An atomistic technique for simulating non-covalent interactions in large ensembles of high-molecular-weight polyaromatics

Molecular modelling is a particularly useful tool for studying the behaviour of high-molecular-weight pitch components, since these very large and irregular compounds are not available in pure form for fundamental experimental studies. To date detailed computational chemistry has been carried out only on small clusters of such large discotics, while simulations of bulk phases have used simple molecular potentials (e.g. Gay-Berne) that cannot be directly related to specific molecular structure. The present paper presents a new isobaric-isothermal Monte Carlo technique for simulating large ensembles of discotic molecules based on massive summation of atomic pair potentials. The technique uses explicit description of atom locations with no adjustable potential parameters and thus allows parametric study of molecular structural features such as non-planar aliphatic rings, irregularly-shaped polyaromatic cores, and methyl groups. Here the technique is described and results presented on two model irregular PAH. The simulations show detailed features of stacking and mesophase formation as a function of temperature, and the phase behaviour is shown to be sensitive to minor perturbations in the molecular structure.