Recent Progress on the Theoretical Treatment of Collisional Energy Transfer between Two Asymmetric Top Rotor Molecules

Quantum-mechanical description of molecule–molecule inelastic scattering, in which two collision partners exchange translational, rotational, and vibrational energy, remains a challenge. Here the full-quantum coupled states (CS) method is extended onto the most general case of collision between two asymmetric top rotors, and the first-ever CS calculations are carried out for the rotationally inelastic scattering of para-H2O + ortho-H2O. It is shown that CS calculations are numerically affordable for reasonably large rotational basis sets and collision energies. Total and partial cross sections are computed and compared to those predicted by mixed quantum/classical theory. Excellent agreement is found, especially for the most intense transitions with large cross sections. It is shown that these processes are driven by long-range dipole–dipole interaction and are dominated by collisions with very large values of total angular momentum, which classically correspond to trajectories with collision impact parameter on the order of 30a0 and even higher.