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 U2S2. We have defined exchange energy by the contribution of order US2. This leads to a simpler expression than the more familiar valence-bond definition of the term. The energy contributions of order U2S2 may be subdivided into two parts, exchange polarization energy and charge transfer energy. The mathematical validity of this subdivision is discussed.