[phenixbb] Is it possible to include a ligand in a rigid_body group?

Is it possible to include a ligand (prosthetic group actually) in a rigid_body group together with protein chain? I have an iron-sulfur cluster (FES) which has the same chain letter as the protein subunit to which it is attached, and the rigid_body is defined by chain and sequence including the FES res number, but the cluster gets left behind when RB refinement yanks the chain into position. The cluster atoms are labeled ATOM not HETATM, in case that matters. In difficult cases including the cluster could be critical to success, since it will define the highest density peak. Ed