I recently ran into a similar case where the SG was P2(1)2(1)2(1) with a
~ b (within a few angstroms), thus emulating a P422 metric symmetry.
Full details here: Pubmed ID: 20057079
As Ian says, sometimes the spot splitting was particularly visible,
sometimes it was not. SAINT was not able to integrate the split spots
separately (it works great for non-merohedral twins though, followed by
TWINABS for scaling). In the end, I ended up using XDS and even the most
severely split spots were nicely integrated in the same integration box.
Putting the pseudo-merohedral twin operator in to SHELXL dropped
R/R-free and improved the GooF and least-squares refinement convergence,
even though the twin fraction was rather low (~ 5%).
HTH,
Jonathan
--
Jonathan Elegheert
Ph.D. Student
Unit for Structural Biology& Biophysics
http://www.lprobe.ugent.be/xray.html
Lab for Protein Biochemistry and Biomolecular Engineering
Department of Biochemistry& Microbiology
Ghent University, Belgium
e-mail: [log in to unmask]
Op 6/23/2010 11:46 AM, Frank von Delft schreef:
>> My experience with pseudo-merohedral twinning (it was actually the
>> reticular case with half the spots overlapped and the other
>> non-overlapped half on a pseudo C-centred lattice) is that the degree
>> of splitting varies widely over the diffraction pattern. In some
>> places there was complete overlap, in others you see elongation of the
>> spots, in others partial separation, and in others complete separation
>> (and of course all shades in-between), with around 50-50 intensity
>> split. In this situation the mosaicity becomes meaningless! I'm not
>> aware of any software that can handle this kind of thing successfully
>> (and certainly the data we did manage to get turned out to be
>> garbage!).
>>
> Both DIRAX or SAINT should be able to handle it, you'll need SADABS to
> scale it. (The latter two are in the Bruker software.)
>
> phx.
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