Hi all, I have a case where a residue is in two alternative conformations for chain A and chain B. This residue (Arg 62) happens to be close to its NCS mate, such that only conformations A for both chains or only conformations B can take place at the same time. I tried to follow guidance from the CNG newsletter 2015/07 and set up the constrained groups as shown below. However, the refinement crashes with an AtomSelectionError. Any ideas what I could have done differently? Cheers, Jan refinement { refine { occupancies { constrained_group { selection = chain A and resseq 62 and altloc A or \ chain B and resseq 62 and altloc A or selection = chain A and resseq 62 and altloc B or \ chain B and resseq 62 and altloc B or } } } } -- Jan Abendroth Emerald Biostructures Seattle / Bainbridge Island, WA, USA home: Jan.Abendroth_at_gmail.com work: JAbendroth_at_embios.com http://www.emeraldbiostructures.com