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Showing papers on "Ab initio quantum chemistry methods published in 2001"


Journal ArticleDOI
TL;DR: The effective fragment potential (EFP) method as discussed by the authors was developed to describe aqueous solvation, by representing Coulombic, induction and repulsive interactions via one-electron terms in the ab initio Hamiltonian.
Abstract: The effective fragment potential (EFP) method is described and its capabilities illustrated using several applications. The original method, EFP1, was primarily developed to describe aqueous solvation, by representing Coulombic, induction and repulsive interactions via one-electron terms in the ab initio Hamiltonian. It is demonstrated, using water clusters, the Menshutkin reaction and the glycine neutral/zwitterion equilibrium, that agreement with both fully ab initio calculations and experiment are excellent. More recently, the model has been extended so that it can treat any solvent, as well as more difficult links across covalent bonds.

579 citations


Journal ArticleDOI
02 Feb 2001-Science
TL;DR: Ab initio studies indicate that Al4(2-) exhibits characteristics of aromaticity with two delocalized pi electrons and a square planar structure and maintains its structural and electronic features in all the MAl4- complexes.
Abstract: Aromaticity is a concept invented to account for the unusual stability of an important class of organic molecules: the aromatic compounds. Here we report experimental and theoretical evidence of aromaticity in all-metal systems. A series of bimetallic clusters with chemical composition MAl4– (M = Li, Na, or Cu), was created and studied with photoelectron spectroscopy and ab initio calculations. All the MAl4– species possess a pyramidal structure containing an M+ cation interacting with a square Al42– unit. Ab initio studies indicate that Al42– exhibits characteristics of aromaticity with two delocalized π electrons (thus following the 4n + 2 electron counting rule) and a square planar structure and maintains its structural and electronic features in all the MAl4– complexes. These findings expand the aromaticity concept into the arena of all-metal species.

560 citations


Journal ArticleDOI
TL;DR: In this paper, an alternative approach to the original Car-Parrinello method where the density matrix elements (instead of the molecular orbitals) are propagated together with the nuclear degrees of freedom is proposed.
Abstract: We propose and implement an alternative approach to the original Car–Parrinello method where the density matrix elements (instead of the molecular orbitals) are propagated together with the nuclear degrees of freedom. Our new approach has the advantage of leading to an O(N) computational scheme in the large system limit. Our implementation is based on atom-centered Gaussian orbitals, which are especially suited to deal effectively with general molecular systems. The methodology is illustrated by applications to the three-body dissociation of triazine and to the dynamics of a cluster of a chloride ion with 25 water molecules.

514 citations


Journal ArticleDOI
TL;DR: In this article, a new hybrid exchange-correlation functional, denoted B97-2, was derived from correlated ab initio electron densities, and the potentials were examined graphically and used in conjunction with conventional thermochemical data to determine a new functional.
Abstract: Multiplicative potentials, appropriate for adding to the non-multiplicative fractional orbital exchange term in the Kohn–Sham equations, are determined from correlated ab initio electron densities. The potentials are examined graphically and are used in conjunction with conventional thermochemical data to determine a new hybrid exchange-correlation functional, denoted B97-2. Calculations using B97-2 are compared with those from (a) the B97-1 functional [J. Chem. Phys. 109, 6264 (1998)], which has the same functional form and fraction of orbital exchange, but was fitted to just thermochemical data; and (b) the widely used B3LYP functional [J. Chem. Phys. 98, 5648 (1993)]. B97-2 atomization energies are close to those from B97-1; total electronic energies and ionization potentials are less accurate, but remain an improvement over B3LYP. Molecular structures from all three functionals are comparable. Static isotropic polarizabilities improve from B3LYP to B97-1 to B97-2; the B97-2 functional underestimates experimental values, which is consistent with the neglect of zero-point vibrational corrections. NMR shielding constants—determined as the conventional second derivative of the electronic energy—improve from B3LYP to B97-1 to B97-2. Shieldings determined directly from these DFT electron densities using the recently proposed MKS approach [Chem. Phys. Lett. 337, 341 (2001)] are two to three times more accurate than the conventional shieldings, and exhibit an analogous improvement across the three functionals. Classical reaction barriers for sixteen chemical reactions improve significantly from B3LYP to B97-1 to B97-2. The introduction of multiplicative potentials into semi-empirical hybrid functional development therefore appears beneficial.

