AB-INITIO STUDY OF THE O-2(X(3)SIGMA(-)(G))-1) VAN-DER-WAALS CLUSTER(HE(S)

Potential energy surface for the He(S-1)+O-2(X (3) Sigma(g)(-)) intera ction is calculated using the supermolecular unrestricted Moller-Pless et perturbation theory approach and is analyzed via the perturbation t heory of intermolecular forces. The latter has been generalized to pro vide a decomposition of the interaction energies into electrostatic, e xchange, induction, and dispersion constituents for monomers described by unrestricted Hartree-Fock determinants. The global minimum occurs for the T-shaped geometry, around 6.0a(0). Our UMP4 estimate of the we ll depth of the global minimum is D-e=27.7 cm(-1). This value is expec ted to be accurate to within a few percent. The potential energy surfa ce reveals also a local minimum for the collinear geometry at about 7. 0a(0). The well depth for the secondary minimum is estimated at D-e=25 .5 cm(-1)(UMP4) and is expected to be accurate within a few percent. T he minima are separated by a barrier of 7.5 cm(-1) The energy partitio ning reveals that the origin of interaction in this complex is typical for van der Waals clusters involving He. The global minimum is determ ined by the minimum in the exchange repulsion in the direction perpend icular to the O-O bond. The secondary linear minimum results from the maximum dispersion attraction and enhanced by a slight flattening of t he electron density near the ends of the interoxygen axis. (C) 1996 Am erican Institute of Physics.