Nuclear Quantum Effects in Multi-Step Condensed Matter Chemistry: A Path Integral Molecular Dynamics Study of Thermal Decomposition

TL;DR

This study investigates the impact of nuclear quantum effects on the thermal decomposition of TATB, revealing that quantum effects accelerate reactions and reduce activation energy, with path integral molecular dynamics providing more accurate insights than classical methods.

Contribution

The paper demonstrates the importance of nuclear quantum effects in condensed matter reactions and compares PIMD, QTB, and classical MD to evaluate their accuracy in modeling thermal decomposition.

Findings

PIMD predicts faster decomposition than classical MD.

Quantum effects reduce activation energy by ~8%.

QTB overestimates quantum acceleration effects.

Abstract

Nuclear quantum effects (NQEs) are often central to a predictive understanding of chemical reactions and rates. While their incorporation in gas-phase reactions is well established, studies involving condensed matter often neglect or approximate such effects. To clarify the role of NQEs in multi-step, multi-molecular reactions in a molecular crystal, we compare atomistic simulations of the thermal decomposition of the energetic material TATB using path integral molecular dynamics (PIMD), the more approximate quantum thermal bath (QTB), and classical MD (ClMD). PIMD samples the quantum canonical distribution by representing each atom as a string of beads (replicas), while QTB uses a frequency-dependent thermostat to reproduce the Bose-Einstein distribution. We find that PIMD results in faster chemical decomposition of the TATB crystal compared to ClMD, as the initial steps involve hydrogen transfer processes. Interestingly, some of the subsequent reactions (e.g. the formation of N2) occur on identical timescales. The PIMD simulations also predict a reduction in overall activation energy by ~8% as compared to the classical result. As observed in model systems and simple unimolecular gas-phase reactions, the QTB significantly overestimates quantum acceleration of chemical reactions and the reduction in activation energy. A comparison of the kinetic energy operator in PIMD and the centroid dynamics provides insight into the physics behind the differences between the QTB and PIMD results.