ANISOTROPIC RIGID ROTOR POTENTIAL-ENERGY FUNCTION FOR H2O-H-2

We have calculated the interaction for H2O-H-2 at 722 points on a five-dimensional surface where both molecules are treated as rigid rotators and we have fitted the ab initio points to a 48-term angular expansion of products of spherical harmonics and rotation matrices. The resulting potential energy function shows strong angle dependence with a large contribution from electrostatic interactions. When averaged over H-2 orientations, the resulting water-atom-like surface is found to have zero crossing and minimum at similar distances to the corresponding H2O-He surface but to be generally more repulsive at short range and more attractive at long range. The isotropic average of the potential has a zero-crossing radius sigma=3.05 Angstrom and a well depth epsilon=49.5 cm(-1) at an intermolecular separation R(m)=3.52 Angstrom.