Models of environmental effects on anion polarizability

This paper deals with three different approaches to the representation of environmental effects on anion polarizability in cubic crystals of the stoichiometry MX, where M is an alkali metal and X is a halogen. Ab initio embedded cluster calculations of the variation in anion polarizability with pressure in a fixed crystal type are presented and compared with experiment. The results are then used in a scaled ab initio model used to predict further values for the pressure dependence of the in-crystal anion polarizability. This scaled model is compared with a fully empirical ‘universal’ model due to Batana et al. based on polarizability change with ionic radius [1997, Molec. Phys., 92, 1029]. The assumptions of the two models differ substantially and the central purpose of this paper is to contrast these differences and highlight their consequences for prediction. Although the empirical model typically overestimates the experimental pressure derivatives, and the ab initio calculations somewhat underestimate them, it is shown that the assumption of incompressible cations in the scaled ab initio-derived model has a firmer physical basis than the empirical picture in which all ions are compressible.     

Analytic gradients for the combined sr-DFT/lr-MP2 method: application to weakly bound systems

The formalism for analytical gradients in short-range density functional schemes with long-range ab initio corrections (sr-DFT/lr-ab initio) is presented. On the density-functional side, both sr-LDA and sr-PBE are available, while on the ab initio side lr-HF and lr-MP2 are possible. Details of the implementation in the Molpro package are given. Results from test calculations using different basis sets on weakly bound systems of the HB6/04, DI6/04, CT7/04 and WI9/04 databases are presented.     

Simulation of Raman spectra of C and C by non-orthogonal tight-binding molecular dynamics

A non-orthogonal tight-binding molecular-dynamics formalism is used to simulate Raman spectra of the fullerene molecules C₆₀ and C₇₀. Two parametrization schemes for the Hamiltonian and the overlap matrix elements are investigated. The considered molecules are excited randomly and the Fourier transform of the displacement autocorrelation function is employed to extract the vibrational properties. Fair agreement with experiment and with force-constant and ab initio calculations is achieved, with comparatively smaller maximum errors in the frequencies than for other molecular dynamics or semi-empirical calculations from the literature. Received 4 February 1999 and Received in final form 28 November 1999     

Is the Ar—Br2(X1Σ+ g ) van der Waals complex linear rather than T-shaped? A study in terms of ab initio based potential energy surfaces

The ground state Ar—Br₂ potential energy surface is predicted from ab initio calculations and from an atom—atom model using empirical ArBr potentials and the (evaluated ab initio) perturbation of the interaction between Ar and Br within Br₂. At all levels of modelling, the surface has a double-minimum topology, with wells for both the linear (L-) and T-shaped geometries. This differs from the single-minimum topology predicted by the commonly used pairwise additive Lennard-Jones potential. For both ab initio and atom—atom model surfaces, the L well is found to be significantly deeper than the T well; this relative behaviour is unchanged by zero-point vibrations. Spectroscopic parameters are predicted for the present surfaces. The final surfaces result from a scaling to reproduce the estimated bond energy of the system. Possible reflections of the surface topology in experimental observables are discussed.     

Significance of model Hamiltonians in energy-band theory

The results of recent ab initio calculations of energy bands are compared with interpolation schemes in order to assess the significance of the latter. When the parameters in the interpolation scheme are required to correspond to those obtainable by an ab initio calculation based on a reasonable one-electron potential, we describe the scheme as a model Hamiltonian. The analytic character of model Hamiltonians for s–p bands (pseudopotential case) or for d bands interacting with s–p bands (resonance case) is summarized. The compatibility of the results of several workers with these analyticity requirements is considered. It is shown that if the empirically-motivated adjustments of parameters are to be regarded as physically significant, these adjustments must satisfy rather strict conditions.     

High-pressure theoretical and experimental study of HgWO4

HgWO₄ at ambient pressure is characterized using a combination of ab initio calculations, X-ray diffraction and Raman scattering measurements. The effect of low pressure and temperature on the structural stability is analysed. Extending our ab initio study to the range of higher pressures, a sequence of stable phases up to 30 GPa is proposed.     

Ab initio calculation of force constants and equilibrium geometries

Force constants of the molecules HF, NH₃, CH₄ and BH₄ ^- have been calculated ab initio by the force method with a 73/3 + 1 gaussian lobe basis set. The results, including a former calculation on H₂O, agree well with experiment: the average relative error is 12 per cent for the diagonal force constants and the average absolute error is 0·06 mdyn/Å for the off-diagonal ones. The trends are also correctly reproduced. It is concluded that ab initio calculations of this accuracy can help to solve a number of spectroscopic problems. Force constants of BH₄ ^- have been determined from a combination of spectroscopic and ab initio information. Geometries have been obtained with little computing work and show good agreement with experiment.     

