Reactive Molecular Dynamics Simulations to Investigate the Shock Response of Liquid Nitromethane

We use molecular dynamics (MD) simulations with the ReaxFF reactive force field to investigate the thermomechanical, chemical, and spectroscopic response of nitromethane (NM) to shock loading. We simulate shocks using the Hugoniostat technique and use four different parametrizations of ReaxFF to assess the sensitivity of the results with respect to the force field. The predicted shock states, for both the unreacted and reacted materials, are in good agreement with experiments, and two of the force fields capture the increase in shock velocity due to exothermic reactions in excellent agreement with experiments. The predicted detonation velocities with these two force fields are also in good agreement with experiments, and the differences in predicted values are linked to the differences in the reaction products. Across all force fields, NM decomposes predominantly via bimolecular reactions and the formation of nitrosomethane (CH3NO) is found as a dominant initiation pathway. We elucidate the mechanisms of secondary reactions leading to stable products, whose predicted populations with all four descriptions are in good agreement with experiments. We also calculated the time-resolved spectra from the trajectories during the shock processes to help correlate the underlying reaction mechanisms with spectral features and enable a one-to-one comparison with laser-driven shock experiments. This study demonstrates the potential of reactive molecular dynamics to describe the physics and chemistry of high-energy density materials under shock loading and complement experimental efforts to derive a definite, validated understanding.