If there is no indication of twinning and your Rmerge is sensible then
it is probably point group P222
Run pointless - that gives you the quality of each of the 2 folds sepera
tely..
Deciding on the spacegroup is a bit trickier.
That depends on absences along h00 0k0 and 00l, and if there is a
non-crystallographic-translation with a 1/2 translation along a b or c
then they can mislead you.
Eg if the pseudo-translation was (0.5,0.3,0.1) then the h00 would have
all h=odd weak and the spacegroup could be P212121 or P 2 21 21
truncate tells you whether there is non-crystallographic translation.
On 07/08/2011 04:30 AM, Raji Edayathumangalam wrote:
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
