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1 Polish Journal of Chemistry

1 1998

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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.