AR-C2H2 - A CHALLENGING SYSTEM FOR AB-INITIO CALCULATIONS

The Ar-HCCH interaction is investigated by the supermolecular Moller-P lesset perturbation theory and coupled-cluster theory in conjunction w ith the perturbation theory of intermolecular forces. The interaction energy in this cluster is dissected into the fundamental components su ch as exchange, induction, and dispersion, and the anisotropy of the o verall surface is analysed in terms of the angular behaviors of these components. The shape of the PES of Ar-HCCH is very sensitive to the l evel of ab initio theory. The coupled-cluster approach which included the single, double, and approximate triple excitations combined with t he aug-cc-pvtz basis set supplemented by bond functions was necessary to obtain the best estimates of D-e (within +/- 5% error) for two mini ma: the global one for a skew T-configuration (118 cm(-1)), and a seco ndary one for the collinear arrangement (115 cm(-1)). The PES generate d at a lower level of calculation was used in diffusion quantum Monte Carlo (DMC) calculations of the ground and first excited bending vibra tional state of the cluster. The expectation values of the coordinates and rotational constants are provided for the ground state. DMC calcu lations are relatively unsensitive to the details of the PES. The natu re and decomposition of three-body interactions in the related cluster Ar-2-HCCH were also investigated. (C) 1997 Elsevier Science B.V.