MOLECULAR-INTERACTIONS - A STUDY OF CHARGE-TRANSFER EFFECTS

A valence bond study of the charge transfer contribution to the interm olecular interaction energy is presented. The charge transfer energy i s calculated by a perturbation method based on a matrix decomposition of the Hamiltonian, relative to a basis of Weyl-Rumer VB structures: t he important effects arise from transfer of one electron between the i nteracting systems, followed by pairing of spins to give at least one intermolecular covalent bond. The method proposed involves only the oc cupied orbitals of the separate fragments in contrast with the usual p erturbation theory in which the charge transfer contribution is define d as a summation of terms involving both the occupied and the virtual orbitals of the two molecules. The system (HF), has been considered as an illustrative example. For this system the charge transfer correcti on gives a contribution of 18 percent to the binding energy and result s in a significant shift of the position of the van der Waals minimum.