Comment on broadening of water microwave lines by collisions with helium atoms

Earlier theoretical descriptions of interaction forces and collision dynamics in the H2O–He system [A. Palma et al., J. Chem. Phys. 89, 1401 (1988); S. Green et al., ibid. 94, 1346 (1991)] are improved by calculating the interaction at additional orientations and by including diffuse functions in the molecular orbital expansion basis set. The improvements make substantial changes to the potential energy surface, but make only small changes in predicted microwave pressure broadening coefficients. The qualitative temperature dependence and relative widths of different lines appear to be converged with respect to successive improvements to the theoretical description of this system. Remaining discrepancies with available data must reflect problems with the experimental values.