Information System (INIS)
Recent progress in the ab initio quantum chemistry study of cathode oxygen reduction on fuel cell catalysts is reviewed with emphasis on density functional theory and ab initio molecular dynamics methods. The capabilities of these methods are illustrated using examples of oxygen adsorption on transition metals and alloys, and the reduction mechanism. Ab initio studies can calculate adsorption geometry, energy, the dissociation energy barrier, reversible potential, activation energy, and potential dependant properties for elementary electron transfer steps. Even though ab initio study in this field is still at an early stage, it has already demonstrated its predictive ability in the trend of adsorption energy on transition metals and alloys, and illustrated its potential in identifying better electrocatalysts
Czech Academy of Sciences Publication Activity Database
Meliá, C.; Ferrer, S.; ?ezá?, Jan; Parisel, O.; Reinaud, O.; Moliner, V.; de la Lande, A.
2013-01-01
Ro?. 19, ?. 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
An Ab Initio Based Potential Energy Surface for Water
Partridge, Harry; Schwenke, David W.; Langhoff, Stephen R. (Technical Monitor)
1996-01-01
We report a new determination of the water potential energy surface. A high quality ab initio potential energy surface (PES) and dipole moment function of water have been computed. This PES is empirically adjusted to improve the agreement between the computed line positions and those from the HITRAN 92 data base. The adjustment is small, nonetheless including an estimate of core (oxygen 1s) electron correlation greatly improves the agreement with experiment. Of the 27,245 assigned transitions in the HITRAN 92 data base for H2(O-16), the overall root mean square (rms) deviation between the computed and observed line positions is 0.125/cm. However the deviations do not correspond to a normal distribution: 69% of the lines have errors less than 0.05/cm. Overall, the agreement between the line intensities computed in the present work and those contained in the data base is quite good, however there are a significant number of line strengths which differ greatly.
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.)
Ab initio study of phase transformations in boron nitride
Yu, W. J.; Lau, W. M.; Chan, S. P.; Liu, Z. F.; Zheng, Q. Q.
2003-01-01
The structural properties and phase stability of the four common polytypes of boron nitride, cubic zinc blende (c-BN), hexagonal (h-BN), wurtzite (w-BN) and rhombohedral (r-BN), are studied by ab initio calculations. Electronic energies are calculated using an ultra soft pseudopotential method under the density-functional theory, and phonon dispersions are calculated using the first-principles force-constant method. The p-T phase diagrams of these four boron nitride phases are constructed with the quasiharmonic approximation. Direct compression simulations are then performed to find probable phase transformation paths among these polytypes, with additional energy calculations of plausible transition structures. The c-BN phase is the most thermodynamically stable in ambient conditions among these four polytypes, and the transformation between r-BN and c-BN has the smallest energy barrier. Direct transformation between h-BN and c-BN is far less favorable than indirect transformation, with w-BN or r-BN as an intermediate. The presence of structural defects is a key attribute in reducing the energy barrier of phase transformation. The results in this work offer theoretical clues to experimental data on c-BN film growth, particularly the absence of w-BN.
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 ?-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 calculations of As-vacancy interactions in silicon
Energy Technology Data Exchange (ETDEWEB)
Xie, J.; Chen, S.P.
1999-04-01
Atomistic simulation of a vacancy-assisted dopant diffusion in silicon needs details of the dopant-vacancy interaction, i.e., the potential as a functional of dopant-vacancy separations. In this paper, the authors present a detailed study on the energetics of As-vacancy reaction in silicon and the lattice distortions surrounding the As-vacancy defect by using an ab initio plane wave pseudopotential method and the density functional theory (DFT). A potential-energy diagram as a function of As-vacancy separation is provided, which can be used in the atomistic diffusion simulations. The authors also calculate the binding energy and the formation energy of different complexes such as AsV, As{sub 2}V and AsV{sub 2} (V represents vacancy). They find that the stable configuration of As{sub 2}V is As-V-As, while the stable configuration of AsV{sub 2} is As-V-V. The nature of the binding between As and vacancy is explained from the lattice distortions and the change of chemical bond configuration introduced by the As-vacancy complex.
Nonempirical ab initio calculations on DNA base pairs
Šponer, Ji?í; Hobza, Pavel
1996-04-01
28 H-bonded DNA base pairs formed by guanine, adenine, thymine and cytosine were optimized by the gradient method at the ab initio HF/MINI-1 level. 10 pairs were found to be nonplanar with propeller structure. Harmonic vibration analysis indicates that all the stationary points found correspond to energy minima. The calculated interaction enthalpies (sum of the SCF interaction energy, the dispersion energy and the zero-point energy) are larger than the respective experimental data due to overestimation of the MINI-1 SCF interaction energy. Buckling and propeller vibrations were found to be the lowest for all the pairs; it is therefore possible to expect that all the pairs are flexible toward buckle and propeller vibrations. The H-bonded structure of the cytosine dimer was found to be considerably more stable than the stacked structures; both structures were studied at the higher MP2/6-31G ? level. The reliability of the procedure used (MP2/6-31G ?) was tested for model clusters (formamide … formamidine, benzene … Ar) at higher levels (MP4, CCSD(T)).
Predicting lattice thermal conductivity with help from ab initio methods
Broido, David
2015-03-01
The lattice thermal conductivity is a fundamental transport parameter that determines the utility a material for specific thermal management applications. Materials with low thermal conductivity find applicability in thermoelectric cooling and energy harvesting. High thermal conductivity materials are urgently needed to help address the ever-growing heat dissipation problem in microelectronic devices. Predictive computational approaches can provide critical guidance in the search and development of new materials for such applications. Ab initio methods for calculating lattice thermal conductivity have demonstrated predictive capability, but while they are becoming increasingly efficient, they are still computationally expensive particularly for complex crystals with large unit cells . In this talk, I will review our work on first principles phonon transport for which the intrinsic lattice thermal conductivity is limited only by phonon-phonon scattering arising from anharmonicity. I will examine use of the phase space for anharmonic phonon scattering and the Grüneisen parameters as measures of the thermal conductivities for a range of materials and compare these to the widely used guidelines stemming from the theory of Liebfried and Schölmann. This research was supported primarily by the NSF under Grant CBET-1402949, and by the S3TEC, an Energy Frontier Research Center funded by the US DOE, office of Basic Energy Sciences under Award No. DE-SC0001299.
Challenges for large scale ab initio Quantum Monte Carlo
Kent, Paul
2015-03-01
Ab initio Quantum Monte Carlo is an electronic structure method that is highly accurate, well suited to large scale computation, and potentially systematically improvable in accuracy. Due to increases in computer power, the method has been applied to systems where established electronic structure methods have difficulty reaching the accuracies desired to inform experiment without empiricism, a necessary step in the design of materials and a helpful step in the improvement of cheaper and less accurate methods. Recent applications include accurate phase diagrams of simple materials through to phenomena in transition metal oxides. Nevertheless there remain significant challenges to achieving a methodology that is robust and systematically improvable in practice, as well as capable of exploiting the latest generation of high-performance computers. In this talk I will describe the current state of the art, recent applications, and several significant challenges for continued improvement. Supported through the Predictive Theory and Modeling for Materials and Chemical Science program by the Office of Basic Energy Sciences (BES), Department of Energy (DOE).
Local Environment Distribution in Ab Initio Liquid Water
Santra, Biswajit; Distasio, Robert A., Jr.; Car, Roberto
2013-03-01
We have analyzed the distribution of local environments in liquid water at ambient conditions and its inherent potential energy surface (IPES) based on state-of-the-art ab initio molecular dynamics simulations performed on 128 molecules implementing hybrid PBE0 exchange [PRB 79, 085102 (2009)] and van der Waals (vdW) interactions [PRL 102, 073005 (2009)]. The local environments of molecules are characterized in terms of the local structure index (LSI) [JCP 104, 7671 (1996)] which is able to distinguish high- and low-density molecular environments. In agreement with simulations based on model potentials, we find that the distribution of LSI is unimodal at ambient conditions and bimodal in the IPES, consistent with the existence of polymorphism in amorphous phases of water. At ambient conditions spatial LSI fluctuations extend up to ~7 Å and their dynamical correlation decays on a time scale of ~3 ps, as found for density fluctuations in a recent study [PRL 106, 037801 (2011)]. DOE: DE-SC0008626, DOE: DE-SC0005180, NSF: CHE-0956500
Ab initio liquid water from PBE0 hybrid functional simulations
Li, Zhaofeng; Wu, Xifan; Car, Roberto
2010-03-01
For reasons of computational efficiency, so far most ab initio molecular dynamics simulations of liquid water have been based on semi-local density functional approximations, such as PBE and BLYP. These approaches yield a liquid structure that, albeit qualitatively correct, is overstructured compared to experiment, even after nuclear quantum effects have been taken into account.footnotetextJ. A. Morrone and R. Car, Phys. Rev. Lett. 101, 017801(2008) A major cause of this inaccuracy is the delocalization error associated to semi-local density functional approximations, which, as a consequence, overestimate slightly the hydrogen bond strength in the liquid. In this work we adopt the PBE0 hybrid functional approximation, which, by mixing a fraction of exact (Hartree-Fock) exchange, reduces significantly the delocalization error of semi-local functionals. Our approach is based on a numerically efficient order-N implementation of exact exchange.footnotetextX. Wu, A. Selloni, and R. Car, Phys. Rev. B 79, 085102(2009) We find that PBE0 systematically improves the agreement of the simulated liquid with experiment. Our conclusion is substantiated by the calculated radial distribution functions, H-bond statistics, and molecular dipole distribution.
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 have been compared to the experimental results of de Wilde and van Tiggelen (1968) who measured the laminar burning velocities for HOCHO flames over a range of stoichiometries and dilution ratios. The modeling predictions are generally satisfactory. The governing reaction mechanisms are outlined based 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 reaction, as well as the fate of HOCO, determines the oxidation rate of formic acid. At lower temperatures HO2, formed from HOCO + O2, is an important chain carrier and modeling predictions become sensitive to the HOCHO + HO2 reaction. © 2014 The Combustion Institute.
Hydrogen adsorption on the ?-graphyne using ab initio calculations
Drogar, J.; Roknabadi, M. R.; Behdani, M.; Modarresi, M.; Kari, A.
2014-11-01
In this study, we use the Siesta ab initio code based on Density Functional Theory (DFT) to determine the feasibility of hydrogen storage on the ?-graphyne. We examined the effect of hydrogen adsorption on the structural properties, the density of states of this nano structure. Calculations were performed in the GGA and LDA approximations. We find that in the case of chemical adsorption of hydrogen, functional exchange-correlation PBE (GGA) in explaining chemical bonds have more accuracy and leads to more stable structures. Adsorption is chemical and the binding energy of single and double hydrogen adsorption is -2.28 eV and -3.48 eV, respectively. Our calculations show that the adsorption of one hydrogen atom induces a magnetic moment as 1 ?B, because of breaking of ?-bonds and generate unpaired electron and system find ferromagnetic configuration. Further analysis indicates that adsorption of second hydrogen eliminates magnetization and the semi metalic ?-graphyne structure attains a band gap of 2 eV upon hydrogenation.
Ab initio calculations of correlated electron dynamics in ultrashort pulses
Feist, Johannes
2010-03-01
The availability of ultrashort and intense light pulses on the femtosecond and attosecond timescale promises to allow to directly probe and control electron dynamics on their natural timescale. A crucial ingredient to understanding the dynamics in many-electron systems is the influence of electron correlation, induced by the interelectronic repulsion. In order to study electron correlation in ultrafast processes, we have implemented an ab initio simulation of the two-electron dynamics in helium atoms. We solve the time-dependent Schr"odinger equation in its full dimensionality, with one temporal and five spatial degrees of freedom in linearly polarized laser fields. In our computational approach, the wave function is represented through a combination of time-dependent close coupling with the finite element discrete variable representation, while time propagation is performed using an Arnoldi-Lanczos approximation with adaptive step size. This approach is optimized to allow for efficient parallelization of the program and has been shown to scale linearly using up to 1800 processor cores for typical problem sizes. This has allowed us to perform highly accurate and well- converged computations for the interaction of ultrashort laser pulses with He. I will present some recent results on using attosecond and femtosecond pulses to probe and control the temporal structure of the ionization process. This work was performed in collaboration with Stefan Nagele, Renate Pazourek, Andreas Kaltenb"ack, Emil Persson, Barry I. Schneider, Lee A. Collins, and Joachim Burgd"orfer.
Ab initio correlation functionals from second-order perturbation theory.
Schweigert, Igor V; Lotrich, Victor F; Bartlett, Rodney J
2006-09-14
Orbital-dependent exchange-correlation functionals are not limited by the explicit dependence on the density and present an attractive alternative to conventional functionals. With the successful implementation of the exact orbital-dependent exchange functional, the challenge lies in developing orbital-dependent approximations for the correlation functional. Ab initio many-body methods can provide such approximations. In particular, perturbation theory with the Kohn-Sham model as the reference [Görling and Levy, Phys. Rev. A 50, 196 (1994)] defines the exact correlation functional via an infinite perturbation series. The second-order term of these series gives the lowest-order approximation to the correlation functional. However, it has been suggested [Bartlett et al., J. Chem. Phys. 122, 034104 (2005)] that the Kohn-Sham Hamiltonian is not the optimal choice for the perturbation expansion and a different reference Hamiltonian may lead to an improved perturbation series and more accurate second-order approximation. Here, we demonstrate explicitly that the modified series can be used to define superior functional and potential. We present results of atomic and molecular calculations with both second-order functionals. Our results demonstrate that the modified functional offers a significantly improved description of the correlation effects as it does not suffer from convergence problems and results in energies and densities that are more accurate than those obtained with second-order Møller-Plesset perturbation theory or generalized-gradient approximation functionals. PMID:16999516
Nature of magnetism in iron pnictides: an ab initio study
Energy Technology Data Exchange (ETDEWEB)
Zhang, Yu-Zhong; Opahle, Ingo; Jeschke, Harald; Valenti, Roser [Goethe-Universitaet Frankfurt, Max-von-Laue-Strasse 1, 60438 Frankfurt am Main (Germany)
2010-07-01
While it is commonly believed that magnetic-mediate pairing is the source of superconductivity in the iron-based superconductors, the nature of magnetism is still under debate. We apply ab initio molecular dynamics to investigate physical properties of LaOFePn, BaFe{sub 2}Pn{sub 2} and LiFePn (Pn=As,Sb), so-called 1111, 122 and 111 compounds, respectively. We find that, with substitution of As by Sb, the stripe-type antiferromagnetic orderings are always enhanced. By calculating Pauli susceptibility, we attribute the enhancement of magnetization to the increase of instability at ({pi},{pi}) when As is substituted by Sb. Furthermore, we study the magnetic and lattice properties of LaOFePn (Pn=P,As,Sb,Bi) as well as ScOFeP, ScOFeAs and YOFeP and argue that LaOFeSb would be a candidate for a superconductor with highest transition temperature among the investigated compounds. We further suggest that the absence of antiferromagnetic phase in LaOFeP and the presence in LaOFeAs are due to the competition of instability in Pauli susceptibility between ({pi},{pi}) and (0,0) and therefore argue that superconductivity can only occur through doping in LaOFeSb.
Ab initio molecular dynamics calculations of ion hydration free energies
Leung, Kevin; von Lilienfeld, Anatole O; 10.1063/1.3137054
2010-01-01
We apply ab initio molecular dynamics (AIMD) methods in conjunction with the thermodynamic integration or "lambda-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 (phi) contributions, we obtain absolute AIMD hydration free energies (Delta G(hyd)) within a few kcal/mol, or better than 4%, of Tissandier 's [J. Phys. Chem. A 102, 7787 (1998)] experimental values augmented with the SPC/E water model phi predictions. The sums of Li+/Cl- and Ag+/Cl- AIMD Delta G(hyd), 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 Delta G(hyd) for unstable radiolysis interme...
Ab Initio Simulation of the Photoelectron Spectrum for Methoxy Radical
Cheng, Lan; Weichman, Marissa L.; Kim, Jongjin B.; Ichino, Takatoshi; Neumark, Daniel; Stanton, John F.
2015-06-01
A theoretical simulation of the photoelectron spectrum for the ground state of methoxy radical is reported based on the quasidiabatic model Hamiltonian originally proposed by Köppel, Domcke, and Cederbaum. The parameters in the model Hamiltonian have been obtained from ab initio coupled-cluster calculations. The linear and quadratic force constants have been calculated using equation-of-motion coupled-cluster ionization potential method with the singles, doubles, and triples (EOMIP-CCSDT) truncation scheme together with atomic natural orbital basis sets of triple-zeta quality (ANO1). The cubic and quartic force constants have been obtained from EOMIP-CCSD calculations with ANO basis sets of double-zeta quality (ANO0), and the spin-orbit coupling constant has been computed at the EOMIP-CCSD/pCVTZ level. The nuclear Schroedinger equation has been solved using the Lanzcos algorithm to obtain vibronic energy levels as well as the corresponding intensities. The simulated spectrum compares favorably with the recent high-resolution slow electron velocity-map imaging experiment for vibronic levels up to 2000 cm-1.
Ab initio transport across bismuth selenide surface barriers
Narayan, Awadhesh
2014-11-24
Â© 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 study of edge sites reactivity on pyrophyllite
International Nuclear Information System (INIS)
The crystal chemistry and surface site reactivity of the phyllosilicate minerals is a key issue for modelling the ion mobility and retardation in clay sediments. Various cation substitutions in the TOT layer of the 2:1 phyllosilicates result in a permanent structural charge near basal plane which is responsible for swelling and the sorption of ions by a cation exchange mechanism. In contrast, the pH-dependent uptake of cations is controlled by protonation/de-protonation reactions on the edge sites of the TOT layer. Various models have been proposed to explain the reactivity of the edge sites based on macroscopic experiments. A detailed understanding of this process on an atomistic level is still missing. We are using electronic structure calculations and ab initio MD simulations in order to understand the mechanism of pH dependent sorption of cations on edge sites of pyrophyllite and the transport properties of aqueous solutions in compacted pyrophyllite. The calculations provide important constrains for the macroscopic surface complexation models of clay minerals. (authors)
Ab initio study of alanine polypeptide chain twisting
DEFF Research Database (Denmark)
Solov'yov, Ilia; Yakubovich, Alexander V.
2006-01-01
We have investigated the potential energy surfaces for alanine chains consisting of three and six amino acids. For these molecules we have calculated potential energy surfaces as a function of the Ramachandran angles ph$ and psi, which are widely used for the characterization of the polypeptide chains. These particular degrees of freedom are essential for the characterization of the proteins folding process. Calculations have been carried out within the ab initio theoretical framework based on the density functional theory and accounting for all the electrons in the system. We have determined stable conformations and calculated the energy barriers for transitions between them. Using a thermodynamic approach, we have estimated the times of characteristic transitions between these conformations. The results of our calculations have been compared with those obtained by other theoretical methods and with the available experimental data extracted from the Protein Data Base. This comparison demonstrates a reasonable correspondence of the most prominent minima on the calculated potential energy surfaces to the experimentally measured angles phi and psi for alanine chains appearing in native proteins. We have also investigated the influence of the secondary structure of polypeptide chains on the formation of the potential energy landscape. This analysis has been performed for the sheet and the helix conformations of chains of six amino acids.
Ab initio examination of ductility features of fcc metals
Kamran, Sami; Chen, Kuiying; Chen, Liang
2009-01-01
Through systematic density-functional theory-based ab initio calculations, various performance indicators such as G/B , the ratio of shear modulus G over bulk modulus B , ?s/?t , the ratio of ideal shear strength ?s over tensile strength ?t , and the Cauchy pressure defined as C12-C44 were evaluated for a selection of fcc metals and assessed in relation to the characteristics of their electronic distributions obtained from the electron localization function. The analysis reveals that the ratio ?s/?t is possibly a better indicator of malleability as it discriminates directionally bonded metals in addition to discerning ductile crystals from brittle ones. Furthermore, Al is found to sustain the largest shear deformation among the scrutinized solids due to its directional bonds. Similarly, the surprisingly long range of distortion of Pd is rationalized as a result of the geometric constraints caused by small electron pockets of comparatively high localization in the interionic region. However, the examination of the peculiar case of Ir suggests that, in general, the extent of shear distortion should be the consequence of at least two factors, namely, the angular characteristics of bonding and the bond strength.
An ab initio non-equilibrium Green's function approach to charge transport: dithiolethine
Schnurpfeil, A; Albrecht, M; Schnurpfeil, Alexander; Song, Bo; Albrecht, Martin
2005-01-01
We present a novel ab initio non-equilibrium approach to calculate the current across a molecular junction. The method rests on a wave function based full ab initio description of the central region of the junction combined with a tight binding approximation for the electrodes in the frame of the Keldysh Green's function formalism. Our procedure is demonstrated for a dithiolethine molecule between silver electrodes. The main conducting channel is identified and the full current-voltage characteristic is calculated.
Ab-Initio Molecular Dynamics Acceleration Scheme with an Adaptive Machine Learning Framework
Botu, Venkatesh; Ramprasad, Rampi
2014-01-01
Quantum mechanics based ab-initio molecular dynamics (MD) simulation schemes offer an accurate and direct means to monitor the time-evolution of materials. Nevertheless, the expensive and repetitive energy and force computations required in such simulations lead to significant bottlenecks. Here, we lay the foundations for such an accelerated ab-initio MD approach integrated with a machine learning framework. The proposed algorithm learns from previously visited configurations in a continuous ...
Development of Linear Scaling ab initio Methods based on Electron Density Matrices
Kußmann, Jörg
2006-01-01
In the last decades ab initio methods have become a standard tool in chemistry, biochemistry, and physics. Nevertheless, the size of treatable systems was limited to the hundred atoms region, even for the less demanding Hartree-Fock (HF) and Kohn-Sham density functional theory (KS-DFT), because of their at least cubic scaling behavior with system size. Ab initio methods of course profit by the fast evolution of computer technology, but their application to larger systems is primarily hampe...
Ab initio Defect Energetics in LaBO3 Perovskite Solid Oxide Fuel Cell Materials
Lee, Yueh-Lin; Morgan, Dane; Kleis, Jesper; Rossmeisl, Jan
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 correlated electron effects. This paper discusses an approach to performing ab initio studies on these materials for SOFC applications and applies the approach to calculate vacancy formation energie...
Ab Initio Calculations and Raman and SERS Spectral Analyses of Amphetamine Species
Berg, Rolf W.; Nørbygaard, Thomas; White, Peter C.; Abdali, Salim
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 Gaussia...
Ab initio simulations of iron-nickel alloys at Earth's core conditions
Cote, A. S.; Vocadlo, L.; Brodholt, J. P.
2012-01-01
We report ab initio density functional theory calculations on iron–nickel (Fe?Ni) alloys at conditions representative of the Earth's inner core. We test different concentrations of Ni, up to ?39 wt% using ab initio lattice dynamics, and investigate the thermodynamic and vibrational stability of the three candidate crystal structures (bcc, hcp and fcc). First of all, at inner core pressures, we find that pure Fe transforms from the hcp to the fcc phase at around 6000 K. Secondly, in agreement ...
Ab Initio Calculations and Raman and SERS Spectral Analyses of Amphetamine Species
Berg, Rolf W.; Nørbygaard, Thomas; White, Peter C.; Abdali, Salim
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 ...
Ab initio quantum chemical design of supermolecule logical devices
Tamulis, Arvydas; Rinkevicius, Zilvinas; Tamuliene, Jelena; Tamulis, Vykintas; Balevicius, Mindaugas L.; Graja, Andrzej
2001-05-01
The geometrical structure of both the ground and excited state of the azo-dyes: Disperse Orange 3 (DO3) and Ch2- C4H4-N=N-C44/H(subscript 4-CH2 molecules have been investigated applying the Hartree Fock (HF), density functional theory (DFT) methods with the Berny geometry optimization and Ames Laboratory determinant (ALDET)single- double-triple-quadra configuration interaction (SDTQ-CI) method. The investigations proved that the above-mentioned molecules can not rotate around the -N=N- bond. Thus, the alternative mechanism of the isomerization of the DO3 molecule per linear transition state has been suggested and investigated. The obtained one- and many- electron energy diagrams have been drawn. According to these diagrams the above mentioned isomerization way is possible. The mechanism of the isomerization of the azo-dyes molecule per linear transition state is suggested and investigated. The results of light induced internal molecular motions in azo-dyes molecules have been used for the design of light driven logically controlled molecular machines composed form photoactive organic molecules such as carbazole, 1,4- phenylenediamine (PhDA) and 4,5-dinitro- 9(dicyanomethylidene)-fluorene (DN9(CN)2F), Dithieno[3,2-b:2',3'-d]thiphene and Ferocene (C10H10Fe) molecules joined with -CH=CH- or -N=N- bridges. Ab initio DFT B3PW91 model using 6-311G** and Watanabe (WBTS) basis sets calculations show the stability of Sc3N and ErSc2 molecules which exist inside endohedral fullerene C80 derivatives: Sc3NC80. Analysis of electronic structure of inside clusters allowed proposing that these endohedral fullerenes might be used for electro- optical and magneto-optical switches and for information storage. We performed design of molecular logical devices based on organic electron donor and electron acceptor molecules, fullerene C60 substituted derivative CH2C60, electron donor-bridge-electron acceptor dyads and triads. Design of new series molecular implementations (MI) of two variable logic functions: AND (NAND), OR (NOR) is based on geometry optimization procedure. Molecular triggers and molecular dynamic memory were designed based on investigations of photo-excitation movements and charge transfer of aza-fullerene supermolecule (NH2)CH-NC59-NC59-CH(NO2). Our ab initio DFT B3PW91/LanL3DZ calculation of HOMO-LUMO gap in CdS nanocluster without four phenyle fragments gives value equal to 3.85 eV and the same method calculation of CdS nanocluster with four phenyle fragments gives HOMO-LUMO gap value equal to 3.66 eV. 121
Ab-initio calculations of the Optical band-gap of TiO2 thin films
Wunderlich, W; Tanemura, M; Tanemura, S; Jin, P; Kaneko, K; Terai, A; Nabatova-Gabin, N; Belkada, R; Wunderlich, Wilfried; Miao, Lei; Tanemura, Masaki; Tanemura, Sakae; Jin, Ping; Kaneko, Kenji; Terai, Asuka; Nabatova-Gabin, Nataliya; Belkada, Rachid
2004-01-01
Titanium dioxide has been extensively studied in recent decades for its important photocatalytic application in environmental purification. The search for a method to narrow the optical band-gap of TiO2 plays a key role for enhancing its photocatalytic application. The optical band gap of epitaxial rutile and anatase TiO2 thin films deposited by helicon magnetron sputtering on sapphire and on SrTiO3 substrates was correlated to the lattice constants estimated from HRTEM images and SAED. The optical band-gap of 3.03 eV for bulk-rutile increased for the thin films to 3.37 on sapphire. The band gap of 3.20 eV for bulk-anatase increases to 3.51 on SrTiO3. In order to interpret the optical band gap expansion for both phases, ab-initio calculations were performed using the Vienna ab-initio software. The calculations for rutile as well anatase show an almost linear increase of the band gap width with decreasing volume or increasing lattice constant a. The calculated band gap fits well with the experimental values. T...
