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 thermodyna mic perturbation theory of polymerization is extended to the case of n onrigid molecules. In this formulation, the free energy in the limit o f complete association is express as a functional of the full molecula r distribution function and the excess contribution is obtained pertur batively 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 d eveloped by Chandler and Pratt. As a first application, the intermolec ular site-site correlations of homonuclear chainlike hard-sphere molec ules are calculated within first-order perturbation theory (TPT1). Qua litative agreement with simulation data is found for short chains at h igh densities but the theory fails for long chains and low densities b ecause of the neglect of the intramolecular excluded volume.