Application of an extended RISM equation to dipolar and quadrupolar fluids

Abstract

A generalization of the RISM integral equation for site–site pair correlation functions previously proposed by us is discussed and applied to model liquids composed of strongly polar diatomic molecules. The nonuniform molecular charge distribution is represented by the introduction of charged interaction sites. The generalization consists of applying closure conditions analogous to those which are known to be reasonable for the description of atomic ionic fluids, and the corresponding renormalization of the contributions arising from long range forces. We discuss both the symmetry properties of the pair correlation functions in special cases and the dielectric properties implied by theory. Applications are presented for three two-site models which differ substantially in the degree of asymmetry of the non-Coulombic potential between the two sites, and for three three-site models for Br2. The two sites models are compared to computer simulation results, and those for Br2 to experimental results. The analysis shows that the integral equation is well balanced in that in every case the qualitative features of the liquids structure which are introduced by polarity are well represented, even in cases where the site–site potentials are individually much larger than kBT. In cases where the molecular shape and polar forces are in competition, the results are of comparable accuracy to the corresponding theory for nonpolar systems. In the extreme case where changes in orientational structure can occur without interfering with packing requirements, the results appear quantitatively less reliable.