Under the assumption of and quantities on the segments 1 and 2, respectively, (4.7) and (4.8) represent a linear equation system for the partial current densities of all involved quantities crossing the interface in normal direction. In these equations
All coefficients , , , and are functions of the simulation time , the process temperature , and the spatial coordinates and . The coefficients and may depend on the concentrations of the quantities on both sides of the interface. The model developer has to provide MDL classes which determine the actual values of the coefficients and their derivatives to the quantity concentrations which are the dependent variables in the PROMIS-NT internal equation system.