Entropy and Pressure of a Liquid with a Charged-Hard-Sphere Reference System

Abstract The Gibbs-Bogoliubov variational method is applied to a simple liquid using a charged-hard-sphere system as a reference system. Simple expressions are derived for the entropy and the pressure. Recently much attention has been focused on the system of charged hard spheres in a uniform neutralising background (hereafter referred to as the CHS system). The CHS system is characterised by two parameters, the packing fraction η = (π/6)nσ3 and the plasma parameter Γ= (Ze)2/akBT where n is the number density, σ the diameter of the hard sphere, Ze the charge of the hard sphere and a is the ion-sphere radius defined by a = (4/37πn)−1/3. The CHS system is considered as the intermediate system between the hard-sphere (HS) system and the classical one-component plasma (OCP), because the CHS system reduces to the HS system when Γ = 0 and to the OCP when η = 0.