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.

A Helmholtz free energy density functional is developed to describe the vapor-liquid interface of associating chain molecules. The functional is based on the statistical associating fluid theory with attractive potentials of variable range ͑SAFT-VR͒ for the homogenous fluid ͓A. Gil-Villegas, A. Galindo, P. J. Whitehead, S. J. Mills, G. Jackson, and A. N. Burgess, J. Chem. Phys. 106, 4168 ͑1997͔͒. A standard perturbative density functional theory ͑DFT͒ is constructed by partitioning the free energy density into a reference term ͑which incorporates all of the short-range interactions, and is treated locally͒ and an attractive perturbation ͑which incorporates the long-range dispersion interactions͒. In our previous work ͓F. J. Blas, E. Martín del Río, E. de Miguel, and G. Jackson, Mol. Phys. 99, 1851 ͑2001͒; G. J. Gloor, F. J. Blas, E. Martín del Río, E. de Miguel, and G. Jackson, Fluid Phase Equil. 194, 521 ͑2002͔͒ we used a mean-field version of the theory ͑SAFT-HS͒ in which the pair correlations were neglected in the attractive term. This provides only a qualitative description of the vapor-liquid interface, due to the inadequate mean-field treatment of the vapor-liquid equilibria. Two different approaches are used to include the correlations in the attractive term: in the first, the free energy of the homogeneous fluid is partitioned such that the effect of correlations are incorporated in the local reference term; in the second, a density averaged correlation function is incorporated into the perturbative term in a similar way to that proposed by Toxvaerd ͓S. Toxvaerd, J. Chem. Phys. 64, 2863 ͑1976͔͒. The latter is found to provide the most accurate description of the vapor-liquid surface tension on comparison with new simulation data for a square-well fluid of variable range. The SAFT-VR DFT is used to examine the effect of molecular chain length and association on the surface tension. Different association schemes ͑dimerization, straight and branched chain formation, and network structures͒ are examined separately. The surface tension of the associating fluid is found to be bounded between the nonassociating and fully associated limits ͑both of which correspond to equivalent nonassociating systems͒. The temperature dependence of the surface tension is found to depend strongly on the balance between the strength and range of the association, and on the particular association scheme. In the case of a system with a strong but very localized association interaction, the surface tension exhibits the characteristic ''s shaped'' behavior with temperature observed in fluids such as water and alkanols. The various types of curves observed in real substances can be reproduced by the theory. It is very gratifying that a DFT based on SAFT-VR free energy can provide an accurate quantitative description of the surface tension of both the model and experimental systems.

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.

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