[phenixbb] Fix coordinates of one single residue

Hi, I'm working on the refinement of a protein mutated with an unnatural amino acid that is present as two rotamers. Whenever I do refinements, one of the rotamers moves away from the electron density map. I therefore would like to fix the coordinates for this amino acid only while continuing the refinement of the rest of the protein. Is there a way in Phenix to do this? Any advice is welcome. Best regards, Anna-Carin