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Dear Wolfram,

First of all you should make sure you have substantial twinning. Is the twin fraction close to zero, then don't use a twin target. Is twinning detected by several test, then there is a good chance your data are twinned.
As for validation, if you do not completely trust your R-factors or maps, spend more time looking at geometry. That is independent of your diffraction data, but still a very good way to see whether your model is plausible.

Hth,
Robbie

Sent from my Windows Phone
Van: wtempel
Verzonden: 13-3-2014 15:19
Aan: [log in to unmask]
Onderwerp: [ccp4bb] twin refinement

Dear colleagues,

this is a request for comments on the evaluation of crystal structures that resulted from twin refinement.

From http://www.ysbl.york.ac.uk/~garib/refmac/Tutorials/refmac_tutorial.pdf:

"Although Rfactors are substantially smaller with twin refinement than without twin 
refinement it does not mean that model also is substantially better. 
[...]
Usually twin maps after twin refinement are more biased 
towards errors in the model."

From phenix.xtriage output:

"Please note however that R-factors from twinned refinement cannot be directly
compared to R-factors without twinning, as they will always be lower when a
twin law is used.  You should also use caution when interpreting the maps from
refinement, as they will have significantly more model bias."

How would you ascertain that inclusion of twin parameters has improved the model? 
How is one to judge the evidentiary strength of such crystal structure in general, given that validation by residuals and maps is further weakened compared to refinement without twinning parameters?

Thank you for your thoughts and time,
Wolfram Tempel