Structure and thermodynamics of square-well and square-shoulder fluids

We have reinvestigated the structural and thermodynamic properties of the square-well and the square-shoulder system using two different theoretical frameworks, i.e., the optimized random-phase approximation and the Rogers-Young integral equation. We discuss the limits of applicability of the respective concepts and compare the results with `exact' Monte Carlo simulation results and with data obtained from a semi-analytic method proposed by Nezbeda for narrow wells. Using these correlation functions, we study in the framework of the modified-weighted-density approximation the isostructural solid-solid transition predicted for narrow wells and narrow shoulders.