Investigation of hydrogen bonding in neat dimethyl sulfoxide and binary mixture (dimethyl sulfoxide + water) by concentration-dependent Raman study and ab initio calculation

In this study,our vibrational spectroscopic analysis is made on hydrogen-bonding between dimethyl sulfoxide and water comprises both experimental Raman spectra and ab initio calculations on structures of various dimethyl sulfoxide/water clusters with increasing water content.The Raman peak position of the v(S=O) stretching mode of dimethyl sulfoxide serves as a probe for monitoring the degree of hydrogen-bonding between dimethyl sulfoxide and water.In addition,the two vibrational modes,namely,the CH 3 symmetric stretching mode and the CH 3 asymmetric stretching mode have been analysed under different concentrations.We relate the computational results to the experimental vibrational wavenumber trends that are observed in our concentration-dependent Raman study.The combination of experimental Raman data with ab initio calculation leads to a better knowledge of the nature of the hydrogen bonding and the structures of the hydrogen-bonded complexes studied.     

Si80笼状分子的结构研究

采用基于密度泛函的第一性原理计算方法,构建了Si₈₀笼状分子的模型,并对其结构和电学特性进行了考察. 研究发现,经过结构优化计算,Si₈₀分子从I_h高对称性下降为T_h,但仍然保持较好的笼状结构. 对Si₈₀笼状分子的稳定性、轨道分布和电荷分布等性质进行了分析和讨论. 关键词： 80')" href="#">Si₈₀ 富勒烯 结构     

Re-examination of the NDDO approximation and introduction of a new model beyond it

The NDDO (neglect of diatomic differential overlap) approximation, a widely used basis for many semi-empirical molecular orbital (MO) approaches, is re-examined based on non-empirical frozen-core calculations on small molecules. An improvement going beyond the NDDO approximation is proposed. Our study shows that under the NDDO approximation, when the remaining non-DDO-type two-electron repulsion integrals (TERIs) are calculated using the basis set from the Löwdin orthogonalization of the valence atomic orbitals, the resulting total energies are much higher than those from the corresponding frozen-core ab initio calculations. On the other hand, when the remaining non-DDO TERIs are calculated using non-orthogonal valence atomic orbitals (similar to the Roby model), for most of the molecules calculated, the total energies are significantly lower than those from the corresponding ab initio calculations. Furthermore, we also find that for some molecules, the total energies thus calculated are higher than the corresponding ab initio results. The nonsystematic variation of the absolute errors in the total energy calculations is due to the fact that the core-electron and the electron-electron interactions are not treated in a balanced way in the NDDO approximation. A new model, which overcomes the deficiencies in the NDDO model, is proposed. In this model, a first-order correction term is added to the electron-electron Coulomb interactions, thereby improving the balance between the core-electron and the electron-electron interactions. Non-empirical test calculations show that the total energies from the new model are consistently higher than those from the ab initio calculation but closer to the ab initio results. We expect that the proposed new model would be useful in developing new high-quality semi-empirical MO approaches.     

Theoretical investigation of magnetic parameters in two-dimensional sheets of pure organic BEDT-TTF and BETS molecules by using ab initio MO and DFT methods

The effective exchange integrals (J _ab(M)) between two cation radicals of title compounds in the Heisenberg model were calculated by ab initio molecular orbital (MO) and density functional theory (DFT) methods, together with hybrid DFT methods. The J _ab(D) values between two dimer mono-cation radicals were also estimated assuming J _ab(D) = J _ab(M)/2. It is found that the spin lattice obtained by the ab initio method is square planar and linear in BEDT-TTF and BETS planes, respectively, although other previous calculations show that spin lattice in the BEDT-TTF plane is a triangular one. The J _ab and overlap integrals (s_ab ) values by the ab initio methods were used to determine transfer integral (t_ab ) and Coulomb repulsion (U_cff ) parameters of the Hubbard model, which were compared with those of the previous results. Implications of the calculated results are discussed in relation to the spin-mediated mechanism for superconductivity.     

Ab initio calculations with small ellipsoidal gaussian basis sets. I

The use of ellipsoidal gaussian type orbitals in ab initio calculations on molecular systems of small and intermediate size is demonstrated, in both non-linear and SCF MO schemes. The method is an extension of Frost's Floating Gaussian Orbital Method. Results for conformational properties (barriers to internal rotation in ethane and 1,3-butadiene) are better than those obtained with basis sets containing only spherical gaussians. The usefulness of very small basis sets is discussed.     

Comment

A new flexible atomic and polarizable potential for water simulation is validated and applied to the study of small clusters of water molecules: (H₂O) _n , n = 4, 6 and 8. The low-energy equilibrium structures obtained are in good agreement with ab initio calculations. Vibrational frequencies are calculated by normal-mode analysis and are in semiquantitative agreement with ab initio data. Binding energies and dipole moments are also satisfactorily reproduced, despite the limited number of parameters of the potential.     

Ab Initio Calculation of Hyperfine Interaction Parameters: Recent Evolutions, Recent Examples

For some years already, ab initio calculations based on Density Functional Theory (DFT) belong to the toolbox of the field of hyperfine interaction studies. In this paper, the standard ab initio approach is schematically sketched. New features, methods and possibilities that broke through during the past few years are listed, and their relation to the standard approach is explained. All this is illustrated by some highlights of recent ab initio work done by the Nuclear Condensed Matter Group at the K.U.Leuven.     