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 ?-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)
Symplectic ab initio no-core shell model
Scientific Electronic Library Online (English)
J.P., Draayer; T., Dytrych; K.D., Sviratcheva; C, Bahri; J.P., Vary.
2008-12-01
Full Text Available En el presente trabajo se confirma la importancia de la simetría simpléctica Sp(3,R) en la dinámica nuclear a través de estudios de interacciones nucleón-nucleón realistas así como de eigenestados calculados en el marco del modelo de capas sin carozo (NCSM, por sus siglas en inglés). Los resultados [...] demuestran para núcleos ligeros un gran traslapo entre la funciones de onda NCSM usando nada más los estados base con simetría Sp(3,R) con mayor deformación, lo cual abre la posibilidad para obtener convergencia de modos colectivos con energías altas y poder describir nucleos más pesados con una extensión del espacio de los estados base NCSM usando estados con simetría Sp(3,R). Además, las interacciones nucleón-nucleón realistas JISP16 y CD-Bonn favorecen la simetría simpléctica lo cual apunta a una explicación a nivel mas fundamental de la simetría simpléctica. Abstract in english The present study confirm the significanc of the symplectic Sp(3,R) symmetry in nuclear dynamics as unveiled, for the firs time, by examinations of realistic nucleon-nucleon (N N) interactions as well as of eigenstates calculated in the framework of the ab initio No-Core Shell Model (NCSM). The resu [...] lts reveal that the NCSM wave functions for light nuclei highly overlap (at the 90% level) with only a few of the most deformed Sp(3,R)-symmetric basis states. This points to the possibility of achieving convergence of higher-lying collective modes and reaching heavier nuclei by expanding the NCSM basis space beyond its current limits through Sp(3,R) basis states. Furthermore, the symplectic symmetry is found to be favored by the JISP16 and CD-Bonn realistic nucleon-nucleon interactions, which points to a more fundamental origin of the symplectic symmetry.
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. Â© 2010 American Chemical Society.
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)
An ab initio-based Er-He interatomic potential in hcp Er
Yang, L.; Ye, Y. T.; Fan, K. M.; Shen, H. H.; Peng, S. M.; Long, X. G.; Zhou, X. S.; Zu, X. T.; Gao, F.
2014-09-01
We have developed an empirical erbium-helium (Er-He) potential by fitting to the results calculated from the ab initio method. Based on the electronic hybridization between Er and He atoms, an s-band model, along with a repulsive pair potential, has been derived to describe the Er-He interaction. The atomic configurations and the formation energies of single He defects, small He interstitial clusters (Hen) and He-vacancy (HenV) clusters obtained by ab initio calculations are used as the fitting database. The binding energies and relative stabilities of the HnVm clusters are studied by the present potential and compared with the ab initio calculations. The Er-He potential is also applied to study the migration of He in hcp Er at different temperatures, and He clustering is found to occur at 600 K in hcp Er crystal, which may be due to the anisotropic migration behavior of He interstitials.
International Nuclear Information System (INIS)
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
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 $S(q)$. 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.
Ab Initio and Phenomenological Modeling of the Phonon Spectrum of Superhard cp-BC2N
Basalaev, Yu. M.; Kopytov, A. V.; Pavlova, T. Yu.; Poplavnoi, A. S.
2015-11-01
The phonon spectrum of hypothetical superhard cp-BC2N is calculated based on ab initio method of density functional in the center of the Brillouin zone and interpolated over the entire Brillouin zone using the Keating phenomenological model. The interaction parameters are determined by optimization of the IR- and Ramanactive frequencies for a phenomenological model by their comparison with the results of ab initio calculations. Numerical values of short-range interaction constants and charges are in agreement with the characteristics of the chemical bond calculated ab initio. These parameters have transparent physical meaning and chemical nature and can further be used for both qualitative estimations of any physical and physico-chemical quantities and quantitative calculations of the phonon spectra of a number of isostructural compounds. The Keating phenomenological model is used to study the genesis of the phonon spectrum from the spectra of sublattices.
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.
Oxidation pathways in Phosphorene: an ab-initio investigation
Paes Lima, Matheus Paes; Fazzio, Adalberto; da Silva, Antonio Jose Roque
2015-03-01
Phosphorene is a recently isolated single layer of Black Phosphorus. In this 2D material, the combination of a direct band gap with a high charge carrier mobility opens up the possibility of its use in nano devices. However, the exposure of Black Phosphorus to air leads to its fast degradation, which indicates the relevance to understand its oxidation processes. In the present work we investigate the initial steps of the oxidation process, focusing on the interaction of a single O2 molecule with the phosphorene layer. We show the existence of oxidation pathways having only a single barrier of 0.13eV occurring between the free O2 (triplet) and the triplet-singlet potential energy surface (PES) crossing point. We estimate a room temperature triplet-singlet transition probability of Pt --> s = 0 . 015 , using the Landau-Zener model. Once the O2 switches to the singlet PES, there is an oxygen incorporation with an energy gain of 4.2eV with respect to the PES crossing point, with the O2 molecule spontaneously dissociating without any barrier. In this process, the final geometry has one O bonded to a P lone pair, and the other located between two P atoms. Our investigations were performed with DFT calculations at the GGA level as implemented in the VASP code.
International Nuclear Information System (INIS)
The ab initio/classical free energy perturbation (ABC-FEP) method proposed previously by Wood et al. [J. Chem. Phys. 110, 1329 (1999)] uses classical simulations to calculate solvation free energies within an empirical potential model, then applies free energy perturbation theory to determine the effect of changing the empirical solute-solvent interactions to corresponding interactions calculated from ab initio methods. This approach allows accurate calculation of solvation free energies using an atomistic description of the solvent and solute, with interactions calculated from first principles. Results can be obtained at a feasible computational cost without making use of approximations such as a continuum solvent or an empirical cavity formation energy. As such, the method can be used far from ambient conditions, where the empirical parameters needed for approximate theories of solvation may not be available. The sources of error in the ABC-FEP method are the approximations in the ab initio method, the finite sample of configurations, and the classical solvent model. This article explores the accuracy of various approximations used in the ABC-FEP method by comparing to the experimentally well-known free energy of hydration of water at two state points (ambient conditions, and 973.15 K and 600 kg/m3). The TIP4P-FQ model [J. Chem. Phys. 101, 6141 (1994)] is found to be a reliable solvent model for use with this method, even at supercritical conditions. Results depend strongly on the ab initio method used: a gradient-corrected density functional theory is not adequate, but a localized MP2 method yields excellent agreement with experiment. Computational costs are reduced by using a cluster approximation, in which ab initio pair interaction energies are calculated between the solute and up to 60 solvent molecules, while multi-body interactions are calculated with only a small cluster (5 to 12 solvent molecules). Sampling errors for the ab initio contribution to solvation free energies are Â±2 kJ/mol or less when 50-200 configurations are used. Using the largest clusters and most accurate ab initio methods, ABC-FEP predicts hydration free energies of water at both state points that agree with equations of state, within the sampling error. These results are the first calculation of a free energy of solvation at extreme conditions from a fully atomistic model with ab initio methods. (c) 2000 American Institute of Physics
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.
Accurate tight-binding Hamiltonian matrices from ab initio calculations: Minimal basis sets
Agapito, Luis A.; Ismail-Beigi, Sohrab; Curtarolo, Stefano; Fornari, Marco; Nardelli, Marco Buongiorno
2016-01-01
Projection of Bloch states obtained from quantum-mechanical calculations onto atomic orbitals is the fastest scheme to construct ab initio tight-binding Hamiltonian matrices. However, the presence of spurious states and unphysical hybridizations of the tight-binding eigenstates has hindered the applicability of this construction. Here we demonstrate that those spurious effects are due to the inclusion of Bloch states with low projectability. The mechanism for the formation of those effects is derived analytically. We present an improved scheme for the removal of the spurious states which results in an efficient scheme for the construction of highly accurate ab initio tight-binding Hamiltonians.
Ab initio Defect Energetics in LaBO3 Perovskite Solid Oxide Fuel Cell Materials
DEFF Research Database (Denmark)
Lee, Yueh-Lin; Morgan, Dane
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 correlated electron effects. This paper discusses an approach to performing ab initio studies on these materials for SOFC applications and applies the approach to calculate vacancy formation energies in LaBO3 (B = Mn, Fe, Co, Ni) compounds.
Ab initio determination of an extended Heisenberg Hamiltonian in CuO2 layers
Calzado, Carmen J.; Malrieu, Jean-Paul
2000-01-01
Accurate ab initio calculations on embedded Cu_4O_{12} square clusters, fragments of the La_2CuO_4 lattice, confirm a value of the nearest neighbor antiferromagnetic coupling (J=124 meV) previously obtained from ab initio calculations on bicentric clusters and in good agreement with experiment. These calculations predict non negligible antiferromagnetic second-neighbor interaction (J'=6.5 meV) and four-spin cyclic exchange (K=14 meV), which may affect the thermodynamic and s...
Ab initio Molecular Dynamics Simulations of Water Under Static and Shock Compressed Conditions
International Nuclear Information System (INIS)
We report herein a series of ab initio simulations of water under both static and shocked conditions. We have calculated the coherent x-ray scattering intensity of several phases of water under high pressure, using ab initio Density Functional Theory (DFT). We provide new atomic scattering form factors for water at extreme conditions, which take into account frequently neglected changes in ionic charge and electron delocalization. We have also simulated liquid water undergoing shock loading of velocities from 5-11 km/s using the Multi-Scale Shock Technique (MSST). We show that Density Functional Theory (DFT) molecular dynamics results compare extremely well to experiments on the water shock Hugoniot
Keegan, Ronan M; Bibby, Jaclyn; Thomas, Jens; Xu, Dong; Zhang, Yang; Mayans, Olga; Winn, Martyn D; Rigden, Daniel J
2015-02-01
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 5â€…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. PMID:25664744
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)
Koyano, Kazuo; Nakano, Tatsuya
2008-01-01
Molecular dynamics and the ab initio fragment molecular orbital method were applied to investigate the interaction of HIV-1 aspartic protease with its inhibitor. The interaction energy of the inhibitor at the active sites of aspartic acid obtained by the ab initio fragment molecular orbital method was as great as 50?kcal?mol?1, coinciding with a tetrahedral transition state.
International Nuclear Information System (INIS)
A computer program has been developed in our laboratories for the parameter determination using energy, gradient and hessian values obtained by ab initio calculations. The empirical potential is fitted to the ab initio PES. (AIP) copyright 1995 American Institute of Physics
Pinto, Anne Cybelle; Ramos, Rommel T J; Silva, Wanderson Marques; Rocha, FlÃ¡via Souza; Barbosa, Silvanira; Miyoshi, Anderson; Schneider, Maria P C; Silva, Artur; Azevedo, Vasco
2012-07-01
Corynebacterium pseudotuberculosis is a bacterium which causes diseases such as caseous lymphadenitis in small ruminants, resulting in large-scale economic losses for agribusiness worldwide. Consequently, this bacterium including its transcriptional profile analysis has been the focus of various studies. Identification of the transcripts that appear under conditions that simulate the environment encountered by this bacterial species in the host is of great importance in discovering new targets for the production of more efficient vaccines. We sequenced the cDNA of Corynebacterium pseudotuberculosis strain 1002, using the SOLiD V3 system, under the following conditions: osmotic stress (2 M), acidity (pH), heat shock (50 Â°C) and control condition (N). To identify the transcripts shared among the stimulons and integrate this information with the results from BLAST and BLAST2GO, we developed the software CoreStImulon (CSI) which allows the user to individually distinguish the genes in terms of their participation in biological processes, their function and cellular location. In the biosynthetic processes, eleven genes represented in the core stimulon and twenty genes in the control were observed. This validates the hypothesis that the organisms strategy for surviving in a hostile environment is through growth reduction. The oxidation reduction process, response to stress process, and cell adhesion are controlled by genes that contribute to bacterial cell maintenance under stress conditions; these could be involved in their pathogenicity. The methodology for identification of transcripts obtained by ab initio assembly and shared among the stimulons permitted candidates selection for vaccine studies. CSI is available at https://sourceforge.net/projects/corestimulon/. PMID:22641428
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
Ro?. 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 molecular dynamics simulation of hydrogen fluoride at several thermodynamic states
DEFF Research Database (Denmark)
Kreitmeir, M.; Bertagnolli, H.; Mortensen, Jens JÃ¸rgen; Parrinello, M.
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...
Ab Initio Investigations of the C2F4S Isomers and of Their Interconversions
DEFF Research Database (Denmark)
Shim, Irene; Vallano-Lorenzo, Sandra; Lisbona-Martin, Pilar; Senning, Alexander
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...
All electron ab initio investigations of the electronic states of the FeC molecule
DEFF Research Database (Denmark)
Shim, Irene; Gingerich, Karl A.
1999-01-01
The low lying electronic states of the molecule FeC have been investigated by performing all electron ab initio multi-configuration self-consistent-field (CASSCF) and multi reference configuration interaction (MRCI) calculations. The relativistic corrections for the one electron Darwin contact te...
All Electron ab initio Investigations of the Electronic States of the MoN Molecule
DEFF Research Database (Denmark)
Shim, Irene; Gingerich, Karl A.
1999-01-01
The low lying electronic states of the molecule MoN have been investigated by performing all electron ab initio multi-configuration self-consistent-field (CASSCF) calculations. The relativistic corrections for the one electron Darwin contact term and the relativistic mass-velocity correction have...
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 mo...
Dispersion Interactions between Rare Gas Atoms: Testing the London Equation Using ab Initio Methods
Halpern, Arthur M.
2011-01-01
A computational chemistry experiment is described in which students can use advanced ab initio quantum mechanical methods to test the ability of the London equation to account quantitatively for the attractive (dispersion) interactions between rare gas atoms. Using readily available electronic structure applications, students can calculate the…
Czech Academy of Sciences Publication Activity Database
Dra?ínský, Martin; Möller, H. M.; Exner, T. E.
2013-01-01
Ro?. 9, ?. 8 (2013), s. 3806-3815. ISSN 1549-9618 R&D Projects: GA ?R 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 Defect Energetics in LaBO3 Perovskite Solid Oxide Fuel Cell Materials
DEFF Research Database (Denmark)
Lee, Yueh-Lin; Morgan, Dane; Kleis, Jesper; Rossmeisl, Jan
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
Ma, D.; Friák, Martin; von Pezold, J.; Raabe, D.; Neugebauer, J.
2015-01-01
Ro?. 85, FEB (2015), s. 53-66. ISSN 1359-6454 Institutional support: RVO:68081723 Keywords : Solid-solution strengthening * DFT * Peierls–Nabarro model * Ab initio * Al alloys Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 4.465, year: 2014
Ab initio quantum-enhanced optical phase estimation using real-time feedback control
DEFF Research Database (Denmark)
Berni, Adriano; Gehring, Tobias; Nielsen, Bo Melholt; Händchen, Vitus; Paris, Matteo G. A.; Andersen, Ulrik Lund
2015-01-01
quantum-enhanced and fully deterministic ab initio phase estimation protocol based on real-time feedback control. Using robust squeezed states of light combined with a real-time Bayesian adaptive estimation algorithm, we demonstrate deterministic phase estimation with a precision beyond the quantum shot...... noise limit. The demonstrated protocol opens up new opportunities for quantum microscopy, quantum metrology and quantum information processing....
Ab initio study of the lattice dynamics of CsNiF3.
Czech Academy of Sciences Publication Activity Database
Legut, Dominik; Wdowik, U. D.
2010-01-01
Ro?. 22, ?. 43 (2010), Art. no. 435402. ISSN 0953-8984 R&D Projects: GA MŠk(CZ) MEB051015 Institutional research plan: CEZ:AV0Z20410507 Keywords : CsNiF3 * 1D ferromagnet * phonon * ab initio Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 2.332, year: 2010
Czech Academy of Sciences Publication Activity Database
Hemzalová, P.; Friák, Martin; Šob, Mojmír; Ma, D.; Udyansky, A.; Raabe, D.; Neugebauer, J.
2013-01-01
Ro?. 88, ?. 17 (2013), Art . no. 174103. ISSN 1098-0121 R&D Projects: GA ?R(CZ) GAP108/12/0311; GA ?R GD106/09/H035; GA AV ?R IAA100100920 Institutional support: RVO:68081723 Keywords : nitrides * ab initio * thermodynamics * elasticity Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 3.664, year: 2013
UGLIENGO, Piero; Corno, Marta
2008-01-01
How does glycine adsorb at hydroxyapatite surfaces? Ab initio simulations based on periodic B3LYP GTO calculations reveal the detailed mechanism of binding to the (001) and (010) surfaces by shedding light on how acid and basic amino acid residues of proteins interact with hydroxyapatite based biomaterials.
DEFF Research Database (Denmark)
Abild-Pedersen, Frank; Nørskov, Jens Kehlet; Rostrup-Nielsen, Jens; Sehested, Jens; Helveg, Stig
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.
Magnetoelastic coupling in epitaxial magnetic films: An ab-initio study
Komelj, Matej; Faehnle, Manfred
2000-01-01
A method is developed which allows to determine the first-order and the second-order magnetoelastic coefficients of a magnetic bulk material from the ab-initio calculation of the magnetocrystalline anisotropy energy as function of a prestrain. Excplicit results are given for bcc Fe, and they agree well with experimental data obtained from the magnetostrictive stress measurements for epitaxial Fe films.
International Nuclear Information System (INIS)
Ab initio MO calculations were applied to the evaluation of repulsion energies in highly charged HBr ions. Net charges of H and Br atoms were obtained from the Mulliken atomic population, and repulsion energies were estimated by a simplified model based on the Coulomb repulsion. The repulsion energy of HBr8+ was calculated to be 71.5 eV. (Auth.)
Ab initio SCF polarisabilities and electron-molecule adiabatic polarisation potentials
International Nuclear Information System (INIS)
Ab initio calculations of the static electric-dipole polarisability and the adiabatic polarisation potential including all multipole terms for electron scattering are reported for H2. The dependence of the polarisation potential on distance to the electron, orientation of the molecule, and internuclear distance is examined. (author)
Ab initio study of electronic structure and crystalline phases of tellurium.
Czech Academy of Sciences Publication Activity Database
Hemzalová, P.; Legut, Dominik; Šob, Mojmír
Bratislava : Univerzita Komenského v Bratislave, 2007, s. 29-31. [Študentská vedecká konferencia. Bratislava (SK), 18.04.2007] R&D Projects: GA AV ?R IAA1041302; GA MŠk OC 147 Institutional research plan: CEZ:AV0Z20410507 Keywords : electronic structure * tellurium * ab initio calculations Subject RIV: BM - Solid Matter Physics ; Magnetism
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)
AB initio investigation of infrared optic phonon modes splitting in TiSe crystal
International Nuclear Information System (INIS)
The calculation technique of a splitting of infrared optical optical phonon frequencies on longitudinal and transverse modes is described. The analysis of splitting at the Brillouin zone center and influence of these splitting on phonon spectrum is carried out on the base of TiSe phonon spectrum derived from ab initio calculations, performed earlier in other papers
Pagliai, Marco; Muniz-Miranda, Maurizio; Cardini, Gianni; Schettino, Vincenzo
2011-05-01
Ab initio molecular dynamics simulations with the Car-Parrinello method have been performed on the spodumene crystal at standard conditions and high pressure. Starting from the computed trajectories, accurate Raman and infrared spectra have been obtained and compared with available experimental measurements in the low and high pressure phases. The structural and spectroscopic changes due to the pressure effects are discussed.
Ab initio electronic properties of dual phosphorus monolayers in silicon
DEFF Research Database (Denmark)
Drumm, Daniel W.; Per, Manolo C.; Budi, Akin; Hollenberg, Lloyd C.L.; Russo, Salvy P.
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, ...
The small index property of automorphism groups of ab-initio generic structures
Ghadernezhad, Zaniar
2015-01-01
Suppose $M$ is a countable ab-initio (uncollapsed) generic structure which is obtained from a pre-dimension function with rational coefficients. We show that if $H$ is a subgroup of $\\mbox{Aut}\\left(M\\right)$ with $\\left[\\mbox{Aut}\\left(M\\right):H\\right
International Nuclear Information System (INIS)
The phonon dispersion relations of bulk hexagonal boron nitride have been determined from inelastic x-ray scattering measurements and analyzed by ab initio calculations. Experimental data and calculations show an outstanding agreement and reconcile the controversies raised by recent experimental data obtained by electron-energy loss spectroscopy and second-order Raman scattering
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.
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.
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
Ro?. 118, ?. 35 (2014), s. 7507-7515. ISSN 1089-5639 R&D Projects: GA ?R 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
Ab initio molecular dynamics approach to a quantitative description of ion pairing in water.
Czech Academy of Sciences Publication Activity Database
Pluha?ová, Eva; Maršálek, Ond?ej; Schmidt, B.; Jungwirth, Pavel
2013-01-01
Ro?. 4, ?. 23 (2013), s. 4177-4181. ISSN 1948-7185 R&D Projects: GA ?R 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
High accuracy ab initio calculations on reactions of OH with 1-alkenes. The case of propene.
Czech Academy of Sciences Publication Activity Database
Izsák, R.; Szöri, Milan; Knowles, P. J.; Viskolcz, B.
2009-01-01
Ro?. 5, ?. 9 (2009), s. 2313-2321. ISSN 1549-9618 Institutional research plan: CEZ:AV0Z40550506 Keywords : multireference ab initio calculations * OH-addition * H- abstraction by OH Subject RIV: CF - Physical ; Theoretical Chemistry Impact factor: 4.804, year: 2009
The Puâ€“Uâ€“Am system: An ab initio informed CALPHAD thermodynamic study
International Nuclear Information System (INIS)
Highlights: â€¢ The ab initio informed CALPHAD assessment of the Amâ€“U system has been realized. â€¢ A strong tendency toward phase separation across the whole composition range is predicted. â€¢ The ab initio informed Puâ€“Uâ€“Am thermodynamic database has been developed. â€¢ The solubility of Am and U in the liquid phase is improved by adding Pu. â€¢ The Î´-Pu (fcc) phase is strongly stabilized by Am, on the contrary to the bcc phase. - Abstract: Phase diagram and thermodynamic properties of the Amâ€“U system, that are experimentally unknown, are calculated using the CALPHAD method with input from ab initio electronic-structure calculations for the fcc and bcc phases. A strong tendency toward phase separation across the whole composition range is predicted. In addition, ab initio informed Puâ€“U and Amâ€“Pu thermodynamic assessments are combined to build a Puâ€“Uâ€“Am thermodynamic database. Regarding the Pu-rich corner of the ternary system, predictions indicate that Am acts as a powerful Î´-Pu (fcc) stabilizer. In the U-rich corner, similar predictions are made but to a lesser extent. In both cases, the bcc phase is destabilized and the fcc phase is enhanced. Finally, results and methodology are discussed and compared with previous assessments and guidelines are provided for further experimental studies
Advances in the ab initio description of nuclear three-cluster systems
Romero-Redondo, Carolina; NavrÃ¡til, Petr; Hupin, Guillaume
2015-01-01
We introduce the extension of the ab initio no-core shell model with continuum to describe three-body cluster systems. We present results for the ground state of 6He and show improvements with respect to the description obtained within the no-core shell model and the no-core shell model/resonating group methods.
Ab initio prediction of vacancy properties in concentrated alloys: The case of fcc Cu-Ni
Zhang, Xi; Sluiter, Marcel H. F.
2015-05-01
Vacancy properties in concentrated alloys continue to be of great interest because nowadays ab initio supercell simulations reach a scale where even defect properties in disordered alloys appear to be within reach. We show that vacancy properties cannot generally be extracted from supercell total energies in a consistent manner without a statistical model. Essential features of such a model are knowledge of the chemical potential and imposition of invariants. In the present work, we derive the simplest model that satisfies these requirements and we compare it with models in the literature. As illustration we compute ab initio vacancy properties of fcc Cu-Ni alloys as a function of composition and temperature. Ab initio density functional calculations were performed for SQS supercells at various compositions with and without vacancies. Various methods of extracting alloy vacancy properties were examined. A ternary cluster expansion yielded effective cluster interactions (ECIs) for the Cu-Ni-Vac system. Composition and temperature dependent alloy vacancy concentrations were obtained using statistical thermodynamic models with the ab initio ECIs. An Arrhenius analysis showed that the heat of vacancy formation was well represented by a linear function of temperature. The positive slope of the temperature dependence implies a negative configurational entropy contribution to the vacancy formation free energy in the alloy. These findings can be understood by considering local coordination effects.
