Static longitudinal dielectric function of model molecular fluids

摘要

The static longitudinal dielectric function egr;L(k) is calculated for several polar interaction site model (ISM) fluids for comparison with related models having arbitrary shorthyphen;range interactions and a set of one or more lowerhyphen;order multipole moments at the centers (OHgr;M models). The requisite averages over the ISM fluids are calculated by the extended reference interaction site method (XRISM) using sitendash;site hypernetted chain (HNC)hyphen;like closures modified to reproduce the correct longhyphen;range behavior of the sitendash;site pair correlation functions. They are compared with averages over the OHgr;M models under the RHNC theory taken from the literature or calculated under the mean spherical approximation. We find for fluids of strong enough polarity that egr;L(k) is negative over a finite range ofk, the low end being in agreement with recent computer simulation studies of both ISM and OHgr;M polar fluids. However, we confirm that the expectedlargehyphen;kbehavior egr;L(k)=1 governs the ISMs, but not the OHgr;M models. Based on an adaptation of the color chargendash;color field techniques of molecular dynamics, we develop the concept of the color longitudinal dielectric function; it provides useful information about the role of the spatial extent of the molecular charge distribution on the behavior of egr;L(k). The ISM fluids we have analyzed include dipolar dumbbells over a wide range of bond length and polarity as well as realistic interaction site models for water and methanol. For the methanol model, we compare our egr;L(k) with recent computer simulation results and find substantial agreement.