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

摘要

Molecules subjected to shock waves will, in general, undergo significant intramolecular distortionand exhibit large amplitude orientational and translational displacements relative to the unshockedmaterial. The analysis of molecular dynamics simulations of strongly perturbed materials iscomplicated, particularly when the goal is to express time-dependent molecular-scale properties interms of structural or geometric descriptors/properties defined for molecules in the equilibriumgeometry. We illustrate the use of the Eckart—Sayvetz condition in a molecular dynamics study ofthe response of crystalline nitromethane subjected to supported shock waves propagating normal to(100). The simulations were performed with the nonreactive but vibrationally accurate force fielddue to Sorescu et al. J. Phys. Chem. B 104, 8406 (2000). Shocks were initiated with impactvelocities of U_p=0.5, 1.0, 2.0, and 3.0 kms~(-1) in crystals at initial temperatures ofT0=50and200 K. Statistical precision in the analysis was enhanced through the use of a spatiotemporalreference frame centered on the advancing shock front, which was located as a function of timeusing the gradient of the kinetic energy along the shock direction. The Eckart—Sayvetz conditionprovides a rigorous approach by which the alignment can be obtained between a coordinate framefor a perturbed molecule and one in a convenient reference frame (e.g., one based on the equilibriumcrystal structure) for analyses of the molecules in the material as the system evolves towardequilibrium. Structural and dynamic properties of the material corresponding to orientation in thelattice, translational symmetry, and mass transport (orientational order parameters, two dimensionalradial distribution functions, and self-diffusion coefficients, respectively) were computed asfunctions of time with 4 fs resolution. The results provide clear evidence of melting for shocksinitiated by impacts of at least U_p=2.0km s~(-1)andprovide insights into the evolution of changes atthe molecular-mode level associated with the onset of the melting instability in shocked crystal.