A molecular dynamics simulation of nitrogen adsorbed on graphite

The adsorption of N2 molecules on a structured graphite surface has been studied using the molecular dynamics technique. N2 is modelled using two Lennard-Jones centres and partial charges to represent the quadrupole moment. The surface, which is represented by a rigid external field, has longitudinal structure and the adsorbate molecules form a √3 × √3 commensurate solid with herringbone orientational ordering in two sublattices. The model exhibits a rotational transition to plastic crystal at 33 K (experiment 27 K). The simulated transition temperature does not vary markedly with coverage. The adsorbate shows little residual herringbone ordering above the phase transition, but a six-fold ordering due to the crystal field of the adsorbate appears.