van der Waals Forces Including Exchange in the Small Overlap Region

A linear response approach based on the many-body Green's function techniques of Martin and Schwinger is employed to obtain an intermediate range theory of interatomic forces between ground state atoms. Electron exchange is included to lowest order in a natural and physically transparent way. Our result for the interaction energy is expressible in terms of local and nonlocal response functions associated with the individual atoms. Moreover, the long-range limit of our expression for the interaction energy is explicitly demonstrated to reduce to the familiar van der Waals dispersion interaction. Finally the extension of the linear response approach to include exchange into the theory of nonadditive three-body interactions is briefly discussed.