In this paper we present a computer simulation study of the phase behavior of the Gay-Berne liquid crystal model, concentrating on the effects of varying the molecular elongation . We study a range of length-to-width parameters 3рр4, using a variety of molecular dynamics and Monte Carlo techniques, obtaining a guide to the phase behavior for each shape studied. We observe vapor (V), isotropic liquid (I), nematic (N), smectic-A (S A ) and smectic-B (S B ) liquid crystal phases. Within the small range of elongation studied, the phase diagram shows significant changes. On increasing , the liquid-vapor critical point moves to lower temperature until it falls below the I-S B coexistence line, around ϭ3.4, where liquid-vapor coexistence proves hard to establish. The liquid-vapor critical point seems to be completely absent at ϭ4.0. Another dramatic effect is the growth of a stable S A ''island'' in the phase diagram at elongations slightly above ϭ3.0. The S A range extends to both higher and lower temperatures as is increased. Also as is increased, the I-N transition is seen to move to lower density ͑and pressure͒ at given temperature. The lowest temperature at which the nematic phase is stable does not vary dramatically with . On cooling, no S B -crystal transition can be identified in the equation of state for any of these elongations; we suggest that, on the basis of simulation evidence, S B and crystal are really the same phase for these models.

We present in this paper a computer simulation study of the phase behaviour of the Gay-Berne liquid crystal model. The effect of the anisotropic attractive interactions on stabilizing orientationally ordered phases is analyzed by varying the anisotropy parameter κ ′ at fixed values of the molecular elongation parameter κ. Molecular dynamics simulations have been performed at constant density and temperature along several isotherms and approximate transition densities are reported. It is found that, for a given value of the molecular elongation κ = 3, smectic order is favoured at lower densities as κ ′ increases. When κ ′ is lowered, the smectic phase is preempted by the nematic phase. As a result, the nematic phase becomes increasingly stable at lower temperatures as κ ′ is decreased. Additionally, we have studied the liquid-vapour coexistence region for different values of κ ′ by using Gibbs ensemble and Gibbs-Duhem Monte Carlo techniques. We have found evidence of a vapourisotropic-nematic triple point for κ ′ = 1 and κ ′ = 1.25. For temperatures below this triple point, we have observed nematic-vapour coexistence as is found for many liquid crystals in experiments.

No abstract

Computer simulations, using the molecular dynamics and Monte Carlo techniques, and employing simple molecular models, yield insight into general features of phase equilibria, structure, and dynamics of liquid crystals. Here, results are reported from extensive simulations of the Gay-Berne family of molecular models, in which potential parameters are adjusted to vary the molecular length-to-width ratio in a systematic way. Attention is paid to the characterization of nematic, smectic-A and smectic-B phases as functions of these parameters.A simulation study of the approach to the isotropic-nematic phase transition, using a large system size and lengthy runs on the T3D parallel supercomputer, is described. Spatially longranged collective orientational correlations develop in the isotropic phase, close to the transition. The direct correlation function has been calculated for these systems, and remains short-ranged, as expected, as the transition is approached. The simulation results are compared with the density functional analysis of isotropic instability relative to the nematic phase.