Dear Crystallographers,
I have not done the statistics, but I have noticed that a large portion of
the inquiries directed to the BB are twinning-related. While I recognize
that many of these issues have case-specific details, it may be that there
are references which might address these inquiries in general and therefore
would be helpful to many people here. Could we compile a list, and I will
put the ref's on the wiki?
Regards,
Jacob Keller
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Jacob Pearson Keller
Northwestern University
Medical Scientist Training Program
Dallos Laboratory
F. Searle 1-240
2240 Campus Drive
Evanston IL 60208
lab: 847.491.2438
cel: 773.608.9185
email: [email protected]
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----- Original Message -----
From: "George M. Sheldrick" <[email protected]>
To: <[email protected]>
Sent: Thursday, July 23, 2009 8:41 AM
Subject: Re: [ccp4bb] is my crystal twinned or not?
It appears that P212121 twinned so that the apparent space
group is P43212 or P41212 is more common than I had realized.
This requires that two axes are approximately equal in length
and that the reflections 00l with l=4n+1 and l=4n+3 are
'accidentally' missing. It is possible that in such cases
translational NCS causes both the twinning and the apparent
absences, and in addition it can disturb the statistical
tests for twinning. These problems are likely to be more
acute when the resolution of the data is relatively low.
George
Prof. George M. Sheldrick FRS
Dept. Structural Chemistry,
University of Goettingen,
Tammannstr. 4,
D37077 Goettingen, Germany
Tel. +49-551-39-3021 or -3068
Fax. +49-551-39-22582
