Symmetry-adapted perturbation theory with regularized Coulomb potential

We present a novel formulation of the symmetry-adapted perturbation theory involving a regularization of the Coulomb singularities in the interaction operator. The perturbation development is performed in two stages. At the first stage, we apply the weak symmetry forcing to a regularized interaction operator and, subsequently, at the second stage we treat the singular, short-range part of the perturbation by employing the strong symmetry forcing characteristic of the Eisenschitz-London-Hirschfelder-van der Avoird theory. The correct asymptotics is recovered at the first stage and the residual, short-range part of the perturbation is sufficiently weakened by the symmetry projection to prevent the divergence of the series appearing at the second stage of our procedure. We tested the method by performing high-order calculations of the interaction energy in the singlet and triplet states resulting from the interaction of two ground-state hydrogen atoms. A large basis set of Gaussian geminals was used to obtain saturated results. It is shown that the proposed regularization procedure eliminates the pathological convergence properties observed earlier for the perturbation expansions involving the weak symmetry forcing and gives very accurate interaction energies in a low-order treatment of the singular part of the potential. (C) 2001 Elsevier Science B.V. All rights reserved.