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 f
irst stage, we apply the weak symmetry forcing to a regularized interaction
operator and, subsequently, at the second stage we treat the singular, sho
rt-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 symme
try projection to prevent the divergence of the series appearing at the sec
ond stage of our procedure. We tested the method by performing high-order c
alculations of the interaction energy in the singlet and triplet states res
ulting from the interaction of two ground-state hydrogen atoms. A large bas
is set of Gaussian geminals was used to obtain saturated results. It is sho
wn that the proposed regularization procedure eliminates the pathological c
onvergence properties observed earlier for the perturbation expansions invo
lving the weak symmetry forcing and gives very accurate interaction energie
s in a low-order treatment of the singular part of the potential. (C) 2001
Elsevier Science B.V. All rights reserved.