[phenixbb] twinning

[Randy Read]

Dear Shun,

Phaser does a test based on the moments of the intensity distribution, after correcting for anisotropy and (if present) translational non-crystallographic symmetry.  However, once a test like that has indicated that twinning is probably present, you will get a better result from running a program like phenix.xtriage, which will compare reflections related by possible twin operators and give a more precise idea of the twin fraction.

Since you’ve managed to get reasonable R-factors (even if they are higher than expected for 1.7A), the twin fraction is probably not too high.  The best thing to do now is probably to run phenix.xtriage to get a suggestion for what the twin operator is, then you can provide that twin operator to phenix.refine, which will then: a) refine the twin fraction to give a much more precise estimate; b) correct for twinning in the refinement.  Detwinning is not recommended any more, because it is better to refine against the original data.

Best wishes,

Randy Read

-----
Randy J. Read
Department of Haematology, University of Cambridge
Cambridge Institute for Medical Research    Tel: +44 1223 336500
Wellcome Trust/MRC Building                         Fax: +44 1223 336827
Hills Road                                                            E-mail: rjr27 at cam.ac.uk
Cambridge CB2 0XY, U.K.                               www-structmed.cimr.cam.ac.uk

On 31 Mar 2015, at 06:18, Shun Liu <sliu.xtal at gmail.com> wrote:

> Dear all,
> Recently I have got a data set that diffracts to 1.7 angstrom. Images look good. During data processing I did not find anything that looks strange. However, when I was doing Phaser-MR, I got a  warning: “ Intensity moments suggest significant twinning (>5%). Tests based on possible twin laws will be more definitive.” What does this mean? A twinning data set? I still found a solution and refined the model step by step, using the data (20-1.7 angstrom). It seems that the final model and map are acceptable. But R-factors are 0.24/0.28, very high. Does the twin cause the high R-factors? Is there a solution to detwin? Or are the R-factors acceptable for a twinning data set? Any suggestions are appreciated and thanks in advance!
> 
> Best,
> 
> Shun
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