Thermodynamic perturbation theory for polydisperse colloidal suspensions using orthogonal polynomial expansions

We present a method for calculating the thermodynamic and structural proper ties of a polydisperse liquid by means of a thermodynamic perturbation theo ry: the optimized random phase approximation (ORPA). The approach is an ext ension of a method proposed recently by one of us for an integral equation application [Phys. Rev. E 54, 4411 (1996)]. The method is based on expansio ns of all sigma-dependent functions in the orthogonal polynomials p(i)(sigm a) associated with the weight function f(Sigma)(sigma), where sigma is a ra ndom Variable (in our case the size of the particles) with distribution f(S igma)(sigma). As in the one-component or general N-component case, one can show that the solution of the ORPA is equivalent to the minimization of a s uitably chosen functional with respect to variations of the direct correlat ion functions. To illustrate the method, we study a polydisperse system of square-well particles; extension to other hard-core or soft-core systems is straightforward.