I have not been able to find any facility to constraint hydrogen geometry, in the manner of the AFIX in ShelXL I mean. Have I missed them or would it need to be implemented? In the latter case, I have tried to figure out how this could be done. I manage to find how position and ADP constraints are handled by constraint objects featuring methods to return the independent variables and the gradient with respect to them. Then cctbx/cctbx/ examples/structure_factor_derivatives_4.py shows how those features can be used. Would Hydrogen geometry constraint be best handled by mimicking those special positions constraint? Cheers, luc