Rémi Delaporte-Mathurin

@KulaginVladimir I think the script would look like ```python my_model.initialise() class CustomSolver: ..... custom_solver = CustomSolver() my_model.h_transport_problem.newton_solver = custom_solver my_model.run()

Yes we just have to move away from NonLinearVariationalSolver and use NewtonSolver instead, right?

@KulaginVladimir do you have a script to reproduce this behaviour? The reason why concentrations are zero is probably because the newton solver converges in zero iterations. Therefore it "solves nothing"...

@KulaginVladimir would you like to give this a look?

@KulaginVladimir thanks for this I think it is much needed to give more flexibility. Would you mind listing the differences - if any - from the user point of view...

Also tagging @Allentro as he was interested in this feature

I would like to keep this open while we test it thoroughly. Playing with the solver can be a bit dangerous so I want to make sure we're not merging...

@JonathanGSDUFOUR glad to see progress on this! Have you tried testing your implementation on an example?

> Would it be better to test each quantity separately or can we test them all in one case ? We want to test each quantity ensuring it computes the...

@JonathanGSDUFOUR I don't know if you are aware but: - the tests don't pass with your changes. You can run the tests locally with `python -m pytest .` - there...