> 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.
>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?
The International Tables / IUCr / NIST convention _is_ a<=b<=c for
orthorhombic so no re-indexing is necessary or desirable. See IT vol.
A 5th ed. (2002), table 9.3.4.1 (p. 758 in my edition) for all the
conventional cells. The problem may be that some programs are not
sticking to the agreed convention - but then the obvious solution is
to fix the program (or use a different one). Is the problem that XDS
is indexing it correctly as P22121 but calling it SG #18 (i.e. instead
of the correct #3018). That would certainly confuse all CCP4 programs
which generally tend to use the space-group number first if it's
available.
I'm not clear what you mean when you say P22121 doesn't exist? It's
clearly shown in my edition of IT (p. 202). Maybe your lab needs to
invest in the most recent edition of IT?
Cheers
-- Ian
|