498 citations


Journal ArticleDOI
TL;DR: In this article, it was found that the most stable geometries arise from a fusion of tetrameric or pentameric rings, while the most unstable geometry arises from either the cuboid or the fused pentamers.
Abstract: Extensive ab initio calculations have been performed using the 6-31G(d,p) and 6-311++G(2d,2p) basis sets for several possible structures of water clusters (H2O)n, n = 8−20. It is found that the most stable geometries arise from a fusion of tetrameric or pentameric rings. As a result, (H2O)n, n = 8, 12, 16, and 20, are found to be cuboids, while (H2O)10 and (H2O)15 are fused pentameric structures. For the other water clusters (n = 9, 11, 13, 14, and 17−19) under investigation, the most stable geometries can be thought of as arising from either the cuboid or the fused pentamers or a combination thereof. The stability of some of the clusters, namely, n = 8−16, has also been studied using density functional theory. An attempt has been made to estimate the basis set superposition error and zero-point energy correction for such clusters at the Hartree−Fock (HF) level using the 6-311++G(2d,2p) basis set. To ensure that a minimum on the potential-energy surface has been located, frequency calculations have been c...

462 citations


Journal ArticleDOI
12 Jan 2001-Science
TL;DR: The compact molecular arrangement, intermolecular overlap integrals of the highest occupied and lowest unoccupied molecular orbitals, and tight-binding electronic band structure calculation revealed that [Ni(tmdt)2] is a three-dimensional synthetic metal composed of planar molecules.
Abstract: Molecular metals normally require charge transfer between two different chemical species. We prepared crystals of [Ni(tmdt) 2 ] (tmdt, trimethylenetetrathiafulvalenedithiolate) and carried out crystal structure analyses and resistivity measurements. The analyses and measurements revealed that these single-component molecular crystals are metallic from room temperature down to 0.6 kelvin. Ab initio molecular orbital calculations suggested that π molecular orbitals form conduction bands. The compact molecular arrangement, intermolecular overlap integrals of the highest occupied and lowest unoccupied molecular orbitals, and tight-binding electronic band structure calculation revealed that [Ni(tmdt) 2 ] is a three-dimensional synthetic metal composed of planar molecules.

428 citations


Journal ArticleDOI
TL;DR: In this paper, the performance of two recent ab initio computational thermochemistry schemes, W1 and W2 theory, was assessed for an enlarged sample of thermochemical data consisting of the ionization potentials and electron affinities in the G2-1 and G2 -2 sets, as well as the heats of formation in the g2- 1 and a subset of the G 2 -2 set.
Abstract: The performance of two recent ab initio computational thermochemistry schemes, W1 and W2 theory [J. M. L. Martin and G. de Oliveira, J. Chem. Phys. 111, 1843 (1999)], is assessed for an enlarged sample of thermochemical data consisting of the ionization potentials and electron affinities in the G2-1 and G2-2 sets, as well as the heats of formation in the G2-1 and a subset of the G2-2 set. We find W1 theory to be several times more accurate for ionization potentials and electron affinities than commonly used (and less expensive) computational thermochemistry schemes such as G2, G3, and CBS-QB3: W2 theory represents a slight improvement for electron affinities but no significant one for ionization potentials. The use of a two-point A+B/L5 rather than a three-point A+B/CL extrapolation for the self-consistent field (SCF) component greatly enhances the numerical stability of the W1 method for systems with slow basis set convergence. Inclusion of first-order spin–orbit coupling is essential for accurate ioniza...

