Showing papers on "Ab initio quantum chemistry methods published in 2006"
TL;DR: An efficient and reliable methodology for crystal structure prediction, merging ab initio total-energy calculations and a specifically devised evolutionary algorithm, which allows one to predict the most stable crystal structure and a number of low-energy metastable structures for a given compound at any P-T conditions without requiring any experimental input.
Abstract: We have developed an efficient and reliable methodology for crystal structure prediction, merging ab initio total-energy calculations and a specifically devised evolutionary algorithm. This method allows one to predict the most stable crystal structure and a number of low-energy metastable structures for a given compound at any P-T conditions without requiring any experimental input. Extremely high (nearly 100%) success rate has been observed in a few tens of tests done so far, including ionic, covalent, metallic, and molecular structures with up to 40 atoms in the unit cell. We have been able to resolve some important problems in high-pressure crystallography and report a number of new high-pressure crystal structures (stable phases: epsilon-oxygen, new phase of sulphur, new metastable phases of carbon, sulphur and nitrogen, stable and metastable phases of CaCO3). Physical reasons for the success of this methodology are discussed.
1,945 citations
TL;DR: An alternative approach is suggested, derived from the Hubbard U correction to solid-state problems, that provides an excellent agreement with correlated-electron quantum chemistry calculations in test cases that range from the ground state of Fe2 and Fe2- to the addition elimination of molecular hydrogen on FeO+.
Abstract: Transition-metal centers are the active sites for a broad variety of biological and inorganic chemical reactions. Notwithstanding this central importance, density-functional theory calculations based on generalized-gradient approximations often fail to describe energetics, multiplet structures, reaction barriers, and geometries around the active sites. We suggest here an alternative approach, derived from the Hubbard U correction to solid-state problems, that provides an excellent agreement with correlated-electron quantum chemistry calculations in test cases that range from the ground state of Fe-2 and Fe-2(-) to the addition elimination of molecular hydrogen on FeO+. The Hubbard U is determined with a novel self-consistent procedure based on a linear-response approach.
513 citations
TL;DR: In this paper, a vibrational analysis using NIR-FT Raman and FT-IR spectra is carried out to understand the structural and electronic contributions to hyperpolarizability in MPHB.
373 citations
350 citations
TL;DR: In this paper, a complete 6-dimensional potential energy surface for the benzene dimer obtained using symmetry-adapted perturbation theory (SAPT) of intermolecular interactions based on Kohn−Sham's description of monomers is presented.
Abstract: We present a complete 6-dimensional potential energy surface for the benzene dimer obtained using symmetry-adapted perturbation theory (SAPT) of intermolecular interactions based on Kohn−Sham's description of monomers. Ab initio calculations were performed for 491 dimer geometries in a triple-ζ-quality basis set supplemented by bond functions. An accurate analytic fit to the ab initio results has been developed and low-energy stationary points on the potential energy surface have been found. We have determined that there are three minima on the surface. Two of them, the tilted T-shape and the parallel-displaced, are nearly isoenergetic with interaction energies of −2.77 and −2.74 kcal/mol, respectively. The third minimum, a twisted edge-to-edge conformation, is significantly less attractive, with the interaction energy equal to −1.82 kcal/mol. Both the T-shape and sandwich geometries, sometimes assumed to be minima, are shown to be only saddle points. The potential energy surface is extremely flat between...
344 citations
344 citations
TL;DR: It is proposed that the standard, quasitetrahedral model of water, although approximate, represents a reasonably accurate description of the local structure of the liquid.
Abstract: We present a series of ab initio calculations of the x-ray absorption cross section (XAS) of ice and liquid water at ambient conditions. Our results show that all available experimental data and theoretical results are consistent with the standard model of the liquid as comprising molecules with approximately four hydrogen bonds. Our simulations of ice XAS including the lowest lying excitonic state are in excellent agreement with experiment and those of a quasitetrahedral model of water are in reasonable agreement with recent measurements. Hence we propose that the standard, quasitetrahedral model of water, although approximate, represents a reasonably accurate description of the local structure of the liquid.
342 citations
TL;DR: The results verify in both theoretical levels that SiCNTs seem to be more suitable materials for hydrogen storage than pure CNTs.
