An efficient ab initio gradient program

An ab initio Hartree-Fock gradient program is described. It is characterized by (1) efficiency of the gradient evaluation, and (2) capability of handling higher angular momentum (d andf) basis functions. The latter are constructed from shifted Cartesian Gaussian p-type primitives. A satisfactory solution is presented for the problems connected with the neglect of small integrals in a gradient program. Methods for increasing the efficiency of the SCF procedure are discussed.     

Some problems est the calculation of ab initio potential energy curves

The non-adiabatic and adiabatic approaches to the calculation of molecular energy levels are presented. The concept of the potential energy curve, its accuracy and limitations are discussed. Simplified approaches to calculations of the intermolecular potential energy curves are also considered.     

Theoretical method for full ab initio calculation of DNA/RNA-ligand interaction energy

In this paper, we further develop the molecular fractionation with conjugate caps (MFCC) scheme for quantum mechanical computation of DNA-ligand interaction energy. We study three oligonuclear acid interaction systems: dinucleotide dCG/water, trinucleotide dCGT/water, and a Watson-Crick paired DNA segment, dCGT/dGCA. Using the basic MFCC approach, the nucleotide chains are cut at each phosphate group and a pair of conjugate caps (concaps) are inserted. Five cap molecules have been tested among which the dimethyl phosphate anion is proposed to be the standard concap for application. For each system, one-dimensional interaction potential curves are computed using the MFCC method and the calculated interaction energies are found to be in excellent agreement with corresponding results obtained from the full system ab initio calculations. The current study extends the application of the MFCC method to ab initio calculations for DNA- or RNA-ligand interaction energies.     

An ab initio calculation of K-spectra in molecules HCl and HF

An ab initio calculation of energies and intensities of K-emission and K-absorption spectra in molecules HCl and HF are carried out. An electronic readjustment due to a hole in a molecular core is taken into account. A vibrational structure of K-emission and K-absorption spectra is also calculated. The calculation shows that it is possible to detect a vibrational structure of valence bonding levels in X-ray emission spectra. A good agreement with experiment is obtained.     

Path integral Monte Carlo method for ab initio calculation

The Feynman path integral method is applied to the many-electron problem. We first give new closure relations in terms of ordinary complex and real numbers, which could be derived from an arbitrary complete set of state vectors. Then, in the path integral form, the partition function of the system and the ensemble average of energy are explicitly expressed in terms of these closure relations. It is impossible to evaluate the path integral by direct numerical integrations because of its huge amount of integration variables. Therefore, we develop an algorithm by the Monte Carlo method with constraints corresponding to the normalization condition of states to calculate the required integral. Finally, the ensemble average of energy for the hydrogen molecule is explicitly evaluated by the quantum Monte Carlo method and results are compared with the result obtained by the ordinary full configuration interaction (CI) method. © 1996 John Wiley & Sons, Inc.     

An ab initio potential energy surface and vibrational energy levels of ZnH2

A three‐dimensional potential energy surface of the electronic ground state of ZnH₂ (${X}^1\sum _g^ +$ ) molecule is constructed from more than 7500 ab initio points calculated at the internally contracted multireference configuration interaction with the Davidson correction (icMRCI+Q) level employing large basis sets. The calculated relative energies of various dissociation reactions are in good agreement with the previous theoretical/experimental values. Low‐lying vibrational energy levels of ZnH₂, ZnD₂, and HZnD are calculated on the three‐dimensional potential energy surface using the Lanczos algorithm, and found to be in good agreement with the available experimental band origins and the previous theoretical values. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010     

An ab initio potential energy surface and predissociative resonances of HArF

A three-dimensional potential energy surface of the ground electronic state HArF is constructed from more than 2000 ab initio points at the multireference averaged quadratic coupled-cluster level employing an augmented large basis set. The calculations indicate that the linear HArF molecule is metastable with a barrier of 0.643 eV in the atomization (HArF --> H + Ar + F) channel and a barrier of 1.017 eV in the dissociation (HArF --> Ar + HF) channel. Variational calculations of low-lying predissociative resonances of both HArF and DArF are performed on the three-dimensional potential energy surface using a complex-symmetric Lanczos propagation method, which yields both positions and widths of the resonance states. The resonance lifetime generally decreases with energy, but strong mode selectivity exists. Reasonably good agreement with experiment confirms the accuracy of our potential. These calculations provide valuable information on the stability and dynamics of HArF/DArF in its ground electronic state.     

An ab initio calculation on proton transfer in the benzoic acid dimer

We have made an ab initio calculation of the barriers for proton transfer in the hydrogen-bonded dimers of benzoic acid and acetic acid. Geometrical optimization values which are closer to the experiment one.     

原子簇量子化学从头算的自洽晶体场模型

本文提出一种收敛的点电荷和Hartree-Fock表面势构成的自洽晶体场Madelung势用于原子簇量子化学从头计算。前者的计算类似于核吸引积分的单电子积分, 后者可以通过点群对称性操作从原子簇内的积分矩阵元得到。点电荷的大小和原子簇内对应的原子电荷相等, 其数目以晶体场势收敛为标准确定。介绍加速计算的程序技巧。该模型用于高温超导体YBa2Cu3O7的全电子从头计算并得到一些新结果。     

An ab initio approach to the mechanism of the Grignard reaction

The mechanism of the Grignard reaction is investigated for the first time in the light of ab initio SCF MO theory. The possible advantage of a single-electron transfer over a polar mechanism is discussed.     

