Perturbation theory for square-well plus sutherland (∞, 6) fluid

The perturbation approach to the theory of fluids, originally introduced by Zwanzig, is applied to the square-well plus Sutherland (∞, 6) fluid. It is shown that the virial coefficients up to the fourth are in close agreement with those for the LJ (12-6) with a hard-sphere cutoff when the bowl width of the square well used is chosen to be equal to 0.25 σ where σ is the hard-sphere diameter. Also, for this suitable choice of the bowl width, the first and the second-order corrections to the free energy have been found to agree reasonably well with those for the square-well fluid of Barker and Henderson.