Computer simulation studies of anisotropic systems

Abstract

We have simulated the behaviour of an ensemble of particles located on a cubic lattice and interacting via anisotropic dispersion forces using the Monte Carlo technique. The system exhibits a transition between an orientationally ordered and a disordered phase. The temperature dependence of the second rank orientational order parameter, the internal energy and the heat capacity have been determined in both phases. Several analogous simulations have employed a simpler anisotropic pair potential which is taken to depend on only the relative orientations of the particles. One of our major objectives was to study the effect of this unrealistic potential on the properties of the liquid crystal like system. To our surprise we find that the differences in behaviour of the system interacting via dispersion forces and that for the simpler potential are slight. The results of our simulation are compared with the predictions of a molecular field theory developed for the lattice system. The theory is found to overestimate the transition temperature, however if allowance is made for this defect then the long range orientational order is well predicted although the short range order is underestimated. Finally, we find that the director is pinned along a lattice axis—a result which should be of considerable practical importance in future simulation studies of liquid crystals.