Well you could have a monoclinic space group with beta = 90.0000...0001, which for everyone would mean 90.0 degrees. You could also have beta = exactly 90 by pure chance. Normally the R-sym values should tell you which of the two possibilities is the correct one.
If you obtain (an example) R-sym = 0.045 for P2(1) and R-sym = 0.052 for P2(1)2(1)2(1), then the orthorhombic space group is most likely the correct one. And a definitive "proof" is solving the structure in P2(1)2(1)2(1). Fred. > Message du 08/07/11 05:31 > De : "Raji Edayathumangalam" > A : CCP4BB@JISCMAIL.AC.UK > Copie à : > Objet : [ccp4bb] Potential Space Group Issue > > 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 >
