Computer simulation of ammonia on graphite. I. Low temperature structure of monolayer and bilayer films

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Abstract
Molecular dynamics computer simulations are reported here for the structures and energies of monolayer and bilayer films of ammonia adsorbed on the basal plane of graphite. A pair‐wise additive intermolecular potential function is used that includes electrostatic interactions by distributions of discrete charges within each molecule. The nonelectrostatic energy is assumed to be given by a sum of site–site energies over the four atoms in each molecule. Parameters for this potential have been chosen to give agreement with experimental data for the bulk NH3. The NH3/graphite potential was also assumed to be site–site potentials summed over the four atoms per molecule and over the C sites in the solid. The simulations yield translational and orientationally ordered layers. A unit cell for the monolayer is deduced which is triangular with an NH3–NH3 spacing in good agreement with experiment. Because of a relatively complex orientational ordering, the unit cell contains four molecules. The orientational ordering is dominated by the electrostatic energies, but differs considerably from that for a purely dipolar monolayer due to the presence of important quadrupolar interactions. The orientational structures of the individual layers in the bilayer films are different from that of the monolayer. The relationship between these observations and the intermolecular potential functions is discussed.