I've seen/heard/read more about automatic differentiation lately, and I think we should aim to include that option for material models in OOFEM as well.

There are many packages out there, and from what I have seen, the best option would seem to be Sacado from the Trilinos package. Sacado is also under LGPL2.1 so that is also a great fit for us (i.e. any commercial partner could still make use of the optional Sacado packages).