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

We have reinvestigated the structural and thermodynamic properties of the s quare-well and the square-shoulder system using two different theoretical f rameworks, i.e., the optimized random-phase approximation and the Rogers-Yo ung integral equation. We discuss the limits of applicability of the respec tive concepts and compare the results with 'exact' Monte Carlo simulation r esults and with data obtained from a semi-analytic method proposed by Nezbe da for narrow wells. Using these correlation functions, we study in the fra mework of the modified-weighted-density approximation the isostructural sol id-solid transition predicted for narrow wells and narrow shoulders.