Many-body interaction free energy to infinite order. I. General susceptibility formulation

Abstract

A unified treatment is presented for the van der Waals interaction between N nonoverlapping molecular systems of arbitrary sizes and electron delocalizations. The theory takes account of many‐body effects and multipolar interactions to all orders, and is valid for any finite temperature. By neglecting correlations between multiple excitations within each system, a closed‐form expression is obtained for the interaction free energy in terms of the charge density susceptibilities of the isolated molecular systems and the Coulomb potentials between their charge distributions. Then an approximation is introduced which allows the free energy shift to be written in simple diagrammatic form. Finally, the general theory is specialized to the case of small molecular systems (or molecular units), and the dipolar interaction is worked out in detail.