@Carl Love Thanks for your answer! You must have looked into that while I edited my start post. I found out about that already! But your answer was still usefull to me, since I didn't know the thing with the infolevel.
I didn't look deep anough into the package and problem I need to solve yet, but in the end I need to minimize a functional F[f, g, h, ...]. The solvers which the package provides, as far as I have seen are only suited to minimize functions f(x, y, z, ...) however. How would I tackle such a problem with a functional?
I suppose the answer is something along the lines of, evaluate the functions in the argument of the functional on a grid, and then treat the functional as function again. But if you would have any suggestions, I would be glad to hear your opinion, as I have never solved a minimization problem so far. (Appart from simple ones of course.)