van der Waals interaction potentials: Many-body effects

Many-body contributions to van der Waals interaction potentials are discussed using the Hartree-Fock approximation as a zero-order model and diagrammatic many-body perturbation theory to describe electron correlation effects. A perturbation series is obtained for the effects of electron correlation on each of the terms in the expansion of the total interaction energy in its n-body components. Generalizations of the function counterpoise technique are used to allow for basis set superposition effects in calculations of many-body components of interaction energies. Prototype calculations are reported for the helium trimer.