Re: [ccp4bb] m-3m vs. m-3

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CCP4bb <-- 1999 <-- November 1999 <-- 30 November 1999

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From: Eleanor Dodson ccp4 {- at -} YSBL {- dot -} YORK {- dot -} AC {- dot -} UK
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Subject: Re: m-3m vs. m-3
From: Eleanor Dodson ccp4 {- at -} YSBL {- dot -} YORK {- dot -} AC {- dot -} UK
Date: 2008-11-04

In the $CHTML/twinning.html it tries to explain:
From the table:

# All *P2i3* and related *2i3* space groups:
(h,k,l) already equivalent to (-h,-k,l) so we only need to check:
real axes: (a,b,c) and (b,a,-c)
reciprocal axes: (a*,b*,c*) and (b*,a*,-c*)

/i.e./ reindex (h,k,l) to (k,h,-l).
# Twinning possible with this operator - apparent symmetry for two fold
perfect twin would be P43 (operator k,h,-l)
space group number space group point group possible twinning operator
195 P23 PG23 k,h,-l
196 F23 PG23 k,h,-l
197 I23 PG23 k,h,-l
198 P213 PG23 k,h,-l
199 I213 PG23 k,h,-l

See if it all makes sense..
Eleanor

yanming Zhang wrote:
> Dear 'old' crystallographers,
>
> During one case of structure solution, the data processing programs output incorrect space group-primitive cubic P4132, which later found out that the correct one should be face centerd cubic f23. This problem was caused by perfect twinning.
>
> Now I'd like to invite you to help me understand, and explain in details,
> WHY, IN CASE OF PERFECT TWINNING, THE LAUE GROUP m-3 WILL BE MIS-INDEXED TO m-3m by some data processing programs? I, sort of, understand the reason behind this is caused by the perfect twin operator which will emulate an additional 2-fold axis. But not fully understand the symmetry in details in this case. Your help and teaching are highly appreciated.
>
> Yanming Zhang
>
>
>
>
>
>

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CCP4bb <-- 1999 <-- November 1999 <-- 30 November 1999

Previous message:
Subject: Group Leader Position in Structural Biology at EMBL Heidelberg
From: Christoph Mueller christoph {- dot -} mueller {- at -} EMBL {- dot -} DE
Date: 2008-11-04

Next message:
Subject: Re: phased MR
From: Eleanor Dodson ccp4 {- at -} YSBL {- dot -} YORK {- dot -} AC {- dot -} UK
Date: 2008-11-04