427 citations


Journal ArticleDOI
TL;DR: In this article, a generalized force field for linear, branched, and cyclic perfluoroalkanes was developed by fitting to conformational profiles from gas phase ab initio calculations (LMP2/cc-pVTZ(-f)//HF/6-31G*) and to experimental data for pure liquids.
Abstract: Classical OPLS-AA force-field parameters are developed for perfluoroalkanes primarily by fitting to conformational profiles from gas-phase ab initio calculations (LMP2/cc-pVTZ(-f)//HF/6-31G*) and to experimental data for pure liquids. The ab initio C−C−C−C profile of n-C4F10 (perfluorobutane) is similar to those from prior high-level calculations and indicates the presence of gauche (g) and ortho (o) minima and of anti (a) minima slightly offset from 180°. Ab initio torsional profiles for n-C5F12 (perfluoropentane) and (CF3)2CFCF2CF3 (perfluoro-2-methylbutane) also show three sets of energy minima. Special OPLS-AA torsional parameters for these three molecules closely match ab initio and experimental geometries, conformational energies (ΔEmin), and conformational energy barriers. These specialized force fields were merged to provide a generalized force field for linear, branched, and cyclic perfluoroalkanes. The resultant parameters yield key ΔEmin values within 0.6 kcal/mol of the ab initio results for t...

404 citations


Journal ArticleDOI
TL;DR: In this article, the structure of the polar surfaces of ZnO was studied using ab initio calculations and surface x-ray diffraction and the experimental and theoretical relaxations were in good agreement.
Abstract: The structures of the polar surfaces of ZnO are studied using ab initio calculations and surface x-ray diffraction. The experimental and theoretical relaxations are in good agreement. The polar surfaces are shown to be very stable; the cleavage energy for the (0001)-Zn and (0001;)-O surfaces is 4.0 J/m(2) comparable to 2.32 J/m(2) for the most stable nonpolar (1010) surface. The surfaces are stabilized by an electronic mechanism involving the transfer of 0.17 electrons between them. This leads to 2D metallic surface states, which has implications for the use of the material in gas sensing and catalytic applications.

396 citations


Journal ArticleDOI
TL;DR: In this paper, the Bader theory of atoms in molecules is also applied for an estimation of electronic densities at bond critical points and Laplacians of these densities, these topological parameters are also used to define H-bond strength measures.
Abstract: Different measures of H-bond strength based on X−H proton donating bond properties and on parameters of H···Y distance (Y−proton acceptor within X−H···Y H bridges) are investigated. Correlations between such measures and H-bond energy are studied. The parameters of H-bonds are taken from geometry of simple complexes optimized within HF/6-311++G** and MP2/6-311++G** levels of theory. The Bader theory of atoms in molecules is also applied for an estimation of electronic densities at bond critical points and Laplacians of these densities, these topological parameters are also used to define H-bond strength measures. Apart from the conventional statistical analysis, the factor analysis is applied to study the properties of H bridges. The results show that the set of geometrical, energetic, and topological variables describing the H bridge may be replaced by one new variable, one factor. It is also shown that the geometrical and topological parameters of the proton donating bond better correlate with the H-bon...

395 citations


Journal ArticleDOI
TL;DR: In this article, the density matrix is propagated with Gaussian orbitals, which facilitates the use of larger time steps for the dynamics process and a rigorous analysis of energy conservation is presented and used to control the deviation of the fictitious dynamics trajectory from the corresponding Born-Oppenheimer dynamics trajectory.
Abstract: A generalization is presented here for a newly developed approach to ab initio molecular dynamics, where the density matrix is propagated with Gaussian orbitals. Including a tensorial fictitious mass facilitates the use of larger time steps for the dynamics process. A rigorous analysis of energy conservation is presented and used to control the deviation of the fictitious dynamics trajectory from the corresponding Born–Oppenheimer dynamics trajectory. These generalizations are tested for the case of the Cl−(H2O)25 cluster. It is found that, even with hydrogen atoms present in the system, no thermostats are necessary to control the exchange of energy between the nuclear and the fictitious electronic degrees of freedom.

Journal ArticleDOI
TL;DR: In this paper, the binding energies of various Cu containing defects believed to play an important role in the embrittlement of pressure vessel steels under radiation are calculated and discussed, and convergence of the results with the simulated system size is examined.
Abstract: Point defects in Fe and dilute FeCu alloys are investigated hy ab initio calculations based on density functional theory. The relaxed vacancy and interstitial and substitutional Cu atom formation energies are determined as well as some migration energies. The binding energies of various Cu containing defects believed to play an important role in the embrittlement of pressure vessel steels under radiation are calculated and discussed. The results are consistent with a Cu transport via a vacancy mechanism. The introduction of a Cu atom decreases the energy difference between the dumbbell and the dumbbell configurations, and should make the dumbbell rotation motion easier. The ab initio results are compared to the figures obtained with empirical interatomic potentials. The convergence of the results with the simulated system size is examined.

