The Asymptotic Behavior of the Site-Site Ornstein-Zernike Equation and RISM-2 Approximations for Polyatomic Molecular Fluids

Abstract

It is shown that the asymptotic properties of the reference interaction site model (RISM)-2 type integral equations for polyatomic molecular fluids are quite similar to those of simple fluids. The exact asymptotic form of the new site-site direct correlation function C_αγ(r), which is defined by the site-site Ornstein-Zernike equation, is given by using the exact asymptotic form of the site-site total correlation function. The result is C_αγ(r)\simeq−U(1, 2)⁄k_BT for nonpolar fluids composed of homonuclear diatomic molecules as well as heteronuclear diatomic molecules, where U(1, 2) is an intermolecular potential, k_B the Boltzmann constant and T the absolute temperature. The asymptotic form of C_αγ(r) given by the RISM-2 approximations is found to be in accordance with the exact result. Thus, the RISM-2 theory is concluded to have no theoretical inconsistency like that in the RISM-1 theory.