Ok. So the error message was helpful in finding the error in the pde's. Concerning the numerical problems I learned in former times, that sometimes a rescaling of the variables (concentration, time and lengths), so that constants have about the same order of magnitude, turns out to make things running smoothly. But I can't make a proposal having not looked intensely at your system.

Regards Hans

No, I don't think that the non-linearity has something to do with the error.

And that is really mystic. I reduced the problem to one dimension AND removed the derivative of C2 in the boundary condition, but nevertheless the error message persists ???????

Look at the notebook.

Regards Hans

Attachments:

Hans, thank you once again. The derivatives by coordinates "x" and "y" are first order while the Laplacian is second order. I am just writing Newmann boundary conditions different from NewmannValue. I wrote then explicitly. Because the reaction term mixes C1 and C2, I am not sure if NewmannValue would be appropriate in that case.

Hans, thank you for your time. Here is the block dedicated only to the reaction diffusion solution. Comments were added to the end of each line to be understood. The workflow is as follows: Definition of the region (rectangular region) Definition of the reaction functions for each specie C1 and C2 Definition of the Diffusion operator (Dt C1 - Laplacian[C1]==f[C1,C2]) eqn1=Derivative by time of C1 - Laplacian2D[C1]== reaction function f[C1,C2] eqn2=Deivative by time of C2 - Laplacian2D[C2]==reaction function g[C1,C2] NDSolve is called for solution.

Hans, thank you for your time. Here is the block dedicated only to the reaction diffusion solution. Comments were added to the end of each line to be understood. The workflow is as follows: Definition of the region (rectangular region) Definition of the reaction functions for each specie C1 and C2 Definition of the Diffusion operator (Dt C1 - Laplacian[C1]==f[C1,C2]) eqn1=Derivative by time of C1 - Laplacian2D[C1]== reaction function f[C1,C2] eqn2=Deivative by time of C2 - Laplacian2D[C2]==reaction function g[C1,C2] NDSolve is called for solution.

Attachments: