Further investigations into the low-density behaviour of the hypernetted chain equation for ionic fluids

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Abstract
We describe the low-density behaviour of the hypernetted chain equation (HNC) for the restricted primitive model (RPM) of ionic fluids. An efficient computational procedure is developed and applied to the study of the thermodynamics and convergency behaviour in the low density and low temperature (or high ionic strength) region in which there is evidence of liquid-gas coexistence. After a careful study, we attribute the divergence found on the liquid side of the coexistence curve to the presence of a spinodal line. In contrast, divergences on the gas side (low density) are unphysical and appear to result from intrinsic inconsistencies in the HNC approximation. We remark upon the effect that the presence of a ‘cavity term’ added to the RPM pair potential can be expected to have upon the phase-separation behaviour in the HNC approximation as well in a more exact analysis.