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 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.