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
>
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