Shock-induced melting of (100)-oriented nitromethane: Structural relaxation

Abstract

Molecules subjected to shock waves will, in general, undergo significant intramolecular distortion and exhibit large amplitude orientational and translational displacements relative to the unshocked material. The analysis of molecular dynamics simulations of strongly perturbed materials is complicated, particularly when the goal is to express time-dependent molecular-scale properties in terms of structural or geometric descriptors/properties defined for molecules in the equilibrium geometry. We illustrate the use of the Eckart-Sayvetz condition in a molecular dynamics study of the response of crystalline nitromethane subjected to supported shock waves propagating normal to (100). The simulations were performed with the nonreactive but vibrationally accurate force field due to Sorescu et al. [J. Phys. Chem. B 104, 8406 (2000)]. Shocks were initiated with impact velocities of U(p)=0.5, 1.0, 2.0, and 3.0 km s(-1) in crystals at initial temperatures of T0=50 and 200 K. Statistical precision in the analysis was enhanced through the use of a spatiotemporal reference frame centered on the advancing shock front, which was located as a function of time using the gradient of the kinetic energy along the shock direction. The Eckart-Sayvetz condition provides a rigorous approach by which the alignment can be obtained between a coordinate frame for a perturbed molecule and one in a convenient reference frame (e.g., one based on the equilibrium crystal structure) for analyses of the molecules in the material as the system evolves toward equilibrium. Structural and dynamic properties of the material corresponding to orientation in the lattice, translational symmetry, and mass transport (orientational order parameters, two dimensional radial distribution functions, and self-diffusion coefficients, respectively) were computed as functions of time with 4 fs resolution. The results provide clear evidence of melting for shocks initiated by impacts of at least U(p)=2.0 km s(-1) and provide insights into the evolution of changes at the molecular-mode level associated with the onset of the melting instability in shocked crystal.