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Re: [ccp4bb]: perfect twinning



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>From my very limited experience, I got the impression that
CNS can handle partial, but not perfect merohedral twinning.

To my knowledge, all structures solved using perfect twins so far
have been refined with SHELXL --

any comments on that?

-- bjoern

> 
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> 
> > Li Minghui wrote:
> >
> > Dear all,
> >
> > I have a perfectly twinned dataset.
> >
> > At first, I determined that the spacegroup was P43212 and could find
> > MR solution using Molrep. There were two molecules in the ASU, and the
> > initial R-factor was about 0.5. After a cycle of rebuilding and
> > refinement the R-factor dropped to 0.36. The density map calculated
> > from the new model was good, and some omitted parts of the initial
> > model also have clear densities. I found there were also many other
> > densities outside the molecules. I could dock part of an other model
> > in them, but the remaining parts would overlap on the existing
> > molecules.
> >
> > I processed the data again in P4 spacegroup and found the data was
> > perfectly twinned with a twinning fraction of 0.48. It is said that
> > perfect twinned data is difficult to detwin.
> >
> > Does anybody have successful experience with perfect twinned data? Any
> > suggestion will be apperciated.
> >
> > Minghui Li
> >
> > National Laboratory of Biomacromolecules
> > Institute of Biophysics, Academia Sinica
> > Tel: 86-10-64888507
> > E-mail: lmh@moon.ibp.ac.cn
> 
>  You can indeed solve a MR search against twinned data - you will find
> two overlapping molecules of course..
> 
>  SHELX and CNS can both refine your models against the twinned data, and
> also refine the twinning factor.
> 
>  You will have difficulties with generating maps for rebuilding though..
> There are ways to detwin the Iobs using the calculated values as a guide
> but I have no experience of how much bias it introduces.
> 
>  Eleanor

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Bjoern Mamat					mamat@mpibp-frankfurt.mpg.de
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