Perturbation theory of three-body exchange nonadditivity and application to helium trimer

The many-body symmetry-adapted perturbation theory (SAPT) of intermolecular interactions has been applied to the nonadditivity of the trimer first-ord er exchange energy. The orbital expressions including intramonomer correlat ion effects up to the second order have been derived. These expressions, to gether with other many-body SAPT terms developed earlier, have been used to compute the nonadditivity of the helium trimer. The computed points have b een fitted by a global potential function. (C) 2000 American Institute of P hysics. [S0021-9606(00)31001-7].