Direct calculation of intermolecular potential energy surfaces

Abstract

A variation perturbation method is presented for the direct calculation of intermolecular interaction energies. The theory is based on valence bond ideas but avoids the full evaluation of the matrix elements by expansion in powers of interchange, a procedure which is valid for small overlap between the systems. The participating excited states are regarded as polarized pseudo-states and are determined by optimizing the long-range multipole-multipole part of the interaction energy. The validity of these ideas is illustrated by a calculation of the He-He interaction. A remarkable simplification is pointed out, in which the interaction energy is given almost exactly by the sum of the repulsive term, calculated as zero + single interchange, plus the long-range interaction.