Square well perturbation theory of fluids

A perturbation theory for simple classical fluids that uses a square well reference system is presented. To first order, the theory expresses the free energy of a simple fluid in terms of the properties of a SW system with suitably selected hard core diameter, range and depth. These effective parameters are shown to have an interpretation as averages of microscopic quantities. The problem of handling the reference SW system is considered and the theory is tested by calculating the properties of a Lennard-Jones system. The results show the SWPT to be highly convergent, and the behaviour of the effective parameters is discussed.