ReaxFF reactive molecular dynamics: Coupling mechanical impact to chemical initiation in energetic materials

We report an approach to large-scale atomistic simulations of chemical initiation processes in shocked energetic materials based on a parallel implementation of the ReaxFF reactive force field. Here we present results of Compressive Shear Reactive Dynamics (CSRD) simulations on compressed PETN and RDX single crystal, conventional high explosives. We show that CSRD can evaluate anisotropy of shock sensitivity along different crystallographic directions in single crystal explosives and provide estimates in agreement with experiment for PETN and RDX. The temperature increase is much faster for shear along the slip planes related to experimentally sensitive shock directions. We also investigate the effect of shear on chemical initiation. The dominant initiation reactions in both systems is NO₂ dissociation whose rate significantly varies along shock and slip directions. All calculations are performed with the massively parallel MD code GRASP enabling multi-million atom reactive MD simulations of chemical processes in many important stockpile materials.