A PERTURBATION DENSITY-FUNCTIONAL THEORY FOR POLYATOMIC FLUIDS .2. FLEXIBLE MOLECULES

The density-functional theory for polyatomic fluids presented earlier [J. Chem. Phys. 97, 9222 (1992)] on the basis of Wertheim's thermodynamic perturbation theory of polymerization is extended to the case of nonrigid molecules. In this formulation, the free energy in the limit of complete association is express as a functional of the full molecular distribution function and the excess contribution is obtained perturbatively from the excess free energy and the structural properties of the reference monomeric fluid at the same density. This defines a set of approximations which can be also related to the general formalism developed by Chandler and Pratt. As a first application, the intermolecular site-site correlations of homonuclear chainlike hard-sphere molecules are calculated within first-order perturbation theory (TPT1). Qualitative agreement with simulation data is found for short chains at high densities but the theory fails for long chains and low densities because of the neglect of the intramolecular excluded volume.