The phase behavior of model linear and bent-core molecules has been studied using isothermal-isobaric Monte Carlo computer simulations. The molecular model consists of seven Lennard-Jones spheres rigidly arranged in a "V" shape, with external bond angle, gamma. With gamma=0 degrees (linear molecules), we find isotropic, nematic, untilted smectic A, and two layered phases in which the molecules are tilted with respect to the layer normal. The latter two phases correspond to distinct branches in the equation of state, and possess different types of ordering within and between the layers; these phases are tentatively assigned as being smectic B and crystal. Apart from the possible existence of a tilted smectic B, the phase behavior of this system is broadly in line with earlier simulation studies on related linear molecular models. In the gamma=20 degrees system, isotropic, nematic, and tilted smectic- B phases are observed. Interestingly, the range of stability of the nematic phase is enhanced compared to the gamma=0 degrees system. In simulations of the gamma=40 degrees system, nematic phases are absent, and only isotropic and tilted phases are in evidence. The in-layer structure of the tilted phases shows a very clear change from smectic- B to smectic- A ordering upon increasing the temperature. In all instances of a tilted phase, the degree of molecular tilt is in the region of 30+/-5 degrees, with respect to the smectic layer normal, which corresponds closely to typical experimental observations in real bent-core liquid crystals. In our model, the tilt provides efficient packing of the spheres and favorable attractive interactions between molecules. The relevance of the present simulation model to real bent-core liquid crystals is discussed critically.

The effects of dipolar interactions and molecular flexibility on the structure and phase behavior of bent-core molecular fluids are studied using Monte Carlo computer simulations. Some calculations of flexoelectric coefficients are also reported. The rigid cores of the model molecules consist of either five or seven soft spheres arranged in a 'V' shape with external bend angle γ. With purely repulsive sphere-sphere interactions and γ = 0 • (linear molecules) the seven-sphere model exhibits isotropic, uniaxial nematic, smectic-A, and tilted phases. With γ ≥ 20 • the smectic-A phase disappears, while the system with γ ≥ 40 • shows a direct tilted smectic-isotropic fluid transition. The addition of electrostatic interactions between transverse dipole moments on the apical spheres is generally seen to reduce the degree of tilt in the smectic and solid phases, destabilize the nematic and smectic-A phases of linear molecules, and destabilize the tilted smectic-B phase of bent-core molecules. The effects of adding three-segment flexible tails to the ends of five-sphere bent-core molecules are examined using configurational-bias Monte Carlo simulations. Only isotropic and smectic phases are observed. On the one hand, molecular flexibility gives rise to pronounced fluctuations in the smectic-layer structure, bringing the simulated system in better correspondence with real materials; on the other hand, the smectic phase shows almost no tilt. Lastly, the flexoelectric coefficients of various nematic phases -with and without attractive sphere-sphere interactions -are presented. The results are encouraging, but the computational effort required is a drawback associated with the use of fluctuation relations.