Rotational excitation in molecule−molecule collisions has been treated for the first time by accurate quantum close−coupling scattering calculations, employing an expansion basis set of two to three rotational levels for each molecule and correctly accounting for exchange of identical particles. Elastic and inelastic cross sections have been computed for collisions of para−para, ortho−ortho, and para−ortho hydrogen molecules assuming an intermolecular potential suggested previously. The accuracy of recent ’’effective potential’’ calculations is demonstrated by comparison with the exact quantum results.

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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