The first off-lattice Monte Carlo kinetics model of interstellar dust-grain surface chemistry is presented .
The positions of all surface particles are determined explicitly , according to the local potential minima resulting from the pair-wise interactions of contiguous atoms and molecules , rather than by a pre-defined lattice structure .
The model is capable of simulating chemical kinetics on any arbitrary dust-grain morphology , as determined by the user-defined positions of each individual dust-grain atom .
A simple method is devised for the determination of the most likely diffusion pathways and their associated energy barriers for surface species .
The model is applied to a small , idealized dust grain , adopting various gas densities and using a small chemical network .
Hydrogen and oxygen atoms accrete onto the grain , to produce H _ { 2 } O , H _ { 2 } , O _ { 2 } and H _ { 2 } O _ { 2 } .
The off-lattice method allows the ice structure to evolve freely ; ice mantle porosity is found to be dependent on the gas density , which controls the accretion rate .
A gas density of 2 \times 10 ^ { 4 } cm ^ { -3 } , appropriate to dark interstellar clouds , is found to produce a fairly smooth and non-porous ice mantle .
At all densities , H _ { 2 } molecules formed on the grains collect within the crevices that divide nodules of ice , and within micropores ( whose extreme inward curvature produces strong local potential minima ) .
The larger pores produced in the high-density models are not typically filled with H _ { 2 } .
Direct deposition of water molecules onto the grain indicates that amorphous ices formed in this way may be significantly more porous than interstellar ices that are formed by surface chemistry .