MANY-BODY THEORY OF EXCHANGE EFFECTS IN INTERMOLECULAR INTERACTIONS -DENSITY-MATRIX APPROACH AND APPLICATIONS TO HE-F-, HE-HF, H2-HF, AND AR-H2 DIMERS

The first-order exchange energy for the interactions of closed-shell m any-electron systems is expanded as a perturbation series with respect to the Muller-Plesset correlation potentials of the monomers. Explici t orbital formulas for the leading perturbation corrections are derive d applying a suitable density matrix formalism. The numerical results obtained using the Moller-Plesset perturbation expansion, as well as n onperturbative, coupled-cluster type procedure, are presented for the interactions of He-F-, He-HF, H-2-HF, and Ar-H-2. It is shown that the correlation part of the first-order exchange energy increases the unc orrelated results by 10% to 30% for the investigated range of configur ations. The analysis of the total interaction energies for selected ge ometries of these systems shows that at the present level of theory th e symmetry-adapted perturbation approach correctly accounts for major intramonomer correlation effects and is capable to accurately reproduc e the empirical potential energy surfaces.