Machine Calculation of Thermodynamic Properties of a Simple Fluid at Supercritical Temperatures

A Monte Carlo method for the evaluation of equilibrium thermodynamic properties of a system of interacting particles is described. The classical configurational partition function at a given density is calculated from the estimated value of the weight function γ(Φ), up to a factor that is independent of temperature; γ(Φ)dΦ is the fraction of all accessible configurations of N particles confined to a fixed volume for which the total potential energy has a value between Φ and Φ+dΦ. The weight function is found to vary approximately as (Φ—Φ0)n, where Φ0 and n are constants; n increases roughly linearly with density. The method is applied to the calculation of thermodynamic properties of gaseous argon at four densities in the temperature range between −100° and 150°C. A Lennard-Jones potential function is used and though the agreement with experiment is good there is evidence of inadequacy in the Leonnard-Jones model. All calculations are made for N=32.