Dear all, I am now dealing with a pseudo-merohedral twin (3.2 A, P1 twinned into C2), and I have a couple of questions concerning the data reduction and refinement: 1) R-free flag in refinement a) For twin refinement, I know the reflections related by the twin law should have the same R-free flag. If I process my data in P1, and generate the R-free flag by phenix.refine using the key word xray_data.r_free_flags.use_lattice_symmetry=True, can phenix generate the R-free flags considering the 2-fold point group symmetry and deal with the twinning properly? b) I read that the R-factor could go down to around 40% in twin refinement in the presence of twinning even if the model is completely wrong. For my understanding, this could be due to the fact that the reflections are correlated by the twin law. If the R-free flags are chosen in the highest lattice symmetry, will the R-free still suffer from this problem? 2) Twin fraction In phenix.xtriage, the predicted twin fraction is about 30%. However, in phenix.refine, the twin_fraction is always between 46% to 50%. Why could this happen? 3) Data reduction I am not very clear about this point, since I just started dealing twinning. But what I am thinking is whether it is a good idea to merge the Friedel pair in the presence of twinning. Since the two twin domains have different hkl index if considering one as the reference, is the Friedel law still valid? This might be a very simple mathematical problem. I really appreciate your answers! Sincerely, Chen