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 > FAX: 33-(0)1 40 61 30 74 > email: ale...@pasteur.fr >
