THEORETICAL-STUDY OF THE OH-(H2O)(2) SYSTEM - NATURE AND IMPORTANCE OF 3-BODY INTERACTIONS

The nature and importance of nonadditive three-body interactions in th e ionic OH-(H2O)(2) cluster have been studied by supermolecule Moller- Plesset (MP) perturbation theory and coupled-cluster method, and by sy mmetry-adapted perturbation theory (SAPT). The convergence of the SAPT expansion was tested by comparison with the results obtained from the supermolecule Moller-Plesset perturbation theory calculations through the fourth order (MP2, MP3, MP4SDQ, MP4), and the coupled-cluster cal culations including single, double, and approximate triple excitations [CCSD(T)]. It is shown that the SAPT results reproduce the converged CCSD(T) results within 10%. The SAPT method has been used to analyze t he three-body interactions in the clusters OH- (H2O)(n), n = 2,3,3,10, with water molecules located either in the first or the second solvat ion shell. It is shown that at the Hartree-Fock level the induction no nadditivity is dominant, but it is partly quenched by the Heitler-Lond on and exchange-induction/deformation terms. This implies that the ind uction energy alone is not a reliable approximation to the Hartree-Foc k nonadditive energy. At the correlated level, the most important cont ributions come from the induction-dispersion and the MP2 exchange ener gies. The exchange-dispersion and dispersion nonadditivities are much smaller, and for some geometries even negligible. This suggests that i t will be difficult to approximate the three-body potential for OH-(H2 O)(4) by a simple analytical expression. The three-body energy represe nts only 4%-7% of the pair CCSD(T) intermolecular energy for the OH-(H 2O)(2) cluster, but can reach as much as 18% for OH-(H2O)(4). Particul ar attention has been paid to the effect of the relaxation of the geom etry of the subsystems. (C) 1998 American Institute of Physics. [S0021 -9606(98)30441-9].