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

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Abstract
The first‐order exchange energy for the interactions of closed‐shell many‐electron systems is expanded as a perturbation series with respect to the Mo/ller–Plesset correlation potentials of the monomers. Explicit orbital formulas for the leading perturbation corrections are derived applying a suitable density matrix formalism. The numerical results obtained using the Mo/ller–Plesset perturbation expansion, as well as nonperturbative, coupled‐cluster type procedure, are presented for the interactions of He–F−, He–HF, H2–HF, and Ar–H2. It is shown that the correlation part of the first‐order exchange energy increases the uncorrelated results by 10% to 30% for the investigated range of configurations. The analysis of the total interaction energies for selected geometries of these systems shows that at the present level of theory the symmetry‐adapted perturbation approach correctly accounts for major intramonomer correlation effects and is capable to accurately reproduce the empirical potential energy surfaces.