Zwanzig's perturbation theory for the triangular-potential fluid

Abstract

The perturbation theory presented by Zwanzig is applied to the evaluation of the thermodynamic properties of a fluid in which molecules interact by a triangular-potential well. Coefficients of the Zwanzig expansion are calculated by the method of Clippe and Evrard, and expressed in terms of a reduced density.

These are numerically evaluated in a particular case for which Monte Carlo data have been reported by Card and Walkley. The density dependence of the coefficients is well represented by expressions given by Clippe and Evrard. Agreement between ‘experimental’ and theoretical results is satisfactory.

In some region of the thermodynamic (density-temperature) plane, some disagreement exists either because of the scarcity of data or because of the truncated nature of the expansions. The convergence of the method is discussed. The results indicate that the good agreement found by Clippe and Evrard for their extension of the Zwanzig method in the case of real fluids is of intrinsic nature and has not been due to the existence of a free parameter (the hard core radius).