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
>
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