LIQUID-CRYSTAL PHASE-DIAGRAM OF THE GAY-BERNE FLUID BY PERTURBATION-THEORY

A thermodynamic linear perturbation theory for the Gay-Berne intermolecular potential has been developed which is able to predict the occurrence of isotropic liquid-vapour coexistence as well as the stability of a nematic phase. The theory can be regarded as a generalisation to molecular fluids of the Weeks, Chandler, Andersen (WCA) pertubative scheme for simple fluids. The reference system, a hard Gaussian overlap model, is analysed within density-functional theory using a standard Onsager-like approach, the "decoupling approximation," where density correlations are treated to all orders (albeit approximately) whereas orientational correlations are approximated by the low density limit. We implement this idea by using an equivalent system of hard spheres to approximate the density-dependent part of the excess free energy. The structure of the reference system is approximated by the radial distribution function of an equivalent system of hard spheres, scaled with the contact distance of the hard core. The phase diagram for the Gay-Berne fluid obtained from the theory is compared with computer simulation data of the same model potential and found to be in fair agreement. In contrast, a mean-field approximation for the perturbative term gives rather poor results. © 1995 American Institute of Physics.