Effects of elongation on the phase behavior of the Gay-Berne fluid

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 kappa. We study a range of length-to-width parameters 3 less than or equal to kappa less than or equal to 4, 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 kappa, the liquid-vapor critical point moves to lower temperature until it falls below the I-S-B coexistence line, around kappa = 3.4, where liquid-vapor coexistence proves hard:to establish. The liquid-vapor critical point seems to be completely absent at kappa = 4.0. Another dramatic effect is the growth of a stable S-A "island" in the phase diagram at elongations slightly above kappa = 3.0. The S-A range extends to both higher and lower temperatures as kappa is increased. Also as kappa is increased, the I-N transition is seen to move to lower density land pressure) at given temperature. The lowest temperature at which the nematic phase is stable does not vary dramatically with kappa. 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.