A hierarchy of approximate methods is developed for solving the equations of motion within framework of the mixed quantum/classical theory (MQCT) of inelastic molecular collisions. Of particular interest is a limiting case – the method in which the classical-like equations of motion for the translational degrees of freedom (scattering) are decoupled from the quantum-like equations for time-evolution of the internal molecular states (rotational, vibrational). In practice, trajectories are pre-computed during the first step of calculations, with driving forces determined solely by the potential energy surface of the entrance channel, which is an adiabatic trajectory approximation. Quantum state-to-state transition probabilities are computed during the second step of calculations, with an expanded basis and very efficient step-size adjustment. Application of this method to H2O + H2 and H2O + H2O rotationally inelastic scattering shows a significant computational speed-up, by two orders of magnitude, relative to the full-coupled MQCT calculations. The results of this approximate propagation scheme are still rather accurate, as demonstrated by benchmarking against more rigorous calculations in which the quantum and classical equations of motion are held coupled, and against the full-quantum coupled-channel calculations from literature. It is concluded that AT-MQCT method represents a promising tool for the computational treatment of molecular collisions and energy exchange for the molecules important in a variety of astro-chemical environments.

Recent progress in the mixed quantum/classical calculations of collisional energy transfer between the molecules of astrochemical relevance

ACS Spring Meeting 2021

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