MANY-BODY THEORY OF EXCHANGE EFFECTS IN INTERMOLECULAR INTERACTIONS -2ND-QUANTIZATION APPROACH AND COMPARISON WITH FULL CONFIGURATION-INTERACTION RESULTS

Explicitly connected many-body perturbation expansion for the energy o f the first-order exchange interaction between closed-shell atoms or m olecules is derived. The influence of the intramonomer electron correl ation is accounted for by a perturbation expansion in terms of the Mol ler-Plesset fluctuation potentials W-A and W-B of the monomers or by a nonperturbative coupled-cluster type procedure. Detailed orbital expr essions for the intramonomer correlation corrections of the first and second order in W-A+W-B are given. Our method leads to novel expressio ns for the exchange energies in which the exchange and hybrid integral s do not appear. These expressions, involving only the Coulomb and ove rlap integrals, are structurally similar to the standard many-body per turbation theory expressions for the polarization energies. Thus, the exchange corrections can be easily coded by suitably modifying the exi sting induction and dispersion energy codes. As a test of our method w e have performed calculations of the first-order exchange energy for t he He-2, (H-2)(2), and He-H-2 complexes. The results of the perturbati ve calculations are compared with the full configuration interaction d ata computed using the same basis sets. It is shown that the Moller-Pl esset expansion of the first-order exchange energy converges moderatel y fast, whereas the nonperturbative coupled-cluster type approximation s reproduce the full configuration interaction results very accurately .