Nuclear Quantum Effects in Multi-Step Condensed Matter Chemistry: A Thermostatted Ring Polymer Molecular Dynamics Study of Thermal Decomposition.

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 multistep, multimolecular reactions in a molecular crystal, we compare atomistic simulations of the thermal decomposition of the energetic material TATB using the path integral-based thermostatted ring polymer molecular dynamics (TRPMD), the more approximate quantum thermal bath (QTB), and classical MD (ClMD). TRPMD 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 TRPMD 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 time scales. The TRPMD 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 TRPMD and the centroid dynamics provides insight into the physics behind the differences between the QTB and TRPMD results.