Scattering of CO with H2O: Statistical and classical alternatives to close-coupling calculations.

Energy transfer in inelastic atom-molecule and molecule-molecule collisions can be described theoretically using the quantum-mechanical close-coupling method. Unfortunately, for bimolecular collisions implying heavy colliders and/or for which the potential energy surface has a deep well, the resulting coupled equations become numerically intractable and approximate methods have to be employed. H2O-CO collisions provide an important example for which close-coupling calculations are not feasible. In this paper, we investigate the accuracy of three approximate methods (the coupled states method, the quasi-classical trajectory method, and the statistical adiabatic channel model) to describe inelastic collisions of H2O with CO. We perform scattering calculations on a recent 5D potential energy surface, and we compare the results of the three approximate methods to fully converged close-coupling calculations at energies below 300 cm-1 and at low values of the total angular momentum. We show that the statistical method provides an attractive alternative to fully quantum mechanical close-coupling calculations at low collision energies, while the quasi-classical method is more advantageous at high energies.