Re: [ccp4bb]: twining about P21, C2 or C2221

[Ulrich Baumann <ulrich.baumann@ibc.unibe.ch>]

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One possibility is pseudo-merohedral twinning in your C2 setting,
emulating an orthorhombic spacegroup. This is possible because the
beta angle is very close to 90 degrees, so the twinning operator would be
h,-k,-l, i.e. 180 deg rotation around a. Since the angle is virtually 90
degs I guess you won't observe spot splitting at large theta angles but
you could compare different crystals (especially small ones) with respect
to their Rsymms in C2 and C222/C222(1) in order to see whether the
orthorhombic symmetry is approximate or exact (hopefully sometimes the
twinning parameter is below 0.500).

There are other twinning operators in monoclinic space groups dependent on
the cell metric as well dependent on the ratio of the cell axes.


***************************************************
**  Ulrich Baumann, Dpt. f. Chemie und Biochemie **
**  Universitaet Bern, Freiestrasse 3            **
**  CH-3012 Bern, Switzerland                    **
**  phone +41-(0)31-631-4320/4324, fax -4887     **
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On Sat, 10 Nov 2001, xinqi liu wrote:

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> ***          CCP4 home page http://www.ccp4.ac.uk         ***
> 
> 
> Dear all,
> 
>    I checked the manual, it seems seldom that twinning
> happens in P21, C2 and C2221 space group, but through
> the truncate and twining check servicer, my data looks
> like pefect-merohedral twinning, <I2>/<I>2 is 1.75. Is
> it possible that twinning happens in this space group?
> My data is one of three space group, I am not sure
> which one is right. 
>  The cell parameter is 
>   P21 : 65.37, 275.29, 65.34, 90, 111.29, 90
>   C2: 107.88, 74.56, 274.90, 90, 90.08, 90
>   C2221: 74.58, 107.90, 274.78, 90, 90, 90.
> 
> If they are twinning, what is the twinning operator,
> and should I do something else special?
> 
> Thank you all.
> 
> xinqi
> 
> HHMI & National Jewish Center.
> 
> 
> 
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