Applications of the RISM equation to diatomic fluids: the liquids nitrogen, oxygen and bromine

Abstract

The reference interaction site model (RISM) integral equation is used to study the equilibrium pair correlation for one-component liquids composed of homonuclear diatomic molecules. The integral equation is first tested by comparing the results obtained from it with those of computer simulation calculations. An internal consistency test is developed which seems to provide an a priori measure of the accuracy of the RISM equation. Then the theory is used to interpret the neutron scattering structure factor data taken on the liquids nitrogen and oxygen. For both of these liquids, a satisfactory explanation of the data is obtained by assuming that the short ranged repulsive forces between molecules are mimicked by a two-site hard core model. However, this simple model does not provide a satisfactory explanation of the data taken from liquid bromine. But, it is shown that a slightly more sophisticated model for the short-ranged repulsion between Br 2 molecules does provide an adequate explanation. With the molecular models determined by fitting the neutron scattering data, the RISM equation provides a method for determining the atom, atom to center-of-mass, and center-of-mass to center-of-mass intermolecular distribution functions in the diatomic liquids. From these functions, the local structures in the three liquids are analyzed. While orientational pair correlations are nearly negligible in both the liquids nitrogen and oxygen, these correlations are fairly substantial in liquid bromine. Furthermore, even when the orientation of a molecule does not greatly influence the orientation of its neighbors, it is found that the orientation of a molecule does have an important effect on the location of its neighbors. Thus, the coupling of translational and orientational coordinates is significant, even in the liquids nitrogen and oxygen.