Inelastic Neutron Scattering and Molecular Dynamics Determination of the Interaction Potential in Liquid ${\mathrm{CD}}_4$

Anisotropic interactions of liquid ${\mathrm{CD}}_4$ are studied in detail by comparison of inelastic neutron Brillouin scattering data with molecular dynamics simulations using up to four different models of the methane site-site potential. We demonstrate that the experimental dynamic structure factor $S(Q,\omega )$ acts as a highly discriminating quantity for possible interaction schemes. In particular, the $Q$ evolution of the spectra enables a selective probing of the short- and medium-range features of the anisotropic potentials. We show that the preferential configuration of methane dimers at liquid densities can thus be discerned by analyzing the orientation-dependent model potential curves, in light of the experimental and simulation results.

Figure 1

${\tilde{S}}_{\mathrm{coh}}(Q,\omega )$ of liquid ${\mathrm{CD}}_4$ at selected $Q$ values. Neutron data (circles with error bars) are compared with the MD results, after instrumental resolution broadening, for the SA (dotted line), RMK (solid line), TUT (full dots) and RP (dashed line) potentials. In the lowest frame, the SA and RMK central peaks coincide.

Figure 2

Three significant configurations of methane dimers (3D graphs in the reference system $XYZ$, reported as insets) and corresponding total ${\mathrm{CD}}_4\mathrm{\text{−}}{\mathrm{CD}}_4$ potential energy, in units of the Boltzmann constant. Different curves refer to the RMK (solid curve), TUT (full circles), RP (dashed curve), and SA (dotted curve) models, plotted as a function of the CC distance along the $X$ axis of the coordinate system. Different energy scales are adopted to enhance, simultaneously, all the details of the attractive well and of the repulsive wall: a linear scale for the former, and a logarithmic scale for the latter.