An ab initio calculation of the molecular structure and properties of hypofluorous acid

An ab initio LCAO MO SCF calculation has been carried out on hypofluorous acid (HOF) in order to study the electronic structure, geometry and other one-electron properties of this molecule. The minimum energy geometry was found to be R_OF = 1.450 Å, R_OH = 1.080 Å, and θ = 100.8°, which is in good agreement with experiment. In general, the bonds in HOF appear to be similar to the analogous bonds in H₂O and F₂O. The first three ionization potentials 13.97, 14.94, and 17.16 eV compare well with recent photoelectron spectroscopic data.     

A periodic ab initio Hartree-Fock calculation on corundum

The electronic structure of corundum (α-Al₂O₃) is calculated at the ab initio Hartree-Fock level. Cohesive energy, total and projected densities of states, atomic multipoles, bond populations and electron charge density distribution maps are given. The oxygen-aluminium bond is found to be partially covalent in nature; the atomic charges are −0.73 e and +1.09 e for O and Al respectively.     

An ab initio study of van der Waals potential energy parameters for silver clusters

We employ ab initio calculations of van der Waals complexes to study the potential energy parameters (C(6) coefficients) of van der Waals interactions for modeling of the adsorption of silver clusters on the graphite surface. Electronic structure calculations of the (Ag(2))(2), Ag(2)-H(2), and Ag(2)-C(6)H(6) complexes are performed using a coupled-cluster approach that includes single, double, and perturbative triple excitations (CCSD(T)), M?ller-Plesset second-order perturbation theory (MP2), and spin-component-scaled MP2 (SCS-MP2) methods. Using the atom pair approximation, the C(6) coefficients for silver-silver, silver-hydrogen, and silver-carbon atom systems are obtained after subtracting the energies of quadrupole-quadrupole interactions from the total electronic energy.     

An ab initio calculation of 14n quadrupole coupling constants

Ab initio SCF-MO calculations of ¹⁴N quadrupole coupling constants are reported for HCN, HNC, CH₃CN, CH₃NC, NH₃, NH₂NH₂, FCN, N₂O, (CN)₂, BrCN, pyridine and pyrazine. There is excellent correlation between calculation and experiment yielding Q = 1.503 ± 0.159 × 10^?26 cm² for the ¹⁴N nuclear quadrupole moment. Dunning sp basis sets are more than adequate for such calculations, STO/4G basis sets yielding almost identical results for pyridine and pyrazine. Unsuccessful attempts were made to correlate coupling constants with electronic population analysis indices.     

An ab initio relativistic calculation for (e113)2

For a molecule containing two heavy atoms a method capable of performing fully ab initio fully relativistic calculations based on the Dirac equation is described and applied to (E113)₂. The wavefunction is mainly composed of the covalent function constructed from the ground 7P? configurations of the atoms although admixture of covalent functions built from 7p orbitais not insignificantly lowers the energy. Ionic terms are unimportant.     

Potential energy function for cation–peptide interactions: An ab initio study

A potential energy function is developed to represent the interaction of small monovalent cations, Li⁺, Na⁺, and K⁺, with the backbone of polypeptides. The results are based on ab initio calculations up to the 6-31G* level of the interactions of the ions with acetamide and N-methylacetamide. Basis set superposition errors are corrected with the counterpoise method. A systematic overestimate of the bond polarities is taken into account by an empirical scaling procedure that uses the ratio of the experimental to ab initio dipole moment. The calculated binding energies obtained with this procedure show consistent convergence with different basis sets and are in good agreement with experimental data on cation–water and cation–dimethylformamide systems. Investigations of the calculated ab initio potential energy surface indicate that the cation–peptide interaction is dominated by electrostatics and includes a nonnegligible contribution from polarization of the peptide group by the ion. The induced polarization results in a steeper-than-Coulombic interaction and cannot be described by fixed ion–peptide partial charges electrostatics. Atomic polarizabilities located on the atoms of the ligand molecule are introduced to account for the induced polarization in the empirical energy function. A ～1/r⁴ attractive interaction appears in the potential function. The resulting radial and angular dependence of the potential energy surface is well reproduced. © 1995 by John Wiley & Sons, Inc.     

Chemisorption of atomic hydrogen on rhodium: An ab initio cluster-model approach

Interaction of atomic hydrogen with cluster models simulating the Rh(111) surface has been studied at the ab initio SCF level using relativistic non-empirical pseudopotentials and moderately large basis sets. From the bonding parameters, it is seen that chemisorption on the threefold open position is five times more favoured than on the threefold eclipsed one, and the bonding picture is rather different for both active sites. Since two different kinds of adsorbed hydrogen atoms should exist according to various experimental techniques, the present work adds further evidence to the existence of these two types of adsorption and reports some structural parameters which can help to a more detailed characterization in the near future.     

An ab initio intermolecular potential energy surface for the F(2) dimer

Two analytical representations for the potential energy surface of the F(2) dimer were constructed on the basis of ab initio calculations up to the fourth-order of M?ller-Plesset (MP) perturbation theory. The best estimate of the complete basis set limit of interaction energy was derived for analysis of basis set incompleteness errors. At the MP4/aug-cc-pVTZ level of theory, the most stable structure of the dimer was obtained at R = 6.82 au, theta(a) = 12.9 degrees , theta(b) = 76.0 degrees , and phi = 180 degrees , with a well depth of 716 microE(h). Two other minima were found for canted and X-shaped configurations with potential energies around -596 and -629 microE(h), respectively. Hexadecapole moments of monomers play an important role in the anisotropy of interaction energy that is highly R-dependent at intermediate intermolecular distances. The quality of potentials was tested by computing values of the second virial coefficient. The fitted MP4 potential has a more reasonable agreement with experimental values.