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2 Polish Journal of Chemistry

2 1998

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Second virial coefficients for atom-molecule complexes from ab initio SAPT potentials

Moszynski R., Korona T., Heijmen T.G.A., Wormer P.E.S., Avoird A. van der, Schramm B.

Polish Journal of Chemistry

1998

Vol. 72, nr 7S

1479-1496

Ab initio potentials, calculated by symmetry-adapted perturbation theory, are applied to compute second virial ciefficients (including first order quantum corrections) for He-CO, Ne-CO, Ar-H2, He-C2H2, and Ar-CH4 mixtures over a wide range of temperatures. The experimental methods are briefly outlined and the errors in the procedure to obtain mixed virial coefficients from the experimental data are discussed. Very good agreement with the majority of the available experimental data is observed for all systems except Ar-CH4 where calculated virial coefficients are slightly too high in comparison with measured data. Possible reasons for discrepancies between theory and experiment at very low temperature are discussed.

Multiple electron exchange in calculations of pairwise nonadditive contribution to trimer interaction energy

Lotrich V.F., Szalewicz K., Jeziorski B.

Polish Journal of Chemistry

1998

Vol. 72, nr 7S

1826-1848

The first order of the symmetry-adapted perturbation theory has been used to compute the pairwise nonadditive contribution to the exchange repulsion energy in the He3, Ne3, Ar2HF, and (H2O)3 trimers. The contributions from single and double electron exchanges, as well as from the cyclic permutations involving three electrons at a time, were calculated seprately and compared with the accurate result involving all possible electron permutations. The intramonomer electron correlation was completely neglected and the Hartree-Fock determinants were used to represent the monomer wave functions. The importance of the resulting three-body contributions, which can be quadratic (single exchanges), cubic (three-electron cycles), and quartic (double exchanges) in a typical intermolecular overlap integral S, is studied for several geomertical configurations of the investigated systems. Whereas the S(2) and S(3) terms are important for all systems, the role of the S(4) terms turns out to be very small for Ar2HF, large for the rare-gas timers at linear configurations, and appreciable for the water trimer. The higher than double exchanges (S(5) and higher terms) are negligible for all systems except for the water trimer at very small intermonomer separations where the expansion in powers of S ceases to be useful. The nonadditivity of the so-called zeroth-order exchange energy, i.e., the difference between the Heitler-London and the first-order energies, has also been investigated. The zeroth-order exchange energy contains only the S(4) and higher terms and becomes significant when the S(4) contribution to the first-order energy is important. The S(2) and S(3) contributions to the first-order exchange nonadditivity in the Ar2HF trimer agree well with the calculations of Cybulski and collaborators[J. Chem. Phys. 101, 10708 (1994), ibid., 106, 3301 (1997)] using their pseudo-dimer model.