Integral equations for simple fluids in a general reference functional approach

The integral equations for the correlation functions of an inhomogeneous fluid mixture are derived using a functional Taylor expansion of the free energy around an inhomogeneous equilibrium distribution. The system of equations is closed by the introduction of a reference functional for the correlations beyond second order in the density difference from the equilibrium distribution. Explicit expressions are obtained for energies required to insert particles of the fluid mixture into the inhomogeneous system. The approach is illustrated by the determination of the equation of state of a simple, truncated Lennard-Jones fluid and the analysis of the behaviour of this fluid near a hard wall. The wall–fluid integral equation exhibits complete drying and the corresponding coexisting densities are in good agreement with those obtained from the standard (Maxwell) construction applied to the bulk fluid. The self-consistency of the approach is examined by analysing the virial/compressibility routes to the equation of state and the Gibbs–Duhem relation for the bulk fluid, and the contact density sum rule and the Gibbs adsorption equation for the hard-wall problem. For the bulk fluid, we find good self-consistency for stable states outside the critical region. For the hard-wall problem, the Gibbs adsorption equation is fulfilled very well near phase coexistence where the adsorption is large. For the contact density sum rule, we find deviations of up to 20% in the ratio of the contact densities predicted by the present method and predicted by the sum rule. These deviations are largely due to a slight disagreement between the coexisting density for the gas phase obtained from the Maxwell construction and from complete drying at the hard wall.