Vanduyfhuys, Louis; Vandenbrande, Steven; Verstraelen, Toon; Schmid, Rochus; Waroquier, Michel; Van Speybroeck, Veronique
2015-05-15
QuickFF is a software package to derive accurate force fields for isolated and complex molecular systems in a quick and easy manner. Apart from its general applicability, the program has been designed to generate force fields for metal-organic frameworks in an automated fashion. The force field parameters for the covalent interaction are derived from ab initio data. The mathematical expression of the covalent energy is kept simple to ensure robustness and to avoid fitting deficiencies as much as possible. The user needs to produce an equilibrium structure and a Hessian matrix for one or more building units. Afterward, a force field is generated for the system using a three-step method implemented in QuickFF. The first two steps of the methodology are designed to minimize correlations among the force field parameters. In the last step, the parameters are refined by imposing the force field parameters to reproduce the ab initio Hessian matrix in Cartesian coordinate space as accurate as possible. The method is applied on a set of 1000 organic molecules to show the easiness of the software protocol. To illustrate its application to metal-organic frameworks (MOFs), QuickFF is used to determine force fields for MIL-53(Al) and MOF-5. For both materials, accurate force fields were already generated in literature but they requested a lot of manual interventions. QuickFF is a tool that can easily be used by anyone with a basic knowledge of performing ab initio calculations. As a result, accurate force fields are generated with minimal effort. © 2015 Wiley Periodicals, Inc. PMID:25740170
Single-ion 4f element magnetism: an ab-initio look at Ln(COT)2(-).
Gendron, Frédéric; Pritchard, Benjamin; Bolvin, Hélène; Autschbach, Jochen
2015-12-14
The electron densities associated with the Ln 4f shell, and spin and orbital magnetizations ('magnetic moment densities'), are investigated for the Ln(COT)2(-) series. The densities are obtained from ab-initio calculations including spin-orbit coupling. For Ln = Ce, Pr the magnetizations are also derived from crystal field models and shown to agree with the ab-initio results. Analysis of magnetizations from ab-initio calculations may be useful in assisting research on single molecule magnets. PMID:26510902
International Nuclear Information System (INIS)
The core structure and stability of the 90 degree sign partial dislocation in diamond is studied within isotropic elasticity theory and ab initio total energy calculations. The double-period reconstruction is found to be more stable than the single-period reconstruction for a broad range of stress states. The analysis of the ab initio results shows further that elasticity theory is valid for dislocation spacings as small as 10-20 Angstrom, thus allowing ab initio calculations to provide reliable parameters for continuum theory analysis. (c) 2000 The American Physical Society
Aryal, Sita Ram
The alumino-silicate solid solution series (Al 4+2xSi2-2 xO10-x) is an important class of ceramics. Except for the end member (x=0), Al2 SiO5 the crystal structures of the other phases, called mullite, have partially occupied sites. Stoichiometric supercell models for the four mullite phases 3Al2O 3 · 2SiO2 · 2Al 2O3 · SiO2, 4 Al2O3· SiO 2, 9Al2O3 · SiO2, and iota-Al2 O3 (iota-alumina) are constructed starting from experimentally reported crystal structures. A large number of models were built for each phase and relaxed using the Vienna ab initio simulation package (VASP) program. The model with the lowest total energy for a given x was chosen as the representative structure for that phase. Electronic structure and mechanical properties of mullite phases were studied via first-principles calculations. Of the various phases of transition alumina, iota-Al 2O3 is the least well known. In addition structural details have not, until now, been available. It is the end member of the aluminosilicate solid solution series with x=1. Based on a high alumina content mullite phase, a structural model for iota- Al2O3 is constructed. The simulated x-ray diffraction (XRD) pattern of this model agrees well with a measured XRD pattern. The iota-Al2 O3 is a highly disordered ultra-low-density phase of alumina with a theoretical density of 2854kg/m3. Using this theoretically constructed model, elastic, thermodynamic, electronic, and spectroscopic properties of iota-Al2 O3 have been calculated and compared it with those of alpha- Al2O3 and gamma- Al2O3. Boron carbide (B4C) undergoes an amorphization under high velocity impacts. The mechanism of amorphization is not clear. Ab initio methods are used to carry out large-scale uniaxial compression simulations on two polytypes of stoichiometric boron carbide (B4C), B 11C-CBC, and B12- CCC where B11C or B12 is the 12-atom icosahedron and CBC or CCC is the three-atom chain. The simulations were performed on large supercells of 180 atoms. Simulated results indicate bending of the three-atom chain leads to the amorphization of the B 11C-CBC (B12-CCC ) at a uniaxial strain s=0.23 (0.22) and with a maximum stress of 168 (151) GPa. The mechanism of amorphization is analyzed with radial pair distribution function (RPDF), total density of states (TDOS), and the distribution of effective charges on atoms.
Crossover of cation partitioning in olivines: a combination of ab initio and Monte Carlo study
Chatterjee, Swastika; Bhattacharyya, Sirshendu; Sengupta, Surajit; Saha-Dasgupta, Tanusri
2011-04-01
We report studies based on a combination of ab initio electronic structure and Monte Carlo (MC) technique on the problem of cation partitioning among inequivalent octahedral sites, M1 and M2 in mixed olivines containing Mg2+ and Fe2+ ions. Our MC scheme uses interactions derived out of ab initio, density functional calculations carried out on measured crystal structure data. Our results show that there is no reversal of the preference of Fe for M1 over M2 as a function of temperature. Our findings do not agree with the experimental findings of Redfern et al. (Phys Chem Miner 27:630-637, 2000), but are in agreement with those of Heinemann et al. (Eur J Mineral 18:673-689, 2006) and Morozov et al. (Eur J Mineral 17:495-500, 2005).
Steinebrunner, Gerold; Dyson, Anthony J.; Kirchner, Barbara; Huber, Hanspeter
1998-08-01
An intermolecular potential energy surface for the carbon dioxide dimer is calculated fully ab initio using a large basis set and including electron correlation. From this potential the dimer structure and the second virial coefficients are determined. In addition, it is applied in molecular dynamics simulations to obtain the fluid structure, the pressure, the internal energy, the thermal pressure coefficient, and the molar heat at constant volume. The results are compared with those from simulations with a previous ab initio potential. In this way we gain information regarding the sensitivity of each property to the quality of the quantum chemically obtained potential. Equilibration of carbon dioxide simulations must be done with great care due to the very slow energy transfer between the intramolecular vibrations and the other degrees of freedom. This point is addressed in some detail.
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.
Ab initio calculations on the magnetic properties of transition metal complexes
Bodenstein, Tilmann; Fink, Karin
2015-12-01
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.
Ab initio ro-vibrational structure of the C2? isotopes of H2O+
International Nuclear Information System (INIS)
The ro-vibrational structures of the C2? isotopes of H2O+ have been calculated from variational solution of the normal coordinate Eckart-Watson Hamiltonian. The calculations use the discrete ab initio potential energy surface of Weis et al. (1989). Where comparisons can be made, the assignment of the vibrational states is in excellent agreement with experiment and with the ab initio variational calculation of Weis et al., who utilised a different force field and an internal coordinate nuclear Hamiltonian (instead of the Eckart-Watson Hamiltonian). Furthermore, the calculated rotational levels of the ground and the first excited vibrational states of H2O+ are in excellent agreement with experiment. 26 refs., 6 tabs., 1 fig
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.
Vibrational properties of a sodium tetrasilicate glass: Ab initio versus Classical Force Fields
Ispas, S; De Wispelaere, S; Kob, W
2003-01-01
We compare some vibrational properties of a sodium tetrasilicate (Na$_{2}$Si$_{4}$O$_{9}$) glass model generated by molecular dynamics simulations. The vibrational study has been carried out using a classical valence force fields approach as well as an {\\it ab initio} approach in the framework of the density functional theory. The total and partial vibrational densities of states (VDOS) are presented, and some characteristics of the vibrational modes (participation ratios, correlation lengths) are determined using both approaches. We find that the shapes of the two calculated VDOS as well as those of their corresponding partial VDOS are quite similar, especially for a low-frequency band below 550 cm${}^{-1}$. For the intermediate- and high-frequency ranges, we observe larger discrepencies between the two calculations. Polarized Raman spectra are also calculated from the {\\it ab initio} and the valence-force-field eigenvectors in the bond-polarizability approximation. We find an overall agreement between the c...
Ab initio study of AlxMoNbTiV high-entropy alloys
Cao, Peiyu; Ni, Xiaodong; Tian, Fuyang; Varga, Lajos K.; Vitos, Levente
2015-02-01
The AlxMoNbTiV (x = 0-1.5) high-entropy alloys (HEAs) adopt a single solid-solution phase, having the body centered cubic (bcc) crystal structure. Here we employ the ab initio exact muffin-tin orbitals method in combination with the coherent potential approximation to investigate the equilibrium volume, elastic constants, and polycrystalline elastic moduli of AlxMoNbTiV HEAs. A comparison between the ab initio and experimental equilibrium volumes demonstrates the validity and accuracy of the present approach. Our results indicate that Al addition decreases the thermodynamic stability of the bcc structure with respect to face-centered cubic and hexagonal close packed lattices. For the elastically isotropic Al0.4MoNbTiV HEAs, the valence electron concentration (VEC) is about 4.82, which is slightly different from VEC ˜ 4.72 obtained for the isotropic Gum metals and refractory—HEAs.
Nuclear quantum effects in ab initio dynamics: theory and experiments for lithium imide
Ceriotti, Michele; Pietropaolo, Antonino; Colognesi, Daniele; Nale, Angeloclaudio; Catti, Michele; Bernasconi, Marco; Parrinello, Michele
2010-01-01
Owing to their small mass, hydrogen atoms exhibit strong quantum behavior even at room temperature. Including these effects in first principles calculations is challenging, because of the huge computational effort required by conventional techniques. Here we present the first ab-initio application of a recently-developed stochastic scheme, which allows to approximate nuclear quantum effects inexpensively. The proton momentum distribution of lithium imide, a material of interest for hydrogen storage, was experimentally measured by inelastic neutron scattering experiments and compared with the outcome of quantum thermostatted ab initio dynamics. We obtain favorable agreement between theory and experiments for this purely quantum mechanical property, thereby demonstrating that it is possible to improve the modelling of complex hydrogen-containing materials without additional computational effort.
Melting of sodium under high pressure. An ab-initio study
González, D. J.; González, L. E.
2015-08-01
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 calculation of valley splitting in monolayer \\delta-doped phosphorus in silicon
Drumm, Daniel W; Per, Manolo C; Russo, Salvy P; Hollenberg, Lloyd C L
2012-01-01
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 tunneling microscope lithography. We develop a plane-wave density functional theory description of these systems which is size-limited due to computational tractability. We then develop a less resource-intensive alternative via localized basis functions, 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 \\delta-layers. In obtaining agreement between plane-wave and delocalized methods, we show that the valley splitting has been overestimated in previous ab initio calculations by more than 50%.
Complex coupled-cluster approach to an ab-initio description of open quantum systems
International Nuclear Information System (INIS)
We develop ab-initio coupled-cluster theory to describe resonant and weakly bound states along the neutron drip line. We compute the ground states of the helium chain 3-10He within coupled-cluster theory in singles and doubles (CCSD) approximation. We employ a spherical Gamow-Hartree-Fock basis generated from the low-momentum N3LO nucleon-nucleon interaction. This basis treats bound, resonant, and continuum states on an equal footing, and is therefore optimal for the description of properties of drip line nuclei where continuum features play an essential role. Within this formalism, we present an ab-initio calculation of energies and decay widths of unstable nuclei starting from realistic interactions
Ab Initio Calculations of Even Oxygen Isotopes with Chiral Two- Plus Three-Nucleon Interactions
Hergert, H; Calci, A; Langhammer, J; Roth, R
2013-01-01
We formulate the In-Medium Similarity Renormalization Group (IM-SRG) for open-shell nuclei using a multi-reference formalism based on a generalized Wick theorem introduced in quantum chemistry. The resulting multi-reference IM-SRG (MR-IM-SRG) is used to perform the first ab initio study of even oxygen isotopes with chiral NN and 3N Hamiltonians, from the proton to the neutron drip lines. We obtain an excellent reproduction of experimental ground-state energies with quantified uncertainties, which is validated by results from the Importance-Truncated No-Core Shell Model and the Coupled Cluster method. The agreement between conceptually different many-body approaches and experiment highlights the predictive power of current chiral two- and three-nucleon interactions, and establishes the MR-IM-SRG as a promising new tool for ab initio calculations of medium-mass nuclei far from shell closures.
{\\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...
International Nuclear Information System (INIS)
The procedure of deriving ab initio effective core potentials (ECP) to incorporate the Coulomb and exchange effects as well as orthogonality constraints from the inner core electrons is extended to include the dominant relativistic effects on the valence orbitals. An ab initio approach is then described which enables the valence electrons in heavy atoms to be treated in a standard nonrelativistic manner by including the effect of the relativistic core-valence interactions directly into the ECP. The starting point for this procedure is the Pauli Hartree-Fock relativistic treatment of Cowan and Griffin. The pseudo-orbital transformation and derivation of the l-dependent effective core potentials are analogous to the nonrelativistic case with certain modifications. Analytic forms for the pseudo-orbitals and ECP's are derived for the U atom, and results of valence electron calculations are presented
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
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...
The Pu-U-Am system: An ab initio informed CALPHAD thermodynamic study
Perron, A.; Turchi, P. E. A.; Landa, A.; SÃ¶derlind, P.; Ravat, B.; Oudot, B.; Delaunay, F.
2015-03-01
Phase diagram and thermodynamic properties of the Am-U system, that are experimentally unknown, are calculated using the CALPHAD method with input from ab initio electronic-structure calculations for the fcc and bcc phases. A strong tendency toward phase separation across the whole composition range is predicted. In addition, ab initio informed Pu-U and Am-Pu thermodynamic assessments are combined to build a Pu-U-Am thermodynamic database. Regarding the Pu-rich corner of the ternary system, predictions indicate that Am acts as a powerful Î´-Pu (fcc) stabilizer. In the U-rich corner, similar predictions are made but to a lesser extent. In both cases, the bcc phase is destabilized and the fcc phase is enhanced. Finally, results and methodology are discussed and compared with previous assessments and guidelines are provided for further experimental studies.
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.
Ab initio molecular dynamics simulation of liquid water by quantum Monte Carlo
Zen, Andrea; Luo, Ye; Mazzola, Guglielmo; Guidoni, Leonardo; Sorella, Sandro
2015-04-01
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.
Bridging a gap between continuum-QCD and ab initio predictions of hadron observables
Directory of Open Access Journals (Sweden)
Daniele Binosi
2015-03-01
Full Text Available 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.
Ab initio determination of an extended Heisenberg Hamiltonian in CuO2 layers
Calzado, C J; Calzado, Carmen J.; Malrieu, Jean-Paul
2000-01-01
Accurate ab initio calculations on embedded Cu_4O_{12} square clusters, fragments of the La_2CuO_4 lattice, confirm a value of the nearest neighbor antiferromagnetic coupling (J=124 meV) previously obtained from ab initio calculations on bicentric clusters and in good agreement with experiment. These calculations predict non negligible antiferromagnetic second-neighbor interaction (J'=6.5 meV) and four-spin cyclic exchange (K=14 meV), which may affect the thermodynamic and spectroscopic properties of these materials. The dependence of the magnetic coupling on local lattice distortions has also been investigated. Among them the best candidate to induce a spin-phonon effect seems to be the movement of the Cu atoms, changing the Cu-Cu distance, for which the variation of the nearest neighbor magnetic coupling with the Cu-O distance is {\\Delta J}/{\\Delta d_{Cu-O}}\\sim 1700 cm^{-1} A^{-1}.
Ab initio determination of an extended Heisenberg Hamiltonian in CuO2 layers
International Nuclear Information System (INIS)
Accurate ab initio calculations on embedded Cu4O12 square clusters, fragments of the La2CuO4 lattice, confirm a value of the nearest neighbor antiferromagnetic coupling (J = 124 meV) previously obtained from ab initio calculations on bicentric clusters and in good agreement with experiment. These calculations predict non negligible antiferromagnetic second-neighbor interaction (J' = 6.5 meV) and four-spin cyclic exchange (K = 14 meV), which may affect the thermodynamic and spectroscopic properties of these materials. The dependence of the magnetic coupling on local lattice distortions has also been investigated. Among them the best candidate to induce a spin-phonon effect seems to be the movement of the Cu atoms, changing the Cu-Cu distance, for which the variation of the nearest neighbor magnetic coupling with the Cu-O distance is ?J/?dCu-O ? 1700 cm -1 A -1. (orig.)
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.
Constant pressure ab initio molecular dynamics with discrete variable representation basis sets
Ma, Zhonghua; Tuckerman, Mark
2010-11-01
The use of discrete variable representation (DVR) basis sets within ab initio molecular dynamics calculations allows the latter to be performed with converged energies and, more importantly, converged forces. In this paper, we show how to carry out ab initio molecular dynamics calculations in the isothermal-isobaric ensemble with fully flexible simulation boxes within the DVR basis set framework. In particular, we derive the appropriate DVR based expression for the pressure tensor when the electronic structure is represented using Kohn-Sham density functional theory, and we examine the convergence of this expression as a function of the basis set size. An illustrative example using 64 silicon atoms in a fully flexible box using a combination of the Martyna-Tobias-Klein [Martyna et al., J. Chem. Phys. 101, 4177 (1994)] and Car-Parrinello [Car and Parinello, Phys. Rev. Lett. 55, 2471 (1985)] algorithms is presented to demonstrate the efficacy of the approach.
B28: the smallest all-boron cage from an ab initio global search
Zhao, Jijun; Huang, Xiaoming; Shi, Ruili; Liu, Hongsheng; Su, Yan; King, R. Bruce
2015-09-01
Our ab initio global searches reveal the lowest-energy cage for B28, which is built from two B12 units and prevails over the competing structural isomers such as planar, bowl, and tube. This smallest boron cage extends the scope of all-boron fullerene and provides a new structural motif of boron clusters and nanostructures.Our ab initio global searches reveal the lowest-energy cage for B28, which is built from two B12 units and prevails over the competing structural isomers such as planar, bowl, and tube. This smallest boron cage extends the scope of all-boron fullerene and provides a new structural motif of boron clusters and nanostructures. Electronic supplementary information (ESI) available: Planar isomer structures of B28 and spatial distributions of front molecular orbitals. See DOI: 10.1039/c5nr04034e
A Hartree-Fock ab initio band-structure calculation employing Wannier-type orbitals
Albrecht, M; Dolg, M; Fulde, P; Stoll, H; Albrecht, Martin; Shukla, Alok; Dolg, Michael; Fulde, Peter; Stoll, Hermann
1998-01-01
An ab initio Wannier-function-based approach to electronic ground-state calculations for crystalline solids is outlined. In the framework of the linear combination of atomic orbitals method the infinite character of the solid is rigorously taken into account. The Hartree-Fock ground-state energy, cohesive energy, lattice constant and bulk modulus are calculated in a fully ab initio manner as it is demonstrated for sodium chloride, NaCl, using basis sets close to the Hartree-Fock limit. It is demonstrated that the Hartree-Fock band-structure can easily be recovered with the current approach and agrees with the one obtained from a more conventional Bloch-orbital-based calculation. It is argued that the advantage of the present approach lies in its capability to include electron correlation effects for crystalline insulators by means of well-established quantum chemical procedures.
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 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.
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 Calculations and Raman and SERS Spectral Analyses of Amphetamine Species
DEFF Research Database (Denmark)
Berg, Rolf W.; Nørbygaard, Thomas; White, Peter C.; Abdali, Salim
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 of the...... 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...
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...
Ab Initio Calculated and Experimentally Measured Raman Spectra of Spodumene (LiAlSi2O6)
Stangarone, C.; Prencipe, M.; Mantovani, L.; Bersani, D.; Tribaudino, M.; Lottici, P. P.
2014-06-01
Polarization Raman measurements on spodumene enabled the identification of all 30 active modes (14 Ag and 16 Bg). Ab initio CRYSTAL14 calculations (WC1LYP hamiltonian) give very good agreement for frequencies (???< 4.8 cm^-1) and intensities.
Ab-initio calculations of diffusion barriers of small vacancy clusters in silicon
Markevich, Alexander
2009-01-01
Esta tese apresenta os resultados de um programa de investigação sobre a difusão da lacuna, bi-lacuna e tri-lacuna em silício utilizando simulações numéricas pelo método da teoria do funcional da densidade. Este método está implementado na forma de um programa informático referido como AIMPRO (Ab Initio Modelling PROgram). Para o cálculo dos pontos cela dos mecanismos de difusão foi usado o método “Nudged Elastic Band”. As condições fronteira dos problemas foram impostas recorrendo à...
Influence of the ab-initio nd cross sections in the critical heavy-water benchmarks
Morillon, B; Carbonell, J
2013-01-01
The n-d elastic and breakup cross sections are computed by solving the three-body Faddeev equations for realistic and semi-realistic Nucleon-Nucleon potentials. These cross sections are inserted in the Monte Carlo simulation of the nuclear processes considered in the International Handbook of Evaluated Criticality Safety Benchmark Experiments (ICSBEP). The results obtained using thes ab initio n-d cross sections are compared with those provided by the most renown international libraries.
All electron ab initio investigations of the electronic states of the FeC molecule
DEFF Research Database (Denmark)
Shim, Irene; Gingerich, Karl A.
1999-01-01
The low lying electronic states of the molecule FeC have been investigated by performing all electron ab initio multi-configuration self-consistent-field (CASSCF) and multi reference configuration interaction (MRCI) calculations. The relativistic corrections for the one electron Darwin contact term and the relativistic mass-velocity correction have been determined in perturbation calculations. The electronic structure of the FeC molecule is interpreted as antiferromagnetic couplings of the local...
All Electron ab initio Investigations of the Electronic States of the MoN Molecule
DEFF Research Database (Denmark)
Shim, Irene; Gingerich, Karl A.
1999-01-01
The low lying electronic states of the molecule MoN have been investigated by performing all electron ab initio multi-configuration self-consistent-field (CASSCF) calculations. The relativistic corrections for the one electron Darwin contact term and the relativistic mass-velocity correction have been determined in perturbation calculations. The electronic ground state is confirmed as being 4 . The chemical bond of MoN has triple bond character due to the approximately fully occupied delocalized...
Two-gap superconductivity in heavily n-doped graphene: Ab initio Migdal-Eliashberg theory
Margine, ER; Giustino, F.
2014-01-01
Graphene is the only member of the carbon family from zero- to three-dimensional materials for which superconductivity has not been observed yet. At this time, it is not clear whether the quest for superconducting graphene is hindered by technical challenges, or else by the fluctuation of the order parameter in two dimensions. In this area, ab initio calculations are useful to guide experimental efforts by narrowing down the search space. In this spirit, we investigate from first principles t...
Gao, Haiyuan; Xu, Yang; Li, Meijiao; Guo, Zhendong; Chen, Hongshen; Jin, Zhonghe; Yu, Bin
2011-01-01
Electronic transport properties of monolayer graphene with extreme physical bending up to 90o angle are studied using ab Initio first-principle calculations. The importance of key structural parameters including step height, curvature radius and bending angle are discussed how they modify the transport properties of the deformed graphene sheet comparing to the corresponding flat ones. The local density of state reveals that energy state modification caused by the physical be...
DEFF Research Database (Denmark)
Bork, Nicolai Christian; Du, Lin; Reiman, Heidi; Kurtén, Theo; Kjærgaard, Henrik Grum
2014-01-01
Models of formation and growth of atmospheric aerosols are highly dependent on accurate cluster binding energies. These are most often calculated by ab initio electronic structure methods but remain associated with significant uncertainties. We present a computational benchmarking study of the Gibbs free binding energies in molecular complexes and clusters based on gas phase FTIR spectroscopy. The acetonitrile-HCl molecular complex is identified via its redshifted H-Cl stretching vibrational mod...
Ab initio study of the migration of intrinsic defects in 3C-SiC
Bockstedte, Michel; Mattausch, Alexander; Pankratov, Oleg
2003-01-01
The diffusion of intrinsic defects in 3C-SiC is studied using an ab initio method based on density functional theory. The vacancies are shown to migrate on their own sublattice. The carbon split-interstitials and the two relevant silicon interstitials, namely the tetrahedrally carbon-coordinated interstitial and the -oriented split-interstitial, are found to be by far more mobile than the vacancies. The metastability of the silicon vacancy, which transforms into a vacancy-an...
Boron and nitrogen impurities in SiC nanoribbons: an ab initio investigation
Costa, C. D.; Morbec, J. M.
2011-01-01
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 occu...
Ab initio studies of the geometrical and electronic structure of nickel porphyrins
Energy Technology Data Exchange (ETDEWEB)
Piqueras, M.C.; Rohlfing, C.M. [Sandia National Labs., Livermore, CA (United States)
1997-12-31
Ab initio calculations on nickel porphyrin (NiP) and nickel tetra(t-butyl) porphrin (NiTtBuP) have been performed to analyze the conformationally dependent photophysical properties of these systems. Geometry optimizations and harmonic frequency analyses have been performed at the Hartree-Fock (HF) and second-order perturbation (MP2) theory level on NiP. For NiTtBuP, HF calculations have been carried out on domed, ruffled and waved conformers.
Estudio teórico de la molécula de hidrogeno calculo ab-initio
N. Quitián
2010-01-01
Utilizando la técnica LCGO-SCF-MO (Combinación Lineal de los Orbitales Moleculares en Orbitales Gausianos, en el método del Campo Auto-Coherente), se determinaron las energías de orbital, la energía electrónica total, la energía de repulsión nuclear y la energía de Hartree-Fock de la molécula de hidrógeno en su estado fundamental, mediante un cálculo ab initio y utilizando una base de funciones gausianas. Los orbitales moleculares fueron desarrollados en términos de una función Is contraída p...
Structural, electronic, and optical properties of ZrO2 from ab initio calculations
Garcia, Joelson Cott; Scolfaro, L. M. R.; Lino, A. T.; Freire, V. N.; Farias, G. A.; Silva, C C; Alves, H. W. Leite; S. C.P. Rodrigues; Silva Jr, E. F. da
2012-01-01
Structural, electronic, and optical properties for the cubic, tetragonal, and monoclinic crystalline phases of ZrO2, as derived from it ab initio full-relativistic calculations, are presented. The electronic structure calculations were carried out by means of the all-electron full potential linear augmented plane wave method, within the framework of the density functional theory and the local density approximation. The calculated carrier effective masses are shown to be highly anisotropic. Th...
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 function technique based on the linear-muffin-tin-orbitals method within the tight-binding and atomic-sphere approximations.
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 amon...
