Should Thermostatted Ring Polymer Molecular Dynamics be used to calculate thermal reaction rates?

TL;DR

This study evaluates Thermostatted Ring Polymer Molecular Dynamics (TRPMD) for calculating thermal reaction rates, comparing its accuracy to other methods across various chemical reactions and temperature regimes.

Contribution

It demonstrates that TRPMD's rate predictions are consistent with quantum theories and explores how friction influences its accuracy in different reaction systems.

Findings

TRPMD's short-time limit matches Quantum Transition-State Theory.

TRPMD rate is insensitive to friction above the crossover temperature.

TRPMD generally underestimates or matches RPMD accuracy for symmetric reactions.

Abstract

We apply Thermostatted Ring Polymer Molecular Dynamics (TRPMD), a recently-proposed approximate quantum dynamics method, to the computation of thermal reaction rates. Its short-time Transition-State Theory (TST) limit is identical to rigorous Quantum Transition-State Theory, and we find that its long-time limit is independent of the location of the dividing surface. TRPMD rate theory is then applied to one-dimensional model systems, the atom-diatom bimolecular reactions H+H$_2$, D+MuH and F+H$_2$, and the prototypical polyatomic reaction H+CH$_4$. Above the crossover temperature, the TRPMD rate is virtually invariant to the strength of the friction applied to the internal ring-polymer normal modes, and beneath the crossover temperature the TRPMD rate generally decreases with increasing friction, in agreement with the predictions of Kramers theory. We therefore find that TRPMD is approximately equal to, or less accurate than, Ring Polymer Molecular Dynamics (RPMD) for symmetric reactions, and for certain asymmetric systems and friction parameters closer to the quantum result, providing a basis for further assessment of the accuracy of this method.