Racemic fluids of hard molecules

The structure in four racemic fluids is explored using two integral equatio n theories: the reference interaction site method (RISM) [D. Chandler and H . C. Andersen, J. Chem. Phys. 57, 1930 (1972)] and the diagrammatically cor rect theory of Chandler, Silbey, and Ladanyi (CSL) [D. Chandler, R. Silbey, and B. M. Ladanyi, Mol. Phys. 46, 1335 (1982)]. Discrimination is measured by comparison of site pair distributions for sites on identical molecules with the corresponding distributions for sites on mirror-image molecules. W e find that discrimination is largest for distributions between the smalles t sites in the molecules. Between racemates, those consisting of more asymm etrical chiral molecules (i.e., with a bigger range of site sizes and bond lengths) show the largest discrimination. The indirect correlation function is shown to be nondiscriminating in racemates. Further, exact relationship s between like-like and like-unlike differences in the other pair functions have been obtained. From these, the importance of the bridge functions in discrimination is evident. The CSL theory always satisfies the exact relati onships, even with approximate bridge diagrams. RISM theory cannot satisfy these exact limits regardless of density and closure relation. We have foun d that RISM theory predicts qualitatively incorrect pair distributions at l ow densities, but that the difference in the distributions is more accurate . All bridge diagrams which contribute to O(rho (o)) have been enumerated a nd evaluated. Inclusion of these diagrams into the CSL theory leads to exac t results at low density. However, we find that the inclusion of the rho (o ) diagrams has dramatically improved the quality of the CSL theory at all d ensities. (C) 2001 American Institute of Physics.