Journal ArticleDOI
TL;DR: Both Raman scattering and ab initio calculations indicate that FeCl(4)(2-) is the predominant iron-containing species in these liquids and both Hartree-Fock and density functional theory methods to compute the optimized structures and vibrational spectra for these species.
Abstract: We have prepared ionic liquids by mixing either iron(II) chloride or iron(III) chloride with 1-butyl-3-methylimidazolium chloride (BMIC). Iron(II) chloride forms ionic liquids from a mole ratio of 1 FeCl2/3 BMIC to almost 1 FeCl2/1 BMIC. Both Raman scattering and ab initio calculations indicate that FeCl42- is the predominant iron-containing species in these liquids. Iron(III) chloride forms ionic liquids from a mole ratio of 1 FeCl3/1.9 BMIC to 1.7 FeCl3/1 BMIC. When BMIC is in excess, Raman scattering indicates the presence of FeCl4-. When FeCl3 is in excess, Fe2Cl7- begins to appear and the amount of Fe2Cl7- increases with increasing amounts of FeCl3. Ionic liquids were also prepared from a mixture of FeCl2 and FeCl3 and are discussed. Finally, we have used both Hartree−Fock and density functional theory methods to compute the optimized structures and vibrational spectra for these species. An analysis of the results using an all-electron basis set, 6-31G*, as well as two different effective core potent...

Journal ArticleDOI
TL;DR: In this paper, the diffusion mechanism of an excess proton in imidazole molecule chains is studied by Car-Parrinello-type ab initio molecular dynamics simulations.

Journal ArticleDOI
TL;DR: In this paper, a first-principles determination of the absolute hydration free energy of the proton ΔGhyd298(H+) by using the latest developments in electronic structure theory including solvation effects is reported.
Abstract: The absolute hydration free energy of the proton, ΔGhyd298(H+), is one of the fundamental quantities for the thermodynamics of aqueous systems. Its exact value remains unknown despite extensive experimental and computational efforts. We report a first-principles determination of ΔGhyd298(H+) by using the latest developments in electronic structure theory including solvation effects. High level ab initio calculations have been performed with a supermolecule-continuum approach based on a recently developed self-consistent reaction field model known as surface and volume polarization for electrostatic interaction (SVPE) or fully polarizable continuum model (FPCM). In the supermolecule-continuum approach, part of the solvent surrounding the solute is treated quantum mechanically and the remaining bulk solvent is approximated by a dielectric continuum medium. With this approach, the calculated results can systematically be improved by increasing the number of quantum mechanically treated solvent molecules. ΔGh...

Journal ArticleDOI
TL;DR: In this paper, a time-dependent density functional theory (TDDFT) is applied to calculate vertical excitation energies of trans-1,3-butadiene, trans−trans-1.3,5,7,9decapentaene, all-trans- 1,3, 5,7-octatetraene, and alltrans 1.3-decaene.
Abstract: Time-dependent density functional theory (TDDFT) is applied to calculate vertical excitation energies of trans-1,3-butadiene, trans−trans-1,3,5-hexatriene, all-trans-1,3,5,7-octatetraene, and all-trans-1,3,5,7,9-decapentaene. Attachment and detachment densities for transitions in butadiene and decapentaene from the ground state to the 2 1Ag and 1 1Bu excited states are also calculated and analyzed. Based on comparisons with experimental results and high level ab initio calculations in the literature, significant improvement over configuration−interaction singles is observed for the 2 1Ag state of the polyenes, which has been known to have significant double excitation character. For the 1 1Bu state, TDDFT underestimates the excitation energy by 0.4−0.7 eV. In this case we have observed a significant difference between the results for TDDFT and TDDFT within the Tamm−Dancoff approximation, both in excitation energies and, at least for butadiene, in the character of the excited state.