Abstract: A multiscale theoretical approach is used for the investigation of hydrogen storage in silicon-carbon nanotubes (SiCNTs). First, ab initio calculations at the density functional level of theory (DFT) showed an increase of 20% in the binding energy of H2 in SiCNTs compared with pure carbon nanotubes (CNTs). This is explained by the alternative charges that exist in the SiCNT walls. Second, classical Monte Carlo simulation of nanotube bundles showed an even larger increase of the storage capacity in SiCNTs, especially in low temperature and high-pressure conditions. Our results verify in both theoretical levels that SiCNTs seem to be more suitable materials for hydrogen storage than pure CNTs.
334 citations
TL;DR: It is demonstrated that the presented method provides much more accurate estimations for the SOC part of ZFS constants than the emerging density functional theory approaches.
Abstract: In this work, an implementation of an approach to calculate the zero-field splitting (ZFS) constants in the framework of ab initio methods such as complete active space self-consistent field, multireference configuration interaction, or spectroscopy oriented configuration interaction is reported. The spin-orbit coupling (SOC) contribution to ZFSs is computed using an accurate multicenter mean-field approximation for the Breit-Pauli Hamiltonian. The SOC parts of ZFS constants are obtained directly after diagonalization of the SOC operator in the basis of a preselected number of roots of the spin-free Hamiltonian. This corresponds to an infinite order treatment of the SOC in terms of perturbation theory. The spin-spin (SS) part is presently estimated in a mean-field fashion and appears to yield results close to the more complete treatments available in the literature. Test calculations for the first- and second-row atoms as well as first-row transition metal atoms and a set of diatomic molecules show accurate results for the SOC part of ZFSs. SS contributions have been found to be relatively small but not negligible (exceeding 1 cm(-1) for oxygen molecule). At least for the systems studied in this work, it is demonstrated that the presented method provides much more accurate estimations for the SOC part of ZFS constants than the emerging density functional theory approaches.
309 citations
TL;DR: The calculated migration energy of He is very low, around 0.06 eV, which indicates that He atoms can be trapped by other He atoms and can explain the formation of He blisters close to the surface of He implanted tungsten.
Abstract: We use state of the art ab initio calculations to obtain the diffusion properties of He in tungsten. The calculated migration energy of He is very low, around 0.06 eV. This value is much lower than the experimental field-ion microscopy results which lead to a migration energy of the order of 0.24-0.32 eV. The reason for this discrepancy is the high propensity for He to form He-He clusters characterized by a very large binding energy of the order of 1 eV. Such a large binding energy indicates that He atoms can be trapped by other He atoms and can explain the formation of He blisters close to the surface of He implanted tungsten.
275 citations
TL;DR: In this article, the elastic, electronic and lattice dynamical properties of chalcogenides (CdS, CdSe, and CdTe) were investigated based on norm-conserving pseudopotentials and density functional theory.
Abstract: Ab initio calculations, based on norm-conserving pseudopotentials and density functional theory, have been performed to investigate elastic, electronic and lattice dynamical properties of chalcogenides (CdS, CdSe, and CdTe). The calculated lattice parameters, elastic constants, band structures, and phonon dispersions are in good agreement with available experimental and theoretical results. We also presented the pressure-dependence of elastic constants and the pressure-dependence of band gaps.
TL;DR: The binuclear [FeIII(bpca)(mu-bpca)Dy(NO3)4], having Single Molecule Magnet (SMM) properties, belonging to a series of isostructural FeIIILnIII complexes and closely related FeIILnIII chain structures, was characterized in concise experimental and theoretical respects.
Abstract: The binuclear [FeIII(bpca)(μ-bpca)Dy(NO3)4], having Single Molecule Magnet (SMM) properties, belonging to a series of isostructural FeIIILnIII complexes (Ln = Eu, Gd, Tb, Dy, Ho) and closely related FeIILnIII chain structures, was characterized in concise experimental and theoretical respects. The low temperature magnetization data showed hysteresis and tunneling. The anomalous temperature dependence of Mossbauer spectra is related to the onset of magnetic order, consistent with the magnetization relaxation time scale resulting from AC susceptibility measurements. The advanced ab initio calculations (CASSCF and spin−orbit) revealed the interplay of ligand field, spin−orbit, and exchange effects and probed the effective Ising nature of the lowest states, involved in the SMM and tunneling effects.
