The equations solved for the transport for each species need to be supplemented by appropriate boundary conditions. This is often accomplished by way of a set of suitable surface reactions, which describe how species are created or consumed on surfaces. Assume a surface mechanism with I surface reactions and K chemical species

where νki are the stoichiometric coefficients and ck is the concentration of the kth species. The surface rate expression (SI unit: mol/(m2·s)) for species k is defined as

where νki = νrki − νfki. The rate of progress of the ith reaction is governed by the law of mass action:

where the following definitions apply for ck.

where wk is the mass fraction of species k, ρ is the gas density, and Mk is the molecular weight of species k. When the Motz-Wise correction option is set to On, the forward rate constant is related to the sticking coefficients via:

where Γtot is the total surface site concentration (SI unit: mol/m2). The sticking coefficient can be a constant or even temperature dependent through an Arrhenius expression:

where a, b, and e are constants. Since γ is a probability it must be between 0 and 1 to be physically meaningful.