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The first off-lattice Monte Carlo kinetics model of interstellar dust-grainsurface chemistry is presented. The positions of all surface particles aredetermined explicitly, according to the local potential minima resulting fromthe pair-wise interactions of contiguous atoms and molecules, rather than by apre-defined lattice structure. The model is capable of simulating chemicalkinetics on any arbitrary dust-grain morphology, as determined by theuser-defined positions of each individual dust-grain atom. A simple method isdevised for the determination of the most likely diffusion pathways and theirassociated energy barriers for surface species. The model is applied to asmall, idealized dust grain, adopting various gas densities and using a smallchemical network. Hydrogen and oxygen atoms accrete onto the grain, to produceH2O, H2, O2 and H2O2. The off-lattice method allows the ice structure to evolvefreely; ice mantle porosity is found to be dependent on the gas density, whichcontrols the accretion rate. A gas density of 2 x 10^{4} cm^{-3}, appropriateto dark interstellar clouds, is found to produce a fairly smooth and non-porousice mantle. At all densities, H2 molecules formed on the grains collect withinthe crevices that divide nodules of ice, and within micropores (whose extremeinward curvature produces strong local potential minima). The larger poresproduced in the high-density models are not typically filled with H2. Directdeposition of water molecules onto the grain indicates that amorphous icesformed in this way may be significantly more porous than interstellar ices thatare formed by surface chemistry.

Three-dimensional off-lattice Monte Carlo kinetics simulations of interstellar grain chemistry and ice structure