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JPK- My boundaries were relatively arbitrary. 5% or 10% might be personal preference. But I agree, why not use it with low twin fraction. I see my mistake here on the perfect twinning. - one cannot detwin perfect data reliably, but that does not mean one cannot use the twin law, thanks Kay for pointing that out. My understanding of the twin law use seems to be incorrect- I was assuming that they detwinned the data, but this is not the case. I appreciate the clarification. So, it seems that using the twin law during refinement is acceptable for all ranges of twinning, and the maps are your ultimate guide. Is it technically/theoretically incorrect to *not* use a twin law for a heavily twinned structure? Is there any reason to think those maps may produce different results as refining with a twin law? Or is either refinement option theoretically acceptable? Thanks all! Teresa On Tue, Mar 22, 2016 at 12:10 AM, Kay Diederichs < [log in to unmask]> wrote: > On Mon, 21 Mar 2016 15:51:20 -0700, Teresa Swanson < > [log in to unmask]> wrote: > > >Hello all, > > > >Our lab has had many discussions on the twinned refinement but we've come > >to a bit of a stalemate, as none of us have an intimate knowledge of > >twinning theory. I wanted to get some clarification on what the current > >consensus is on processing twinned data based on the twin fraction. I have > >a few questions here, so I'll try to be as clear as possible. For all of > >this, I'm assuming MR for phases, which finds at least one viable > solution. > >We're also assuming that space group assignment as been done correctly, as > >well as twin law assignment. > > > >It is my understanding that the way we refine twinned data heavily depends > >on the estimated twin fraction, and it (roughly) breaks down as such: > > > > - *1-10%: *You can probably refine this data *without* taking the twin > > law in to account and it should be just fine. As long as the R factors > are > > good and your maps look reasonable, no twin law necessary. > > I agree (maybe except with the "As long as the R factors are good" > half-sentence: what exactly does that mean??) > > > - *11-45%: *Twin law *should* be used during refinement. If you do not > > use the twin law, your R factors will probably stall and your maps may > not > > be reliable. Be aware of increased model bias and artificially lowered > R > > factors due to the twin law. > > I agree. > 'increased model bias': the somewhat subtle reason why this applies is > that in the limit of alpha going towards 0.5, this reduces the number of > independent observations by a factor of 2. This corresponds to the same > observations-to-parameters ratio at a resolution of untwinned data which is > cubic-root-of-2=1.26 worse than that of the perfectly twinned data (e.g. 2A > twinned is like 2.5A untwinned in this respect). > > > - *45-50%: *This is a grey area. I assume most people just throw these > > data sets out. However, if you were to process this, you *cannot* use a > > twin law, since the data is (likely) perfectly twinned, and the twin > law > > cannot discriminate between each lattice. > > there is no reason to have a separate category here. If you wanted to > de-twin the data, yes this would not be possible with alpha>0.4 or so. But > the refinement programs do _not_ de-twin the data; rather, they "twin the > I_calc to match the I_obs". > > So no reason to throw the data away if you can solve and refine the > structure, and properly document it. > > > > >In the cases of perfect twinning, how do you correctly refine your data > >set? With or without the twin law? How would you determine if you can > trust > > with twin law > > >your maps? > > omit maps > > > > >Whether or not you deploy the twin law during the refinement of a twinned > >crystal, are reasonable R factors, reasonable physiological > interpretation, > >and reasonably placed anomalous signal enough to claim a solved structure? > > yes, if everything is consistent and the model is good > > > > >I appreciate all insight on these questions, thanks! > > > > HTH, > > Kay > > > >Teresa Swanson > > >