TL;DR: In this article, a structure model for a broad boron sheet is discussed, which is the analog of a single graphite sheet, and the precursor of borin nanotubes.
Abstract: Based on a numerical ab initio study, we discuss a structure model for a broad boron sheet, which is the analog of a single graphite sheet, and the precursor of boron nanotubes. The sheet has linear chains of $sp$ hybridized $\ensuremath{\sigma}$ bonds lying only along its armchair direction, a high stiffness, and anisotropic bonds properties. The puckering of the sheet is explained as a mechanism to stabilize the $sp$ $\ensuremath{\sigma}$ bonds. The anisotropic bond properties of the boron sheet lead to a two-dimensional reference lattice structure, which is rectangular rather than triangular. As a consequence the chiral angles of related boron nanotubes range from 0\ifmmode^\circ\else\textdegree\fi{} to 90\ifmmode^\circ\else\textdegree\fi{}. Given the electronic properties of the boron sheets, we demonstrate that all of the related boron nanotubes are metallic, irrespective of their radius and chiral angle, and we also postulate the existence of helical currents in ideal chiral nanotubes. Furthermore, we show that the strain energy of boron nanotubes will depend on their radii, as well as on their chiral angles. This is a rather unique property among nanotubular systems, and it could be the basis of a different type of structure control within nanotechnology.
TL;DR: A method for modeling infrared solvent shifts using the electrostatic field generated by the solvent is presented and the generated map, which takes into account the electric field and its gradients at four molecular positions, is tested in a number of common solvents.
Abstract: A method for modeling infrared solvent shifts using the electrostatic field generated by the solvent is presented. The method is applied to the amide I vibration of N-methyl acetamide. Using ab initio calculations the fundamental frequency, anharmonicity, and the transition dipoles between the three lowest vibrational states are parametrized in terms of the electrostatic field. The generated map, which takes into account the electric field and its gradients at four molecular positions, is tested in a number of common solvents. Agreement of solvent shift and linewidths with experimental Fourier transform infrared (FTIR) data is found to within seven and four wave numbers, respectively, for polar solvents. This shows that in these solvents electrostatic contributions dominate solvation effects and the map is transferable between these types of solvents. The effect of motional narrowing arising from the fast solvent fluctuations is found to be substantial for the FTIR spectra. Also the two-dimensional infrared (2DIR) spectra, simulated using the constructed map, reproduce experimental results very well. The effect of anharmonicity fluctuations on the 2DIR spectra was found to be negligible.
TL;DR: In this article, the authors present electrochemical and spectroscopic evidence indicating that reductive decomposition of the bis(trifluoromethanesulfonyl)amide (TFSA) anion begins at −2.0 V vs. Fc+/Fc, well before the reported cathodic limit.
Abstract: The bis(trifluoromethanesulfonyl)amide (TFSA) anion is widely studied as an ionic liquid (IL) forming anion which imparts many useful properties, notably electrochemical stability. Here we present electrochemical and spectroscopic evidence indicating that reductive decomposition of the bis(trifluoromethanesulfonyl)amide (TFSA) anion begins at ~ −2.0 V vs. Fc+/Fc, well before the reported cathodic limit for many of these ILs. These processes are shown to be dependent upon the electrode substrate and are influenced by the water content of the IL. Supporting ab initio calculations are presented which suggest a possible mechanism for the anion decomposition. The products appear to passivate the electrode surface and the implications of this behaviour are discussed.
TL;DR: An overview of wavefunction-based correlation methods generalised for the application to solids is presented in this paper, where the authors focus on the so-called method of increments where the correlation energy of the solid is expanded in terms of localised orbitals or of a group of localized orbitals.
TL;DR: A local ab initio density matrix renormalization group algorithm is devised to describe multireference correlations in large systems and can obtain an exact characterization of correlation for long molecules that are extended in one of their spatial dimensions with a cost that scales only quadratically with the size of the system.
Abstract: We have devised a local ab initio density matrix renormalization group algorithm to describe multireference correlations in large systems. For long molecules that are extended in one of their spatial dimensions, we can obtain an exact characterization of correlation, in the given basis, with a cost that scales only quadratically with the size of the system. The reduced scaling is achieved solely through integral screening and without the construction of correlation domains. We demonstrate the scaling, convergence, and robustness of the algorithm in polyenes and hydrogen chains. We converge to exact correlation energies (in the sense of full configuration interaction, with 1–10μE_h precision) in all cases and correlate up to 100 electrons in 100 active orbitals. We further use our algorithm to obtain exact energies for the metal-insulator transition in hydrogen chains and compare and contrast our results with those from conventional quantum chemical methods.
