Structural and thermodynamic properties of argon liquid as a square-well fluid based on a modification of the ORPA theory

The structural and thermodynamic properties of the argon fluid were investigated by means of a modification of the optimised random phase approximation (ORPA) theory and the square-well (SW) potential. The results were obtained at the SW potential values 1.25 lambda<2, and along different isotherms at a wide range of densities. The radial distribution function, g(r), was calculated, and the results obtained were extrapolated to give the appropriate contact values g(sigma), g(lambda sigma (-)) and g(lambda sigma (+)). The results were compared with the simulation and experimental data available in the literature. The results calculated for the Helmholtz free energy (A) and internal energy (E) are in good agreement with the experimental data at the sub- and super-critical temperatures. The ORPA version considered here provided good results for the compressibility factor Z from the Virial route with the SW attractive range lambda=1.7 at high temperatures and low densities.