International Nuclear Information System (INIS)
The melting and lattice dynamics of sodium are studied by quantum molecular dynamics simulation, i.e., with allowance for anharmonicity, at pressures up to 1 Mbar and temperatures up to 1000 K. The simulation results agree well with the experimental data and our earlier calculation performed ab initio in the quasi-harmonic approximation. The simulation results demonstrate that anharmonic interactions weakly affect the melting curve and the phonon frequencies of Na up to near-melting temperatures.
The Projector Augmented Wave Method ab-initio molecular dynamics with full wave functions
Blöchl, P E; Schimpl, J; Bl\\"ochl, Peter E.; Schimpl, Johannes
2002-01-01
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 the 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.
The role of ab initio electronic structure calculations in multiscale modelling of materials.
Czech Academy of Sciences Publication Activity Database
Šob, Mojmír
Cambridge, England : Woodhead Publishing and Maney Publishing, 2007 - ( Guo , Z.), s. 1-24 ISBN 978-1-84569-071-7 R&D Projects: GA ?R GA202/03/1351; GA AV ?R IAA1041302; GA MŠk OC 147 Institutional research plan: CEZ:AV0Z20410507 Keywords : multiscale modelling * electronic structure * ab initio calculations Subject RIV: BM - Solid Matter Physics ; Magnetism
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.; Hansen, Poul Erik
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-Bornâˆ’Oppenheimer approximation. This...... the structures in solution are different from those in the crystal and that solvation plays an important role in weakening the hydrogen bonds....
Pair potentials for alumina from ab initio results on the Al2O3 molecule
International Nuclear Information System (INIS)
We use results from an ab initio investigation by Chang et al. on energetically low-lying stationary points of the Al2O3 molecule to determine interionic potentials for the Al-O, O-O and Al-Al pairs. Our results are discussed in the perspective of previous studies of the condensed phases of alumina, with special regard to the structure of its molten state. (author)
Glowacki, Leszek; Migdalek, Jacek
2006-04-01
The relativistic configuration-interaction (CI) method with numerical Dirac-Fock wavefunctions generated in the field of ab initio screened model potential is used to compute oscillator strengths for some spin-allowed and spin-forbidden ns2 1S0-nsnp 1P1, 3P1 transitions in light systems with two valence electrons: beryllium, magnesium and zinc. The present results are discussed and compared with other theoretical and experimental data available.
Relativistic ab initio study of forbidden transitions of singly ionized Zinc using CCSD(T)
Dixit, Gopal; Sahoo, B. K.; Chaudhuri, R. K.; Majumder, Sonjoy
2007-01-01
In this work, the {\\it ab initio} calculations have been carried out to study the oscillator strengths corresponding to `forbidden transitions' of astrophysically important electromagnetic transitions of singly ionized zinc, Zn II. Many important electron correlations are considered to all-orders using the relativistic coupled-cluster theory. Calculated ionization potentials are compared with the experimental values, wherever available. To our knowledge, oscillator strengths...
Operator evolution for ab initio electric dipole transitions of 4He
Schuster, Micah D.; Quaglioni, Sofia; Johnson, Calvin W.; Jurgenson, Eric D.; Navratil, Petr
2013-01-01
A goal of nuclear theory is to make quantitative predictions of low-energy nuclear observables starting from accurate microscopic internucleon forces. A major element of such an effort is applying unitary transformations to soften the nuclear Hamiltonian and hence accelerate the convergence of ab initio calculations as a function of the model space size. The consistent simultaneous transformation of external operators, however, has been overlooked in applications of the theo...
Ab initio configuration interaction calculations on the hydrogen molecular excimer, H4*
International Nuclear Information System (INIS)
Results of ab initio configuration interaction calculations are presented for a simple model of molecular excimers, (H4)*. Preliminary SCF calculations as well as several levels of CI are discussed for the lowest three singlet and triplet states of H4. Square, rectangular, and regular trapezoidal geometries of H4 are examined. The results of this study are significantly better in a quantitative sense than earlier work on the excited states of H4. (Auth.)
Ab initio I-V characteristics of short C-20 chains
DEFF Research Database (Denmark)
Roland, C.; Larade, B.; Taylor, Jeremy Philip; Guo, H.
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 orientation and distance between the individual C-20 molecules. Transport through the molecular chains is accompanied by a significant amount of charge transfer, which remains localized at the electrode/molec...
Ab initio study of electronic structure and crystalline phases of tellurium.
Czech Academy of Sciences Publication Activity Database
Hemzalová, P.; Legut, Dominik; Šob, Mojmír
Brno : Masarykova univerzita, 2007 - (Trnková, L.; Janderka, P.; Kizek, R.), s. 57-58 ISBN 978-80-210-4235-3. [Pracovní setkání fyzikálních chemik? a elektrochemik? /7./. Brno (CZ), 29.01.2007-30.01.2007] R&D Projects: GA AV ?R IAA1041302; GA MŠk OC 147 Institutional research plan: CEZ:AV0Z20410507 Keywords : electronic structure * tellurium * ab initio calculations Subject RIV: BM - Solid Matter Physics ; Magnetism
Czech Academy of Sciences Publication Activity Database
Ehala, Sille; Dybal, Ji?í; Makrlík, E.; Kaši?ka, Václav
2009-01-01
Ro?. 32, ?. 4 (2009), s. 597-604. ISSN 1615-9306 R&D Projects: GA ?R(CZ) GA203/06/1044; GA ?R(CZ) GA203/08/1428; GA AV ?R 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
Experimental IR study and ab initio modelling of ethylene adsorption in a MFI - type host zeolite
Zvereva-Loëte, Natalia; Ballandras, Anthony; Weber, Guy; ROTGER, Maud; Boudon, Vincent
2009-01-01
Abstract Different ab initio methods and experimental results are used to investigate the effect of the adsorption of one ethylene molecule on silicalite-1, a MFI type zeolite. We used simplified models to simulate a portion of a straight or sinusoidal channel of silicalite-1 at a quantum level. The calculated absorption spectra of the models are qualitatively in good agreement with the experimental FTIR spectrum of silicalite-1. Additionally we simulate the FTIR spectrum of the ...
Ab initio calculations for S=-2, A=4-6 s-shell hypernuclei with strangeness
International Nuclear Information System (INIS)
We describe full-coupled-channel ab initio calculations for S=-2, A=4-6 s-shell hypernuclei. The wave function includes explicit ??, ??, ?? and ?? channels. Minnesota ??, D2' ?? and Nijmegen model D (or F) simulated ?? interactions are used. Bound state solutions of the ?? hypernuclei, ??4H, ??5H, ??5He and ??6He, are obtained. This is the first attempt to explore the few-body problem of the full-coupled channel scheme for these systems. (author)
Ab-initio structural, elastic, and vibrational properties of carbon nanotubes
Sanchez-Portal, D.; Artacho, E.; Soler, J.M.; Rubio, A.; P. Ordejon
1998-01-01
A study based on ab initio calculations is presented on the estructural, elastic, and vibrational properties of single-wall carbon nanotubes with different radii and chiralities. We use SIESTA, an implementation of pseudopotential-density-functional theory which allows calculations on systems with a large number of atoms per cell. Different quantities like bond distances, Young moduli, Poisson ratio and the frequencies of different phonon branches are monitored versus tube radius. The validit...
Ab initio GW electron-electron interaction effects in Quantum Transport
Darancet, Pierre; Ferretti, Andrea; Mayou, Didier; Olevano, Valerio
2006-01-01
We present an ab initio approach to electronic transport in nanoscale systems which includes electronic correlations through the GW approximation. With respect to Landauer approaches based on density-functional theory (DFT), we introduce a physical quasiparticle electronic-structure into a non-equilibrium Green's function theory framework. We use an equilibrium non-selfconsistent $G^0W^0$ self-energy considering both full non-hermiticity and dynamical effects. The method is applied to a real ...
Pernot, Pascal
2008-01-01
Bayesian Model Calibration is used to revisit the problem of scaling factor calibration for semi-empirical correction of ab initio calculations. A particular attention is devoted to uncertainty evaluation for scaling factors, and to their effect on prediction of observables involving scaled properties. We argue that linear models used for calibration of scaling factors are generally not statistically valid, in the sense that they are not able to fit calibration data within t...
Transition pathway in GaAs under uniaxial stress: an ab initio study
International Nuclear Information System (INIS)
We study the behaviour of GaAs under a uniaxial compression using an ab initio constant-pressure technique and find that GaAs undergoes a first-order phase transition to a side-disordered orthorhombic Imm 2 structure via an intermediate state having the space group of I4-barm2. The transition pathway and mechanism under uniaxial stress are found to be considerably different from those under the hydrostatic compression
Properties of defect centres on nanothick silica layers: an ab initio investigation
COLUCCIA, Salvatore; Martra, Gianmario
2007-01-01
The structural, vibrational and electronic properties of the nanothick surfaces of mesoporous molecular sieves have been investigated at a microscopic level by ab-initio molecular dynamics. Simulations were performed on a series of 1-nm thick layers of a-SiO2 exhibiting different kinds of defects and on two reference samples of bulk amorphous silica. Our results show that the nanometre thickness of the mesoporous silica walls strictly influences the properties of the defect centres.
Arnaud, Roger; Barone, Vincenzo; Olivella, Santiago; Sole, Albert
1985-08-01
Addition of methyl radicals to ethylene and acetylene has been studied by ab initio molecular-orbital calculations. In agreement with experimental data, we find that, although addition to ethylene is characterized by a lower activation energy, addition to acetylene is faster due to the opposite and larger trend of pre-exponential factors. The reaction barriers have been analyzed by the energy decomposition scheme proposed by Morokuma.
Ab initio potential energy surface for the highly nonlinear dynamics of the KCN molecule
Energy Technology Data Exchange (ETDEWEB)
Párraga, H.; Arranz, F. J., E-mail: fj.arranz@upm.es; Benito, R. M., E-mail: rosamaria.benito@upm.es [Grupo de Sistemas Complejos, ETSI Agrónomos, Universidad Politécnica de Madrid, 28040 Madrid (Spain); Borondo, F., E-mail: f.borondo@uam.es [Departamento de Química and Instituto de Ciencias Matemáticas (ICMAT), Universidad Autónoma de Madrid, Cantoblanco, 28049-Madrid (Spain)
2013-11-21
An accurate ab initio quantum chemistry study at level of quadratic configuration interaction method of the electronic ground state of the KCN molecule is presented. A fitting of the results to an analytical series expansion was performed to obtain a global potential energy surface suitable for the study of the associated vibrational dynamics. Additionally, classical Poincaré surfaces of section for different energies and quantum eigenstates were calculated, showing the highly nonlinear behavior of this system.
An ab initio study of core-valence correlation. [in atoms
Partridge, H.; Bauschlicher, C. W., Jr.; Walch, S. P.; Liu, B.
1983-01-01
Especially in the cases of the first two columns of the periodic table, the inclusion of core-valence correlation in ab initio CI calculations yields a contraction of the atomic valence shell and improves both calculated atomic ionization potentials and atomic energy separations. For the alkali dimers Na2, K2, and Rb2, the presently calculated bond lengths are in excellent agreement with experiments when core-valence is included. In addition, the valence dissociation energies are accurate.
Ab-initio calculation of the effect of stress on the chemical activity of graphene
de andres, P. L.; Verges, J. A.
2009-01-01
Graphene layers are stable, hard, and relatively inert. We study how tensile stress affects $\\sigma$ and $\\pi$ bonds and the resulting change in the chemical activity. Stress affects more strongly $\\pi$ bonds that can become chemically active and bind to adsorbed species more strongly. Upon stretch, single C bonds are activated in a geometry mixing $120^{o}$ and $90^{o}$; an intermediate state between $sp^{2}$ and $sp^{3}$ bonding. We use ab-initio density functional theory to study the adsor...
Ab initio molecular dynamics study of liquid sodium and cesium up to critical point
Yuryev, Anatoly A.; Gelchinski, Boris R.
2015-08-01
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.
Pride, Prejudice, and Penury of {\\it ab initio} transport calculations for single molecules
Evers, F.; K. Burke
2006-01-01
Recent progress in measuring the transport properties of individual molecules has triggered a substantial demand for {\\it ab initio} transport calculations. Even though program packages are commercially available and placed on custom tailored to address this task, reliable information often is difficult and very time consuming to attain in the vast majority of cases, namely when the molecular conductance is much smaller than $e^2/h$. The article recapitulates procedures for molecular transpor...
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.; Hansen, Poul Erik
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 method enables the determination of both the electronic and the protonic (deuteronic) wave functions simultaneously and can directly calculate the geometrical difference induced by H/D isotope effects. The...
i-PI: A Python interface for ab initio path integral molecular dynamics simulations
Ceriotti, M.; More, J; Manolopoulos, DE
2014-01-01
Recent developments in path integral methodology have significantly reduced the computational expense of including quantum mechanical effects in the nuclear motion in ab initio molecular dynamics simulations. However, the implementation of these developments requires a considerable programming effort, which has hindered their adoption. Here we describe i-PI, an interface written in Python that has been designed to minimise the effort required to bring state-of-the-art path integral techniques...
Expanded phase of ZrO2: An ab initio constant-pressure study
International Nuclear Information System (INIS)
The stability of ZrO2 in the tensile regime is studied using an ab initio constant-pressure technique. A first-order phase transformation from the baddeleyite structure to an anatase-like structure (I41/amd) is predicted through the constant-pressure simulation. A transformation mechanism at the atomistic level is discussed. Furthermore this phase transformation is studied from the energy-volume calculations. (author)
Expanded phase of ZrO{sub 2}: An ab initio constant-pressure study
Energy Technology Data Exchange (ETDEWEB)
Durandurdu, M. [Texas at El Paso - El Paso Univ., Dept. of Physics, TX (United States); Ahi Evran Univ., Fizik Bolumu, Kirsehir (Turkey)
2009-12-15
The stability of ZrO{sub 2} in the tensile regime is studied using an ab initio constant-pressure technique. A first-order phase transformation from the baddeleyite structure to an anatase-like structure (I4{sub 1}/amd) is predicted through the constant-pressure simulation. A transformation mechanism at the atomistic level is discussed. Furthermore this phase transformation is studied from the energy-volume calculations. (author)
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 calculation of the ${}^6Li$ binding energy with the Hybrid Multideterminant scheme
Puddu, Giovanni
2010-01-01
We perform an ab-initio calculation for the binding energy of ${}^6Li$ using the CD-Bonn 2000 NN potential renormalized with the Lee-Suzuki method. The many-body approach to the problem is the Hybrid Multideterminant method. The results indicate a binding energy of about $31 MeV$, within a few hundreds KeV uncertainty. The center of mass diagnostics are also discussed.
Projector augmented wave method: ab initio molecular dynamics with full wave functions
Indian Academy of Sciences (India)
Peter E BlÃ¶chl; Clemens J FÃ¶rst; 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.
Ab initio calculation of Quasi-equilibrium microcracks in cubic boron nitride
International Nuclear Information System (INIS)
A crack in cubic boron nitride crystal was simulated within ab initio density-functional theory. The lattice constant, critical stress intensity factor Kc, which describes crack creation stress, and fracture surface energy ? were calculated from the elongated 2 x 6 x 1 supercell and these quantities agree quite well with the experimental data. We conclude that density-functional theory can be used to estimate crack's material constants. (author)
Ab initio study of the work functions of elemental metal crystals
Fall, Caspar
1999-01-01
This thesis is concerned with the theoretical study of the physical characteristics of metallic surfaces. Ab initio quantum calculations are performed to determine the electronic properties of clean elemental metal surfaces, fully accounting for the material's atomic structure. We aim to understand the atomicscale mechanisms responsible for the dependence of the work function on the surface geometry, including the crystallographic orientation of the surface, the atomic relaxation and reconstr...
Recent developments in ab-initio few-body scattering calculations including the Coulomb interaction
International Nuclear Information System (INIS)
Recent progress on the solution of ab-initio three- and four-nucleon scattering equations in momentum space that include the correct treatment of the Coulomb interaction is reviewed; results for specific observables in reactions initiated by p + d, p + 3He and n + 3He are shown. In addition three-body calculations of d + 12C reactions are compared with equivalent CDCC calculations. Presented at the 20th Few-Body Conference, Pisa, Italy, 10-14 September 2007 (author)
Ab initio calculations of ideal tensile strength and mechanical stability in copper.
Czech Academy of Sciences Publication Activity Database
?erný, M.; Šob, Mojmír; Pokluda, J.; Šandera, P.
2004-01-01
Ro?. 16, ?. 7 (2004), s. 1045-1052. ISSN 0953-8984 R&D Projects: GA ?R GA202/03/1351; GA AV ?R IAA1041302 Grant ostatní: GA MŠk1(CZ) 262100002 Institutional research plan: CEZ:AV0Z2041904 Keywords : ab initio calculations * electronic structure * theoretical tensile strength Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 2.049, year: 2004
Ab initio study of phase transformations in transition-metal disilicides.
Czech Academy of Sciences Publication Activity Database
Ká?a, Tomáš; Šob, Mojmír; Vitek, V.
2011-01-01
Ro?. 19, ?. 7 (2011), s. 919-926. ISSN 0966-9795 R&D Projects: GA AV ?R 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
Advances and applications in the FIREBALL ab initio tight-binding molecular-dynamics formalism.
Czech Academy of Sciences Publication Activity Database
Lewis, J.P.; Jelínek, Pavel; Ortega, J.; Demkov, A.A.; Trabada, D.G.; Haycock, B.; Wang, H.; Adams, G.; Tomfohr, J.K.; Abad, E.; Wang, Ho.; Drabold, D.A.
2011-01-01
Ro?. 248, ?. 9 (2011), 1989-2007. ISSN 0370-1972 R&D Projects: GA ?R GA202/09/0545; GA ?R GAP204/10/0952 Grant ostatní: AV?R(CZ) M100100904 Institutional research plan: CEZ:AV0Z10100521 Keywords : DFT * ab initio molecular-dynamics Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 1.316, year: 2011
Modelling of nuclear glasses by classical and ab initio molecular dynamics
International Nuclear Information System (INIS)
A calcium aluminosilicate glass of molar composition 67 % SiO2 - 12 % Al2O3 - 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)
International Nuclear Information System (INIS)
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 up 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 vibrational free energy of the FCC phase of dense hydrogen at 300 K is also calculated with an AI-PIMD thermodynamic integration method, which gives a result of about 0.51 eV/proton at a density of rs=0.912
Energy Technology Data Exchange (ETDEWEB)
Geng, Hua Y., E-mail: huay.geng@gmail.com [National Key Laboratory of Shock Wave and Detonation Physics, Institute of Fluid Physics, CAEP, P.O. Box 919-102, Mianyang, Sichuan, 621900 (China); Department of Chemistry and Chemical Biology, Cornell University, Baker Laboratory, Ithaca, NY 14853 (United States)
2015-02-15
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 up 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 vibrational free energy of the FCC phase of dense hydrogen at 300 K is also calculated with an AI-PIMD thermodynamic integration method, which gives a result of about 0.51 eV/proton at a density of r{sub s}=0.912.
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...
Geng, Hua Y.
2015-02-01
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 up 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 vibrational free energy of the FCC phase of dense hydrogen at 300 K is also calculated with an AI-PIMD thermodynamic integration method, which gives a result of about 0.51 eV/proton at a density of rs = 0.912.
Ab initio study of the reaction of ozone with bromide ion.
Gladich, Ivan; Francisco, Joseph S; Buszek, Robert J; Vazdar, Mario; Carignano, Marcelo A; Shepson, Paul B
2015-05-14
Surface level ozone destruction in polar environments may be initiated by oxidation of bromide ions by ozone, ultimately leading to Br2 production. Ab initio calculations are used to support the development of atmospheric chemistry models, but errors can occur in study of the bromide-ozone reaction due to inappropriate treatment of the many-electron species and the ionic nature of the reaction. In this work, a high level ab initio study is used to take into account the electronic correlation and the polarization effects. Our results show three possible pathways for the reaction. In particular, we find that this process, though endothermic on the singlet spin state surface, can be energetically feasible on the triplet surface. The triplet surface can be reached through photoexcitation of ozone or by the spin crossing of the potential energy surface. Because this process is known to occur in the dark, it may be that it occurs after intersystem crossing to a triplet surface. This paper also provides a starting point calibration for any future ab initio calculation studies of the bromide-ozone reaction, from the gas to the condensed phase. PMID:25642913
Thermodynamic evaluation of the Np-Zr system using CALPHAD and ab initio methods
Xiong, Wei; Xie, Wei; Morgan, Dane
2014-09-01
A new thermodynamic description of Np-Zr alloys is developed using the CALPHAD (CALculation of PHAse Diagrams) method based on available experimental information on phase equilibria, supplemented by ab initio energetics for end members that are not available in the standard CALPHAD unary database. The present thermodynamic description shows improvements compared to previous models in the predicted phase diagram when comparing to assessed reliable experimental data. Ab initio Density Functional Theory (DFT) calculations are also performed on all known stable solid phases of Np-Zr alloys and the end member Np and Zr metals. Comparing to the formation energetics predicted from the CALPHAD models of both this work and a previous study we find that DFT with the Generalized Gradient Approximation (GGA) to the exchange-correlation potential overestimates the formation enthalpies of Np and Np-Zr by about 0.15 eV/atom, and the so-called DFT + U approach with a Ueff of near 0.65 eV can reduce this error by about 0.07-0.10 eV. Our comprehensive comparison between existing CALPHAD, ab initio and experimental results for Np-Zr indicates a need for further experiments on the phase equilibrium.
Modeling of nuclear glasses by classical and ab initio molecular dynamics
International Nuclear Information System (INIS)
A calcium aluminosilicate glass of molar composition 67 % SiO2 - 12 % Al2O3 - 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 aluminum 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)
International Nuclear Information System (INIS)
Accurate ab initio multireference configuration interaction (CI) calculations with large correlation-consistent basis sets are performed for HOCl. After extrapolation to the complete basis set limit, the ab initio data are precisely fit to give a semiglobal three-dimensional potential energy surface to describe HOCl?Cl+OH from high overtone excitation of the OH-stretch. The average absolute deviation between the ab initio and fitted energies is 4.2thinspcm-1 for energies up to 60 kcal/mol relative to the HOCl minimum. Vibrational energies of HOCl including the six overtones of the OH-stretch are computed using a vibrational-Cl method on the fitted potential and also on a slightly adjusted potential. Near-spectroscopic accuracy is obtained using the adjusted potential; the average absolute deviation between theory and experiment for 19 experimentally reported states is 4.8thinspcm-1. Very good agreement with experiment is also obtained for numerous rotational energies for the ground vibrational state, the ClO-stretch fundamental, and the fifth overtone of the OH-stretch. copyright 1998 American Institute of Physics
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)
Ab initio rovibrational structure of the lowest singlet state of O2-O2
Carmona-Novillo, Estela; Bartolomei, Massimiliano; Hernández, Marta I.; Campos-Martínez, José; Hernández-Lamoneda, Ramón
2012-09-01
Rovibrational bound states of the O_2(^3? ^-_g,v = 0)-O_2(^3? ^-_g,v = 0) dimer in its singlet electronic state have been obtained by solving the time-independent Schrödinger equation for the nuclear degrees of freedom. We have employed two different ab initio potential energy surfaces, based on high level multiconfigurational methods, which are expected to give upper and lower bounds for the real values of the interaction. Results are compared with spectroscopy experiments as well as with calculations using other semi ab initio and empirical interaction potentials. For the two ab initio potentials studied here, the ground vibrational state has a rectangular geometry and behaves as a semi-rigid molecule. The associated rotational constant is found in very good agreement with high resolution spectra. However, the computed dissociation energy and the frequency of the torsion mode are larger than previous experimental determinations, and possible reasons for these discrepancies are discussed. On the other hand, we have computed the splitting between the rovibrational states of the singlet and triplet electronic states and have found a fair agreement with measurements of the dimer spectra in a solid rare gas host.
Ab initio nuclear structure and reactions with chiral three-body forces
Energy Technology Data Exchange (ETDEWEB)
Langhammer, Joachim; Roth, Robert; Calci, Angelo [Institut fuer Kernphysik - Theoriezentrum, TU Darmstadt (Germany); Navratil, Petr [TRIUMF, Vancouver (Canada)
2014-07-01
One major ambition of ab initio nuclear theory is the description of nuclear-structure and reaction observables on equal footing. This is accomplished by combining the no-core shell model (NCSM) with the resonating-group method (RGM) to a unified ab initio approach to bound and continuum states, which is developed further to the no-core shell model with continuum (NCSMC). We present the formal developments to include three-nucleon interactions in both the NCSM/RGM and NCSMC formalism. This provides the possibility to assess the predictive power of chiral two- and three-nucleon forces in the variety of scattering observables. We study three-nucleon force effects on phase-shifts, cross sections and analyzing powers in first ab-initio studies of nucleon-{sup 4}He scattering with chiral two- and three-nucleon forces. Finally, we focus on heavier target nuclei using the NCSMC, e.g., in neutron-{sup 8}Be scattering and study the impact of the continuum on the spectrum of {sup 9}Be.
Atomic-like shell models for alkali trimers derived from ab initio calculations
International Nuclear Information System (INIS)
Full text: Alkali metal clusters have received great attention due to their role as bridge between atomic and solid state physics. Among the smallest clusters, the trimers are of special interest, since these systems provide complex spectra including Jahn-Teller distortions, yet the spectra are well defined and still accessible via ab initio calculations. The experimental spectra, as well as ab initio calculations, show a regular pattern of electronic states. High level ab initio calculations [CCSD(T), CASPT2] provide detailed information about the participating electronic orbitals, and allow us to rationalize the observed patterns in terms of simplified shell models. For the low-spin states of K3 the standard electron-droplet model offers a qualitative explanation. In this simplified picture the electronic states are interpreted as single electron excitations into delocalized molecular orbitals with typical atomic-like shape. For the description of the quartet manifolds of K3, K2Rb, KRb2 and Rb3 we utilize the eigenstates of a harmonic oscillator in a quantum-dot-like confining potential. (author)
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.
