[phenixbb] Unknown atoms in refinement

What's the best way of dealing with unknown atoms (UNK) in phenix.refine? I have a long disordered loop that runs basically the length of the protein on the back side and there is a single "amino acid" in the middle that I'm trying to model as UNK. I can run a round of refmac and auto generate a lib file with my ligand and the unk atoms but phenix.refine balks at that with respect to the Van der Waals radii. Thanks for the help, -Jeff