The Theory of Intermolecular Forces in the Region of Small Orbital Overlap

A general theory is given of the intermolecular energy when there is small overlap between orbitals of different molecules. The energy is expanded as a power series in the intermolecular potential U and the overlap integral S, and expressions are given for the contributions to the energy up to order U$^2$S$^2$. We have defined exchange energy by the contribution of order US$^2$. This leads to a simpler expression than the more familiar valence-bond definition of the term. The energy contributions of order U$^2$S$^2$ may be subdivided into two parts, exchange polarization energy and charge transfer energy. The mathematical validity of this subdivision is discussed.