Structure and thermodynamics of the dipolar hard sphere fluid from the reference-hypernetted chain equation with minimized free energy

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Abstract
The reference-hypernetted chain equation, generalized to molecular fluids, is optimized by choosing the reference system so as to minimize the free energy. This procedure, which assures a significant improvement in the internal thermodynamic consistency of the theory, is here applied to a fluid of dipolar hard spheres, using both the complete dipolar potential and one with a reaction field (RF) truncation. We confirm that a recent reformulation of the relation between the dielectric constant ε and the mean square dipole moment for the RF geometry indeed brings ε for the truncated potential into reasonably good agreement with the infinite-range values, but that the important correlation functions nevertheless differ qualitatively in their long-range behavior. The method of solving the molecular integral equation, developed earlier, can be applied to other multipolar potentials, or alternatively, to molecules with distributed point charges.