A RISM analysis of structural data for tetrahedral molecular systems

Abstract

A systematic study has been made of available X-ray and neutron diffraction data for liquids and amorphous solids composed of tetrahedral molecules. The intermolecular function, D M(Q), has been compared with predictions computed by the reference interaction site model (RISM) in order to determine the effective hard core radii which characterize the molecular contours. It is found that the RISM formulation gives a satisfactory first-order representation of the data in most cases but there are significant discrepancies which prevent the model being used to give a detailed quantitative description. Specific features are noted in the case of P4 and As4 systems which suggest that a central auxiliary site is required to give agreement with the experimental results. The geometrical packing is considered in relation to orientational correlations between adjacent molecules and the systematic behaviour examined for a range of materials.