In principle, these models can be run a la 'ab initio' (from first principles) so no fitting is required. Of course, there are subtleties and unknowns, so some

In principle, these models can be run a la 'ab initio' (from first principles) so no fitting is required. Of course, there are subtleties and unknowns, so some fitting is needed, but overfitting can be checked in the usual ways.