TL;DR: The magnetic interactions in organic diradicals, dinuclear inorganic complexes and ionic solids are presented from a unified point of view and density functional theory calculations within the broken symmetry approach are critically revised showing the contradictions of this procedure when applied to molecules and solids without the guidelines of the appropriate mapping.
Abstract: The magnetic interactions in organic diradicals, dinuclear inorganic complexes and ionic solids are presented from a unified point of view. Effective Hamiltonian theory is revised to show that, for a given system, it permits the definition of a general, unbiased, spin model Hamiltonian. Mapping procedures are described which in most cases permit one to extract the relevant magnetic coupling constants from ab initio calculations of the energies of the pertinent electronic states. Density functional theory calculations within the broken symmetry approach are critically revised showing the contradictions of this procedure when applied to molecules and solids without the guidelines of the appropriate mapping. These concepts are illustrated by describing the application of state-of-the-art methods of electronic structure calculations to a series of representative molecular and solid state systems.
TL;DR: In this article, the phase stability of supersaturated c-Ti1−xAlxN not only depends on the chemical composition but also on the Al distribution of the metal sublattice.
Abstract: Ti1−xAlxN films and/or their alloys are employed in many industrial applications due to their excellent mechanical and thermal properties. Synthesized by plasma-assisted vapor deposition, Ti1−xAlxN is reported to crystallize in the cubic NaCl (c) structure for AlN mole fractions below 0.4–0.91, whereas at larger Al contents the hexagonal ZnS-wurtzite (w) structure is observed. Here we use ab initio calculations to analyze the effect of composition and Al distribution on the metal sublattice on phase stability, structure, and elastic properties of c-Ti1−xAlxN and w-Ti1−xAlxN. We show that the phase stability of supersaturated c-Ti1−xAlxN not only depends on the chemical composition but also on the Al distribution of the metal sublattice. An increase of the metastable solubility limit of AlN in c-Ti1−xAlxN from 0.64 to 0.74 is obtained by decreasing the number of Ti–Al bonds. This can be understood by considering the Al distribution induced changes of the electronic structure, bond energy, and configuration...
TL;DR: The present study indicates that the nature of the potential surfaces for the phosphoryl transfer reactions in solution and proteins is quite complicated and cannot be determined in a conclusive way without the use of careful theoretical studies that should, of course, reproduce the available experimental information.
Abstract: The nature of the hydrolysis of phosphate monoester dianions in solutions and in proteins is a problem of significant current interest. The present work explores this problem by systematic calculations of the potential surfaces of the reactions of a series of phosphate monoesters with different leaving groups. These calculations involve computational studies ranging from ab initio calculations with implicit solvent models to ab initio QM/MM free energy calculations. The calculations reproduce the observed linear free energy relationship (LFER) for the solution reaction and thus are consistent with the overall experimental trend and can be used to explore the nature of the transition state (TS) region, which is not accessible to direct experimental studies. It is found that the potential surface for the associative and dissociative paths is very flat and that the relative height of the associative and dissociative TS is different in different systems. In general, the character of the TS changes from associ...
TL;DR: Good agreement has been found with condensed phase experimental results using parameters derived from gas phase ab initio calculations and the coordination of the calcium cation is influenced by the size of the periodic water box, a recurrent issue in first principles molecular dynamics studies.
Abstract: Molecular dynamics simulations were performed using a modified amoeba force field to determine hydration and dynamical properties of the divalent cations Ca2+ and Mg2+. The extension of amoeba to divalent cations required the introduction of a cation specific parametrization. To accomplish this, the Thole polarization damping model parametrization was modified based on the ab initio polarization energy computed by a constrained space orbital variation energy decomposition scheme. Excellent agreement has been found with condensed phase experimental results using parameters derived from gas phase ab initio calculations. Additionally, we have observed that the coordination of the calcium cation is influenced by the size of the periodic water box, a recurrent issue in first principles molecular dynamics studies.
