Journal of Molecular Structure (THEOCHEM) 307, 135-151 (1994)

Tatiana Cwioka, Bogumil Jeziorskib,a, Wlodzimierz Kolosa, Robert Moszynskib,a, and Krzysztof Szalewiczc

a Department of Chemistry, University of Warsaw, Pasteura 1, 02093 Warsaw, Poland
b Institute of Theoretical Chemistry, University of Nijmegen, Toernooiveld, 6525 ED Nijmegen, Netherlands
c Department of Physics and Astronomy, University of Delaware, Newark, DE 19716, USA

Symmetry-adapted perturbation theory of potential-energy surfaces for weakly bound molecular complexes


The symmetry-adapted perturbation theory (SAPT) expansions for the intermolecular 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 ground-state hydrogen atoms. As has been shown for the case of the interaction of a hydrogen atom with a proton, the HS theory converges very well. At low order this theory provides results very close to those of the symmetrized Rayleigh-Schrödinger (SRS) approach. In particular, the differences are negligible at the second order, i.e. at the level which can be practically applied to larger systems. Our results indicate that these two SAPT methods properly account for the electron exchange effects. The singlet-triplet splitting at the van der Waals minimum, obtained from the SRS exchange energy through second order, is considerably more accurate than that obtained from the asymptotically exact Herring 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-H2, 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.

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