Re: [ccp4bb] Shifting twin fraction with refinement - finally zero

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

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Subject: Re: Rsym problems...maybe???
From: Eleanor Dodson ccp4 {- at -} YSBL {- dot -} YORK {- dot -} AC {- dot -} UK
Date: 2010-04-26

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Subject: Re: Shifting twin fraction with refinement - finally zero
From: Eleanor Dodson ccp4 {- at -} YSBL {- dot -} YORK {- dot -} AC {- dot -} UK
Date: 2010-04-26

A few comments -

you dont give your cell dimensions but if they are more or less the same
for the P212121 and P43 21 2 I dont see how a 43 set of absences can
turn into a 21 set ..

However if there is non-crystallographic translation, then absences an
mislead.. Is that true for you -

Twinning tests can also be very deceptive if there is
non-crystallographic translation, but I admit they usually hide it
rather than display false twinning...

Eleanor

hari jayaram wrote:
> I am refining a twinned dataset in possible spacegroup P212121 . Pointless
> thinks it is P43212 , but based on reading this posting (
> http://www.phenix-online.org/pipermail/phenixbb/2007-September/000501.html)
> I think it is P212121.
>
> The starting R/Rfree after molecular replacement ( single site mutant) was
> 34/38 to 2.2 A
>
> After an initial round of restrained refinement ( without twin refinement)
> and minimal rebuilding I got the R/Rfree to 30/34
>
> Then I did an amplitude based twin refinement - The twin fraction was 0.48
> k,h,-l and 0.52 h,k,l and The r/rfree became 24/29
>
> After a little more rebuilding ( a few residues out of 800 residues in ASU)
> and another twin refinement I got an r/rfree of 22/27 . Now the twin
> fraction was 0.87 (h,k,l) and 0.13 (k,h,-1)
>
> The maps looked a little better allowing me to fix a few more residues
>
> Finally the same twin refinement gives me no twin operators and the R/Rfree
> is 22/26
>
>
> All the twinning tests indicate a serious twinning in my crystal. Any ideas
> why I am seeing this
>
> Hari
>

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

Previous message:
Subject: Re: Rsym problems...maybe???
From: Eleanor Dodson ccp4 {- at -} YSBL {- dot -} YORK {- dot -} AC {- dot -} UK
Date: 2010-04-26

Next message:
Subject: Re: YATQ (yet another twinning question) - may involve pirates
From: Eleanor Dodson ccp4 {- at -} YSBL {- dot -} YORK {- dot -} AC {- dot -} UK
Date: 2010-04-26