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I think you should be able to detect twinning from the intensity statistics regardless of the spacegroup you have processed in... Have you tried SFCHECK or looked at the truncate plots? A word of warning - FreeRs and Rfactors for twinned data are somewhat different to those for untwinned data, and it iss hard to compare them. For one thing you would need to assign your Freer flags in the P222 point group, and then extend them to P21 or Pq1 to get an unbiased set.. Eleanor Yuan, Hua wrote: > Dear Colleagues, > > I'd like to share with you with a twinning puzzle that's been > troubling me for a while. > I have a ~2.2 A data merged in p212121 space group with unit cell: > 41.8300 117.4490 134.4840 90.0000 90.0000 90.0000 > Translational NCS was detected but structure solution was able to be > obtained by phased molecular replacement as we have a Au derivative > data set. Obtaining experimental phase or MR alone didn't work. > > However, with this model and data set, the refinement didn't work > (with R-free ~40% or higher if with more cycles). Different twinning > tests have been carried out and no sign of twinning. All possible > p222 space groups have been tried but no luck or even worse. > Considering that translational symmetry and perfect twinning may make > things deceiving, the data was merged into p21 and a twin-refinement > was performed in phenix.refine. After 3 cycles of rigid-body and > group ADP, a R-free of ~30% was obtained and the output map looks OK. > It looks like we got it. But then things get interesting when we > tested other two axis as the p21 unique axis. With all the three > choices plus corresponding twin-refinement, we could get a similar > R-free (30%). Besides, the three maps all look OK except for one > missing some side chains. > > Then the structure was solve in p1 and Xtriage was used with the hope > that one axis would stand out. However, the RvsR for all three axis > are almost the same. Did I miss anything here? Of course, we could > pick up the one axis we like best and get the structure solved ^---^. > However, this puzzle would eat me alive for a very long time. > > Best, > > Hua > >