Molecular dynamics simulations of calamitic and discotic liquid crystals

Significant progress has been made in recent years in modelling liquid crystal phases using the Monte Carlo and molecular dynamics simulation techniques. We describe the technique of molecular dynamics in the microcanonical ensemble that we have used in simulations of liquid crystal systems. A review and discussion of some of the important simulations that have been performed to date on non-spherical hard particle models, soft anisotropic single site models, and realistic atom-atom based models is presented.

We report the results of molecular dynamics simulation studies of a system of particles interacting via an anisotropic potential proposed by Luckhurst and Romano, scaled by part of the well depth formulation employed by Gay and Berne. The resultant hybrid Gay-Berne Luckhurst-Romano (HGBLR) potential has an approximately spherical hard core with anisotropic long range attractive interactions with a dependency on the intermolecular vector joining a pair of sites. The spherical hard core nature of individual HGBLR centres notwithstanding we have parameterised single-site HGBLR centres to represent both calarnitic and discotic mesogens. Both systems are shown to exhibit a range of mesophases on cooling from the isotropic liquid to form a crystal, including uniaxial-nematic and columnar-like phases. Unlike previous hard particle studies these ordered phases obtain because of the presence of the long range attractive interactions. A comparison between the different structures formed with the two different parameterisations is presented including graphical representations of the simulation cell .

In order to more closely represent the short range anisotropic interactions of real mesogens, a 3-HGBLR-site model has been parameterised to represent the mesogen para-terphenyl. Details of the parameterisation are discussed. Two versions of this model,, a twisted central site 3-HGBLR-site site model obtained from a molecular mechanics minimum energy conformation of para-terphenyl, and an all coplanar 3-HGBLR-site site model have been studied using the molecular dynamics technique. The resultant models are found to be biaxial unlike previous anisotropic single site studies utilising soft potentials. Both models appear to exhibit a variety of uniaxial and biaxial mesophases but inclusion of the twisted site appears to promote the formation of biaxial phases. A comparison of the two models is made.