The IUCr standard is to make a<b<c, see
http://nvl.nist.gov/pub/nistpubs/jres/107/4/j74mig.pdf
http://nvl.nist.gov/pub/nistpubs/jres/106/6/j66mig.pdf
and P 2 21 21 is a perfectly valid space group
Pointless will reindex within the same point group, or you can choose the a<b<c convention (SETTING CELL_BASED) or the "reference" setting P 21 21 2 (SETTING SYMMETRY_BASED), so you have a choice
Most programs are perfectly happy with space group P 2 21 21 (I'm not sure about [auto]Sharp)
Phil
On 31 Mar 2011, at 15:28, Anita Lewit-Bentley wrote:
> Dear all,
>
> I would like to share my experiencde with a rather unexpected problem of indexing conventions. Perhaps I can save people some time....
>
> I have a crystal in the more unusual P21212 space-group (No 18). Its unit cell lengths are b>a>c (please note). I systematically use XDS for data integration, since so far it was able to handle even the most horrible-looking spots.
>
> Now XDS indexed my data in space-group 18, but with the axes order a<b<c! It had, in fact, "invented" a space-group P22121, which does not exist. I did not realise this until I had spent a couple of weeks with beautiful peaks in rotation functions, but hopeless results in translation functions. It wasn't until I looked more closely into the definition of the screw axes that I realised the problem.
>
> POINTLESS does not allow a reindexing of reflexions within the same space-group, but fortunately REINDEX did the trick at the level of intensities, because I like to use SCALA for careful scaling of my data.
>
> So, basically, beyond just warning people who might encounter similar problems, I was wo,dering if XDS could perhaps reindex reflexions according to Int. Table conventions once the screw axes of a crystal system have been identified?
>
> With best wishes,
>
> Anita
>
>
> Anita Lewit-Bentley
> Unité d'Immunologie Structurale
> CNRS URA 2185
> Département de Biologie Structurale & Chimie
> Institut Pasteur
> 25 rue du Dr. Roux
> 75724 Paris cedex 15
> FRANCE
>
> Tel: 33- (0)1 45 68 88 95
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>
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