A PHASE-DIAGRAM FOR NONSPHERICAL MOLECULES WITH SPHERICAL SQUARE WELLS - ISOTROPIC AND NEMATIC PHASES

The phase diagram for a model fluid of hard ellipsoids with an encircl ing square well potential has been calculated using a van der Waals li ke theory. In the present theory for the Helmholtz free energy, the ha rd repulsive interactions have been treated using 'exact' second and t hird virial coefficients, combined with a resummation of the higher or der terms in accord with the Carnahan and Starling hard-sphere limit. Attractive interactions in the Helmholtz free energy have been retaine d to lowest order by the second virial coefficient alone. The present theory accommodates prolate and oblate bodies with roughly the same le vel of accuracy. Further, the predicted vapour-liquid coexistence curv es are in much closer agreement with the simulations of de Miguel and Allen than that of our earlier mean spherical approximation based theo ry.