Efficient computational methods for rovibrational transition rates in molecular collisions

TL;DR

This paper introduces efficient approximate methods based on the coupled-states approximation for calculating rovibrational transition rates in molecular collisions, significantly reducing computational effort while maintaining accuracy.

Contribution

It presents the NNCC method that includes first-order Coriolis coupling and combines it with MC-DWBA for faster, accurate rate coefficient calculations in molecular collision modeling.

Findings

NNCC improves accuracy over CSA in rovibrational transition calculations.

Combining NNCC with MC-DWBA reduces CPU time without significant accuracy loss.

Validated methods on CO₂-He collision data.

Abstract

Astrophysical modeling of processes in environments that are not in local thermal equilibrium requires the knowledge of state-to-state rate coefficients of rovibrational transitions in molecular collisions. These rate coefficients can be obtained from coupled-channel (CC) quantum scattering calculations which are very demanding, however. Here we present various approximate, but more efficient methods based on the coupled-states approximation (CSA) which neglects the off-diagonal Coriolis coupling in the scattering Hamiltonian in body-fixed coordinates. In particular, we investigated a method called NNCC (nearest-neighbor Coriolis coupling) [D. Yang, X. Hu, D. H. Zhang, and D. Xie, J. Chem. Phys. 148, 084101 (2018)] that includes Coriolis coupling to first order. The NNCC method is more demanding than the common CSA method, but still much more efficient than full CC calculations, and it is substantially more accurate than CSA. All of this is illustrated by showing state-to-state cross sections and rate coefficients of rovibrational transitions induced in CO$_2$ by collisions with He atoms. It is also shown that a further reduction of CPU time, practically without loss of accuracy, can be obtained by combining the NNCC method with the multi-channel distorted-wave Born approximation (MC-DWBA) that we applied in full CC calculations in a previous paper.