Model grand potential for a nonuniform classical fluid

Classical equilibrium statistical mechanics of one‐dimensional hard cores in an arbitrary external field is reviewed. The grand canonical potential is shown to be an extension of local thermodynamics in which suitable averages of density and pressure are used. A general three‐dimensional system with arbitrary internal and external potentials is modeled in corresponding form, and the external potential represented as a functional of the density. Comparison with assumed bulk data permits evaluation of the required averages. It is shown that the wall equation of state for a wall‐bounded fluid is satisfied exactly. Application is also made to the self‐maintained density profile of the two‐phase fluid at a first order phase transition.