Predicting the Thermodynamic Properties of the Modified Lennard-Jones Fluid from the Lennard-Jones Equation of State

The Lennard-Jones (LJ) potential has been widely used, even beyond fluid science, as a standard form to express interparticle interaction. However, numerical computation and simulation requires some convention for dealing with the potential's infinite tail. This delicate problem can be circumvented by adopting a truncated potential. The functional form of the latter should mimic that of the original potential as closely as possible, while simultaneously minimizing artificial effects caused by the resultant discontinuities. The advantage of choosing such a truncated LJ potential, called the modified Lennard-Jones (mLJ) potential, is that the thermodynamic behavior of fluids can be predicted from the extensive knowledge base regarding the LJ fluid. In this study, the properties of the mLJ fluid were treated as perturbations of the LJ fluid case, and the discrepancies were expressed as density series expansions. The second virial coefficient of the mLJ fluid was well reproduced by this method. Reproduction of the liquid--vapor coexisting envelope was also satisfactory, except in the vicinity of the critical point.