Ab initio study of the van der Waals interaction of NH(X3Σ-) with Ar(1S)

The potential energy surface for the Ar(S-1)+NH(X (3)Sigma(-)) interaction is calculated using the supermolecular unrestricted Moller-Plesset (UMP) perturbation theory approach and analyzed via the perturbation theory of intermolecular forces. The global minimum occurs for the approximate T-shaped geometry with Ar skewed toward the H atom at about Theta = 67 degrees and R =6.75 a(0). Our UMP4 estimate of the well depth of the global minimum is D-e = 100.3 cm(-1) and the related ground state dissociation energy obtained by rigid-body diffusion quantum Monte Carlo calculations (RBDQMC) is D-0 = 71.5 +/- 0.1 cm(-1). These values are expected to be accurate to within a few percent. The potential energy surface also features a wide plateau in the proximity of Ar-N-H collinear geometry, at ca. 7.0 a(0). RBDQMC calculations reveal nearly a free rotation of the NH subunit in the complex. (C) 1998 American Institute of Physics.