7Be(p,gamma)8B S-factor from Ab Initio Wave Functions
Energy Technology Data Exchange (ETDEWEB)
Navratil, P; Bertulani, C A; Caurier, E
2006-10-12
There has been a significant progress in ab initio approaches to the structure of light nuclei. Starting from realistic two- and three-nucleon interactions the ab initio no-core shell model (NCSM) predicts low-lying levels in p-shell nuclei. It is a challenging task to extend ab initio methods to describe nuclear reactions. We present here a brief overview of the first steps taken toward nuclear reaction applications. In particular, we discuss our calculation of the {sup 7}Be(p,{gamma}){sup 8}B S-factor. We also present our first results of the {sup 3}He({alpha},{gamma}){sup 7}Be S-factor and of the S-factor of the mirror reaction {sup 3}H({alpha},{gamma}){sup 7}Li. The {sup 7}Be(p,{gamma}){sup 8}B and {sup 3}He({alpha},{gamma}){sup 7}Be reactions correspond to the most important uncertainties in solar model predictions of neutrino fluxes.
International Nuclear Information System (INIS)
The deuterium quadrupole coupling constant, ?D, in the PDO32- anion has been measured in solution by NMR spin-lattice (T1) relaxation time measurements and it has been calculated via ab initio methods. The experimental value of 94.7 ± 0.5 kHz is in excellent agreement with the ab initio value of 95.0 kHz. The activation energy for the ion reorientation is 2.23 ± 0.01 kJ mol-1
Heuvel, Willem Van den; Calvello, Simone; Soncini, Alessandro
2015-01-01
We present an ab initio methodology dedicated to the determination of the electronic structure and magnetic properties of ground and low-lying excited states, i.e., the crystal field levels, in lanthanide(III) complexes. Currently, the most popular and successful ab initio approach is the CASSCF/RASSI-SO method, consisting of the optimization of multiple complete active space self-consistent field (CASSCF) spin eigenfunctions, followed by full diagonalization of the spin--orbit coupling (SOC)...
Kamarulzaman Abdul Ghani; Mohammad Shiham Mahfuz; Abdul Jamir Md. Saad; Nik Mohd. Rahimi Nik Yusoff
2014-01-01
This study was aimed to investigate the relationship between Language Learning Strategies (LLS) and the level of proficiency of students in Arabic Ab Initio language. There were also several studies done to investigate the achievement of students and their frequent usage of these Language Learning Strategies. Meanwhile, the level of student proficiency was measured by their results based on college-level exam results before they were registered in the Arabic Ab Initio classes. 162 year 1 stud...
Physical origin of satellite in photoemission of doped graphene: An ab-initio GW plus cumulant study
Lischner, Johannes; Vigil-Fowler, Derek; Louie, Steven G.
2013-01-01
We calculate the photoemission spectra of suspended and epitaxial doped graphene using an ab initio cumulant expansion of the Green's function based on the GW self-energy. Our results are compared to experiment and to standard GW calculations. For doped graphene on a silicon carbide substrate, we find, in contrast to earlier calculations, that the spectral function from GW only does not reproduce experimental satellite properties. However, ab initio GW plus cumulant theory c...
International Nuclear Information System (INIS)
The interaction potential energy surface (PES) of H4 is of great importance for quantum chemistry, as a test case for molecule--molecule interactions. It is also required for a detailed understanding of certain astrophysical processes, namely, collisional excitation and dissociation of H2 in molecular clouds, at densities too low to be accessible experimentally. Accurate ab initio energies were computed for 6046 conformations of H4, using a multiple reference (single and) double excitation configuration interaction (MRD-CI) program. Both systematic and ''random'' errors were estimated to have an rms size of 0.6 mhartree, for a total rms error of about 0.9 mhartree (or 0.55 kcal/mol) in the final ab initio energy values. It proved possible to include in a self-consistent way ab initio energies calculated by Schwenke, bringing the number of H4 conformations to 6101. Ab initio energies were also computed for 404 conformations of H3; adding ab initio energies calculated by other authors yielded a total of 772 conformations of H3. (The H3 results, and an improved analytic PES for H3, are reported elsewhere.) Ab initio energies are tabulated in this paper only for a sample of H4 conformations; a full list of all 6101 conformations of H4 (and 772 conformations of H3 ) is available from Physics Auxiliary Publication Service (PAPS), or from the authors
Iron -chromium alloys and free surfaces: from ab initio calculations to thermodynamic modeling
International Nuclear Information System (INIS)
Ferritic steels possibly strengthened by oxide dispersion are candidates as structural materials for generation IV and fusion nuclear reactors. Their use is limited by incomplete knowledge of the iron-chromium phase diagram at low temperatures and of the phenomena inducing preferential segregation of one element at grain boundaries or at surfaces. In this context, this work contributes to the multi-scale study of the model iron-chromium alloy and their free surfaces by numerical simulations. This study begins with ab initio calculations of properties related to the mixture of atoms of iron and chromium. We highlight complex dependency of the magnetic moments of the chromium atoms on their local chemical environment. Surface properties are also proving sensitive to magnetism. This is the case of impurity segregation of chromium in iron and of their interactions near the surface. In a second step, we construct a simple energy model for high numerical efficiency. It is based on pair interactions on a rigid lattice to which are given local chemical environment and temperature dependencies. With this model, we reproduce the ab initio results at zero temperature and experimental results at high temperature. We also deduce the solubility limits at all intermediate temperatures with mean field approximations that we compare to Monte Carlo simulations. The last step of our work is to introduce free surfaces in our model. We then study the effect of ab initio calculated bulk and surface properties on surface segregation.Finally, we calculate segregation isotherms. We therefore propose an evolution model of surface composition of iron-chromium alloys as a function of bulk composition. which are given local chemical environment and temperature dependencies. With this model, we reproduce the ab initio results at zero temperature and experimental results at high temperature. We also deduce the solubility limits at all intermediate temperatures with mean field approximations that we compare to Monte Carlo simulations. The last step of our work is to introduce free surfaces in our model. We then study the effect of ab initio calculated bulk and surface properties on surface segregation. Finally, we calculate segregation isotherms. We therefore propose an evolution model of surface composition of iron-chromium alloys as a function of bulk composition. (author)
Vogel, Eckhard; Hellmann, Robert; Bich, Eckard
2008-01-01
Abstract A neon-neon interatomic potential energy curve was derived from quantum-mechanical ab initio calculations using basis sets of up to t-aug-cc-pV6Z quality supplemented with bond functions and ab initio methods up to CCSDT(Q). In addition, corrections for relativistc effects were determined. An analytical potential function was fitted to the ab initio values and utilized to calculate the rovibrational spectra. The quality of the interatomic potential function was tested by c...
A new ab initio ground-state dipole moment surface for the water molecule.
Lodi, Lorenzo; Tolchenov, Roman N; Tennyson, Jonathan; Lynas-Gray, A E; Shirin, Sergei V; Zobov, Nikolai F; Polyansky, Oleg L; Császár, Attila G; van Stralen, Joost N P; Visscher, Lucas
2008-01-28
A valence-only (V) dipole moment surface (DMS) has been computed for water at the internally contracted multireference configuration interaction level using the extended atom-centered correlation-consistent Gaussian basis set aug-cc-pV6Z. Small corrections to these dipole values, resulting from core correlation (C) and relativistic (R) effects, have also been computed and added to the V surface. The resulting DMS surface is hence called CVR. Interestingly, the C and R corrections cancel out each other almost completely over the whole grid of points investigated. The ground-state CVR dipole of H(2) (16)O is 1.8676 D. This value compares well with the best ab initio one determined in this study, 1.8539+/-0.0013 D, which in turn agrees well with the measured ground-state dipole moment of water, 1.8546(6) D. Line intensities computed with the help of the CVR DMS shows that the present DMS is highly similar to though slightly more accurate than the best previous DMS of water determined by Schwenke and Partridge [J. Chem. Phys. 113, 16 (2000)]. The influence of the precision of the rovibrational wave functions computed using different potential energy surfaces (PESs) has been investigated and proved to be small, due mostly to the small discrepancies between the best ab initio and empirical PESs of water. Several different measures to test the DMS of water are advanced. The seemingly most sensitive measure is the comparison between the ab initio line intensities and those measured by ultralong pathlength methods which are sensitive to very weak transitions. PMID:18247946
Ab initio theories of electric transport in solid systems with reduced dimensions
International Nuclear Information System (INIS)
Ab initio theories of electric transport in solid systems with reduced dimensions, i.e., systems that at best are characterized by two-dimensional translational invariance, are reviewed in terms of a fully relativistic description of the Kubo-Greenwood equation. As the use of this equation requires concepts such as collinearity and non-collinearity in order to properly define resistivities or resistances corresponding to particular magnetic configurations, respective consequences of the (local) density functional theory are recalled in quite a detailed manner. Furthermore, since theoretical descriptions of solid systems with reduced dimensions require quantum mechanical methods different from bulk systems (three-dimensional periodicity), the so-called Screened Korringa-Kohn-Rostoker (SKKR-) method for layered systems is introduced together with a matching coherent potential approximation (inhomogeneous CPA). The applications shown are mainly meant to illustrate various aspects of electric transport in solid systems with reduced dimensions and comprise not only current-in-plane (CIP) experiments, but also current perpendicular to the planes of atoms geometries, consequences of tunneling, and finite nanostructures at or on metallic substrates. In order to give a more complete view of available ab initio methods also a non-relativistic approach based on the Tight Binding Linear Combination of muffin tin orbitals (TB-LMTO-) method and the so-called Kubo-Landauer equation in terms of transmission and reflection matrices is presented. A compilation of references with respect to ab-initio type approaches not explicitly discussed in here finally concludes the discussion of electric properties in solid systems with reduced dimensions
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(1s?g)2, H2+(1s?g), H2+(2p?u) and (LiH2)+ energy surfaces are also presented. The new parametric fits for H2, H2+(1s?g) are shown to be improvements over the well-known Morse potentials for these surfaces
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
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
Formation and stability of cellular carbon foam structures: an ab initio study.
Zhu, Zhen; Tománek, David
2012-09-28
We use ab initio density functional calculations to study the formation and structural as well as thermal stability of cellular foamlike carbon nanostructures. These systems with a mixed sp(2)/sp(3) bonding character may be viewed as bundles of carbon nanotubes fused to a rigid contiguous 3D honeycomb structure that can be compressed more easily by reducing the symmetry of the honeycombs. The foam may accommodate the same type of defects as graphene, and its surface may be stabilized by terminating caps. We postulate that the foam may form under nonequilibrium conditions near grain boundaries of a carbon-saturated metal surface. PMID:23030101
Ab initio study of spin-dependent transport in carbon nanotubes with iron and vanadium adatoms
DEFF Research Database (Denmark)
Fürst, Joachim Alexander; Brandbyge, Mads; Jauho, Antti-Pekka; Stokbro, Kurt
2008-01-01
We present an ab initio study of spin-dependent transport in armchair carbon nanotubes with transition metal adsorbates: iron or vanadium. The method based on density functional theory and nonequilibrium Green's functions is used to compute the electronic structure and zero-bias conductance. The presence of the adsorbate causes scattering of electrons of mainly one spin type. The scattering is shown to be due to a coupling of the two armchair band states to the metal 3d orbitals with matching sy...
Directory of Open Access Journals (Sweden)
Paulo Meneghelli
2001-01-01
Full Text Available Possible reasons of low reactivities of some N-(2-X-benzoyl imidazoles utilising ab initio Hartree-Fock calculations have been studied in imidazole-catalyzed hydrolysis. The elucidation of its molecular structures display that the inhibition of resonance, between the carbonyl group and the aromatic moiety, due to steric effects are very much important than that a possible stabilization of the reactant state by a donor-acceptor effects interaction between the orto nitro group of benzene ring and the imidazole ring.
Ab initio calculation of electronic and optical properties of metallic tin
International Nuclear Information System (INIS)
The electronic and optical properties of the metallic bcc and ?-Sn phases of tin are studied using density functional theory. The effects of spin-orbit coupling are examined and significant splittings are found in the band structures for both phases. Based on ab initio band structures we calculate the anisotropic optical response of ?-Sn. Both intra- and interband contributions are included and the plasma frequencies for both the ordinary and extraordinary optical axis are calculated. The theoretical results are found to be in excellent agreement with experimental spectra for the anisotropic optical response. We identify the electronic transitions responsible for the dominant interband resonances in the near-infrared response.
Ab initio studies of the tunneling magneto-Seebeck effect: influence of magnetic material
Heiliger, Christian; Franz, Christian; Czerner, Michael
2013-01-01
We found a strong influence of the composition of the magnetic material on the temperature dependence of the tunneling magneto-Seebeck effect in $MgO$ based tunnel junctions. We use \\textit{ab initio} alloy theory to consider different $Fe_xCo_{1-x}$ alloys for the ferromagnetic layer. Even a small change of the composition leads to strong changes in the magnitude or even in the sign of the tunneling magneto-Seebeck effect. This can explain differences between recent experim...
Shaughnessy, M C; Jones, R E
2016-02-01
We develop and demonstrate a method to efficiently use density functional calculations to drive classical dynamics of complex atomic and molecular systems. The method has the potential to scale to systems and time scales unreachable with current ab initio molecular dynamics schemes. It relies on an adapting dataset of independently computed Hellmann-Feynman forces for atomic configurations endowed with a distance metric. The metric on configurations enables fast database lookup and robust interpolation of the stored forces. We discuss mechanisms for the database to adapt to the needs of the evolving dynamics, while maintaining accuracy, and other extensions of the basic algorithm. PMID:26669825
Fertitta, E; 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 orbitals in the Li and Be rings and show that the transition bond length can be detected using orbital entropy functions. Also, the effect of different orbital basis on the effectiveness of the DMRG procedure is analyzed comparing the convergence behavior.
Thermodynamic and ab initio investigation of the A1-H-Mg system
Baricco, Marcello
2007-01-01
A coupled ab initio and thermodynamic study of the Al-H-Mg system has been carried out and a self-consistent thermodynamic database has been obtained. Magnesium alanate Mg(AlH4)2, a candidate material for hydrogen storage, has been included into the database. According to Density Functional first principles calculations, the alanate is an insulator and its thermodynamic properties have been obtained at room temperature. This compound has been found metastable at 298.15 K and 1 bar. The alanat...
Ab initio calculations of {sup 14}N and {sup 15}N hyperfine structures
Energy Technology Data Exchange (ETDEWEB)
Joensson, P [Center for Technology Studies, Malmoe University, 205-06 Malmoe (Sweden); Carette, T; Godefroid, M [Chimie Quantique et Photophysique, CP160/09, Universite Libre de Bruxelles, Av. F.D. Roosevelt 50, B-1050 Brussels (Belgium); Nemouchi, M, E-mail: mrgodef@ulb.ac.b [Laboratoire d' Electronique Quantique, Faculte de Physique, USTHB, BP32, El-Alia, Algiers (Algeria)
2010-06-14
Hyperfine structure parameters are calculated for the 2p{sup 2}({sup 3}P)3s {sup 4}P{sub J}, 2p{sup 2}({sup 3}P)3p {sup 4}P{sup o}{sub J} and 2p{sup 2}({sup 3}P)3p {sup 4}D{sup o}{sub J} levels, using the ab initio multiconfiguration Hartree-Fock method. The theoretical hyperfine coupling constants are in complete disagreement with the experimental values of Jennerich et al deduced from the analysis of the near-infrared Doppler-free saturated absorption spectra.
A study on defect energy in UO{sub 2} lattice using ab initio DFT
Energy Technology Data Exchange (ETDEWEB)
Kim, Sung Ryul; Kim, Yong Uhn [Chungbuk National University, Cheongju (Korea, Republic of); Kim, Hee Moon; Park, Chang Je; Jin, Young Gwan; Ahn, Sang Bok [Korea Atomic Energy Research Institute, Daejeon (Korea, Republic of); Yun, Youn Suk [Paul Scherrer Institute, Villigen (Switzerland); Park, Kwang Heon [Kyunghee University, Yongin (Korea, Republic of)
2012-05-15
The behavior of fission gases is an important limiting factor for the nuclear fuel performance, as fission gas release affects the pressure and temperature of the fuel rod. The point defects are a major diffusion channel for fission gases. The defect energies and mobility energies are needed to know the fission gas mobility in point defect. In this study, we performed ab initio total energy calculations to investigate the defect energies and employed a large super cell containing 96 atoms to decrease defect-defect interaction. The formation energy of intrinsic defects indicates that the accuracy of the present modeling is significantly improved, compared with previous studies
Ab Initio Cosmological Simulations of CR7 as an Active Black Hole
Smidt, Joseph; Wiggins, Brandon K.; Johnson, Jarrett L.
2016-01-01
We present the first ab initio cosmological simulations of a CR7-like object which approximately reproduce the observed line widths and strengths. In our model, CR7 is powered by a massive (2.23 x 10^7 M_Sun), accreting (0.25 Eddington) supermassive black hole (BH). Our model takes into account multi-dimensional effects, X-ray feedback, secondary ionizations and primordial chemistry. We estimate Ly-alpha line widths by post-processing simulation output with Monte Carlo radiative transfer and ...
An Ab Initio MP2 Study of HCN-HX Hydrogen Bonded Complexes
Araújo Regiane C.M.U.; Ramos Mozart N.
1998-01-01
An ab initio MP2/6-311++G** study has been performed to obtain geometries, binding energies and vibrational properties of HCN-HX H-bonded complexes with X = F, Cl, NC, CN and CCH. These MP2/6-311++G** results have revealed that: (i) the calculated H-bond lengths are in very good agreement with the experimental ones; (ii) the H-bond strength is associated with the intermolecular charge transfer and follows the order: HCN-HNC ~ HCN-HF > HCN-HCl ~ HCN-HCN > HCN-HCCH; (iii) BSSE correction introd...
Synthesis and Ab Initio/DFT Studies on 2-(4-methoxyphenyl)benzo[d]thiazole
ÃƒÂ–ztekin AlgÃƒÂ¼l; Hakan Arslan
2007-01-01
2-aminothiophenol and 4-methoxybenzaldehyde were cyclized under microwave irridation and solvent free conditions to synthesize 2-(4-methoxyphenyl)benzo[d]thiazole. The molecular structure and vibrational frequencies of the title compound in the ground state have been investigated with ab initio (HF) and density functional theory methods (BLYP, B3LYP, B3PW91 and mPW1PW91) implementing the standard 6–311G(d,p) basis set. Comparison of the observed fundamental vibrational frequencies of title co...
Ab initio calculation of the vibrationally resolved O 1s photoelectron spectrum of CO2
Hahne, J. A.; Carroll, T. X.; Thomas, T. D.
1998-06-01
The oxygen 1s photoelectron spectrum for CO2 has been calculated using ab initio electronic structure theory, a localized hole simulated by the equivalent-cores approximation, and the harmonic-oscillator approximation. The predicted spectrum is found to be in good agreement with a recently reported experimental spectrum, and is in better agreement with the experimental spectrum than is an earlier prediction by Domcke and Cederbaum. The equivalence of the approach taken here and that used by Cederbaum and Domcke is discussed, as well as the reasons for the differences in the calculated results.
International Nuclear Information System (INIS)
Using Kubo's linear response theory, we derive expressions for the frequency-dependent electrical conductivity (Kubo-Greenwood formula), thermopower, and thermal conductivity in a strongly correlated electron system. These are evaluated within ab initio molecular dynamics simulations in order to study the thermoelectric transport coefficients in dense liquid hydrogen, especially near the nonmetal-to-metal transition region. We also observe significant deviations from the widely used Wiedemann-Franz law, which is strictly valid only for degenerate systems, and give an estimate for its valid scope of application toward lower densities.
International Nuclear Information System (INIS)
The electronic transport properties of monolayer graphene with extreme physical deformation are studied using ab initio calculations. The aim is to explore the influence of physical bending on transport properties and identify the most important geometrical parameter. The transmission spectra are relatively insensitive to the geometrical parameters in low-energy regions-even in the extreme case of uniaxial bending. The results suggest that graphene, with its superb electromechanical robustness, could serve as a viable nanoscale material platform in a wide spectrum of applications such as photovoltaics, flexible electronics, and 3D carbon chips.
International Nuclear Information System (INIS)
We present exemplary results of extensive studies of mechanical, electronic and transport properties of covalent functionalization of graphene monolayers (GML). We report new results of ab initio studies for covalent functionalization of GML with ?NH2 groups up to 12.5 % concentration. Our studies are performed in the framework of the density functional theory (DFT) and non-equilibrium Green’s function (NEGF). We discuss the stability (adsorption energy), elastic moduli, electronic structure, band gaps, and effective electron masses as a function of the density of the adsorbed molecules. We also show the conductance and current – voltage I(V) characteristics for these systems
Ab-initio molecular treatment of the symmetric Mg2+-Mg charge transfer process
Amami, M.; Moussa, A.; Zaidi, A.; Lahmar, S.; Bacchus-Montabonel, M. C.
2016-02-01
Potential energy curves of the MgMg2+ molecular system in the lowest electronic states as well as radial and rotational couplings between these states are obtained from ab-initio calculations at the CASSCF/MRCI level of theory using large basis sets. These data are used to investigate, via the semi-classical molecular close coupling method, the single symmetric charge transfer processes of Mg2+-Mg collisions in the [1.0-650] keV laboratory energy range. Total and partial cross sections for the expected capture channels are calculated and compared with the available experimental and theoretical results.
Testing the density matrix expansion against ab initio calculations of trapped neutron drops
International Nuclear Information System (INIS)
Microscopic input to a universal nuclear energy density functional can be provided through the density matrix expansion (DME), which has recently been revived and improved. Several DME implementation strategies are tested for neutron drop systems in harmonic traps by comparing to Hartree-Fock (HF) and ab initio no-core full configuration (NCFC) calculations with a model interaction (Minnesota potential). The new DME with exact treatment of Hartree contributions is found to best reproduce HF results and supplementing the functional with fit Skyrme-like contact terms shows systematic improvement toward the full NCFC results.
Testing the density matrix expansion against ab initio calculations of trapped neutron drops
Bogner, S K; Hergert, H; Kortelainen, M; Maris, P; Stoitsov, M; Vary, J P
2011-01-01
Microscopic input to a universal nuclear energy density functional can be provided through the density matrix expansion (DME), which has recently been revived and improved. Several DME implementation strategies are tested for neutron drop systems in harmonic traps by comparing to Hartree-Fock (HF) and ab initio no-core full configuration (NCFC) calculations with a model interaction (Minnesota potential). The new DME with exact treatment of Hartree contributions is found to best reproduce HF results and supplementing the functional with fit Skyrme-like contact terms shows systematic improvement toward the full NCFC results.
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.; Shim, Irene; Thompson, J.M.
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 calculations. The previously reported anti and gauche forms of the [C(4)mim](+) cation have been observed, and this study reveals this to be a general feature of the long-chain I-alkyl derivatives. Analysis o...
Ab initio theory of NMR chemical shifts in solids and liquids
Mauri, F; Louie, S G; Mauri, Francesco; Pfrommer, Bernd G.; Louie, Steven G.
1996-01-01
We present a theory for the ab-initio computation of NMR chemical shifts (sigma) in condensed matter systems, using periodic boundary conditions. Our approach can be applied to periodic systems such as crystals, surfaces, or polymers and, with a super-cell technique, to non-periodic systems such as amorphous materials, liquids, or solids with defects. We have computed the hydrogen sigma for a set of free molecules, for an ionic crystal, LiH, and for a H-bonded crystal, HF, using density functional theory in the local density approximation. The results are in excellent agreement with experimental data.
Ab initio calculation of the spectrum and structure of $^{16}$O
Epelbaum, Evgeny; Lähde, Timo A; Lee, Dean; Meißner, Ulf-G; Rupak, Gautam
2013-01-01
We present ab initio lattice calculations of the low-energy even-parity states of $^{16}$O using chiral nuclear effective field theory. We find good agreement with the empirical energy spectrum, and with the electromagnetic properties and transition rates. For the ground state, we find that the nucleons are arranged in a tetrahedral configuration of alpha clusters. For the first excited spin-0 state, we find that the predominant structure is a square configuration of alpha clusters, with rotational excitations that include the first spin-2 state.
Exploration for Two-Dimensional Electrides via Database Screening and Ab Initio Calculation
Inoshita, Takeshi; Jeong, Sehoon; Hamada, Noriaki; Hosono, Hideo
2014-07-01
Inspired by the recent demonstration that Ca2N is a two-dimensional electride in which electrons near the Fermi level float between ionic layers, we have conducted a systematic search for 2D electrides by an approach combining crystal structure databases and ab initio electronic structure calculations. Starting from a set of working hypotheses, we have identified six 2D metallic electrides, of which one is nonmagnetic and five are nearly ferromagnetic ferrimagnets with a distinct spin texture. The latter five materials are the first magnetic electrides reported at ambient pressure.
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.; Shim, Irene; Thompson, J.M.
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...... calculations. The previously reported anti and gauche forms of the [C(4)mim](+) cation have been observed, and this study reveals this to be a general feature of the long-chain I-alkyl derivatives. Analysis of mixtures Of [C(6)mim]Cl and [C(6)mim][PF6] has provided information on the nature of the hydrogen...
Tuning magnetic properties of antiferromagnetic chains by exchange interactions: ab initio studies.
Tao, Kun; Guo, Qing; Jena, Puru; Xue, Desheng; Stepanyuk, Valeri S
2015-10-21
The possibility of using exchange interactions to manipulate the spin state of an antiferromagnetic nanostructure is explored using ab initio calculations. By considering M (M = Mn, Fe, Co) mono-atomic chains supported on Cu2N islands on a Cu(001) surface as a model system, it is demonstrated that two indistinguishable Néel states of an antiferromagnetic chain can be tailored into a preferred state by the exchange interaction with a magnetic STM tip. The magnitude and direction of the anisotropy for antiferromagnetic chains can also be tuned by exchange coupling upon varying the tip-chain separation. PMID:26387802
"Ab initio" studies of hydrogen-enhanced oxygen diffusion in silicon
Scientific Electronic Library Online (English)
R. B., Capaz; L. V. C., Assali; L. C., Kimerling; K., Cho; J. D., Joannopoulos.
1999-12-01
Full Text Available A novel microscopic mechanism for hydrogen-enhanced oxygen diffusion in p-doped silicon is proposed. A path for joint diffusion of O and H is obtained from an ab-initio molecular dynamics "kick" simulation. The migration pathway consists of a two-step mechanism, with a maximum energy of 1.46 eV. Thi [...] s path represents a 0.54 eV reduction in the static barrier when compared with the diffusion of isolated O in Si, in excellent agreement with experiments.
