Interesting. My IT, both volume I and volume A (1983) only have P21212 for space group #18. Do I have to purchase a new volume A every year to keep up with the new conventions?
Cheers, Bernie On Thu, March 31, 2011 12:57 pm, Ian Tickle wrote: >> 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 >
