A THEORY FOR THE LIQUID-CRYSTALLINE PHASE-BEHAVIOR OF THE GAY-BERNE MODEL

A simple yet reasonably accurate perturbation theory for the Gay-Berne model, capable of describing the uniform isotropic and nematic phases , as well as the layered smectic-A phase, is presented. The theory, in line with a previously proposed theory, is based on a perturbative sc heme, but the reference system, a hard Gaussian overlap model, is trea ted using the nonlocal approximation of Somoza and Tarazona. This appr oximate scheme, which reduces to the well-known decoupling approximati on for nematics, is a simple generalization of the decoupling approxim ation designed to include smectic structures. The attractive free ener gy is calculated using a mean-field approximation. Underestimation of the attractive energy implied by this approximation is alleviated by i ntroducing some scale factors, set to reproduce the critical point and two triple points involving the smectic phase. The choice of scale fa ctors, which is valid for a particular set of molecular parameters, is shown to reproduce accurately the phase diagram corresponding to othe r parameter values. The theory is used to examine the global liquid-cr ystalline phase behavior of the Gay-Berne model, paying particular att ention to the effect of the anisotropy attraction parameter kappa' on the location of the various phase boundaries. Comparison of the result s with the available computer simulations for this system indicates th at the theory leads to qualitatively correct predictions. The theory c ould be useful to predict the phase behavior of realistic systems with respect to molecular elongation and energy anisotropy. (C) 1998 Ameri can Institute of Physics. [S0021-9606(98)50230-9].