Ab initio study of the O-2(X(3)Sigma(-)(g))+He(S-1) van der Waals cluster

Potential energy surface for the He(S-1)+O-2(X (3) Sigma(g)(-)) interaction is calculated using the supermolecular unrestricted Moller-Plesset perturbation theory approach and is analyzed via the perturbation theory of intermolecular forces. The latter has been generalized to provide a decomposition of the interaction energies into electrostatic, exchange, 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 well depth of the global minimum is D-e=27.7 cm(-1). This value is expected to be accurate to within a few percent. The potential energy surface 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. The minima are separated by a barrier of 7.5 cm(-1) The energy partitioning reveals that the origin of interaction in this complex is typical for van der Waals clusters involving He. The global minimum is determined by the minimum in the exchange repulsion in the direction perpendicular to the O-O bond. The secondary linear minimum results from the maximum dispersion attraction and enhanced by a slight flattening of the electron density near the ends of the interoxygen axis. (C) 1996 American Institute of Physics.