Ab initio calculation of coverage-dependent adsorption properties of H on Pd(001)
International Nuclear Information System (INIS)
The preferential adsorption sites, bond lengths, and vibration frequencies for H on the Pd (001) surface are determined by use of the ab initio pseudopotential local-orbital method. From the calculated total energies we derive a coverage-dependent adsorption energy in very good agreement with experiment. We establish that at low coverages (theta?1), H occupies fourfold hollow sites. At higher coverages (1< theta?2), islands of bridge-bonded H (local coverage theta = 2) are predicted to coexist with regions of H in hollow sites
Enhancing mechanical toughness of aluminum surfaces by nano-boron implantation: An ab initio study
Zhu, Zhen; Kwon, Dae-Gyeon; Kwon, Young-Kyun; Tománek, David
2015-01-01
Searching for ways to enhance surface hardness of aluminum, we study the equilibrium structure, stability, elastic properties and formation dynamics of a boron-enriched surface using ab initio density functional calculations. We used molecular dynamics simulations to model the implantation of energetic boron nanoparticles in Al and identify structural arrangements that optimize the formation of strong covalent Bsbnd Al bonds. Nano-indentation simulations based on constrained optimization suggest that presence of boron nanostructures in the subsurface region enhances significantly the mechanical hardness of aluminum surfaces.
Ab initio studies of the electronic and transport properties of topological insulator-metal contacts
Spataru, Catalin; Leonard, Francois
2014-03-01
Topological insulators (TI) hold great promise for novel applications in electronics and optoelectronics. For such device applications, TIs need to be contacted with a metal for electron injection. Depending on the character and strength of the interaction, a metal contact can modify the properties of TI surface states and induce new states at the interface. In this work, we study via ab initio Density Functional Theory the electronic and transport properties of realistic interfaces between a thin film TI and several magnetic and non-magnetic metal surfaces. We will discuss how band topology, band bending and hybridization effects affect charge injection and the contact properties (Schotkky versus ohmic) of the interface.
ab initio calculations on alkali atom - rare gas Van Der Waals clusters
Energy Technology Data Exchange (ETDEWEB)
Lee, Bo Soon; Lee, Sung Yul [Kyunghee Univ., Yongin (Korea, Republic of)
2000-06-01
ab initio calculations are presented for M-Rg and M-Rg{sub 2} (M=Li, Na, Rg=He, Ar) can der Waals clusters. Internuclear distances and binding energies of LiHe, LiAr and NaAr obtained by all-electron MP2/(6-311++G(3df, 3pd)) method are in good agreement with experimental values. Calculated properties of LiHe{sub 2}, LiAr{sub 2}, NaHe{sub 2} and NaAr{sub 2} are also reported.
Atomic defects in D03-Ni3Sb: An ab initio study
Bester, G.; Meyer, B.; Fähnle, M.
1998-05-01
The effective formation energies and the effective formation volumes of atomic defects in the ordered compound NixSb1-x in the D03 structure are calculated by the statistical mechanics in combination with the ab initio electron theory. For the Sb-rich compounds the structural defect is the Ni vacancy on the ? sublattice. The stoichiometric compound is a pentadefect system where on the average four Ni vacancies on the ? sublattice and one Ni antistructure atom are generated. The calculations strongly support the interpretation of results from neutron-scattering experiments.
Ab initio no core calculations of light nuclei and preludes to Hamiltonian quantum field theory
Energy Technology Data Exchange (ETDEWEB)
Vary, J.P.; Maris, P.; /Iowa State U.; Shirokov, A.M.; /Iowa State U. /SINP, Moscow; Honkanen, H.; li, J.; /Iowa State U.; Brodsky, S.J.; /SLAC; Harindranath, A.; /Saha Inst.; Teramond, G.F.de; /Costa Rica U.
2009-08-03
Recent advances in ab initio quantum many-body methods and growth in computer power now enable highly precise calculations of nuclear structure. The precision has attained a level sufficient to make clear statements on the nature of 3-body forces in nuclear physics. Total binding energies, spin-dependent structure effects, and electroweak properties of light nuclei play major roles in pinpointing properties of the underlying strong interaction. Eventually,we anticipate a theory bridge with immense predictive power from QCD through nuclear forces to nuclear structure and nuclear reactions. Light front Hamiltonian quantum field theory offers an attractive pathway and we outline key elements.
Ab initio many-body calculations of the 4He photo-absorption cross section
Schuster, Micah D; Johnson, Calvin W; Jurgenson, Eric D; Navratil, Petr
2013-01-01
A major goal of nuclear theory is to make quantitative calculations of low-energy nuclear observables starting from microscopic internucleon forces. Computationally, this is complicated by the large model spaces needed to reach convergence in many-body approaches, such as the no-core shell model (NCSM). In recent years, the similarity renormalization group (SRG) has provided a powerful and versatile means to soften interactions for ab initio structure calculations, thus leading to convergence within smaller model spaces. Here we compute the 4He total photo absorption cross section and study, for the first time, the consistency of the SRG approach in a continuum observable.
Ab initio calculations of the optical properties of ions in glass
International Nuclear Information System (INIS)
There is interest in the optical properties of ions in insulating glasses for various applications. This paper addresses the task of predicting optical properties of ions in glass from first principles given only the physics of the constituent atoms. The prospects for developing such ab initio-computation procedures are gleaned from examining the progress made in understanding the properties of dopant ions in crystals. It is probable that in 20 years, there will be reliable procedures for predicting optical properties of ions in crystals, but the development of corresponding procedures for complex glasses is a herculean task. The use of computer simulations is addressed
Heavy Metal Compounds and Hydrogen Storage Materials from Ab Initio Calculations
Sun, Weiwei
2013-01-01
In principle, most of the properties of solids can be determined by their electronic structures. So the understanding of electronic structures is essential. This thesis presents two classes of materials using ab initio method based on density functional theory. One is heavy metal compounds like Ta2AlC, ThO and the other one is hydrogen storage material namely MgH2 surfaces. The study of correlation and relativistic effects in Ta2AlC are presented. Based on our results, Ta2AlC is a weakly corr...
Ab initio study of magnesium alanate, Mg(AlH4)2
Setten, M.J. van; de Wijs, G. A.; Popa, V.A.; Brocks, G.
2005-01-01
Magnesium alanate Mg(AlH4)2 has recently raised interest as a potential material for hydrogen storage. We apply ab initio calculations to characterize structural, electronic and energetic properties of Mg(AlH4)2. Density functional theory calculations within the generalized gradient approximation (GGA) are used to optimize the geometry and obtain the electronic structure. The latter is also studied by quasi-particle calculations at the GW level. Mg(AlH4)2 is a large band gap...
Second generation wave-function thermostat for ab-initio molecular dynamics
Blöchl, P E
2001-01-01
A rigorous two-thermostat formulation for ab-initio molecular dynamics using the fictitious Lagrangian approach is presented. It integrates the concepts of mass renormalization and temperature control for the wave functions. The new thermostat adapts to the instantaneous kinetic energy of the nuclei and thus minimizes its influence on the dynamics. Deviations from the canonical ensemble, which are possible in the previous two-thermostat formulation, are avoided. The method uses a model for the effective mass of the wave functions, which is open to systematic improvement.
The hydrogen diffusion in liquid aluminum alloys from ab initio molecular dynamics.
Jakse, N; Pasturel, A
2014-09-01
We study the hydrogen diffusion in liquid aluminum alloys through extensive ab initio molecular dynamics simulations. At the microscopic scale, we show that the hydrogen motion is characterized by a broad distribution of spatial jumps that does not correspond to a Brownian motion. To determine the self-diffusion coefficient of hydrogen in liquid aluminum alloys, we use a generalized continuous time random walk model recently developed to describe the hydrogen diffusion in pure aluminum. In particular, we show that the model successfully accounts the effects of alloying elements on the hydrogen diffusion in agreement with experimental features. PMID:25194378
Elastic Properties of CaSiO3 Perovskite from ab initio Molecular Dynamics
Shigeaki Ono
2013-01-01
Ab initio molecular dynamics simulations were performed to investigate the elasticity of cubic CaSiO3 perovskite at high pressure and temperature. All three independent elastic constants for cubic CaSiO3 perovskite, C11, C12, and C44, were calculated from the computation of stress generated by small strains. The elastic constants were used to estimate the moduli and seismic wave velocities at the high pressure and high temperature characteristic of the Earth’s interior. The dependence of temp...
Fully ab initio finite-size corrections for charged-defect supercell calculations.
Freysoldt, Christoph; Neugebauer, Jörg; Van de Walle, Chris G
2009-01-01
In ab initio theory, defects are routinely modeled by supercells with periodic boundary conditions. Unfortunately, the supercell approximation introduces artificial interactions between charged defects. Despite numerous attempts, a general scheme to correct for these is not yet available. We propose a new and computationally efficient method that overcomes limitations of previous schemes and is based on a rigorous analysis of electrostatics in dielectric media. Its reliability and rapid convergence with respect to cell size is demonstrated for charged vacancies in diamond and GaAs. PMID:19257218
Experimental and ab initio study of the photofragmentation of DNA and RNA sugars
International Nuclear Information System (INIS)
The photoelectron-photoion-photoion coincidence method is used to measure the photodissociation of doubly charged D-ribose (C5H10O5), the RNA sugar molecules, and 2-deoxy-D-ribose (C5H10O4), the DNA sugar molecules, following normal Auger decay after initial C 1s and O 1s core ionizations. The fragment identification is facilitated by measuring isotopically labeled D-ribose, such as D-ribose deuterated at C(1), and with 13C at the C(5) position. Ab initio quantum chemistry calculations are used to gain further insight into the abundant appearance of the CHO+ fragment.
Mechanical properties of graphite oxides: Ab initio simulations and continuum theory
Incze, Andrei; Pasturel, Alain; Peyla, Philippe
2004-12-01
The mechanical properties of oxidized graphitic layers are calculated by atomic ab initio simulations and continuum theory of elasticity. The adsorption of atomic oxygen atoms induces a nanocorrugation as well as a local bonding reinforcement that modify the bending coefficient of a graphite layer. As a matter of fact, a surface oxygen composition of 12.5% leads to an increase of the rigidity coefficient by more than a factor of 40. We thus demonstrate that oxidation also has a considerable impact on the mechanical properties of graphite structures.
Ab initio Path Integral Molecular Dynamics Based on Fragment Molecular Orbital Method
Fujita, Takatoshi; Watanabe, Hirofumi; Tanaka, Shigenori
2009-10-01
We have developed an ab initio path integral molecular dynamics method based on the fragment molecular orbital method. This “FMO-PIMD” method can treat both nuclei and electrons quantum mechanically, and is useful to simulate large hydrogen-bonded systems with high accuracy. After a benchmark calculation for water monomer, water trimer and glycine pentamer have been studied using the FMO-PIMD method to investigate nuclear quantum effects on structure and molecular interactions. The applicability of the present approach is demonstrated through a number of test calculations.
Ab initio study of the positron affinity of LiH
International Nuclear Information System (INIS)
Ab initio calculations, within an LCAO-MO-SCF framework, are presented demonstrating that a positron can bind to the LiH molecule giving rise to a stable [LiH; e+] complex. The binding of the positron in [LiH; e+] is compared to that of the 'extra' electron in LiH-. In both cases the charge density of the 'extra' particle is dominated by the dipole field of the neutral molecule. In particular, both the positron affinity and the electron affinity are found to vary nearly linearly with the dipole moment which in turn varies nearly linearly with the Li-H bond length. (author)
International Nuclear Information System (INIS)
High-level ab initio studies using the 6-311G(3d/f, 2p) basis set with electron correlation included at the second-order Moller Plesset perturbation theory are reported. At this level, full geometry optimization was performed, followed by calculation of the vibrational IR spectrum. Predicted molecular parameters (bond lengths, bond angles, dipole moment, rotational constants) and vibrational IR spectra (harmonic wave numbers, absolute intensities) of formaldehyde and its deuterated species agree very well with the experimental data
Balta, B.; Gianturco, F. A.
2000-04-01
Ab initio calculations are carried out, at various levels of treatment of correlation forces, for the orientation and distance dependence of the interaction between a helium atom and the structure of the ionic core found earlier in protonated He cluster studies, i.e. the linear (HeHHe) + triatomic molecular ion. Several aspects of the potential energy surface are discussed and specific details of the stable configuration analysed. The final energy surface is spline-fitted for further use in the stochastic calculations for the ground vibrational states of the small protonated clusters presented in a following paper.
Ab initio calculations and analysis of the torsional spectra of dimethylamine and dimethylphosphine
Senent Díez, María Luisa; Smeyers, Y. G.
1996-01-01
In the present paper, ab initio calculations at MP2/RHF level are performed with different basis sets 6-31G(d,p), 6-311G(d,p), and 6-311(df,p) to determine the potential energy functions, the kinetic parameters, and the dipole moment components as a function of the double methyl rotation in dimethylamine (DMA) and dimethylphosphine (DMP). From the potential energy and kinetic parameters, the torsional energy levels and torsional functions are determined, and from the dipole moment variations,...
Ab Initio Investigations of the C2F4S Isomers and of Their Interconversions
DEFF Research Database (Denmark)
Shim, Irene; Vallano-Lorenzo, Sandra
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, Møller-Plesset second order perturbation calculations and by Gaussian-3 theory. The results show that the unobserved isomerization reactions are feasible. Furthermore, all three isomers should exist as stable species, but the unknown isomer, 3, is considerably less stable than the known isomers, 1 and 2.
Ab initio study of the dissociative recombination of HCNH{sup +}
Energy Technology Data Exchange (ETDEWEB)
Ngassam, V [University of California Davis, Davis, CA (United States); Centre for Atomic Molecular Physics and Quantum Optics, Douala (Cameroon); Orel, A E [University of California Davis, Davis, CA (United States); Suzor-Weiner, A [Universite Paris-Sud, Orsay (France)
2005-01-01
Dissociative recombination of protonated hydrogen cyanide HCNH{sup +} is a very important process in dark interstellar molecular clouds. The dominant mechanism that drives this process, either 'direct' through a resonance, or 'indirect' through Rydbergs is currently an issue of controversy. Only qualitative conclusions for the branching ratio between the HCN and HNC fragments is available. We will report ab initio electron scattering calculations using the complex Kohn variational method for low energy electron scattering from HCNH{sup +} using a correlated wave function for the target. Resonance energies and widths were abstracted and their behavior as a function of the internuclear geometry is studied.
Overlap model and ab initio cluster calculations of polarisabilities of ions in solids
Domene, C; Fowler, PW; Madden, PA; Wilson, M.; Wheatley, RJ
1999-01-01
A recently developed overlap model for exchange-induction is used to simulate in-crystal anion polarisabilities for alkali halides and chalcogenides (LiF, NaF, KF, LiCl, NaCl, KCl, LiBr, NaBr, KBr, MgO, CaO, SrO, MgS, CaS and SrS) in overall qualitative agreement with results of ab initio cluster calculations and experiment. Extension to AgF supports the proposal that crystal-field splitting causes significant enhancement of cation polarisability for d10 systems, in contrast to the demonstrat...
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 Dunningâ€™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.
Charge carrier motion in disordered conjugated polymers: a multiscale ab-initio study
Energy Technology Data Exchange (ETDEWEB)
Vukmirovic, Nenad; Wang, Lin-Wang
2009-11-10
We developed an ab-initio multiscale method for simulation of carrier transport in large disordered systems, based on direct calculation of electronic states and electron-phonon coupling constants. It enabled us to obtain the never seen before rich microscopic details of carrier motion in conjugated polymers, which led us to question several assumptions of phenomenological models, widely used in such systems. The macroscopic mobility of disordered poly(3- hexylthiophene) (P3HT) polymer, extracted from our simulation, is in agreement with experimental results from the literature.
Lithium incorporation at the MoS2/graphene interface: an ab initio investigation
Miwa, Roberto H.; Scopel, Wanderla L.
2013-01-01
Based on ab initio calculations, we examine the incorporation of Li atoms in the MoS2/graphene interface. We find that the intercalated Li atoms are energetically more stable than Li atoms adsorbed on the MoS2 surface. The intercalated atoms interact with both graphene sheet and MoS2 layer, increasing the Li binding energies. However, the equilibrium geometries are ruled by the MoS2 layer, where the intercalated Li atoms lie on the top (Li_T) and hollow (Li_H) sites of the MoS2 layer. We calc...
Ramsdellite-structured LiTiO 2: A new phase predicted from ab initio calculations
Koudriachova, M. V.
2008-06-01
A new phase of highly lithiated titania with potential application as an anode in Li-rechargeable batteries is predicted on the basis of ab initio calculations. This phase has a composition LiTiO2 and may be accessed through electrochemical lithiation of ramsdellite-structured TiO2 at the lowest potential reported for titanium dioxide based materials. The potential remains constant over a wide range of Li-concentrations. The new phase is metastable with respect to a tetragonally distorted rock salt structure, which hitherto has been the only known polymorph of LiTiO2.
Kolmogorov, A.N.; Shah, S; Margine, E. R.; Bialon, A. F.; Hammerschmidt, T.; Drautz, R.
2011-01-01
New candidate ground states at 1:4, 1:2, and 1:1 compositions are identified in the well-known Fe-B system via a combination of ab initio high-throughput and evolutionary searches. We show that the proposed oP12-FeB2 stabilizes by a break up of 2D boron layers into 1D chains while oP10-FeB4 stabilizes by a distortion of a 3D boron network. The uniqueness of these configurations gives rise to a set of remarkable properties: oP12-FeB2 is expected to be the first semiconducting...
Ab initio quantum chemistry: A source of ideas for lattice gauge theorists
International Nuclear Information System (INIS)
Ab initio quantum chemistry is an emerging computational area that is fifty years ahead of lattice gauge theory, a principal competitor for supercomputer time, and a rich source of new ideas and new approaches to the computation of many fermion systems. An overview of the history, current prospects and future frontiers of quantum chemistry is given, with special emphasis on lessons for lattice gauge theory. Particular reference is given to the role of Gaussian basis functions (in place of grids) and analytic (as opposed to Monte Carlo) methods. The main recommendation to lattice gauge theorists is for greater emphasis on infinite momentum frame studies, using Gaussian basis functions. (orig.)
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; Berg, Rolf W.
2011-01-01
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 sets. The vibrations in the region 1600–1000 cm?1 were found to comprise variousmixed modes including in-plane stretching and bending of various C–C,N–N, C–N and C–Obonds and angles in the molecule. Be...
Dislocation constriction and cross-slip in Al and Ag: an ab initio study
Lu, Gang; Bulatov, Vasily V.; Kioussis, Nicholas
2002-01-01
A novel model based on the Peierls framework of dislocations is developed. The new theory can deal with a dislocation spreading at more than one slip planes. As an example, we study dislocation cross-slip and constriction process of two fcc metals, Al and Ag. The energetic parameters entering the model are determined from ab initio calculations. We find that the screw dislocation in Al can cross-slip spontaneously in contrast with that in Ag, which splits into partials and cannot cross-slip w...
Current rectification by simple molecular quantum dots: an ab-initio study
Larade, B.; Bratkovsky, A. M.
2003-01-01
We calculate a current rectification by molecules containing a conjugated molecular group sandwiched between two saturated (insulating) molecular groups of different length (molecular quantum dot) using an ab-initio non-equilibrium Green's function method. In particular, we study S-(CH2)m-C10H6-(CH2)n-S dithiol with Naphthalene as a conjugated central group. The rectification current ratio ~35 has been observed at m = 2 and n = 10, due to resonant tunneling through the molecular orbital (MO) ...
International Nuclear Information System (INIS)
Full text: Silicon is often described as the prototype covalent material, and when it comes to developing atomistic models this situation is well described by the sentiment that 'everything works for silicon'. The same cannot be said for carbon though, where the interaction potential has always proved problematical, be it with empirical, tight-binding or ab initio methods. Thus far the most decisive contributions to understanding amorphous carbon networks have come from ab initio simulations using the Car-Parrinello method, where the fully quantum treatment of the valence electrons has provided unexpected insight into the local structure. However such first principles calculations are restricted spatially and temporally to systems with approximately 100 atoms and times of order one picosecond. There is therefore demand for less expensive techniques capable of resolving important questions whose solution can only to found with larger simulations running for longer times. In the case of tetrahedral amorphous carbon, such issues include the release of compressive stress through annealing, the origin of graphitic surface layers and the nature of the film growth process and thermal spike. Against this background tight-binding molecular dynamics has emerged as a popular alternative to first principles methods, and our group has an ongoing program to understand film growth using one of the efficient variants of tight-binding. Another direction of research is a new empirical potential based on the Environment Dependent Interaction Potential (EDIP) recently developed for silicon. The EDIP approach represents a promising direction for empirical potentials through its use of ab initio data to motivate the functional form as well as the more conventional parametrisation. By inverting ab initio cohesive energy curves the authors of EDIP arrived at a pair potential expression which reduces to the well-known Stillinger-Weber form at integer coordination, while providing interpolation and extrapolation to situations where the coordination was either fractional or beyond the fitting data set. The application of EDIP to carbon has thus far been quite encouraging, in particular the two-body pair potential which exhibits a remarkably good degree of transferability to coordinations two and six, despite being fitted to only the graphene and diamond structures. Current development work is concentrating on the c-axis ?-repulsion and dihedral rotation penalties which are absent from the original EDIP. Preliminary molecular dynamics simulations using this modified EDIP provide a visual and intuitive demonstration of why these effects are so important when modelling amorphous carbons
Pietrucci, Fabio; Andreoni, Wanda
2011-08-01
Social permutation invariant coordinates are introduced describing the bond network around a given atom. They originate from the largest eigenvalue and the corresponding eigenvector of the contact matrix, are invariant under permutation of identical atoms, and bear a clear signature of an order-disorder transition. Once combined with ab initio metadynamics, these coordinates are shown to be a powerful tool for the discovery of low-energy isomers of molecules and nanoclusters as well as for a blind exploration of isomerization, association, and dissociation reactions.
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...... transmission in large energy ranges. The effect is due to the spin-polarized zig-zag edge terminating each graphene flake causing a spin-splitting of the graphene pi(z) bands, and the chain states. Transmission occurs when the graphene p-states resonate with similar states in the strongly hybridized edges and...
Transport coefficients of helium-argon mixture based on ab initio potential.
Sharipov, Felix; Benites, Victor J
2015-10-21
The viscosity, thermal conductivity, diffusion coefficient, and thermal diffusion factor of helium-argon mixtures are calculated for a wide range of temperature and for various mole fractions up to the 12th order of the Sonine polynomial expansion with an ab initio intermolecular potential. The calculated values for these transport coefficients are compared with other data available in the open literature. The comparison shows that the obtained transport coefficients of helium-argon mixture have the best accuracy for the moment. PMID:26493894
Ab initio simulation of effects of structural singularities in aerogel absorption potential
International Nuclear Information System (INIS)
In the present work simulation of Van der Waals potential between helium atom and part of silica aerogel strand by means of ab initio methods was performed. Cell with alpha quartz structure was used as building block of aerogel strand, because it is the most stable structure at low temperature, and only the surface layer of aerogel has been considered. For modeling absorption potential field in plane, summation of potential from individual building blocks has been provided. Two dimensional Van der Waals energy field was calculated for different geometries of aerogel strands. A rather deep potential well has been found in the corner formed due to aerogel strand crossing.
Electronic structure of superconducting gallium-doped germanium from ab-initio calculations
Energy Technology Data Exchange (ETDEWEB)
Lebegue, S. [Laboratoire de Cristallographie, Resonance Magnetique et Modelisations (CRM2, UMR CNRS 7036), Institut Jean Barriol, Nancy Universite, BP 239, Boulevard des Aiguillettes, 54506 Vandoeuvre-les-Nancy (France)
2009-10-15
Using ab-initio calculations, we study the electronic structure of gallium-doped germanium, which was found recently to be a superconductor, with a critical temperature of 0.5 K, and a particularly low density of Cooper pairs. The calculations of the electronic properties reveal that no sign of an impurity band is observed, and that the Fermi level lies in the valence band of Ge. Moreover, the calculation of the phonon frequencies shows that a new mode associated to the Ga atom is appearing, around 175 cm{sup -1}. (copyright 2009 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)
Periodic arrays of intercalated atoms in twisted bilayer graphene: an \\it{ ab initio} investigation
Miwa, R. H.; Venezuela, P.; Morell, E. Suárez
2015-01-01
We have performed an \\it {ab initio} investigation of transition metals (TMs = Mo, Ru, Co, and Pt) embedded in twisted bilayer graphene (tBG) layers. Our total energy results reveal that, triggered by the misalignment between the graphene layers, Mo and Ru atoms may form a quasi-periodic (triangular) array of intercalated atoms. In contrast, the formation of those structures is not expected for the other TMs, Co and Pt atoms. The net magnetic moment (m) of Mo and Ru atoms ma...
Calzado, Carmen J.
2009-01-01
Abstract A series of ab initio quantum chemistry calculations on a trinuclear mixed-valence system [(NH3)6Cu3O2]+3 have been performed in order to simultaneously evaluate its magnetic and electronic parameters, namely the magnetic coupling constants J1 and J2, the electron transfer integrals t1 and t2, and the exchange-transfer terms h1 and h2. The procedure is based on the use of the effective Hamiltonian theory. The results evidence the presence of two ferromagnetic interactions ...
Pride, Prejudice, and Penury of {\\it ab initio} transport calculations for single molecules
Evers, F
2006-01-01
Recent progress in measuring the transport properties of individual molecules has triggered a substantial demand for {\\it ab initio} transport calculations. Even though program packages are commercially available and placed on custom tailored to address this task, reliable information often is difficult and very time consuming to attain in the vast majority of cases, namely when the molecular conductance is much smaller than $e^2/h$. The article recapitulates procedures for molecular transport calculations from the point of view of time-dependent density functional theory. Emphasis is describing the foundations of the ``standard method''. Pitfalls will be uncovered and the domain of applicability discussed.
