CHARMM Development Project Forums User Discussion & Questions Parameter Set Discussion Dihedrals in colchicine (rings)

I expect the flexing of the central 7-membered ring (is that what you call the B-ring?) to be quite important too

I don't know. I haven't looked at it yet, but this central 7-membered ring is part of two other rings, and that's why I would predict that it is quite limited in what conformations it can assume.