STATIC LONGITUDINAL DIELECTRIC FUNCTION OF MODEL MOLECULAR FLUIDS

The static longitudinal dielectric function epsilon(L) (k) is calculated for several polar interaction site model (ISM) fluids for comparison with related models having arbitrary short-range interactions and a set of one or more lower-order multipole moments at the centers (OMEGA-M models). The requisite averages over the ISM fluids are calculated by the extended reference interaction site method (XRISM) using site-site hypernetted chain (HNC)-like closures modified to reproduce the correct long-range behavior of the site-site pair correlation functions. They are compared with averages over the OMEGA-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 epsilon(L)(k) is negative over a finite range of k, the low end being in agreement with recent computer simulation studies of both ISM and OMEGA-M polar fluids. However, we confirm that the expected large-k behavior epsilon(L)(k) = 1 governs the ISMs, but not the OMEGA-M models. Based on an adaptation of the color charge-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 epsilon(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 epsilon(L)(k) with recent computer simulation results and find substantial agreement.