Most of the all-quantum-mechanical calculations of line shape relaxation cross sections that have been previously published were based on the "impact approximation" allowing us to express off-diagonal elements in terms of collision theory S-matrices. Two consequences of that approximation are pointed out. The first is that impact cross sections do not obey detailed balance except in limited cases. The second, which is shown of more severe consequence for line coupling applications, is the loss of a sum rule relating the off-diagonal elements (coupling a given line to all the others) to the diagonal one. Finally, it appears clearly that off-the-energy shell calculations with realistic potential surfaces are absolutely necessary for ab initio computing line shapes of overlapping lines.