SYMMETRY-ADAPTED PERTURBATION-THEORY OF POTENTIAL-ENERGY SURFACES FORWEAKLY-BOUND MOLECULAR-COMPLEXES

The symmetry-adapted perturbation theory (SAPT) expansions for the int ermolecular interaction energies can provide potential energy surfaces for weakly bound complexes such as van der Waals molecules or systems forming hydrogen bonds. The convergence properties of SAPT expansions are discussed. New results are presented for the Hirschfelder-Silbey (HS) method applied through high order to the interaction of two groun d-state hydrogen atoms. As has been shown for the case of the interact ion of a hydrogen atom with a proton, the HS theory converges very wel l. At low order this theory provides results very close to those of th e symmetrized Rayleigh-Schrodinger (SRS) approach. In particular, the differences are negligible at the second order, i.e. at the level whic h can be practically applied to larger systems. Our results indicate t hat these two SAPT methods properly account for the electron exchange effects. The singlet-triplet splitting at the van der Waals minimum, o btained from the SRS exchange energy through second order, is consider ably more accurate than that obtained from the asymptotically exact He rring and Flicker formula. The many-body version of the SRS theory is briefly discussed and the results of its applications to several many- electron systems (He2, He-K+, Ar-H-2, He-HF and Ar-HF) are presented. In all cases a very good agreement, generally within a few per cent or less, between theoretical and experimental binding energies was found .