Elise Kochanski

Ab initio calculations on the H4 van der Waals dimer: perturbation dispersion energy versus CI treatment

Ab initio SCF-CI calculations are performed on the (H2)2 dimer, including all singly and doubly-excited configurations with respect to the fundamental SCF configuration. The CI contribution to intermolecular energy is compared to the dispersion energy obtained from a perturbation treatment. Both procedures give a similar qualitative description, the CI values being slightly less attractive than the dispersion energies. The higher excitation contribution might be non-negligible in the CI treatment.

Theoretical study of the OH−(H2O)2 system: Nature and importance of three-body interactions

The nature and importance of nonadditive three-body interactions in the ionic OH−(H2O)2 cluster have been studied by supermolecule Mo/ller–Plesset (MP) perturbation theory and coupled-cluster method, and by symmetry-adapted perturbation theory (SAPT). The convergence of the SAPT expansion was tested by comparison with the results obtained from the supermolecule Mo/ller–Plesset perturbation theory calculations through the fourth order (MP2, MP3, MP4SDQ, MP4), and the coupled-cluster calculations including single, double, and approximate triple excitations [CCSD(T)]. It is shown that the SAPT results reproduce the converged CCSD(T) results within 10%. The SAPT method has been used to analyze the three-body interactions in the clusters OH−(H2O)n, n=2,3,4,10, with water molecules located either in the first or the second solvation shell. It is shown that at the Hartree–Fock level the induction nonadditivity is dominant, but it is partly quenched by the Heitler–London and exchange-induction/deformation terms. ...

Non-additivity of SCF interaction energies in H3O+(H2O)2

Abstract Three-body contributions have been studied in the system H 3 O + H 2 O) 2 from supermolecule ab initio SCF calculations. Atom-atom analytical formulae are proposed to reproduce the SCF results. Special attention is paid to the possibility of relating three-body effects to effective pair potentials.

Ab initio studies of the intermolecular interactions between two hydrogen molecules near the van der Waals minimum from a perturbation procedure using biorthogonal orbitals

The intermolecular energy between two hydrogen molecules near the van der Waals minimum has been computed from a perturbative procedure using biorthogonal orbitals. The zeroth-order hamiltonian possesses the proper symmetry with respect to the intermolecular electron permutation. This procedure allows us to study separately the effect of the overlap and the effect of the exchange of the electrons on the different terms of the perturbation series. It is seen that the overlap cannot be neglected in the first-order term at the van der Waals minimum, and can affect the dispersion energy by as much as 20 per cent. This method can be generalized for larger systems and reliable approximations are considered in order to shorten the computation time.

Decades of Theoretical Work on Protonated Hydrates

Abstract Theoretical studies on protonated hydrates (PH) are illustrative of the progress realized in theoretical chemistry over several decades. The evolution of such studies is presented. The main methods used (quantum chemistry, Monte Carlo or Molecular Dynamics calculations…) and the problems encountered are briefly recalled. Some of the results obtained are commented.

Theoretical studies of sulfuric acid monohydrate: Neutral or ionic complex?

Abstract The formation of a complex between one water and one sulfuric acid molecule has been studied using ab initio SCF-MO-LCGO calculations. A stable structure has been found for the neutral complex, with a stabilization energy of about 16 kcal/mol and an intermolecular OO distance of about 2.656 A. Similar studies on the complex H3O+-HSO4−, with a geometry suggested by the experimental monohydrate crystal structure, have shown that, although the stability is considerable (at least 137 kcal/mol) with respect to the isolated ions, the ionic complex is less stable than the isolated neutral molecules.

Use of some predominant intermolecular (or interatomic) energy contributions to characterize van der waals molecules. Ab initio calculations on two neon atoms

Abstract In previous papers, we presented an evaluation of the dispersion energy, using a double perturbation scheme and an “Epstein-Nesbet partition” of the individual molecular hamiltonians. The purpose of the present work is to check whether such dispersion energies, added to the SCF supermolecule energies, are able to give a reasonable description of the intermolecular (or interatomic) interactions. In order to compare our values with experimental data or with other ab initio and and semi-empirical work, we have studied the Ne + Ne system. The depth of the van der Waals minimum obtained is about 71% of the value derived from experimental data.

Ab initio SCF-CI studies of the intermolecular interaction between two hydrogen molecules near the Van der Waals minimum

SCF-CI calculations have been used to study the intermolecular energy between two hydrogen molecules in four different geometrical configurations. The CI matrix was diagonalized using perturbation techniques. The importance of the perturbation expansion order upon the intermolecular energy could therefore be studied. The wave function includes all singly and doubly excited configurations. The natural orbitals have been determined and their relative importance on the intermolecular energy is considered.

Ab initio calculation of the intermolecular energy between two hydrogen molecules near the van der Waals minimum

The interaction energy between two hydrogen molecules near the van der Waals minimum is computed as the sum of the SCF interaction energy of the supermolecule and the so-called “Hartree-Fock dispersion energy”. The most stable configuration is the perpendicular planar one (T configuration), this configuration being stable through the first order term. The energy averaged over the four configurations is in agreement with the available experimental data. The perturbative polarization energy is negligible near the van der Waals minimum but it seems that the charge transfer energy must be taken into account.

Dispersion energy in rare gas dimers from an ab initio perturbative procedure: Ne + Ne, Ar + Ar

Abstract The dispersion energy between two neon and two argon atoms is computed from an ab initio perturbative procedure not based on the multipole expansion. A comparison with the multipole expansion provides C 6 = 5.36 for Ne and 76.6 for Ar. It is seen that one d polarization function provides the main part of the C 6 R −6 contribution, the exponent of this function probably being related to the polarizability of the molecule. The multipole expansion seems acceptable up to the van der Waals minimum but quite invalid for smaller distances, and doubtful at the van der Waals minimum itself.