Analytical approach to molecular liquids. I. Site–site interaction model using an extended mean‐spherical approximation

The observation that the intramolecular direct‐correlation‐function matrix is very closely approximated by its large‐r form for rigid diatomics suggests an approximation such that the site–site Ornstein–Zernike equation becomes analytically solvable for several molecular models of interest. Alternatively, approximation of the particle–particle direct correlation function by its asymptotic form directly yields the same approximation, in particle–particle (in contrast to site–site) language. Applications of this approximation, which can be regarded as an extended mean spherical approximation when viewed in particle–particle terms, are made to rigid hard diatomics, dipolar diatomics, and a tetrahedral molecular model. The close but subtle connections between our approximation procedure and earlier approximations are discussed.