Rate coefficients for the O+H2 and O+D2 reactions: How well Ring Polymer Molecular Dynamics accounts for tunneling

Abstract

We present here extensive calculations of the O(³P)+H₂ and O(3P)+D₂ reaction dynamics spanning the temperature range from 200K to 2500 K. The calculations have been carried out using fully converged time-independent quantum mechanics (TI QM), quasiclassical trajectories (QCT) and ring polymer molecular dynamics (RPMD) on the two lowest lying adiabatic potential energy surfaces (PESs), 1³A′ and 1³A′′, calculated by Zanchet et al. [J. Chem. Phys., 2019, 151, 094307]. TI QM rate coefficients were determined using the cumulative reaction probability formalism on each PES including all of the total angular momenta and the Coriolis coupling, and can be considered to be essentially exact within the Born-Oppenheimer approximation. The agreement between the rate coefficients calculated by QM and RPMD is excellent for the reaction with D₂ in almost the whole temperature range. For the reaction with H2 although the agreement is very good above 500 K, the deviations are significant at lower temperatures. In contrast, the QCT calculations largely underestimate the rate coefficients for the two isotopic variants due to its inability to account for tunneling. The differences found in the disagreements between RPMD and QM rate coefficients for both isotopologue reactions are indicative of the ability of RPMD method to accurately describe systems where tunneling plays a relevant role. Considering that both reactions are dominated by tunneling below 500 K, the present results show that RPMD is a very powerful tool for determining rate coefficients. The present QM rate coefficients calculated on adiabatic PESs slightly underestimate the best global fits of the experimental measurements, which we attribute to the intersystem crossing with the singlet 1¹A′ PES.