High-pressure neutron diffraction on fluid methane

Abstract

High pressure neutron diffraction measurements of the structure factor of fluid methane (C${\mathrm{D}}_{4}$) at $T=370$ K and three supercritical densities are presented, and the intra- and intermolecular structures are determined. The variation of the density has only a weak effect on the intermolecular distribution functions. Statistical mechanical calculations according to the extended reference interaction site model formalism were performed, and an intermolecular potential of a (12-6) Lennard-Jones type with partial charges on the atoms was deduced that reproduces the experimental total atom correlation functions at all investigated thermodynamic states. Additionally, the corresponding structure factor of statistically orientated methane molecules was calculated. As the agreement between experiment and theory is very good, it is concluded that no orientational correlation exists in fluid methane. Reverse Monte Carlo calculations (RMC) were performed with the recently developed optimization algorithm great deluge [G. Dueck, J. Comput. Phys. 104, 86 (1993)]. This is a non-Boltzmann sampling method specially used for minimizing the difference between the experimental and the simulated intermolecular distribution function. The results, obtained by RMC, confirm the statistical mechanical calculations and show that the distribution of the molecules in fluid methane is randomly distributed and no orientational correlation between the molecule axes exists.