Dear All, I have tried an occupancy refinement for a partially modifed residue. I have split the residue in altloc A and B and the modification (PMSF) is attached on B. I included the PMS moiety as part of the residue and gave them altloc B. I created the cif link. The refinement resulted in a strange mixture of occupancys for PMS around 0.8 and both Serins 0.23 an 0.77. So PMS is not part of the residue. I am not sure about the correct syntax for the constrained group. At the moment I have used constrained_group { selection = chain A and resseq 130 and altloc A selection = chain A and resname PMS and altloc B } However the Serine pat of altloc B refines to a value not compatible to the PMS values or the Altloc A value of the serine. How do I incorporate the altloc B for the serine? Thanks in advance for your help Christian