Analysis of Collisional Properties in Molecular Dynamics Simulations of Shockwaves

Non-equilibrium molecular dynamics simulation (NEMD) provides a complete description of how shockwaves form and propagate in liquids and solids. However, despite many decades of study, the highly non-equilibrium transport processes that drive entropy production within shockwaves are still poorly understood. The standard NEMD practice of projecting the raw particle trajectories onto a static reduced dimensionality representation of the shockwave obscures the non-equilibrium nature of this process. In particular, we show that the thermal anisotropy observed in previous studies is an artifact caused by commingling velocity distributions of pre- and post-shock particle populations. NEMD simulations of shockwave structure can be better understood by sampling the collisional velocity correlation function (CVCF) averaged over all the individual particles. This has been made convenient by recent enhancements to the LAMMPS code. We use this approach to directly observe shockwave collisional processes and shockwave structure in several important model materials, including the Lennard-Jones liquid and FCC crystal. The analysis of shockwaves using CVCF is general and can be equally applied to more realistic simulations of diverse materials using quantum-accurate machine-learning interatomic potentials.