Journal ArticleDOI
TL;DR: In this paper, a spin-flip configuration interaction (SF-CI) model is presented, which is based on the spinflip approach and describes the spin flipping excitations from the reference Hartree-Fock high-spin, e.g. determinant.

Journal ArticleDOI
TL;DR: In this paper, high resolution, vibrationally resolved, near-edge x-ray absorption fine structure (NEXAFS) spectra at the C 1 s and N 1 s ionization thresholds of pyridine and deuterated d5-pyridine in the gas phase have been recorded.
Abstract: High resolution, vibrationally resolved, near-edge x-ray absorption fine structure (NEXAFS) spectra at the C 1s and N 1s ionization thresholds of pyridine and deuterated d5-pyridine in the gas phase have been recorded. The high resolution of 65 meV (150 meV) at the C s (N 1s) ionization thresholds reveals vibrational structures in the spectra. Detailed ab initio and density functional theory (DFT) calculations were performed to interpret the experimental spectra and to assign the observed peaks. In particular we focused on the previously unexplained intensity ratio for the two components of the C 1s→1π* transition. For this transition the vibrational structure is included through a linear coupling model in the DFT calculations and leads to the experimentally observed ∼2:3 intensity ratio between the two π* components in the C 1s spectrum rather than the ∼3:2 ratio obtained without vibrational effects. After inclusion of relaxation effects in the excited states, in addition to the vibrational effects, both theoretical methods yield almost perfect agreement with experiment.

Journal ArticleDOI
TL;DR: In this article, the indirect nuclear spin-spin coupling constants of C2H4, CH2NH, CH 2O, and CH2S were investigated by means of correlated ab initio calculations at the level of second order polarization propagator approximation (SOPPA) and with coupled cluster singles and doubles amplitudes using large basis sets, which are optimized for the calculation of coupling constants.
Abstract: The indirect nuclear spin–spin coupling constants of C2H4, CH2NH, CH2O, and CH2S were investigated by means of correlated ab initio calculations at the level of the second order polarization propagator approximation (SOPPA) and the second order polarization propagator approximation with coupled cluster singles and doubles amplitudes—SOPPA(CCSD) using large basis sets, which are optimized for the calculation of coupling constants. It is found that at the self-consistent-field (SCF) level CH2NH and CH2S exhibit triplet instabilities whereas CH2CH2 and CH2O show triplet quasi-instabilities, which renders the SCF results meaningless. Our best results deviate between 0.3 and 2.7 Hz from the experimental values. We find that although the one-bond C–H and Y–H couplings as well as the two- and three-bond H–H couplings are dominated by the Fermi contact term, significant contributions of the orbital paramagnetic and sometimes even spin–dipolar terms are observed for the one-bond C–Y and two-bond C–H and Y–H coupli...

Journal ArticleDOI
TL;DR: In this paper, the authors investigated all possible vacancy configurations within a 40-atom unit cell, and found that the minimum energy structure has all the vacancies located at octahedral sites.
Abstract: $\ensuremath{\gamma}$-alumina is usually described as a defect spinel structure with cation site vacancies distributed randomly between octahedral and tetrahedral positions. Its structure and properties are still not well understood. Using ab initio calculations based on density functional theory and plane-wave pseudopotentials, we investigate all possible vacancy configurations within a 40-atom unit cell, which are consistent with the crystallographic data available. We find that the minimum energy structure has all the vacancies located at octahedral sites. Results are presented for the atomic configuration, bulk modulus, and electronic density of states of this structure.

Journal ArticleDOI
TL;DR: In this paper, the MP2/6-311++G∗∗ level of theory was used to study the resonance assisted hydrogen bonds and the topological properties of the electron density distributions for O-H⋯O intramolecular bridges.

