Full-Dimensional Quantum Dynamics Studies of Ro-vibrationally Inelastic Scattering of H 2 O with Ar: A Benchmark Test of the Rigid-Rotor Approximation.

While the rigid-rotor (RR) approximation is usually considered to be accurate for describing pure rotationally inelastic scattering involving diatoms in their ground or low-lying vibrational states, its validity in scattering involving polyatomic molecules has not been fully examined. The existence of soft/anharmonic vibrational modes in polyatomic molecules could make rotational-vibrational energy transfer rather efficient, thus undermining the premise of the RR approximation. In this work, we conduct a benchmark test of the RR approximation in the rotationally inelastic scattering of the H 2 O( v 2 = 0, 1) + Ar system by comparing with full-dimensional quantum scattering calculations. We demonstrate that the error in the RR rate coefficient for v 2 = 0 is less than 5%, while it can reach up to 20% for some initial states within the v 2 = 1 manifold. These results indicate that the RR approximation gradually deteriorates with increasing quantum number v 2 . Vibrational relaxation dynamics of this system was also studied, and it is found that transitions from initial states with a large rotational quantum number of projection on the a principal axis are more efficient. These results shed valuable light on ro-vibrationally inelastic scattering involving polyatomic molecules.