Dear all, I refine a structure of a protein to 1.4 A and found a few SO4-Ions. I tried to refine this sulfurs and some of these seems to be not fully occupied, as the negative difference density suggest. I tried to refine the occupancy of the sulfate ions to get an idea how strong the different binding sites might be occupied. All sulfate ions are in Chain C of my pdb. I tried to refine the occupancy individually using refine.occupancy.individual SO4 but this lead to different occupnacies for every atom for every atom and theix cant be the case for sure. Then I tried and group occupancy with resname=SO4 but now every sulfate ion has the same occupancy, which is indicated from my previous refinements also not the case, because two ions have no negative difference density. At the momentI can not figure out how I should combine the keywords for the individual occupancy refinement of the different sulfate ions. In my structure. Should I define for every ion a different chain or how I could overcome this problem? I would be very happy if anyone has a idea to sove this problem or tell me what keyword I misunderstood in the documentation. Best regards Christian