Phase Diagram of the Lennard-Jones System of Particles from the Cell Model and Thermodynamic Perturbation Theory

Alan Bizjak, Tomaž Urbič* and Vojko Vlachy

Faculty of Chemistry and Chemical Technology, University of Ljubljana, Aškerčeva 5, 1000 Ljubljana, Slovenia
* Corresponding author: E-mail: tomaz.urbic@fkkt.uni-lj.si

Abstract
The thermodynamic perturbation approach and the cell theory are used to determine the complete phase diagram of a system of particles interacting via the Lennard-Jones potential. The Barker-Henderson perturbation theory (J. A. Barker, D. Henderson, J. Chem. Phys. 1967, 47, 4714–4721) is invoked to calculate the liquid-vapour line, while the cell model approach is utilized to evaluate the solid-vapour and solid-liquid equilibria lines. The resulting phase diagram along with the triple point and the critical point conditions are determined. Although the liquid-vapour line is predicted quite well, the triple point parameters, T ^*_t and P^*_t are in relatively poor agreement with the available computer simulation data. This finding, together with our other calculations, suggest that a simple cell theory may not be adequate for characterizing the solid-fluid equilibrium in systems involving strongly correlated molecules.