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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.