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Re: [ccp4bb]: Non-merohedral twinning



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On Wed, 17 Apr 2002, Roeland Boer wrote:

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>
> Hi there,
>
> I'm working on a dataset from a sample that is non-merohedrally twinned.
> I got the twinning operation and can process the reflections of the
> major component of the twin, but got a rather high Rint (11%). I thought
> that the two lattices would not overlap too much, but apparently they
> do. I'd like to detwin the data, and was wondering if somebody can point
> out programs that can handle NON-merohedral twinned data.
>
> Thanks a lot,
> Roeland.

If this is a molecular replacement problem I'd just go on with MR and
structure refinement. The Rint is indeed a bit high but the discrepancies
between Fobs and Fcalc are dominated by imperfections in the model not the
data and I think you can get a perfectly acceptable model if data
completeness and resolution are ok. This is NOT an excuse for sloppy data
collection and it will be fatal during experimental MAD/MIR phasing, but if
you have the data already and can't easily get better data from a non-twinned
crystal then you have my blessing.

The alternative route would require that you can identify reflection overlaps
between your twins. I don't know if there are programs available for general
use to do this. Basically, I think you'll have to process the second lattice
(in mosflm, just predict the spots from the first lattice and hand pick spots
from the other lattice for indexing). The resulting MTZ files will have the
spot position on the detector as well as the image number. You'll then have to
write a small program, if none are available, to detect and reject reflections
that overlap. If this is of general use I could consider adding such an option
to SFTOOLS.

Bart

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