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.

Though there are numerous studies of solute molecule rotational dynamics in ordinary liquids and supercritical fluids, few such studies have focused on the molecules of a neat supercritical fluid. In the present study, we investigate the structure and rotational dynamics of supercritical carbon dioxide at a temperature 1% above its critical temperature and over a 20-fold range of densities about the critical density. Our simulations reveal the presence of density inhomogeneities and thus regions of modest local density enhancement in this fluid over a significant range of bulk densities. Although these inhomogeneities account for the weak bulk density dependence of the CO 2 orientational correlation times in the near-critical regime, they are not reflected in a concomitant variation in the observed total rotational friction. Caution is warranted, however, in interpreting the usual diffusive dynamics parameters in this system because only at densities in excess of the critical density may the dynamics be characterized as diffusive; only at lower densities are orientational correlations found to decay on a time scale equal to or shorter than that characterizing the angular velocity correlations.

A study of the structural relaxation of nitromethane subsequent to shock loading normal to the (100) crystal plane performed using molecular dynamics and a nonreactive potential was reported recently [J. Chem. Phys. 131, 064503 (2009)]. Starting from initial temperatures of T(0)=50 and 200 K, shocks were simulated using impact velocities U(p) ranging from 0.5 to 3.0 km s(-1); clear evidence of melting was obtained for shocks initiated with impacts of 2.0 km s(-1) and higher. Here, we report the results of analyses of those simulation data using a method based on the Eckart frame normal-mode analysis that allows partitioning of the kinetic energy among the molecular degrees of freedom. A description of the energy transfer is obtained in terms of average translational and rotational kinetic energies in addition to the rates of individual vibrational mode heating. Generally, at early times postshock a large superheating of the translational and rotational degrees of freedom (corresponding to phonon modes of the crystal) is observed. The lowest frequency vibrations (gateway modes) are rapidly excited and also exhibit superheating. Excitation of the remaining vibrational modes occurs more slowly. A rapid, early excitation of the symmetric C-H stretch mode was observed for the shock conditions T(0)=50 K and U(p)=2.0 km s(-1) due to a combination of favorable alignment of molecular orientation with the shock direction and frequency matching between the vibration and shock velocity.

A coexisting solid-liquid (s-l) system of nitromethane is created by surface-induced melting. A nitromethane crystal with a free surface is simulated by molecular dynamics (MD) in the constant-volume and -energy (NVE) ensemble for initial conditions generated by short MD simulations of the constant-volume and -temperature (NVT) ensemble at temperatures slightly above the melting point. Melting starts at the surface, initiating a solid-liquid interface, and the temperature drops as the system moves toward a state of equilibrium in which the solid and liquid phases coexist. The temperature at which the coexisting solid and liquid reach equilibrium is taken to be the melting point. The melting points of crystals with exposed (100), (010), and (001) crystallographic faces are predicted to be 238, 245, and 242 K, respectively. The predicted melting points are in good agreement with experiment (244.7 K) and previous simulations. The approach to equilibrium during the NVE simulation is monitored by calculating the orientational order parameter, diffusion coefficient, and density, which provide insights into the melting mechanism. The Sorescu-Rice-Thompson [J. Phys. Chem. B 2000, 104, 8406] force field, which accurately describes the inter-and intramolecular motions, was used.

The melting of nitromethane initiated at solid-vacuum interfaces has been investigated using molecular dynamics nvt simulations with a realistic force field [D. C. Sorescu et al., J. Phys. Chem. B 104, 8406 (2000)]. The calculated melting point (251+/-5 K) is in good agreement with experiment (244.73 K) and values obtained previously (approximately 255.5 and 266.5+/-8 K) using other simulation methods [P. M. Agrawal et al., J. Chem. Phys. 119, 9617 (2003)]. Analyses of the molecular orientations and diffusion during the simulations as functions of the distance from the exposed surfaces show that the melting is a direct crystal-to-liquid transition, in which the molecules first gain rotational freedom, then mobility. There is a slight dependence of the melting temperature on the exposed crystallographic face.

Studies of rotational relaxation dynamics provide particular insight into local solution structures and consequently into the interactions between species in a solution. We report here the results of molecular dynamics simulations describing a neat CO 2 supercritical fluid and an infinitely dilute solution of toluene in supercritical CO 2 . Over a period of 0.1-0.2 ps, the rotation of the near-critical solvent molecules is relatively unhindered, becoming purely diffusive only on a time scale that is long compared with the decay of the orientational correlations. As expected, the rotational relaxation rate of a toluene molecule is found to increase with increasing solvent density, although the simulation results imply some anomalous behavior near the critical point that may be associated with the appearance of long-range spatial correlations. We also show that a system consisting of a nonpolar toluene analogue experiences an isotropic rotational friction environment, unlike the anisotropic environment in which a real toluene molecule is found when dissolved in supercritical CO 2 .

Large fluorescent molecules have been encapsulated in pyrogallol[4]arene nano-capsules, thus exploring the limits of molecular encapsulation; the orientation of these probe molecules within these nano-capsules has been examined with additional computational approaches in one case.

The crystallization of nitromethane, CH(3)NO(2), from the melt on the (100), (010), (001), and (110) crystal surfaces at 170, 180, 190, 200, 210, and 220 K has been investigated using constant-volume and -temperature (NVT) molecular dynamics simulations with a realistic, fully flexible force field [D. C. Sorescu, B. M. Rice, and D. L. Thompson, J. Phys. Chem. B 104, 8406 (2000)]. The crystallization process and the nature of the solid-liquid interface have been investigated by computing the molecular orientations, density, and radial distribution functions as functions of time and location in the simulation cell. During crystallization the translational motion of the molecules ceases first, after which molecular rotation ceases as the molecules assume proper orientations in the crystal lattice. The methyl groups are hindered rotors in the liquid; hindrance to rotation is reduced upon crystallization. The width of the solid-liquid interface varies between 6 and 13 Å (about two to five molecular layers) depending on which crystal surface is exposed to the melt and which order parameter is used to define the interface. The maximum rate of crystallization varies from 0.08 molecules ns(-1) Å(-2) for the (010) surface at 190 K to 0.41 molecules ns(-1) Å(-2) for the (001) surface at 220 K.