Modeling high rate impact sensitivity of perfect RDX and HMX crystals by ReaxFF reactive dynamics

We report a methodology for rapid assessment of impact sensitivity of energetic materials which uses the ReaxFF reactive force field in reactive dynamics (RD) simulations of the high rate compression/expansion of a perfect energetic crystal. This approach is validated here to study the high rate impact sensitivity of 1,3,5-trinitrohexahydro-striazine (RDX) crystal and octahydro-1,3,5,7-tetrazocine (HMX) crystal at different phases (α, β, γ and δ). These simulations found that for a compression rate of ~8.76km/s along the [100] direction, RDX crystals at lower volume compression ratios (x=30, 35, 38%) led only to a few RDX molecules decomposing and only into primary products. However, at higher compression ratios (x ≥ 40%), all RDX molecules in the crystal decompose very quickly, leading to both primary and secondary decomposition reactions, including various intermediates such as NO₂, NO, HONO, and OH and final products such as H₂O, N₂, CO, and CO₂. For the various phases of HMX, these ReaxFF RD simulations found noticeably higher impact sensitivity for the d-phase than for other three phases (α, β, and γ). At the same compression ratio x=40%, all HMX molecules in δ-phase decompose leading to both primary and secondary reaction. However, at 40%, only few HMX molecules in α-, β-, and γ-phases decompose. For a higher compression ratio (x=42%), increased HMX decomposition is observed for all four phases. These simulation results for both RDX and HMX crystals agree qualitatively with experiment observations. We also observe a variation of the strain energy in different HMX phases induced by the high rate compression, which could be related to the sensitivity difference of HMX phases. These simulations typically took less than 18 h to run on a single 3.0-GHz processor, demonstrating that the fast compression approach by MD simulations with the ReaxFF force field can be used for a quick evaluation of the sensitivity of energetic materials.