Theoretical rate constants among the lowest 45 para and 45 ortho rotational levels of water in collisions with He atoms have been calculated for temperatures between 20 and 2000 K using a recently improved theoretical interaction potential. These values are about 30%-40% larger than those reported previously (Palma et al. 1988b) but relative sizes of different state-to-state rates have not changed significantly. Successive improvements to the theoretical description of this system now appear to have converged.

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We have calculated the interaction for H2O–H2 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 H2 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 σ=3.05 Å and a well depth ε=49.5 cm−1 at an intermolecular separation Rm=3.52 Å.

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