Dear Jacob, I'm not an expert on the topic, but from my experiences with twinning I can agree with you. I recently solved my second twinned structure by MR (twin fraction of 0.43, as estimated by Xtriage). Performing twin refinement in Refmac or phenix.refine dropped the R-factors, as expected, but worsened the geometry considerably without a noticeable improvement in the maps. For this reason, I opted *not* to go with the twin refinement... I don't know if others would make the same choice, though it seemed reasonable to me. Besides, my Rwork/Rfree is down to 0.25/0.29, which ain't too shabby for 2.6 A resolution.
Cheers, Chris On Tue, Oct 11, 2016 at 2:15 AM, Keller, Jacob <kell...@janelia.hhmi.org> wrote: > Dear Crystallographers, > > > > Based on some data sets I have looked at and anecdotal-type evidence here > and there I have gotten the impression that detwinning does not help in > structure solution. (Please let me know if you have a case where detwinning > saved the day.) Is there a clear answer to this enigma anywhere, to > anyone’s knowledge? Wouldn’t it seem that **any** detwinning would be > better than **no** detwinning? I understand that the errors explode as > one approaches 50% twins and does detwinning, but still, I don’t think one * > *loses** information by detwinning, right? Take the case of a 33% twin: > since the twin-reflections are on average about half the intensity of the > non-twin, and since they are generally not correlated in intensity, isn’t > this like having noise added at 50% of the measured intensity? So why does > detwinning make things worse generally? Is there something wrong in the > assumptions underlying the detwinning algorithm, or perhaps something about > the calculation that throws things off? > > > > A related sub-enigma: why is MR generally immune to twinning, but > anomalous methods are susceptible? > > > > All the best, > > > > Jacob Keller > > > > ******************************************* > > Jacob Pearson Keller, PhD > > Research Scientist > > HHMI Janelia Research Campus / Looger lab > > Phone: (571)209-4000 x3159 > > Email: kell...@janelia.hhmi.org > > ******************************************* > > >
