A perturbation density-functional theory for polyatomic fluids. I. Rigid molecules

We present a density-functional theory of nonuniform fluids composed of polyatomic species. It is derived from Wertheim’s first-order perturbation theory of polymerization by taking the limit of complete association. In the case of the uniform fluid, an explicit expression of the complete angle-dependent pair distribution function is obtained which shows that the theory yields nontrivial results for the intermolecular structure. However, features of the long-wavelength structure, such as the existence of an orientational order, are not described. Illustrative applications of the theory are given in the case of diatomic and linear tetra-atomic molecules formed by tangent hard spheres of the same diameter.