A refined estimate of the bond length of methane

The atmospheric reaction of H₂S with Cl was investigated using high level ab initio calculations and Canonical Variational Transition State Theory (CVTST). The adduct formation step is the dynamical bottleneck, and the rate constant was calculated to be 1.2 × 10^?9 cm³ molecule^?1 s^?1, which is around ten times greater than the upper experimental value. Additional ab initio classical trajectory calculations show that the adduct formed in the initial collision can easily dissociate, recrossing the variational transition state. The stabilization of this species depends on the vibrational excitation of H₂S molecule, which requires an almost collinear SH-Cl collision. These dynamical effects provide an explanation for the substantial error in the rate constant obtained using CVTST.     

A general pseudopotential model for molecules with many valence electrons

Criteria are discussed for the practical application of ‘valence-electron only’ calculations aimed at simulating ab initio all-electron calculations on molecules containing heavy atoms with many core electrons. A theoretical model is outlined based on the use of pseudopotentials derived from atomic SCF calculations. The model has been used in calculations for NaH, Na₂, H₂S, S₂, H₂S₂, HCl, Cl₂ and NaCl, and has been tested by comparison with reference all-electron calculations. Most of these tests employ double-zeta basis sets and SCF wavefunctions, although polarization functions and complete valence-electron CI wavefunctions have been used in some cases. The model is generally successful for ground-state equilibrium structures, charge distributions and orbital energies.     

Improved techniques for the calculation of ab initio ion-neutral interaction potentials: application to coinage metal ions interacting with rare gas atoms

Ab initio values for the potential energy functions for ion–neutral interactions can be tested by comparison with gaseous ion transport coefficients, but only if special care is taken to compute the interaction potentials accurately over wide ranges of internuclear separation. This is illustrated here by a reanalysis of the ab initio values for the coinage metal ions interacting with rare gas atoms, precise calculations of the transport cross sections over extremely wide ranges of energy, and similarly precise calculations of the zero-field ion mobilities as functions of gas temperature and the field-dependent ion mobilities at various fixed temperatures. The calculations indicate that the mobilities for Ag⁺(¹S) moving in Ne or Ar can distinguish between the existing, very similar ab initio potentials. They also show that substantial differences exist among the mobilities of the coinage metal anions and the ground and excited states of the cations. The techniques implemented are recommended for future ab initio calculations.     

How to simulate a structural phase transition by the first-principles method?

We outline the procedure and approximations used to study the structural phase transition. The standard ab initio programs provide the Hellmann-Feynman forces, which allow, by the direct method, to describe the dispersion curves including the soft mode. The values of the anharmonic terms of the order parameter and order parameter-strain coupling terms, found from symmetry analysis, can be calculated within the ab initio total energy approach, by carring on the calculations for a variety of atom configurations. The method is used to describe the cubic-tetragonal phase transition in SrTiO₃, for which a soft mode and anharmonic coefficients have been found.     

Coupled ab initio potential energy surfaces for the two lowest 2A′ electronic states of the C2H molecule

Realistic two-valued potential energy surfaces for the reaction C(³P) + CH(X²Π) → C₂ + H have been constructed from a set of high level ab initio data describing the first two ²A′ electronic states of the C₂H system. These states have linear equilibrium configurations, known as the X ²Σ⁺ and A²Π states, and are coupled by a conical intersection. They lead to the formation of C₂(X¹Σ⁺ _g) and C₂(a³Π_u) considering an adiabatic dissociation process. The ab initio calculations are of the multireference configuration interaction variety and were carried out using a polarized triple-zeta basis set. Using the ab initio adiabatic energies and the matrix elements of the dipole moment, a 2 × 2 diabatic representation of the electronic Hamiltonian was built. Each element of this Hamiltonian matrix was expressed within the double many-body expansion (DMBE) scheme which is based, in this case, on the extended Hartree-Fock approximate correlation energy model (EHFACE). The analytical adiabatic potential energy surfaces are then obtained as the eigenvalues of this matrix, and display correctly the Σ/Π conical intersection. Moreover, the non-adiabatic couplings given by our analytical model are compared with the ab initio ones, and good qualitative agreement is observed.     

Investigation of inter-molecular hydrogen bonding in the binary mixture (acetone + water) by concentration dependent Raman study and ab initio calculations

This paper reports that vibrational spectroscopic analysis on hydrogen-bonding between acetone and water comprises both experimental Raman spectra and ab initio calculations on structures of various acetone/water complexes with changing water concentrations. The optimised geometries and wavenumbers of the neat acetone molecule and its complexes are calculated by using ab initio method at the MP2 level with 6-311+G(d,p) basis set. Changes in wavenumber position and linewidth (fullwidth at half maximum) have been explained for neat as well as binary mixtures with different mole fractions of the reference system, acetone, in terms of intermolecular hydrogen bonding. The combination of experimental Raman data with ab initio calculation leads to a better knowledge of the concentration dependent changes in the spectral features in terms of hydrogen bonding.