CONVERGENCE OF SYMMETRY-ADAPTED PERTURBATION-THEORY EXPANSIONS FOR PAIRWISE NONADDITIVE INTERATOMIC INTERACTIONS

Convergence properties of symmetry-adapted perturbation expansions for nonadditive interactions are tested by performing high-order calculat ions for three spin-aligned hydrogen atoms. It is shown that the stron g symmetry forcing characteristic of the Hirschfelder-Silbey theory le ads to a rapidly convergent perturbation series. The symmetrized Rayle igh-Schrodinger perturbation theory employing a weak symmetry forcing is shown to provide in low orders accurate approximations to the nonad ditive part of the interaction energy. In very high orders the converg ence of this perturbation expansion becomes very slow, and the series converges to an unphysical limit, very close to the exact interaction energy. The nonadditive part of the interaction energy for the lowest quartet state of H-3 is interpreted in terms of the first-order exchan ge, induction, exchange-induction, exchange-dispersion, induction-disp ersion, and dispersion contributions. It is shown that even for such a simple trimer the correct description of these components is necessar y to obtain quantitative agreement with variational full configuration interaction results. (C) 1996 American Institute of Physics.