On the theory and computer simulation of dipolar fluids

In this paper we describe a perturbation approximation which can be used to calculate the pair correlation function for an infinite dipolar fluid if ‘exact’ results for a spherically truncated potential are known. Integral equation and Monte Carlo calculations are used to show that a good approximation for the static dielectric constant, ε, of simple dipolar systems can be obtained in this way. Also we reconsider the mean reaction field (MRF) method sometimes applied in the computer simulation of dipolar fluids and derive a rigorous result relating ε and the mean square moment of the sample for these boundary conditions. The formula obtained differs from that previously assumed to be correct and leads to significantly different estimates of the dielectric constant. We carry out a systematic investigation of the MRF method using both integral equations and Monte Carlo calculations and conclude that good estimates of ε can be obtained. For dipolar hard spheres at μ*2=2·0 and ρ*=0·8 the perturbation treatment and Monte Carlo (MRF) calculations give similar values for the dielectric constant. The result obtained (ε≈31) is considerably lower than the value given by the LHNC and QHNC integral equation theories.