Post-shock relaxation in crystalline nitromethane.

Molecular dynamics simulations of shocked (100)-oriented crystalline nitromethane were carried out to determine the rates of relaxation behind the shock wave. The forces were described by the fully flexible non-reactive Sorescu-Rice-Thompson force field [D. C. Sorescu, B. M. Rice, and D. L. Thompson, J. Phys. Chem. B 104, 8406 (2000)]. The time scales for local and overall thermal equilibration in the shocked crystal were determined. The molecular center-of-mass and atomic kinetic energy distributions rapidly reach substantially different local temperatures. Several picoseconds are required for the two distributions to converge, corresponding to establishment of thermal equilibrium in the shocked crystal. The decrease of the molecular center-of-mass temperature and the increase of the atomic temperature behind the shock front exhibit essentially exponential dependence on time. Analysis of covalent bond distance distributions ahead of, immediately behind, and well behind the shock front showed that the effective bond stretching potentials are essentially harmonic. Effective force constants for the C-N, C-H, and N-O bonds immediately behind the shock front are larger by factors of 1.6, 2.5, and 2.0, respectively, than in the unshocked crystal; and by factors of 1.2, 2.2, and 1.7, respectively, compared to material sufficiently far behind the shock front to be essentially at thermal equilibrium.