Ab initio calculations and modeling of three-body forces in Ar2H2O

The nonadditive interactions in the Ar2H2O cluster, in a variety of geometries, are studied using the supermolecular Moller-Plesset (MP) perturbation theory in conjunction with the perturbation theory of intermolecular forces. The three-body effect is dissected into several components: the Heitler-London exchange, induction, dispersion, and second-order supermolecular MP perturbation nonadditivity. The latter term is further analyzed within the framework of pseudodimer treatment. The Heitler-London exchange is dissected into the SE (single exchange) and TE (triple exchange) parts. Analytical representations of the nonadditive terms are discussed. For the exchange (SE) and induction, the multicenter expansions work very well. However, in the case of the three-body dispersion term, the single-center dipole-dipole-dipole plus dipole-dipole-quadrupole expression proves to be more suitable. (C) 2002 Wiley Periodicals, Inc.