TL;DR: Ab initio calculations of the migration barrier for H are presented, yielding a barrier of less than approximately 0.5 eV, indicating isolated hydrogen is mobile at low temperature and that thermally stable H-related donors must logically be trapped at other defects.
Abstract: Zinc oxide, a wide-gap semiconductor, typically exhibits $n$-type conductivity even when nominally undoped. The nature of the donor is contentious, but hydrogen is a prime candidate. We present ab initio calculations of the migration barrier for H, yielding a barrier of less than $\ensuremath{\sim}0.5\text{ }\text{ }\mathrm{eV}$. This indicates isolated hydrogen is mobile at low temperature and that thermally stable H-related donors must logically be trapped at other defects. We argue this is also true for other oxides where H is a shallow donor.
TL;DR: Compared to recent ab initio MD simulations based on PW basis sets, less significant overstructuring was found in the RDFs and the distributions of hydrogen bond angles, suggesting that previous plane-wave and Gaussian basis set calculations have exaggerated the tendency toward over Structural properties.
Abstract: Structural properties of liquid water at ambient temperature were studied using Car-Parrinello [Phys. Rev. Lett.55, 2471 (1985)] ab initiomolecular dynamics (CPAIMD) simulations combined with the Kohn-Sham (KS) density functional theory and the BLYP exchange-correlation functional for the electronic structure. Unlike other recent work on the same subject, where plane-wave (PW) or hybrid Gaussian/plane-wave basis sets were employed, in the present paper, a discrete variable representation (DVR) basis set is used to expand the KS orbitals, so that with the real-space grid adapted in the present work, the properties of liquid water could be obtained very near the complete basis set limit. Structural properties of liquid water were extracted from a 30 ps CPAIMD-BLYP/DVR trajectory at 300 K . The radial distribution functions (RDFs), spatial distribution functions, and hydrogen bond geometry obtained from the CPAIMD-BLYP/DVR simulation are generally in good agreement with the most up to date experimental measurements. Compared to recent ab initioMD simulations based on PW basis sets, less significant overstructuring was found in the RDFs and the distributions of hydrogen bond angles, suggesting that previous plane-wave and Gaussian basis set calculations have exaggerated the tendency toward overstructuring.
TL;DR: In this article, the magnetic and electronic properties and phase stabilities of the Heusler compounds Co2MSi (with M=Ti,V,Cr,Mn,Fe,Co,Ni) were investigated.
Abstract: By means of density functional calculations, the magnetic and electronic properties and phase stabilities of the Heusler compounds Co2MSi (with M=Ti,V,Cr,Mn,Fe,Co,Ni) were investigated. Based on the calculated results, we predict the ferromagnetic phases of the compounds Co2TiSi, Co2VSi, and Co2CrSi to be half metals. Of particular interest is Co2CrSi because of its high density of majority-spin states at Fermi energy in combination with a reasonably high estimated Curie temperature of 747K. The compounds Co2TiSi and Co2VSi are thermodynamically stable, whereas Co2CrSi is of a metastable phase which might be stabilized by suitable experimental techniques.
TL;DR: In this article, the presence of metal-→borane interactions in the resulting square-pyramidal complexes is highlighted by structural analyses and DFT calculations, and it is shown that Diphosphanylborane derivative 1 behaves as a tridentate, ambiphilic ligand towards rhodium(I) fragments.
Abstract: To and fro: Diphosphanylborane derivative 1 behaves as a tridentate, ambiphilic ligand towards rhodium(I) fragments (see picture). The presence of metal-→borane interactions in the resulting square-pyramidal complexes is highlighted by structural analyses and DFT calculations. (Chemical Equation Presented). © 2006 Wiley-VCH Verlag GmbH & Co. KGaA.
TL;DR: The results show very good agreement with density functional theory calculations, reproducing the individual CSOV energy contributions for a given interaction as well as the B3LYP total interaction energies with errors below kBT at room temperature.
