Hi All, 1) phenix.refine has a standard set of geometry restrains as other refinement programs: bond, angle, planarity, chirality, dihedral, non-bonded. Clash score is not something that is included into refinement as a target and then used directly in refinement. Clash scores may be bad at the beginning of structure refinement and should be good at the end. 2) Using H atoms as a riding model should normally help in fixing clashes, however it's not 100% guaranteed: some clashes may be "locked" so the refinement cannot overcome the barriers even with H atoms. In this case what's Luca Jovine suggesting (see one of the emails in this thread) is an excellent thing to do (to add to his receipt: you may also consider using more than default number of macro-cycles). Running simulated annealing is also helpful (use "simulated_annealing=true" flag for this). 3) Since handling of H atoms in phenix.refine is significantly improved in newer versions, I would warmly recommend to use the latest phenix.refine available (currently from CCI Apps: http://www.phenix-online.org/download/cci_apps/). 4) Also, clashes related concerns may be a result of inoptimal weight between Xray target and restrains. Although one can play with the weight adjusting scales manually by trying different values of "wxc_scale" parameter ("wxu_scale" is analog for b-factors refinement) the new version of phenix.refine has an automatic procedure for finding the best weight (here "the best" means the weight that leads to the lowest Rfree) (Carsten's suggestion). To use this option: % phenix.refine model.pdb data.mtz your_parameters.par optimize_wxc=true Note this this may take a while to run (depending on resolution and structure size) so consider running it overnight. If none of the above helps, I would be *very* interested to look at the data and model myself to 1) help you find the best strategy to overcome this problem, and 2) find and fix problem in phenix.refine (if any). As always, I promise to keep all data confidential. Pavel.