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Five models for the site-site intermolecular pair interactions of methane are compared in some detail and used to investigate both structural and dynamical properties of the dense liquid deuteromethane by means of molecular dynamics (MD) simulations. The orientational distribution probabilities of molecular pairs are carefully analyzed for each anisotropic potential model. We propose a revision of existing classification methods used to group the innumerable relative orientations of methane-methane pairs into six basic geometries. With this new approach, our results for the probability of the six basic categories as a function of the intermolecular distance are different from the ones present in the literature, where the role of the angular spread on the anisotropic interaction energy is not taken in full consideration and certain configurations with no significant change in the pair-potential are assigned to different categories. The analysis of the static orientational correlations in liquid methane and the prevalence of certain configurations in different ranges guide the subsequent discussion of the MD model-dependent results for the dynamic structure factor. Comparison with our inelastic neutron scattering results for liquid CD4 at the nanometer and picosecond space and time scales allows us to confirm the full adequacy of the Tsuzuki, Uchimaru and Tanabe model of 1998 with respect to more recent potentials. [ABSTRACT FROM AUTHOR]

We performed large scale molecular dynamics simulations of liquid gallium at 320 and 970 K, exploiting a phenomenological model of the ion-pair potential depending only on the electron density. The simulations provided a description of the structure and of several dynamic properties of the system, in quantitative agreement with the experimental findings. The analysis of the intermediate self-correlation function indicated the presence of a second process, other than the diffusive process, driving the dynamics of liquid gallium at short times. This fast process, having a characterizing decay time of the order of a fraction of a picosecond, is here identified with a cage motion. [ABSTRACT FROM AUTHOR]

The low-momentum dynamics of liquid mercury has been investigated by means of inelastic neutron scattering and molecular dynamics (MD) simulations. Because of the rather high incoherent cross-section and the high mass of mercury, the measured dynamic structure factor is dominated by the self-correlation function, although the low-momentum collective dynamics are made accessible by the very good energy resolution of the experiment. Collective modes are clearly visible against the incoherent scattering up to a momentum transfer of 0.4 Å[sup -1] and the associated velocity is found to be 2200=200 m s[sup -1], well in excess of the sound velocity which is equal to 1470 m s[sup -1]. The self-dynamics, as resulting from either the experiment or the MD simulations, turn out to be characterized by two time scales. [ABSTRACT FROM AUTHOR]

The presence of a fast-sound mode in the microscopic dynamics of the rare-gas mixture He-Ne, predicted by theoretical studies and molecular-dynamics simulations, was demonstrated by an inelastic neutron scattering experiment. In order to study the transition between the fast and the normal acoustic modes in the hydrodynamic regime, k values lower by about one order of magnitude than in the usual experiments have to be probed. We describe here the results of the first neutron Brillouin scattering experiment performed with this purpose on the same system already investigated at larger k. The results of both experiments, together with those of a new molecular-dynamics simulation, provide a complete and consistent description, still missing so far, of the onset of fast-sound propagation in a binary mixture. [ABSTRACT FROM AUTHOR]

The velocity autocorrelation function (VAF), a key quantity in the atomic-scale dynamics of fluids, has been the first paradigmatic example of a long-time tail phenomenon, and much work has been devoted to detecting such long-lasting correlations and understanding their nature. There is, however, much more to the VAF than simply the evidence of this long-time dynamics. A unified description of the VAF from very short to long times, and of the way it changes with varying density, is still missing. Here we show that an approach based on very general principles makes such a study possible and opens the way to a detailed quantitative characterization of the dynamical processes involved at all time scales. From the analysis of molecular dynamics simulations for a slightly supercritical Lennard-Jones fluid at various densities, we are able to evidence the presence of distinct fast and slow decay channels for the velocity correlation on the time scale set by the collision rate. The density evolution of these decay processes is also highlighted. The method presented here is very general, and its application to the VAF can be considered as an important example. [ABSTRACT FROM AUTHOR]