Abstract: Total intermolecular interaction energies are determined with a first version of the Gaussian electrostatic model (GEM-0), a force field based on a density fitting approach using s-type Gaussian functions. The total interaction energy is computed in the spirit of the sum of interacting fragment ab initio (SIBFA) force field by separately evaluating each one of its components: electrostatic (Coulomb), exchange repulsion, polarization, and charge transfer intermolecular interaction energies, in order to reproduce reference constrained space orbital variation (CSOV) energy decomposition calculations at the B3LYP/aug-cc-pVTZ level. The use of an auxiliary basis set restricted to spherical Gaussian functions facilitates the rotation of the fitted densities of rigid fragments and enables a fast and accurate density fitting evaluation of Coulomb and exchange-repulsion energy, the latter using the overlap model introduced by Wheatley and Price [Mol. Phys. 69, 50718 (1990)]. The SIBFA energy scheme for polarization and charge transfer has been implemented using the electric fields and electrostatic potentials generated by the fitted densities. GEM-0 has been tested on ten stationary points of the water dimer potential energy surface and on three water clusters (n=16,20,64). The results show very good agreement with density functional theory calculations, reproducing the individual CSOV energy contributions for a given interaction as well as the B3LYP total interaction energies with errors below kBT at room temperature. Preliminary results for Coulomb and exchange-repulsion energies of metal cation complexes and coupled cluster singles doubles electron densities are discussed.
TL;DR: XPS spectra were calculated for various cluster models which are designed to describe different possibilities of stabilizing the polar O-terminated ZnO(0001) surface by the adsorption of H atoms and are in excellent agreement with the available experimental data for all systems included in the present study.
Abstract: O1s core level binding energies of oxygen atoms in bulk ZnO, at different ZnO surfaces, and in some Zn oxo compounds were calculated by means of wave function based quantum chemical ab initio methods. Initial and final state effects were obtained by Koopmans’ theorem and at the ΔSCF level, respectively. After correction for scalar relativistic effects and electron correlation, the calculated XPS peak positions are in excellent agreement with the available experimental data for all systems included in the present study. The O1s core level shifts between an isolated H2O molecule and the Zn oxo compounds or ZnO, as well as between oxygen atoms in bulk ZnO and at various ZnO surfaces, can be understood by means of Madelung potentials and electronic relaxation or screening. XPS spectra were calculated for various cluster models which are designed to describe different possibilities of stabilizing the polar O-terminated ZnO(000) surface by the adsorption of H atoms. The experimental spectra are only compatible with the theoretical results for the fully hydroxylated H–ZnO(000) surface exhibiting a (1 × 1) surface structure.
TL;DR: In this article, the diameter and chirality dependences of the binding energies, sizes, and bright-dark splittings of excitons in semiconducting single-wall carbon nanotubes were calculated.
Abstract: We calculate the diameter and chirality dependences of the binding energies, sizes, and bright-dark splittings of excitons in semiconducting single-wall carbon nanotubes. Using results and insights from ab initio calculations, we employ a symmetry-based variational method within the effective-mass and envelope-function approximations using tight-binding wave functions. Binding energies and spatial extents show a leading dependence on diameter as $1∕d$ and $d$, respectively, with chirality corrections providing a spread of roughly 20% with a strong family behavior. Bright-dark exciton splittings show a $1∕{d}^{2}$ leading dependence. We provide analytical expressions for the binding energies, sizes, and splittings that should be useful to guide future experiments.
TL;DR: This work demonstrates that ab initio structure solution of these nanostructured materials is feasible using diffraction data in combination with distance geometry methods, and presents and validate two algorithms for structure reconstruction from precise unassigned interatomic distances for a range of clusters.
Abstract: Advances in materials science and molecular biology followed rapidly from the ability to characterize atomic structure using single crystals. Structure determination is more difficult if single crystals are not available. Many complex inorganic materials that are of interest in nanotechnology have no periodic long-range order and so their structures cannot be solved using crystallographic methods. Here we demonstrate that ab initio structure solution of these nanostructured materials is feasible using diffraction data in combination with distance geometry methods. Precise, sub-angstrom resolution distance data are experimentally available from the atomic pair distribution function (PDF). Current PDF analysis consists of structure refinement from reasonable initial structure guesses and it is not clear, a priori, that sufficient information exists in the PDF to obtain a unique structural solution. Here we present and validate two algorithms for structure reconstruction from precise unassigned interatomic distances for a range of clusters. We then apply the algorithms to find a unique, ab initio, structural solution for C60 from PDF data alone. This opens the door to sub-angstrom resolution structure solution of nanomaterials, even when crystallographic methods fail.