Fluids of hard diatomic molecules. I

Abstract

The problem addressed is that of finding the complete orientation-dependent pair distribution function of a fluid of hard diatomic molecules. The treatment decomposes the Mayer f function into several terms and uses a graph-theoretical treatment. The basic idea is the explicit use of molecular geometry in constructing approximations. The fugacity expansion is found to be superior to the density expansion for this purpose. The neglect of large classes of graphs is shown to be exact for extremely asymmetric diatomics and is introduced as a new approximation for the less asymmetric case. Topological reduction of the reduced expansion leads to a result that resembles the density expansion, but contains an additional parameter. An integral equation of Ornstein–Zernike type is derived, and an approximation for closing the equation is suggested. The resulting equations are calculationally tractable and can be used to obtain both the centers correlation function and the higher harmonics of the pair distribution function. The relation to RAM perturbation theory is discussed briefly.