First‐Order perturbation treatment of the short‐range repulsion in a system of many closed‐shell atoms or molecules

Abstract Assuming a determinantal form for the wave‐functions of free molecules, explicit formula for the first‐order interaction energy of many closed‐shell molecules has been derived. Provided that the determinants describing the free molecules are constructed from the Hartree‐Fock orbitals, the two‐, three‐, and four‐body effects predicted by the first‐order perturbation theory are closely related to those which one obtains in the framework of the Löwdin LCAO MO ‐type approach. The results are illustrated by numerical calculations for the system of three ground‐state helium atoms.