Ab initio calculations of 14N and 15N hyperfine structures
JÃ¶nsson, P; Nemouchi, M; Godefroid, M
2010-01-01
Hyperfine structure parameters are calculated for the 2p2(3P)3s 4P_J, 2p2(3P)3p 4Po_J and 2p2(3P)3p 4Do_J levels, using the ab initio multiconfiguration Hartree-Fock method. The theoretical hyperfine coupling constants are in complete disagreement with the experimental values of Jennerich et al. (EPJD 40(2006), 81) deduced from the analysis of the near-infrared Doppler-free saturated absorption spectra.
Ab initio-Theory of Point Defects and Defect Complexes in SiC
Mattausch, Alexander
2005-01-01
The work investigates the intrinsic and extrinsic defects in SiC by means of an ab initio approach based on density functional theory. This includes the study of the migration of the defects as well as the impact of the intrinsic defects on the annealing of the implantation damage and on the dopant diffusion. The vibrational signatures of carbon-related defects and defect aggregates are obtained, allowing a comparison with the photoluminescence data. We show in this work that the silicon and ...
Exploring proton transfer in 1,2,3-triazole-triazolium dimer with ab initio method
Energy Technology Data Exchange (ETDEWEB)
Li, Ailin; Yan, Tianying; Shen, Panwen [Department of Material Chemistry, Institute of New Energy Material Chemistry, Nankai University, Tianjin, 300071 (China)
2011-02-01
Ab initio calculations are utilized to search for transition state structures for proton transfer in the 1,2,3-triazole-triazolium complexes on the basis of optimized dimers. The result suggests six transition state structures for single proton transfer in the complexes, most of which are coplanar. The energy barriers, between different stable and transition states structures with zero point energy (ZPE) corrections, show that proton transfer occurs at room temperature with coplanar configuration that has the lowest energy. The results clearly support that reorientation gives triazole flexibility for proton transfer. (author)
Simple synthesis, structure and ab initio study of 1,4-benzodiazepine-2,5-diones
Jadidi, Khosrow; Aryan, Reza; Mehrdad, Morteza; Lügger, Thomas; Ekkehardt Hahn, F.; Ng, Seik Weng
2004-04-01
A simple procedure for the synthesis of pyrido[2,1-c][1,4] benzodiazepine-6,12-dione ( 1) and 1,4-benzodiazepine-2,5-diones ( 2a- 2d), using microwave irradiation and/or conventional heating is reported. The configuration of 1 was determined by single-crystal X-ray diffraction. A detailed ab initio B3LYP/6-31G* calculation of structural parameters and substituent effects on ring inversion barriers (? G#) and also free energy differences (? G0) for benzodiazepines are reported.
Illg, Christian; Haag, Michael; Mueller, Benedikt Y.; Czycholl, Gerd; Fähnle, Manfred
2015-11-01
Transition matrix elements for electron-phonon scattering are discussed. The approximate expression for the matrix element derived by the combination of a phenomenological theory of electronic and ionic screening of the electron-electron interaction with a microscopic perturbation theory for the matrix element is compared with unscreened matrix elements from the ab initio electron theory. The matrix elements calculated in these two ways differ very strongly. A situation is investigated (ultrafast demagnetization of a ferromagnetic film after excitation with a femtosecond laser pulse) in which nevertheless both types of matrix elements yield similar results for the demagnetization rate.
Cobalt-doped ZnO nanocrystals: quantum confinement and surface effects from ab initio methods.
Schoenhalz, Aline L; Dalpian, Gustavo M
2013-10-14
Cobalt-doped ZnO nanocrystals were studied through ab initio methods based on the Density Functional Theory. Both quantum confinement and surface effects were explicitly taken into account. When only quantum confinement effects are considered, Co atoms interact through a superexchange mechanism, stabilizing an antiferromagnetic ground state. Usually, this is the case for high quality nanoparticles with perfect surface saturation. When the surfaces were considered, a strong hybridization between the Co atoms and surfaces was observed, strongly changing their electronic and magnetic properties. Our results indicated that the surfaces might qualitatively change the properties of impurities in semiconductor nanocrystals. PMID:23942732
Ab initio simulation of particle momentum distributions in high-pressure water
International Nuclear Information System (INIS)
Applying pressure to water reduces the average oxygen-oxygen distance, and facilitates the delocalisation of protons along the hydrogen bond. This pressure-induced delocalisation is further enhanced by the quantum nature of hydrogen nuclei, which is very significant even well above room temperature. Here we will evaluate the quantum kinetic energy and the particle momentum distribution of hydrogen and oxygen nuclei in water at extreme pressure, using ab initio path integral molecular dynamics. We will show that (transient) dissociation of water molecules induce measurable changes in the kinetic energy hydrogen atoms, although current deep inelastic scattering experiments are probably unable to capture the heterogeneity of the sample
Initial oxidation of TiAl: An ab-initio investigation
International Nuclear Information System (INIS)
We present ab-initio investigation of oxygen adsorption up to two monolayer coverage on the stoichiometric TiAl(100) surface to illustrate the initial oxidation stage. The formation of band gap near the Fermi level demonstrates the transformation from metal to oxide surface with increasing oxygen coverage. The oxidation of Ti rather than Al is observed from our electronic structure calculations. The energy barriers of oxygen diffusion between different sites on surface as well as in subsurface and bulk region are derived. It is shown that the diffusion of oxygen is much easier on the surface than that into the subsurface region
Initial oxidation of TiAl: An ab-initio investigation
Energy Technology Data Exchange (ETDEWEB)
Bakulin, Alexander V., E-mail: bakulin@ispms.tsc.ru; Kulkova, Svetlana E., E-mail: bakulin@ispms.tsc.ru [Institute of Strength Physics and Materials Science SB RAS, Tomsk, 634055, Russia and National Research Tomsk State University, Tomsk, 634050 (Russian Federation); Hu, Qing-Miao; Yang, Rui [Institute of Metal Research, Chinese Academy of Sciences, Shenyang, 110016 (China)
2014-11-14
We present ab-initio investigation of oxygen adsorption up to two monolayer coverage on the stoichiometric TiAl(100) surface to illustrate the initial oxidation stage. The formation of band gap near the Fermi level demonstrates the transformation from metal to oxide surface with increasing oxygen coverage. The oxidation of Ti rather than Al is observed from our electronic structure calculations. The energy barriers of oxygen diffusion between different sites on surface as well as in subsurface and bulk region are derived. It is shown that the diffusion of oxygen is much easier on the surface than that into the subsurface region.
An adaptive finite-element method for large-scale ab initio molecular dynamics simulations.
Tsuchida, Eiji; Choe, Yoong-Kee; Ohkubo, Takahiro
2015-12-21
We present the current status of the finite-element method for large-scale atomistic simulations based on the density-functional theory. After a brief overview of our formulation, we describe recent developments, including the optimal choice of adaptive coordinates, an efficient implementation of the ground-state calculations, and a remedy for the eggbox effect. As a new application of our formulation, we present ab initio molecular dynamics simulations on sulfonated poly(4-phenoxybenzoyl-1,4-phenylene) (SPPBP), which is a typical example of polymer electrolyte membranes for fuel cells. PMID:25891523
Polarization effects in semiconductors from ab initio theory to device applications
Wood, Colin
2007-01-01
This book presents the latest understanding of the solid physics, electronic implications and practical applications of the unique spontaneous or pyro-electric polarization charge of hexagonal semiconductors, and the piezo-electric effects in thin film hetero-structures, which are used in wide forbidden band gap sensor, electronic and opto-electronic semiconductor devices. The book covers ab initio theory of polarization in cubic and hexagonal semiconductors, growth of thin film GaN, and GaN/AlGaN GaAlN/ AlGaInN and other nitride, and SiC hetero-structures; graded structures for distributed pi
Initial stages of salt crystal dissolution determined with ab initio molecular dynamics.
Liu, Li-Min; Laio, Alessandro; Michaelides, Angelos
2011-08-01
The initial stages of NaCl dissolution in liquid water have been examined with state-of-the-art ab initio molecular dynamics and free energy sampling techniques. Our simulations reveal a complex multi-step process triggered by the departure of Cl ions from the lattice, with a well-defined intermediate state wherein departing ions are partially solvated but remain in contact with the crystal. The polarizability of Cl(-) is identified as the source of the anion's preferential initial dissolution, an effect which leads a forcefield based description of NaCl dissolution to fail to identify a preference for Cl over Na dissolution. PMID:21637872
Rodríguez Martínez, Dayté Dayana
2013-01-01
Els actuals mètodes ab initio per a la resolució d'estructures macromoleculars estan generalment limitats a casos de proteïnes petites que difracten a resolució atòmica (1.2Å màxim) llevat que continguin àtoms pesats. ARCIMBOLDO1 és un mètode general per a la resolució d'estructures amb dades de difracció de fins 2Å que s'executa en un entorn de multiso lució. Està basat en l'ús del programa Phaser2, per a la localització de petits fragments model com ?-hèlixs (la seva presència es pot predir...
Renison, C Alicia; Fernandes, Kyle D; Naidoo, Kevin J
2015-07-01
This article describes an extension of the quantum supercharger library (QSL) to perform quantum mechanical (QM) gradient and optimization calculations as well as hybrid QM and molecular mechanical (QM/MM) molecular dynamics simulations. The integral derivatives are, after the two-electron integrals, the most computationally expensive part of the aforementioned calculations/simulations. Algorithms are presented for accelerating the one- and two-electron integral derivatives on a graphical processing unit (GPU). It is shown that a Hartree-Fock ab initio gradient calculation is up to 9.3X faster on a single GPU compared with a single central processing unit running an optimized serial version of GAMESS-UK, which uses the efficient Schlegel method for s- and l-orbitals. Benchmark QM and QM/MM molecular dynamics simulations are performed on cellobiose in vacuo and in a 39 Å water sphere (45 QM atoms and 24843 point charges, respectively) using the 6-31G basis set. The QSL can perform 9.7 ps/day of ab initio QM dynamics and 6.4 ps/day of QM/MM dynamics on a single GPU in full double precision. © 2015 Wiley Periodicals, Inc. PMID:25975864
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 calculation of the ro-vibrational spectrum of H2F+
Kyuberis, Aleksandra A.; Lodi, Lorenzo; Zobov, Nikolai F.; Polyansky, Oleg L.
2015-10-01
An ab initio study of the rotation-vibrational spectrum of the electronic ground state of the (gas-phase) fluoronium ion H2F+ is presented. A new potential energy surface (PES) and a new dipole moment surface (DMS) were produced and used to compute rotation-vibrational energy levels, line positions and line intensities. Our computations achieve an accuracy of 0.15 cm-1 for the fundamental vibrational frequencies, which is about 50 times more accurate than previous ab initio results. The computed room-temperature line list should facilitate the experimental observations of new H2F+ lines, in particular of yet unobserved overtone transitions. The H2F+ molecular ion, which is isoelectronic to water, has a non-linear equilibrium geometry but a low-energy barrier to linearity at about 6000 cm-1. As a result the effects of so-called quantum monodromy become apparent already at low bending excitations. An analysis of excited bends in terms of quantum monodromy is presented.
Ab initio estimate of Hubbard model parameters: A simple procedure applied to BEDT-TTF salts
International Nuclear Information System (INIS)
We describe a simple procedure to extract the parameters of the one-band Hubbard model from a set of ab initio calculations on a dimeric unit. The method applies to narrow band solids with negligible orbital relaxation. In this paper we consider a specific system, ?-(BEDT-TTF)2Cu[N(CN)2]Br, but the proposed procedure is of general applicability. From a detailed discussion of the ab initio data, we extract a few general criteria of wide applicability about the relative magnitude of the empirical model parameters. We prove that the Hubbard model, including both on-site and inter-site electron-electron interactions, is a reliable model for systems with negligible orbital relaxation, apart from particular geometrical arrangements. The hopping parameter (t), as well as the intersite electronic repulsion (V), are barely affected by the chemical environment. On the contrary, the on-site energy and the on-site repulsion (the Hubbard h and U) show an appreciable dependence on the chemical environment, through a dependence of the frontier and core energies on the surrounding potential. copyright 1997 The American Physical Society
Deviational simulation of phonon transport in graphene ribbons with ab initio scattering
International Nuclear Information System (INIS)
We present a deviational Monte Carlo method for solving the Boltzmann-Peierls equation with ab initio 3-phonon scattering, for temporally and spatially dependent thermal transport problems in arbitrary geometries. Phonon dispersion relations and transition rates for graphene are obtained from density functional theory calculations. The ab initio scattering operator is simulated by an energy-conserving stochastic algorithm embedded within a deviational, low-variance Monte Carlo formulation. The deviational formulation ensures that simulations are computationally feasible for arbitrarily small temperature differences, while the stochastic treatment of the scattering operator is both efficient and exhibits no timestep error. The proposed method, in which geometry and phonon-boundary scattering are explicitly treated, is extensively validated by comparison to analytical results, previous numerical solutions and experiments. It is subsequently used to generate solutions for heat transport in graphene ribbons of various geometries and evaluate the validity of some common approximations found in the literature. Our results show that modeling transport in long ribbons of finite width using the homogeneous Boltzmann equation and approximating phonon-boundary scattering using an additional homogeneous scattering rate introduces an error on the order of 10% at room temperature, with the maximum deviation reaching 30% in the middle of the transition regime.
Ab initio molecular dynamics study of dissociation of water under an electric field
Saitta, A Marco; Giaquinta, Paolo V
2012-01-01
The behavior of liquid water under an electric field is a crucial phenomenon in science and engineering. However, its detailed description at a microscopic level is difficult to achieve experimentally. Here we report on the first ab initio molecular-dynamics study on water under an electric field. We observe that the hydrogen-bond length and the molecular orientation are significantly modified at low-to-moderate field intensities. Fields beyond a threshold of about 0.35 V/\\AA are able to dissociate molecules and sustain an ionic current via a series of correlated proton jumps. Upon applying even more intense fields (1.0 V/\\AA), a 15-20% fraction of molecules are instantaneously dissociated and the resulting ionic flow yields a conductance of about 7.8 $\\Omega^{-1}cm^{-1}$, in good agreement with experimental values. This result paves the way to quantum-accurate microscopic studies of the effect of electric fields on aqueous solutions and, thus, to massive applications of ab initio molecular dynamics in neurob...
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.
Ground state analytical ab initio intermolecular potential for the Cl2-water system
Hormain, Laureline; Monnerville, Maurice; Toubin, Céline; Duflot, Denis; Pouilly, Brigitte; Briquez, Stéphane; Bernal-Uruchurtu, Margarita I.; Hernández-Lamoneda, Ramón
2015-04-01
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 Cl2 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 Cl2 - H2O complex. The potential fitted to reproduce the three minima structures of 1:1 complex is validated by the comparison of ab initio results of Cl2 interacting with an increasing number of water molecules. Finally, the model potential is used to study the physisorption of Cl2 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.
Ground state analytical ab initio intermolecular potential for the Cl2-water system
International Nuclear Information System (INIS)
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 Cl2 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 Cl2 ? H2O complex. The potential fitted to reproduce the three minima structures of 1:1 complex is validated by the comparison of ab initio results of Cl2 interacting with an increasing number of water molecules. Finally, the model potential is used to study the physisorption of Cl2 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
Experimental and ab initio investigations on textured Liâ€“Mnâ€“O spinel thin film cathodes
Energy Technology Data Exchange (ETDEWEB)
Fischer, J., E-mail: Julian.Fischer@kit.edu [Karlsruhe Institute of Technology (KIT), Institute for Applied Materials (IAM), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen (Germany); Music, D. [RWTH Aachen University, Materials Chemistry, Kopernikusstrasse 10, 52074 Aachen (Germany); Bergfeldt, T.; Ziebert, C.; Ulrich, S.; Seifert, H.J. [Karlsruhe Institute of Technology (KIT), Institute for Applied Materials (IAM), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen (Germany)
2014-12-01
This paper describes the tailored preparation of nearly identical lithiumâ€“manganeseâ€“oxide thin film cathodes with different global grain orientations. The thin films were synthesized by rf magnetron sputtering from a LiMn{sub 2}O{sub 4}-target in a pure argon plasma. Under appropriate processing conditions, thin films with a cubic spinel structure and a nearly similar density and surface topography but different grain orientation, i.e. (111)- and (440)-textured films, were achieved. The chemical composition was determined by inductively coupled plasma optical emission spectroscopy and carrier gas hot extraction. The constitution- and microstructure were evaluated by X-ray diffraction and Raman spectroscopy. The surface morphology and roughness were investigated by scanning electron and atomic force microscopy. The differently textured films represent an ideal model system for studying potential effects of grain orientation on the lithium ion diffusion and electrochemical behavior in LiMn{sub 2}O{sub 4}-based thin films. They are nearly identical in their chemical composition, atomic bonding behavior, surface-roughness, morphology and thickness. Our initial ab initio molecular dynamics data indicate that Li ion transport is faster in (111)-textured structure than in (440)-textured one. - Highlights: â€¢ Thin film model system of differently textured cubic Liâ€“Mnâ€“O spinels. â€¢ Investigation of the Liâ€“Mnâ€“O thin film mass density by X-ray reflectivity. â€¢ Ab initio molecular dynamics simulation on Li ion diffusion in LiMn{sub 2}O{sub 4}.
Field theoretic approach to dynamical orbital localization in ab initio molecular dynamics
International Nuclear Information System (INIS)
Techniques from gauge-field theory are employed to derive an alternative formulation of the Car-Parrinello ab initio molecular-dynamics method that allows maximally localized Wannier orbitals to be generated dynamically as the calculation proceeds. In particular, the Car-Parrinello Lagrangian is mapped onto an SU(n) non-Abelian gauge-field theory and the fictitious kinetic energy in the Car-Parrinello Lagrangian is modified to yield a fully gauge-invariant form. The Dirac gauge-fixing method is then employed to derive a set of equations of motion that automatically maintain orbital locality by restricting the orbitals to remain in the 'Wannier gauge'. An approximate algorithm for integrating the equations of motion that is stable and maintains orbital locality is then developed based on the exact equations of motion. It is shown in a realistic application (64 water molecules plus one hydrogen-chloride molecule in a periodic box) that orbital locality can be maintained with only a modest increase in CPU time. The ability to keep orbitals localized in an ab initio molecular-dynamics calculation is a crucial ingredient in the development of emerging linear scaling approaches
Horní?ek, Jan; Kaprálová, Petra; Bou?, Petr
2007-08-01
An algorithm allowing simulating vibrational spectra from classical time-dependent trajectories was applied for infrared absorption, vibrational circular dichroism, Raman, and Raman optical activity of model harmonic systems. The implementation of the theory within the TINKER molecular dynamics (MD) program package was tested with ab initio harmonic force fields in order to determine the feasibility for more extended MD simulations. The results suggest that sufficiently accurate frequencies can be simulated with integration time steps shorter than about 0.5fs. For a given integration time step, lower vibrational frequencies (˜0-2000cm-1) could be reproduced with a higher accuracy than higher-frequency vibrational modes (e.g., O-H and C-H stretching). In principle, the algorithm also provides correct intensities for ideal systems. In applied simulations, however, the intensity profiles are affected by an unrealistic energy distribution between normal modes and a slow energy relaxation. Additionally, the energy fluctuations may cause weakening of the intensities on average. For ab initio force fields, these obstacles could be overcome by an arbitrary normal mode energy correction. For general MD simulations, averaging of many shorter MD trajectories started with randomly distributed atomic velocities provided the best spectral shapes. ?-pinene, D-gluconic acid, formaldehyde dimer, and the acetylprolineamide molecule were used in the tests.
Ab Initio Calculations and Raman and SERS Spectral Analyses of Amphetamine Species
DEFF Research Database (Denmark)
Berg, Rolf W.; NØrbygaard, Thomas
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 of the conformational energy of the S-plus-amphetamine molecule and the S-plus-amphetamine-H+ ion. The harmonic frequency calculations provide information about the characteristic features of the Raman spectra and the nature of the bonding in the molecule. It is concluded that vibrational bands from salt anions 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, discrimination between the free and protonated forms of amphetamine salts can be observed. Here, we provide evidence for this difference and show experimentally how it has been overseen.
Evolved Chiral NN+3N Hamiltonians for Ab Initio Nuclear Structure Calculations
Roth, Robert; Langhammer, Joachim; Binder, Sven
2013-01-01
We discuss the building blocks for a consistent inclusion of chiral three-nucleon (3N) interactions into ab initio nuclear structure calculations beyond the lower p-shell. We highlight important technical developments, such as the similarity renormalization group (SRG) evolution in the 3N sector, a JT-coupled storage scheme for 3N matrix elements with efficient on-the-fly decoupling, and the importance truncated no-core shell model with 3N interactions. Together, these developments make converged ab initio calculations with explicit 3N interactions possible also beyond the lower p-shell. We analyze in detail the impact of various truncations of the SRG-evolved Hamiltonian, in particular the truncation of the harmonic-oscillator model space used for solving the SRG flow equations and the omission of the induced beyond-3N contributions of the evolved Hamiltonian. Both truncations lead to sizable effects in the upper p-shell and beyond and we present options to remedy these truncation effects. The analysis of th...
Evolved chiral NN +3N Hamiltonians for ab initio nuclear structure calculations
Roth, Robert; Calci, Angelo; Langhammer, Joachim; Binder, Sven
2014-08-01
We discuss the building blocks for a consistent inclusion of chiral three-nucleon (3N) interactions into ab initio nuclear structure calculations beyond the lower p shell. We highlight important technical developments, such as the similarity renormalization group (SRG) evolution in the 3N sector, a JT-coupled storage scheme for 3N matrix elements with efficient on-the-fly decoupling, and the importance-truncated no-core shell model with 3N interactions. Together, these developments make converged ab initio calculations with explicit 3N interactions possible also beyond the lower p shell. We analyze in detail the impact of various truncations of the SRG-evolved Hamiltonian, in particular the truncation of the harmonic-oscillator model space used for solving the SRG flow equations and the omission of the induced beyond-3N contributions of the evolved Hamiltonian. Both truncations lead to sizable effects in the upper p shell and beyond and we present options to remedy these truncation effects. The analysis of the different truncations is a first step towards a systematic uncertainty quantification of all stages of the calculation.
Ab initio quantum-enhanced optical phase estimation using real-time feedback control
DEFF Research Database (Denmark)
Berni, Adriano; Gehring, Tobias
2015-01-01
Optical phase estimation is a vital measurement strategy that is used to perform accurate measurements of various physical quantities including length, velocity and displacements(1,2). The precision of such measurements can be greatly enhanced by the use of entangled or squeezed states of light as demonstrated in a variety of different optical systems(3-8). Most of these accounts, however, deal with the measurement of a very small shift of an already known phase, which is in stark contrast to ab initio phase estimation where the initial phase is unknown(9-12). Here, we report on the realization of a quantum-enhanced and fully deterministic ab initio phase estimation protocol based on real-time feedback control. Using robust squeezed states of light combined with a real-time Bayesian adaptive estimation algorithm, we demonstrate deterministic phase estimation with a precision beyond the quantum shot noise limit. The demonstrated protocol opens up new opportunities for quantum microscopy, quantum metrology and quantum information processing.
Ab initio simulation of elastic and mechanical properties of Zn- and Mg-doped hydroxyapatite (HAP).
Aryal, Sitaram; Matsunaga, Katsuyuki; Ching, Wai-Yim
2015-07-01
Hydroxyapatite (HAP) is an important bioceramic which constitutes the mineral components of bones and hard tissues in mammals. It is bioactive and used as bioceramic coatings for metallic implants and bone fillers. HAP readily absorbs a large amount of impurities. Knowledge on the elastic and mechanical properties of impurity-doped HAP is a subject of great importance to its potential for biomedical applications. Zn and Mg are the most common divalent cations HAP absorbs. Using density function theory based ab initio methods, we have carried out a large number of ab initio calculations to obtain the bulk elastic and mechanical properties of HAP with Zn or Mg doped in different concentration at the Ca1 and Ca2 sites using large 352-atom supercells. Detailed information on their dependece on the concetraion of the substitued impurity is obtained. Our results show that Mg enhances overall elastic and bulk mechanical properties whereas Zn tends to degrade except at low concentrations. At a higher concentration, the mechanical properties of Zn and Mg doped HAP also depend significantly on impurity distribution between the Ca1 and Ca2 sites. There is a strong evidence that Zn prefers Ca2 site for substituion whereas Mg has no such preference. These results imply that proper control of dopant concentration and their site preference must carefully considered in using doped HAP for specific biomedical applications. PMID:25918859
Shi, Lin; Xu, Ke; Wang, Lin-Wang
2015-05-01
Nonradiative carrier recombination is of both great applied and fundamental importance, but the correct ab initio approaches to calculate it remain to be inconclusive. Here we used five different approximations to calculate the nonradiative carrier recombinations of two complex defect structures GaP :Z nGa-OP and GaN :Z nGa-VN , and compared the results with experiments. In order to apply different multiphonon assisted electron transition formalisms, we have calculated the electron-phonon coupling constants by ab initio density functional theory for all phonon modes. Compared with different methods, the capture coefficients calculated by the static coupling theory are 4.30 ×10-8 and 1.46 ×10-7c m3/s for GaP :Z nGa-OP and GaN :Z nGa-VN , which are in good agreement with the experiment results, (4-1+2) ×10-8 and 3.0 ×10-7c m3/s , respectively. We also provided arguments for why the static coupling theory should be used to calculate the nonradiative decays of semiconductors.
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...
Mössbauer spectroscopy, magnetic, and ab initio study of the Heusler compound Fe2NiGa
Nejadsattari, Farshad; Stadnik, Zbigniew M.; Przewo?nik, Janusz; Buschow, Kurt H. J.
2015-11-01
The structural, electronic, magnetic, elastic, and hyperfine-interaction properties of Fe2NiGa have been determined by means of X-ray diffraction, 57Fe Mössbauer spectroscopy, and magnetic measurements and ab initio calculations. The compound studied crystallizes in the cubic space group F 4 bar 3 m with lattice constant a = 5.7961(4) A ? . Evidence is provided for the presence of significant structural disorder in the compound. Fe2NiGa is predicted to be half-metallic with covalent chemical bonding. It orders ferromagnetically with the Curie temperature TC = 586.0(7) K. The saturation magnetization per formula unit and the estimated Fe magnetic moments at the A and B sites are 3.00, 1.87(2), and 2.25(2) ? B, respectively. The ab initio calculations overestimate the values of the A- and B-site Fe magnetic moments. It is observed that the magnetic properties of Fe2NiGa are very strongly dependent on its heat treatment. The calculated hyperfine-interaction parameters show general agreement with the experimental ones. It is demonstrated that the compound studied decomposes when heated and kept at temperatures above around 500 K. The Debye temperature of Fe2NiGa is found to be 378(5) K.
