Biaxial nematic liquid crystal phases of a soft ellipsoid-string fluid

We have calculated the phase diagram of a fluid, the molecules of which are composed of a string of partially overlapping oblate soft ellipsoid intera ction sites, by molecular dynamics simulation. The molecules are elongated in one direction and flattened in the perpendicular direction so that they become biaxial. We find that when the fluid consisting of 8-site or 9-site molecules is compressed, a discotic uniaxial nematic phase is formed. Furth er compression causes a transition to a biaxial phase. The same phase behav iour is found by compressing a fluid composed of molecules with 10 or 11 si tes, but in this case the uniaxial phase is calamitic. The 9-site and 10-si te molecules consequently bracket the Landau bicritical point where there i s a continuous transition between the isotropic phase and the biaxial nemat ic phase. This behaviour is consistent with the Landau-de Gennes theory and mean field theories. However, the molecular dimensions for which the Landa u point appears differ from the theoretical predictions of the mean field t heories.