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As luck would have it, I have put together a "challenge" data set series here:
http://smb.slac.stanford.edu/~holton/challenge/twin/

Currently, the top contender is SHELXD, which can find all 12 sites using data with a twin fraction of 0.44.  Phasing, however, still fails at this twin fraction.  Nobody has been able to solve this structure using data with alpha=0.30 or higher.  But, if you "cheat" and know the right answer, then all 12 sites are clearly resolved up to a 50:50 twin.  So, theoretically, there is no reason why you couldn't write a program to do this, but to the best of my knowledge nobody has.

Any takers?

-James Holton
MAD Scientist

On 4/3/2016 2:05 AM, Eleanor Dodson wrote:
[log in to unmask]" type="cite">
hmmm - in the ideal world with perfect data and a twin factor < 0.5 you would detwin your data before trying sad or mad.. 

But detaining inevitably increases the Sigmas so a weak signal can be lost.. 
The lab philosophy was - if you need to do expel phasing, and the data are twinned look for another crystal … But that may be too negative.

You could try it and see if the phasing improved.

Eleanor

On 1 April 2016 at 21:40, Keller, Jacob <[log in to unmask]> wrote:
Dear Crystallographers,

It occurred to me that SAD/MAD would work much better by incorporating twinning into the HA finding/refining/phasing stage, since the Bijvoet differences could be split according to the twinning fraction and/or the HA's would be transformed by the twin operator and weighted by alpha and (1-alpha). I believe twinning is not currently modeled during this stage, even though for twinned structures this would dramatically improve things almost to the level of untwinned data, I think. Twin fractions could be determined at the HA stage, then everything would go more smoothly. Any thoughts of such an implementation? Perhaps Shelx does this since 1969 or something?

I thought of this because I saw some really deep "holes" in the Phaser output from a twinned dataset, and think these might be the evil other twin sneaking in?

JPK


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Jacob Pearson Keller, PhD
Looger Lab/HHMI Janelia Research Campus
19700 Helix Dr, Ashburn, VA 20147
email: [log in to unmask]
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