Computational studies of the structure of monolayers of chlorine physisorbed on the basal plane of graphite

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Abstract
Results are reported of energy minimizations and molecular-dynamics calculations for chlorine physisorbed on the basal plane of graphite. The dispersion-repulsion component of the intermolecular potential is modelled by an anisotropic site-site potential and the permanent electrostatic potential is represented by distributed multipoles at the atomic nuclei. The molecule-surface interaction includes the anisotropic polarizability of the graphite, image-multipole interactions and substrate mediation of the dispersion interaction. The energy minimizations show that the adsorbed molecules are likely to favour a two-sublattice, herringbone structure; a four-sublattice pinwheel arrangement is also a possibility if image interactions play a sufficiently important role. Difficulties in reproducing the experimental X-ray scattering pattern are discussed. The molecular-dynamics calculations suggest that the low-symmetry structure postulated by Klee et al., (1986, Surf. Sci., 171, 103) is unstable, and melting of the simulated monolayers occurs at temperatures at least 100K below the claimed experimental value. There is a clear need for further thermodynamic and structural studies of this system.