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The molecular dynamics technique has been used to simulate patches of solid nitrogen adsorbed on the basal plane of graphite. A model of the molecule-surface interaction involving an isotropic site-site interaction predicts a melting transition 9 K below the experimentally observed temperature and leads to an adsorbed liquid patch on melting. By including the anisotropy in the site-site interaction the lateral surface barriers are increased and a site-site potential of the form r ~ ~ u(,Nc) = 4 Nc E1 + -cos 0)1 -+ A(1 -cos 0)1with ?a = 0-4, ~a = --1'05 will reproduce the experimental transition temperature. The nature of the phase above the melting transition cannot be determined with certainty. The simulation does not give a uniform density distribution for the anisotropic model, but we speculate that the spreading of the patch occurs on a time-scale longer than the simulation. There is clear evidence for a periodic density fluctuation in the adsorbed liquid associated with the molecule-surface potential.

The structure of an adsorbed layer of CS2 on graphite has been determined by X-ray diffraction. The experimentally determined unit cell has lattice parameters a = 0.807nm, b = 0.602nm, and fl = 90 ~ There are two molecules in the unit cell arranged in a herringbone structure and aligned at angles of + and -33 ~ with respect to the short side of the unit cell.The structure has also been calculated using a three centre interaction site model and different quadrupolar interactions. The favoured configuration of the isolated molecule was found to be with the carbon atom over a saddlepoint and the two sulphurs in the centre of adjacent hexagons. However, the most favourable structure for the whole layer was found to be incommensurate. Two stable structures were found whose relative energies depended on the value chosen for the quadrupole moment, 0. One of them, where 0 has the value 11 x 10-4~ 2, is close to the experimental structure, with lattice parameters a = 0.785 nm, b = 0.625 nm, and fl = 90 ~ and two molecules per unit cell inclined at angles of + and -29 ~ to the shorter side.