Micro-Monte-Carlo Models

Definition

In a microscopic Monte Carlo model, when used for grain chemistry, the positions of each atom or molecule on the grain mantle can be followed. Thus, for an ice mantle consisting of 50 monolayers, the approach will follow all of the atoms and molecules as well as unoccupied sites for each of the 50 monolayers. In the simplest approach, each monolayer is modeled as a rectangular grid of potential wells in lattice sites with or without occupation. The atoms or molecules move from lattice well to well and possibly react when they land in the same site, while the species in the outermost layer also desorb into the gas, as gas-phase species accrete onto the outermost monolayer. The choice of which process happens in which time period depends upon the approach utilized. In the continuous time random-walk approach, a kinetic Monte Carlo approach used most commonly in the astrochemical literature, a Poisson (or waiting time) distribution (ψ_a(t) = k_aexp(−k_at)) is used, where k_ais...