Hi Pavel, I'm digging in docs and list serv but could not find out, and figured you probably know faster. I need to make an occ.params file for some groups of non adjacent side chains which interact with each other, and associated waters. 1) How do I select ACs 2) How do I constrain water occupancies so that their Q + that of coupled protein atomes does not exceed 1. If I match the AC flags for the waters and side chains will this take care of it. e.g. if residues 2 and 21 have coupled ACs and waters 101 and 102 move in when residue 21 is in one of its ACs how would I define below? refinement { refine { occupancies { constrained_group { selection = chain A and resseq 2 selection = chain A and resseq 21 selection = chain W and resseq 101 selection = chain W and resseq 102 } } Thanks !!! -- +++++++++++++++++++++++++++++++++++++++++++++++++++++++ Mark Mayer Ph.D.