Tilocca, Antonio
2007-12-01
Ab initio (Car-Parrinello) molecular dynamics simulations were carried out to investigate the melt precursor of a modified phosphosilicate glass with bioactive properties, and to quench the melt to the vitreous state. The properties of the 3000K liquid were extensively compared with those of the final glass structure. The melt is characterized by a significant fraction of structural defects (small rings, undercoordinated and overcoordinated ions), often combined together. The creation or removal of these coordinative defects in the liquid (through Si-O bond formation or dissociation) reflects frequent exchanges within the silicate first coordination shell, which in turn dynamically modify the intertetrahedral connectivity of silicate groups. The observed dynamical variation in both the identity and the number of silicate groups linked to a tagged Si ( Qn speciation) are considered key processes in the viscous flow of silicate melts [I. Farnan and J. F. Stebbins, Science 265, 1206 (1994)]. On the other hand, phosphate groups do not show an equally marked exchange activity in the coordination shell, but can still form links with Si. Once formed, these Si-O-P bridges are rather stable, and in fact they are retained in the glass phase obtained after cooling; their formation within the present full ab initio melt-and-quench approach strongly supports their presence in melt-derived phosphosilicate glasses with bioactive applications. On the other hand, the simulations show that the fraction of structural defects rapidly decreases during the cooling, and the glass is essentially free of miscoordinated ions and small rings.
Energy Technology Data Exchange (ETDEWEB)
Kimberg, Victor, E-mail: victor.kimberg@pks.mpi.de [Max Planck Institute for the Physics of Complex Systems, NÃ¶thnitzer StraÃŸe 38, 01187 Dresden (Germany); Miron, Catalin, E-mail: miron@synchrotron-soleil.fr [Synchrotron SOLEIL, lâ€™Orme des Merisiers, Saint-Aubin, BP 48, FR-91192 Gif-sur-Yvette Cedex (France)
2014-08-15
Highlights: â€¢ Some studies related to the vibrational wave functions mapping phenomenon are reviewed. â€¢ The core-excited vibrational wave functions were mapped using dissociative and bound final states. â€¢ High-resolution experimental data is accompanied by ab initio calculations. â€¢ The mapping phenomenon allows one to extract constants of the molecular potentials. â€¢ The mapping techniques are general and can be applied for the study of many systems. - Abstract: The recent development of high brightness 3{sup rd} generation soft X-ray sources and high energy resolution electron spectrometers made it possible to accurately trace quantum phenomena associated to the vibrational dynamics in core-excited molecules. The present paper reviews the recent results on mapping of vibrational wave functions and molecular potentials based on electron spectroscopy. We discuss and compare the mapping phenomena in various systems, stressing the advantages of the resonant X-ray scattering for studying of the nuclear dynamics and spectroscopic constants of small molecules. The experimental results discussed in the paper are most often accompanied by state-of-the-art ab initio calculations allowing for a deeper understanding of the quantum effects. Besides its fundamental interest, the vibrational wave function mapping is shown to be useful for the analysis of core- and valence-excited molecular states based on the reflection principle.
A theoretical study of Ti nanoparticle effect on sodium water reaction: Using ab initio calculation
International Nuclear Information System (INIS)
Highlights: • We report effectiveness and possible mechanisms of Na atom and Ti nanoparticle surface interaction to the sodium water reaction. • Chemical interaction between Na atom and Ti nanoparticle surface was evaluated by ab initio calculation with density functional theory. • Covalent like metallic Na adsorbed atomic layer onto Ti(0 0 0 1) surface was found and its strength was evaluated as 1.65 eV. • We postulate that strong chemical interaction between Na atoms and Ti NP surface increase both hydration energy and activation energy required initiate SWR. - Abstract: The sodium–water reaction (SWR) is a serious impediment to development of sodium fast cooled nuclear reactors. The reaction can be suppressed by suspending 2 at% of 10-nm Ti nanoparticles (NPs) in liquid Na, but the mechanisms by which the Ti NPs suppress the SWR are not known. In this study, ab initio calculation revealed a strong chemical bond with interaction strength 1.65 eV (64.5 kT at 298 K) between Na atoms and the Ti NP surface. We postulate that this interaction with Ti atoms causes a layer of Na atoms to form on the Ti NP surface, and that this layer increases both hydration energy and the activation energy required to initiate SWR. As a result, these interactions suppress the SWR by reducing effective reaction area
Ab Initio Classical Dynamics Simulations of CO_2 Line-Mixing Effects in Infrared Bands
Lamouroux, Julien; Hartmann, Jean-Michel; Tran, Ha; Snels, Marcel; Stefani, Stefania; Piccioni, Giuseppe
2013-06-01
Ab initio calculations of line-mixing effects in CO_2 infrared bands are presented and compared with experiments. The predictions were carried using requantized Classical Dynamics Molecular Simulations (rCDMS) based on an approach previously developed and successfully tested for CO_2 isolated line shapes. Using classical dynamics equations, the force and torque applied to each molecule by the surrounding molecules (described by an ab initio intermolecular potential) are computed at each time step. This enables, using a requantization procedure, to predict dipole and isotropic polarizability auto-correlation functions whose Fourier-Laplace transforms yield the spectra. The quality of the rCDMS calculations is demonstrated by comparisons with measured spectra in the spectral regions of the 3?_3 and 2?_1+2?_2+?_3 Infrared bands. J.-M. Hartmann, H. Tran, N. H. Ngo, et al., Phys. Rev. Lett. A {87} (2013), 013403. H. Tran, C. Boulet, M. Snels, S. Stefani, J. Quant. Spectrosc. Radiat. Transfer {112} (2011), 925-936.
Ab initio prediction of the critical thickness of a precipitate.
Sampath, S; Janisch, R
2013-09-01
Segregation and precipitation of second phases in metals and metallic alloys is an important phenomenon that has a strong influence on the mechanical properties of the material. Models exist that describe the growth of coherent, semi-coherent and incoherent precipitates. One important parameter of these models is the energy of the interface between matrix and precipitate. In this work we apply ab initio density functional theory calculations to obtain this parameter and to understand how it depends on chemical composition and mechanical strain at the interface.Our example is a metastable Mo-C phase, the body-centred tetragonal structure, which exists as a semi-coherent precipitate in body-centred cubic molybdenum. The interface of this precipitate is supposed to change from coherent to semi-coherent during the growth of the precipitate. We predict the critical thickness of the precipitate by calculating the different contributions to a semi-coherent interface energy by means of ab initio density functional theory calculations. The parameters in our model include the elastic strain energy stored in the precipitate, as well as a misfit dislocation energy that depends on the dislocation core width and the dislocation spacing. Our predicted critical thickness agrees well with experimental observations. PMID:23896820
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...
Globally accurate ab initio based potential energy surface of H2O+(X 4Aâ€³)
Song, Yu-Zhi; Zhang, Yuan; Zhang, Lu-Lu; Gao, Shou-Bao; Meng, Qing-Tian
2015-06-01
A globally accurate potential energy surface is reported for the electronic ground-state H2O+. The ab initio energies utilized to map the potential energy surface are calculated at the multireference configuration interaction method employing the aug-cc-pVQZ basis set and the full valence complete active space wave function as reference. In order to improve accuracy of the resulting raw ab initio energies, they are then extrapolated to the complete basis set limit and most importantly to the full configuration-interaction limit by semiempirically correcting the dynamical correlation using the double many-body expansion-scaled external correlation method. The topographical features of the current potential energy surface were examined in detail, which agree nicely with those of other theoretical work. Project supported by the National Natural Science Foundation of China (Grant Nos. 11304185 and 11074151), China Postdoctoral Science Foundation (Grant No. 2014M561957), the Postdoctoral Innovation Project of Shandong Province, China (Grant No. 201402013), and the Shandong Provincial Natural Science Foundation, China (Grant No. ZR2014AM022).
Ab initio quantum chemistry in parallel-portable tools and applications
International Nuclear Information System (INIS)
In common with many of the computational sciences, ab initio chemistry faces computational constraints to which a partial solution is offered by the prospect of highly parallel computers. Ab initio codes are large and complex (O(105) lines of FORTRAN), representing a significant investment of communal effort. The often conflicting requirements of portability and efficiency have been successfully resolved on vector computers by reliance on matrix oriented kernels. This proves inadequate even upon closely-coupled shared-memory parallel machines. We examine the algorithms employed during a typical sequence of calculations. Then we investigate how efficient portable parallel implementations may be derived, including the complex multi-reference singles and doubles configuration interaction algorithm. A portable toolkit, modeled after the Intel iPSC and the ANL-ACRF PARMACS, is developed, using shared memory and TCP/IP sockets. The toolkit is used as an initial platform for programs portable between LANS, Crays and true distributed-memory MIMD machines. Timings are presented. 53 refs., 4 tabs
Ab initio calculations of the optical properties of crystalline and liquid InSb
Directory of Open Access Journals (Sweden)
Haruyuki Sano
2015-11-01
Full Text Available Ab initio calculations of the electronic and optical properties of InSb were performed for both the crystalline and liquid states. Two sets of atomic structure models for liquid InSb at 900 K were obtained by ab initio molecular dynamics simulations. To reduce the effect of structural peculiarities in the liquid models, an averaging of the two sets of the calculated electronic and optical properties corresponding to the two liquid models was performed. The calculated results indicate that, owing to the phase transition from crystal to liquid, the density of states around the Fermi level increases. As a result, the energy band gap opening near the Fermi level disappears. Consequently, the optical properties change from semiconductor to metallic behavior. Namely, owing to the melting of InSb, the interband transition peaks disappear and a Drude-like dispersion is observed in the optical dielectric functions. The optical absorption at a photon energy of 3.06 eV, which is used in Blu-ray Disc systems, increases owing to the melting of InSb. This increase in optical absorption is proposed to result from the increased optical transitions below 2 eV.
Ab initio determination of an extended Heisenberg Hamiltonian in CuO{sub 2} layers
Energy Technology Data Exchange (ETDEWEB)
Calzado, C.J.; Malrieu, J.P. [Lab. de Physique Quantique, IRSAMC, Univ. Paul Sabatier, Toulouse (France)
2001-06-01
Accurate ab initio calculations on embedded Cu{sub 4}O{sub 12} square clusters, fragments of the La{sub 2}CuO{sub 4} lattice, confirm a value of the nearest neighbor antiferromagnetic coupling (J = 124 meV) previously obtained from ab initio calculations on bicentric clusters and in good agreement with experiment. These calculations predict non negligible antiferromagnetic second-neighbor interaction (J' = 6.5 meV) and four-spin cyclic exchange (K = 14 meV), which may affect the thermodynamic and spectroscopic properties of these materials. The dependence of the magnetic coupling on local lattice distortions has also been investigated. Among them the best candidate to induce a spin-phonon effect seems to be the movement of the Cu atoms, changing the Cu-Cu distance, for which the variation of the nearest neighbor magnetic coupling with the Cu-O distance is {delta}J/{delta}d{sub Cu} {sub -} {sub O} {proportional_to} 1700 cm {sup -1} A {sup -1}. (orig.)
Directory of Open Access Journals (Sweden)
K. F. Sichilongo
2015-10-01
Full Text Available The gas phase ion chemistry of coumarins using electron ionization (EI, positive chemical ionization (PCI and negative chemical ionization (NCI in a time of flight and quadrupole mass spectrometer (qMS coupled to a gas chromatograph is outlined. The observations in NCI mode were complimented with Ab initio calculations. This was because NCI gave enhanced signals in contrast to EI and PCI and therefore could serve as a potential ionization method for quantitative analysis. For the two classes of analytes that were examined here, i.e. methyl and acetyl coumarin derivatives, experimental data showed that the methyl derivatives underwent dissociative electron capture to produce ions of the type [M–H]¯ while the acetyl derivatives underwent resonance electron capture to produce ions of the type M?¯. The M?¯ type of ions were more intense than the earlier. Ab initio calculations showed higher electron affinities in the acetyl than the methyl derivatives. DOI: http://dx.doi.org/10.4314/bcse.v29i3.15
International Nuclear Information System (INIS)
A new full-dimensional global potential energy surface (PES) for the acetylene-vinylidene isomerization on the ground (S0) electronic state has been constructed by fitting ?37 000 high-level ab initio points using the permutation invariant polynomial-neural network method with a root mean square error of 9.54 cm?1. The geometries and harmonic vibrational frequencies of acetylene, vinylidene, and all other stationary points (two distinct transition states and one secondary minimum in between) have been determined on this PES. Furthermore, acetylene vibrational energy levels have been calculated using the Lanczos algorithm with an exact (J = 0) Hamiltonian. The vibrational energies up to 12 700 cm?1 above the zero-point energy are in excellent agreement with the experimentally derived effective Hamiltonians, suggesting that the PES is approaching spectroscopic accuracy. In addition, analyses of the wavefunctions confirm the experimentally observed emergence of the local bending and counter-rotational modes in the highly excited bending vibrational states. The reproduction of the experimentally derived effective Hamiltonians for highly excited bending states signals the coming of age for the ab initio based PES, which can now be trusted for studying the isomerization reaction
Energy Technology Data Exchange (ETDEWEB)
Han, Huixian [Department of Chemistry and Chemical Biology, University of New Mexico, Albuquerque, New Mexico 87131 (United States); School of Physics, Northwest University, Xi’an, Shaanxi 710069 (China); Li, Anyang; Guo, Hua, E-mail: hguo@unm.edu [Department of Chemistry and Chemical Biology, University of New Mexico, Albuquerque, New Mexico 87131 (United States)
2014-12-28
A new full-dimensional global potential energy surface (PES) for the acetylene-vinylidene isomerization on the ground (S{sub 0}) electronic state has been constructed by fitting ?37 000 high-level ab initio points using the permutation invariant polynomial-neural network method with a root mean square error of 9.54 cm{sup ?1}. The geometries and harmonic vibrational frequencies of acetylene, vinylidene, and all other stationary points (two distinct transition states and one secondary minimum in between) have been determined on this PES. Furthermore, acetylene vibrational energy levels have been calculated using the Lanczos algorithm with an exact (J = 0) Hamiltonian. The vibrational energies up to 12 700 cm{sup ?1} above the zero-point energy are in excellent agreement with the experimentally derived effective Hamiltonians, suggesting that the PES is approaching spectroscopic accuracy. In addition, analyses of the wavefunctions confirm the experimentally observed emergence of the local bending and counter-rotational modes in the highly excited bending vibrational states. The reproduction of the experimentally derived effective Hamiltonians for highly excited bending states signals the coming of age for the ab initio based PES, which can now be trusted for studying the isomerization reaction.
Ab initio study on electron excitation and electron transfer in tryptophan-tyrosine system
International Nuclear Information System (INIS)
In this article, ab initio calculation has been performed to evaluate the transition energy of electronic excitation in tryptophan and tyrosine by using semiempirical molecular orbital method AM1 and complete active space self-consistent field method. The solvent effect has been considered by means of the conductor-like screening model. After geometric optimizations of isolated tryptophan and tyrosine, and their corresponding radicals and cations, reaction heat of these electron transfer reactions have been obtained by the means of complete active space self-consistent field method. The transition energies from the ground state, respectively, to the lowest excited state and to the lowest triplet state of these two amino acids are also calculated and compared with the experimentally observed values. The ionization potential and electron affinity are also calculated for tryptophan and tyrosine employing Koopmans' theorem and ab initio calculation. Compared with the experimental measurements, the theoretical results are found satisfactory. Theoretical results give good explanations on the experimental phenomena that N3· can preferably oxide the side chain of tryptophan residue and then the electron transfer from tyrosine residue to tryptophan residue follows in peptides involving tryptophan and tyrosine
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
Ab initio many-body calculations of nucleon-nucleus scattering
Quaglioni, Sofia
2009-01-01
We develop a new ab initio many-body approach capable of describing simultaneously both bound and scattering states in light nuclei, by combining the resonating-group method with the use of realistic interactions, and a microscopic and consistent description of the nucleon clusters. This approach preserves translational symmetry and Pauli principle. We outline technical details and present phase shift results for neutron scattering on 3H, 4He and 10Be and proton scattering on 3He and 4He, using realistic nucleon-nucleon (NN) potentials. Our A=4 scattering results are compared to earlier ab initio calculations. We find that the CD-Bonn NN potential in particular provides an excellent description of nucleon-4He S-wave phase shifts. On the contrary, the experimental nucleon-4He P-wave phase shifts are not well reproduced by any NN potential we use. We demonstrate that a proper treatment of the coupling to the n-10Be continuum is successful in explaining the parity-inverted ground state in 11Be.
Ab initio calculations of non-radiative carrier trapping due to deep impurity levels
Wang, Lin-Wang; Shi, Lin
2013-03-01
Non-radiative carrier decay due to deep impurity levels in semiconductors is an important process which affects the efficiencies of devices from solar cells to light emitting diode. This process is due to multiple phonon emission. Despite of the fact the analytical formalisms have been derived long time ago, so far there is no direct ab initio calculations due to the high cost of calculating all the electron-phonon coupling constants. Here we introduce an algorithm which calculates all the electron-phonon coupling constants at once, hence allows the ab initio calculations of such processes. Another approximation is introduced to calculate the phonon modes of a given impurity system. We use a ZnGa-VN paired defect in GaN as an example to study this process. We found that while most of the promoting phonon modes (used to promote the transition with the electron-phonon coupling) come from the optical modes, the accepting phonon modes (used to satisfy the energy conservation) come mostly from the acoustic phonons. This work is supported by SC/BES/MSED of the U.S. Department of Energy (DOE) under Contract No. DE-AC02-05CH11231, and by the National High Technology Research and Development Program of China (863 Program) (No. 2011AA03A103)
Malerba, L.; Ackland, G. J.; Becquart, C. S.; Bonny, G.; Domain, C.; Dudarev, S. L.; Fu, C.-C.; Hepburn, D.; Marinica, M. C.; Olsson, P.; Pasianot, R. C.; Raulot, J. M.; Soisson, F.; Terentyev, D.; Vincent, E.; Willaime, F.
2010-11-01
The objective of the FP6 Perfect Project was to develop a first example of integrated multiscale computational models, capable of describing the effects of irradiation in nuclear reactor components, namely vessel and internals. The use of ab initio techniques to study, in the most reliable way currently possible, atomic-level interactions between species and defects, and the transfer of this knowledge to interatomic potentials, of use for large scale dynamic simulations, lie at the core of this effort. The target materials of the Project were bainitic steels (vessel) and austenitic steels (internals), i.e. iron alloys. In this article, the advances made within the Project in the understanding of defect properties in Fe alloys, by means of ab initio calculations, and in the development of interatomic potentials for Fe and Fe alloys are overviewed, thereby providing a reference basis for further progress in the field. Emphasis is put in showing how the produced data have enhanced our level of understanding of microstructural processes occurring under irradiation in model alloys and steels used in existing nuclear power plants.
Ab initio rotation-vibration energies and intensities for the H 2F + molecule
Bunker, P. R.; Jensen, Per; Wright, J. S.; Hamilton, I. P.
1990-12-01
In a previous publication [I. D. Petsalakis, G. Theodorakopoulos, J. S. Wright, and I. P. Hamilton, J. Chem. Phys., 92, 2440-2449 (1990)] we reported the ab initio multireference configuration interaction calculation of the three-dimensional potential energy surface of the H 2F + molecule in the ground X˜1A 1 electronic state at 119 nuclear geometries spanning an energy range up to about 50 000 cm -1 above equilibrium. We fitted the 71 points within 33 000 cm -1 of equilibrium to an analytic expression and performed variational calculation of the vibrational energies in Jacobi coordinates using the Discrete Variable Representation and Distributed Gaussian Basis functions (DVR-DGB) technique. In the present paper we examine the effect on the vibrational energies of using a surface obtained by fitting through 52 points within 25 000 cm -1 of equilibrium. We use this surface in a variational calculation of the J = 0, 1, and 2 rotation-vibration energies using the Morse Oscillator Rigid Bender Internal Dynamics Hamiltonian [P. Jensen, J. Mol. Spectrosc., 128, 478-501 (1988); J. Chem. Soc. Faraday Trans. 2, 84, 1315-1340 (1988)]. The vibrational energies obtained are compared with those obtained by the DVR-DGB technique. We also calculate ab initio the dipole moment function and rotation-vibration intensities, and we simulate the ?2 band, which has not yet been observed.
Ab initio molecular dynamics simulations of ion–solid interactions in zirconate pyrochlores
International Nuclear Information System (INIS)
In this study, an ab initio molecular dynamics method is employed to study low energy recoil events in zirconate pyrochlores (A2Zr2O7, A = La, Nd and Sm). It shows that both cations and anions in Nd2Zr2O7 and Sm2Zr2O7 are generally more likely to be displaced than those in La2Zr2O7. The damage end states mainly consist of Frenkel pair defects, and the Frenkel pair formation energies in Nd2Zr2O7 and Sm2Zr2O7 are lower than those in La2Zr2O7. These results suggest that the order–disorder structural transition more easily occurs in Nd2Zr2O7 and Sm2Zr2O7 resulting in a defect-fluorite structure, which agrees well with experimental observations. Our calculations indicate that oxygen migration from 48f and 8b to 8a sites is dominant under low energy irradiation. A number of new defects, including four types of cation Frenkel pairs and six types of anion Frenkel pairs, are revealed by ab initio molecular dynamics simulations. The present results may help to advance the fundamental understanding of the irradiation response behavior of zirconate pyrochlores
International Nuclear Information System (INIS)
Highlights: • Some studies related to the vibrational wave functions mapping phenomenon are reviewed. • The core-excited vibrational wave functions were mapped using dissociative and bound final states. • High-resolution experimental data is accompanied by ab initio calculations. • The mapping phenomenon allows one to extract constants of the molecular potentials. • The mapping techniques are general and can be applied for the study of many systems. - Abstract: The recent development of high brightness 3rd generation soft X-ray sources and high energy resolution electron spectrometers made it possible to accurately trace quantum phenomena associated to the vibrational dynamics in core-excited molecules. The present paper reviews the recent results on mapping of vibrational wave functions and molecular potentials based on electron spectroscopy. We discuss and compare the mapping phenomena in various systems, stressing the advantages of the resonant X-ray scattering for studying of the nuclear dynamics and spectroscopic constants of small molecules. The experimental results discussed in the paper are most often accompanied by state-of-the-art ab initio calculations allowing for a deeper understanding of the quantum effects. Besides its fundamental interest, the vibrational wave function mapping is shown to be useful for the analysis of core- and valence-excited molecular states based on the reflection principle
Ionization dynamics of the water trimer: A direct ab initio MD study
International Nuclear Information System (INIS)
Highlights: ? We calculated ionization dynamics of water trimer. ? Direct ab initio molecular dynamics (MD) method is applied. ? Proton transfer dynamics were discussed. ? The proton transfer process calculated are well reproduced in recent experiments. - Abstract: Ionization dynamics of the cyclic water trimer (H2O)3 have been investigated by means of direct ab initio molecular dynamics (AIMD) method. Two reaction channels, complex formation and OH dissociation, were found following the ionization of (H2O)3. In both channels, first, a proton was rapidly transferred from H2O+ to H2O (time scale is ?15 fs after the ionization). In complex channel, an ion–radical contact pair (H3O+–OH) solvated by the third water molecule was formed as a long-lived H3O+(OH)H2O complex. In OH dissociation channel, the second proton transfer further takes place from H3O+(OH) to H2O (time scale is 50–100 fs) and the OH radical is separated from the H3O+. At the same time, the OH dissociation takes place when the excess energy is efficiently transferred into the kinetic energy of OH radical. The OH dissociation channel is significantly minor, and almost all product channels were the complex formation. The reaction mechanism was discussed on the basis of theoretical results
A theoretical study of Ti nanoparticle effect on sodium water reaction: Using ab initio calculation
Energy Technology Data Exchange (ETDEWEB)
Kim, Soo Jae; Park, Gunyeop [Department of Mechanical Engineering, POSTECH, Pohang, Gyungbuk 790-784 (Korea, Republic of); Kim, Moo Hwan [Korea Institute of Nuclear Safety, Yuseong, Daejeon 305-338 (Korea, Republic of); Park, Hyun Sun, E-mail: hejsunny@postech.ac.kr [Division of Advanced Nuclear Engineering, POSTECH, Pohang, Gyungbuk 790-784 (Korea, Republic of); Baek, JeHyun [Department of Mechanical Engineering, POSTECH, Pohang, Gyungbuk 790-784 (Korea, Republic of)
2015-01-15
Highlights: â€¢ We report effectiveness and possible mechanisms of Na atom and Ti nanoparticle surface interaction to the sodium water reaction. â€¢ Chemical interaction between Na atom and Ti nanoparticle surface was evaluated by ab initio calculation with density functional theory. â€¢ Covalent like metallic Na adsorbed atomic layer onto Ti(0 0 0 1) surface was found and its strength was evaluated as 1.65 eV. â€¢ We postulate that strong chemical interaction between Na atoms and Ti NP surface increase both hydration energy and activation energy required initiate SWR. - Abstract: The sodiumâ€“water reaction (SWR) is a serious impediment to development of sodium fast cooled nuclear reactors. The reaction can be suppressed by suspending 2 at% of 10-nm Ti nanoparticles (NPs) in liquid Na, but the mechanisms by which the Ti NPs suppress the SWR are not known. In this study, ab initio calculation revealed a strong chemical bond with interaction strength 1.65 eV (64.5 kT at 298 K) between Na atoms and the Ti NP surface. We postulate that this interaction with Ti atoms causes a layer of Na atoms to form on the Ti NP surface, and that this layer increases both hydration energy and the activation energy required to initiate SWR. As a result, these interactions suppress the SWR by reducing effective reaction area.