A failing of coupled-states calculations for inelastic and pressure-broadening cross sections:: Calculations on CO2-Ar

Fully quantal benchmark calculations of pressure-broadening cross sections for infrared and Raman lines of CO2 perturbed by Ar are carried out using both close-coupling (CC) and coupled-states (CS) calculations. CS calculations are found to underestimate the cross sections by up to 15%. The effect occurs even for isotropic Raman cross sections, which are not affected by reorientation contributions. The discrepancy arises mostly for collisions with large orbital angular momenta l, occurring on the long-range part of the potential. It may be attributed to collisions that are adiabatic rather than sudden in nature. A hybrid computational method, employing CS calculations for low l and decoupled l-dominant (DLD) calculations for high l, offers a promising solution. (C) 1999 American Institute of Physics. [S0021-9606(99)00735-7].