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Hello Everyone, I have a 3.1 Ang dataset for which I'd like to get to the bottom of what the correct space group is. The current unit cell in p212121 is 98.123 101.095 211.201 90.000 90.000 90.000 I fed the reflection data into Xtriage to look for twinning and pseudotranslational NCS and there is no indication for either issue in the Xtriage output. Also, all odd 00h, 00k, 00l reflections are systematically absent as they should be for p212121. However, my colleague who is also working on the same dataset recently reprocessed the data in P21. Here's the cell in p21: 98.010 100.940 210.470 90.00 90.04 90.00 p21 I am not sure if BETA=90.04 is significant enough to treat as p21 (0.04% deviation of beta angle from ideal lattice for p212121). I don't think so but I could be wrong. Could someone please clarify? Also, what kind of twinning and twinning operators can relate a p212121 cell to a p21 cell with almost identical unit cell parameters as that of the p212121 cell and leave all systematic absences intact? Thanks much. Raji ----------- Raji Edayathumangalam Instructor in Neurology, Harvard Medical School Research Associate, Brigham and Women's Hospital Visiting Research Scholar, Brandeis University