New x-ray diffraction data for liquid methane near the triple point are presented. The data are analyzed using molecular scattering factors for methane which permit extraction of the carbon–carbon structure and correlation functions. Several site–site pair potentials are used with the reference interaction site model (RISM) to predict the structure and correlation functions for a dense methane-like fluid, and these are compared to the structure and correlation functions derived from the diffraction study. In addition, the accuracy of RISM is assessed by comparison with published computer simulation calculations on methane. Our results indicate that although the RISM distribution functions for ’’tetrahedral’’ molecules such as methane hae some physically unrealistic features, they compare well with molecular dynamics results. Site–site interaction potentials give an adequate description of liquid methane, provided that attractive forces are included in the potential. Although methane molecules are ’’seen’’ by x rays as spherically symmetric, orientational correlations between pairs of neighboring molecules are clearly present in the liquid near the triple point.

Habenschuss, A., Johnson, E., & Narten, A. H. (1981). X-ray diffraction study and models of liquid methane at 92 K. The Journal of Chemical Physics, 74(9), 5234–5241.