Nuclear Quantum Effects in Multi-Step Condensed Matter Chemistry: A Path Integral 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 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.

Figures (8)

• Figure 1: (a) Chemical and (b) crystallographic structure of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB). Atoms are colored as follows: carbon (gray), hydrogen (white), oxygen (red), and nitrogen (blue).
• Figure 2: Average molecular energy at 300 K using PIMD as a function of the number of replicas compared with classical (ClMD) and quantum thermal bath (QTB) simulations. Quantum weighted density of states (qDOS) predictions were calculated directly from ClMD trajectories.
• Figure 3: (a) Average molecular energy and (b) specific heat of TATB at various temperatures from PIMD, QTB, and ClMD simulations, as well as qDOS predictions. For temperatures below 300 K, P was increased to 128 replicas to accommodate for convergence.
• Figure 4: Potential energy (top) and species fraction of TATB (bottom) over time under isothermal-isochoric decomposition at temperatures of 1000, 1500, and 2000 K.
• Figure 5: Arrhenius kinetics plots for TATB decomposition with corresponding activation energies ($E_a$). The characteristic time to determine the rates is defined as the point at which 50% of the TATB molecules remain.