Computer simulation study of the phase behavior of a nematogenic lattice-gas model

The phase behavior of a mesogenic lattice-gas model consisting of freely rotating spins located at the sites of a three-dimensional cubic lattice has been studied using grand canonical Monte Carlo simulations. When two neighboring sites are occupied, the spin vectors interact via the extensively studied anisotropic Lebwohl-Lasher potential, plus an isotropic term of variable strength. The interaction between occupied and empty sites and two empty sites is taken to be zero. If the parameter governing the strength of the isotropic term is zero, the model exhibits an isotropic fluid-nematic transition, which becomes increasingly stronger as the temperature is lowered. The additional isotropic term is found to be important if the model is to reproduce experimental phase behavior, that is, to exhibit both nematic-vapor coexistence at low temperature and isotropic-vapor coexistence at higher temperatures.