  For the first time, we report a unified microscopic-macroscopic Monte Carlo
simulation of gas-grain chemistry in cold interstellar clouds in which both the
gas-phase and the grain surface chemistry are simulated by a stochastic
technique. The surface chemistry is simulated with a microscopic Monte Carlo
method in which the chemistry occurs on an initially flat surface. The surface
chemical network consists of 29 reactions initiated by the accreting species H,
O, C, and CO. Four different models are run with diverse but homogeneous
physical conditions including temperature, gas density, and
diffusion-barrier-to-desorption energy ratio. As time increases, icy
interstellar mantles begin to grow. Our approach allows us to determine the
morphology of the ice, layer by layer, as a function of time, and to ascertain
the environment or environments for individual molecules. Our calculated
abundances can be compared with observations of ices and gas-phase species, as
well as the results of other models.