Journal ArticleDOI
TL;DR: It is shown that a recently synthesized Ga4-organometallic compound also contains an aromatic -Ga4(2-)- unit, analogous to the gaseous clusters.
Abstract: We investigated the electronic structure and chemical bonding of two bimetallic clusters NaGa4- and NaIn4- Photoelectron spectra of the anions were obtained and compared with ab initio calculations We found that the ground state of the two anions contains a square planar dianion interacting with a Na+ cation The Ga42- and In42- dianions both possess two delocalized π electrons and are considered to be aromatic, similar to that recently found in Al42- Using calculations for a model compound, we showed that a recently synthesized Ga4−organometallic compound also contains an aromatic −Ga42-− unit, analogous to the gaseous clusters

Journal ArticleDOI
TL;DR: A new correlation-consistent pseudopotential valence basis set for Cu derived at the second-order Møller-Plesset level suppresses considerably the basis set superposition error in Cu-Cu interactions compared to the standard Hartree-Fock optimized valence base set.
Abstract: Cuprophilic interactions in neutral perpendicular model dimers of the type (CH 3 CuX) 2 (X = OH 2 , NH 3 , SH 2 , PH 3 , N 2 , CO, CS, CNH, CNLi) were analyzed by ab initio quantum-chemical methods. The basis set superposition error for the weakly interacting CH 3 CuX subunits is significant and is discussed in detail. A new correlation-consistent pseudopotential valence basis set for Cu, derived at the second-order Moller - Plesset level suppresses considerably the basis set superposition error in Cu-Cu interactions compared to the standard Hartree-Fock optimized valence basis set. This allowed the first accurate predictions of cuprophilicity, which has been the subject of considerable debate in the past. The dependence of the strength of cuprophilic interactions on the nature of the ligand X was addressed. The Cu-Cu interaction increases with increasing o-donor and π-acceptor capability of the ligand and is approximately one-third of the well-documented aurophilic interactions. By fitting our potential-energy data to the Hershbach-Laurie equation, we determined a relation between the Cu-Cu bond length and the Cu-Cu force constant; this is important for future studies on vibrational behaviour. The role of relativistic effects on the structure and the interaction energy is also discussed. Finally we investigated cuprophilic interactions in (CH 3 Cu) 4 as a model species for compounds isolated and characterized by X-ray diffraction.

Journal ArticleDOI
TL;DR: In this paper, an ab initio Car-Parrinello molecular dynamics simulation of a Li+ ion in water has been carried out using the density-functional theory with Becke-Lee-Yang-Yang−Parr (BLYP) functional and ultrasoft Vanderbildt pseudopotentials.
Abstract: Ab initio Car–Parrinello molecular dynamics simulations of a Li+ ion in water have been carried out using the density-functional theory with Becke–Lee–Yang–Parr (BLYP) functional and ultrasoft Vanderbildt pseudopotentials. Both structural and dynamical properties of Li+ have been studied in detail and compared with available neutron scattering and spectroscopic data. Excellent agreement is obtained with the existing experimental data for the structure of the first hydration shell around the Li+ ion. Spectral features of bound water are compared to those of bulk water. Reasonable agreement is obtained with IR and Raman experiments. The ab initio simulation results have also been used to derive a Li+–water interaction potential. The best fit of the data gave a simple single-exponential potential function, which reproduces very well the liquid structure from the original ab initio simulations. This potential model, together with the simple point charge (SPC) water model, was applied to calculate the hydratio...

Journal ArticleDOI
TL;DR: The results indicate that the strength of the pi-H interaction is enhanced as one progresses from CH(4) to HF, and both electrostatic and inductive energies are important in the description of the aromatic pi-h interaction.
Abstract: The nature and origin of the π−H interaction in both the ethene (olefinic) and benzene (aromatic) complexes of the first-row hydrides (BH3, CH4, NH3, H2O, and HF) has been investigated by carrying out high level ab initio calculations. The results indicate that the strength of the π−H interaction is enhanced as one progresses from CH4 to HF. Unlike conventional H-bonds, this enhancement cannot be simply explained by the increase in electrostatic interactions or the electronegativity of the atom bound to the π H-bonded proton. The contributions of each of the attractive (electrostatic, inductive, dispersive) and repulsive exchange components of the total binding energy are important. Thus, the inductive energy is highly correlated to the olefinic π−H interaction as we progress from CH3 to HF. On the other hand, both electrostatic and inductive energies are important in the description of the aromatic π−H interaction. In either case, the contribution of dispersion energies is vital to obtain an accurate est...

