OPTIMIZED PERTURBED HARD-SPHERE EXPRESSIONS FOR THE STRUCTURE AND THERMODYNAMICS OF LENNARD-JONES FLUIDS

First-order perturbed hard sphere fluid expressions are fitted to Lennard-Jones fluid compressibility factor, Z, and internal energy, U, data taking the two Lennard-Jones constants as adjustable parameters. The results are used to distinguish the utility of alternative perturbative algorithms over a wide density and temperature range. First-order Barker-Henderson perturbative expressions are found to systematically underestimate temperature dependent changes in thermodynamic properties while the Weeks-Chandler-Andersen model is found to reasonably reproduce simulation results throughout the liquid and supercritical fluid domain, with only a few per cent adjustment of the effective Lennard-Jones parameters. Closed analytical expressions for the Barker-Henderson and Weeks-Chandler-Andersen hard sphere diameters as well as a hard fluid reference system radial distribution function model are presented. These are shown to accurately reproduce hard sphere diameters and thermodynamic properties of Lennard-Jones fluids. Efficient algorithms for determining effective Lennard-Jones parameters for real fluids are demonstrated using fits to supercritical fluid N2 compressibility factor data.