Ring polymer molecular dynamics in gas-surface reactions: tests on initial sampling and potential energy landscape

Abstract

Given the exponential scaling of the standard quantum wavepacket method, accurate quantum dynamics calculations for gas-surface scattering remain a daunting challenge. Recent development of the ring polymer molecular dynamics (RPMD) approach has shed some light on calculating microcanonical dynamical quantities in non-thermal conditions, although most studied systems feature a notable barrier. Here we investigate the performance of RPMD reaction probability calculations in two gas-surface systems, one is an example for weakly-activated reactions (H2+Co(0001)) and the other is a prototypical non-activated reaction (H2+Pd(111)). Initial conditions of the molecule have been obtained with the Boltzmann sampling on the realistic potential energy surface or the approximate harmonic potential. The two sampling schemes lead to little difference in the weakly-activated H2+Co(0001) system, both giving results in good agreement with the quantum ones. However, they yield distinguishable dynamics in the non-activated system, where harmonic Boltzmann sampling-based RPMD behaves similarly as the quasi-classical trajectory method and underestimates the reactivity at low energies. In comparison, the realistic Boltzmann sampling-based RPMD overestimates the reactivity but reasonably captures the trapping-mediated reaction mechanism. Possible reasons of this deficiency have been discussed.