Molecular interactions: a study of charge transfer effects Original Research Article

A valence bond study of the charge transfer contribution to the intermolecular interaction energy is presented. The charge transfer energy is calculated by a perturbation method based on a matrix decomposition of the Hamiltonian, relative to a basis of Weyl-Rumer VB structures: the important effects arise from transfer of one electron between the interacting systems, followed by pairing of spins to give at least one intermolecular covalent bond. The method proposed involves only the occupied orbitals of the separate fragments in contrast with the usual perturbation theory in which the charge transfer contribution is defined as a summation of terms involving both the occupied and the virtual orbitals of the two molecules. The system (HF)2 has been considered as an illustrative example. For this system the charge transfer correction gives a contribution of 18 percent to the binding energy and results in a significant shift of the position of the van der Waals minimum.