Molecular Dynamics Simulations of Shock Induced Chemistry in Organic Materials

The chemistry of energetic materials (EM) is characterized by cascades of exothermic reactions on the ps to ns timescales, which generate numerous intermediates between the explosive and final products. Experiments have struggled to resolve the intermediates (species and reaction rates), which dictate the state of the material at longer time scales. It is imperative to obtain a quantitative understanding of the reaction pathways as a function of temperature and pressure during the early stages of detonation, so that we can move beyond the use of heavily calibrated, though non-transferable, rate laws in mesoscale simulations.

Molecular dynamics simulations (MD) are well suited to resolving the reactions that occur during detonation, and the density functional tight-binding (DFTB) method offers a good compromise: DFTB inherently includes a description of the electronic structure of materials, critical to capture bond breaking and remaking processes, but remains computationally inexpensive compared to standard DFT, allowing us to simulate systems with hundreds of atoms, over hundreds of picoseconds, with good accuracy. The DFTB-lanl parameterizations for CHNO-based molecules are found to provide accuracy approaching that of regular DFT theory for our systems.

In this work, we performed MD simulations of reactions as a function of temperature and pressure for two EM: nitromethane and pentaerythritol tetranitrate (PETN), in order to identify the types and rates of reactions as a function of the initial conditions. Our analyses allowed us to identify pressure-dependent reaction pathways for these two EM. Further developments, such as the inclusion of long-range dispersion forces and the use of accelerated MD techniques to reach ns timescales, will also be discussed.