Journal ArticleDOI
TL;DR: In this paper, the absorption spectra of Ag5-8 have been determined in the framework of the linear response equation-of-motion coupled cluster method and related techniques employing 11-electron relativistic effective core potential.
Abstract: The absorption spectra of Ag5–8 have been determined in the framework of the linear response equation-of-motion coupled cluster method and related techniques employing 11-electron relativistic effective core potential. In these treatments electron correlation effects for 11 electrons per atom are included, providing an accurate description of excited states of silver clusters. The calculations of transition energies and oscillator strengths have been carried out in a large energy interval for the stable structures and for the isomeric forms higher in energy. This allowed us to investigate the influence of structural properties on the spectroscopic patterns and to determine the role of d-electrons. Inclusion of d-electrons in the correlation treatment is mandatory to obtain accurate values for transition energies, but the excitations of s-electrons are primarily responsible for the spectroscopic patterns. They are characterized by the interference phenomena known in molecular spectroscopy which lead to a s...

Journal ArticleDOI
TL;DR: In this article, a density-functional method that employs linear combinations of numerical atomic orbitals as basis sets, nonlocal norm-conserving pseudopotentials, and the local spin density approximation for exchange and correlation is presented.
Abstract: We report ab initio calculations of ${\mathrm{Fe}}_{n}$ clusters up to $n=17.$ We used a density-functional method that employs linear combinations of numerical atomic orbitals as basis sets, nonlocal norm-conserving pseudopotentials, and the local spin density approximation for exchange and correlation. Our results for $nl~7$ generally agree quite well with those obtained in previous density-functional studies. The structural predictions for $ng7$ are also generally in keeping with information inferred from chemical probe and time-of-flight mass spectrometry experiments. The origin of the discrepancies arising in some cases is discussed.

Journal ArticleDOI
TL;DR: In this paper, the authors examined quantum effects in liquid water at ambient conditions by performing path-integral molecular dynamics simulations of a flexible, polarizable water model that was parameterized from ab initio calculations.
Abstract: We examine quantum effects in liquid water at ambient conditions by performing path-integral molecular dynamics simulations of a flexible, polarizable water model that was parameterized from ab initio calculations. The quantum liquid is less structured and has a smaller binding energy, in accord with previous simulations. The difference between the quantum and classical liquid binding energies (∼1.5 kcal/mol) is in reasonable agreement with a simple harmonic model, and is somewhat larger than previous estimates in the literature. Quantum effects do not appear to significantly modify the average induced dipole moment for a polarizable model, although the distribution is broader, especially for the component along the C2 axis of symmetry.

Journal ArticleDOI
TL;DR: The lattice dynamics of a wurtzite GaN single crystal by inelastic x-ray scattering is investigated, validating the correctness of the eigenvectors.
Abstract: We have investigated the lattice dynamics of a wurtzite GaN single crystal by inelastic x-ray scattering. Several dispersion branches and phonons at high-symmetry points have been measured, including the two zone-center Raman- and infrared-inactive silent modes. The experiments have been complemented by ab initio calculations. They are in very good agreement with our measurements, not only for phonon energies, but also for scattering intensities, thus validating the correctness of the eigenvectors. Other phenomenological and ab initio theories exhibit significant differences.

Journal ArticleDOI
TL;DR: The diameter of the smallest dot is found, which could emit photoluminescence in the visible region of the spectrum, and is in excellent agreement with recent and earlier experimental data on oxygen-free samples.
Abstract: Using state of the art time-dependent density functional theory and multireference second-order perturbation theory, we have accurately calculated (within 03 eV) in real space the optical gap of small silicon quantum dots, with diameters up to 25 A Our results, which support the quantum confinement hypothesis, are in excellent agreement with recent and earlier experimental data on oxygen-free samples and the conclusions of Wilcoxon et al [Phys Rev B 60, 2704 (1999)] We have found that the diameter of the smallest dot, which could emit photoluminescence in the visible region of the spectrum, is around 22 A Our work can resolve existing controversies and bridge diverse experimental and theoretical results