The species transport is described by the following conservation equation:

C = concentration

D = diffusion coefficient

ρ = density (temperature dependent)

Three boundary types are available for the variable
**Concentration**:

*Dirichlet*: A fixed value for the concentration along a certain line: C = constant.*Neumann*: A fixed species flux along a certain line: q = constant.*Poincaré*: A flux dependent on the value of the concentration itself at the boundary line: q = k ( C - C_{0}) .This condition might be taken to describe a species reaction along a line. In this case k is some kind of reaction rate.

*Temperature dependent Dirichlet*: The same as*Dirichlet*, but the value is specified as a function of the temperature on the line C(t).*Temperature dependent Poincaré*: A flux dependent on the value of the concentration itself at the boundary line: q = k(T) ( C - C_{0}(T) ) . The transfer-coefficient K(T) and the reference concentration C_{0}(T) can be specified as functions of the temperature. A typical application is evaporation of a species on the line where k(T) corresponds to the sticking coefficient and C_{0}(T) is the equilibrium partial pressure of the species (Hertz-Knudsen-equation).