Yes, they are "de-twinned". There is really no other way to do it.
Without de-twinning you'd be looking at a map of two overlapping
structures. How do you turn a single observation of the sum of
intensities from two different hkls into two different Iobs values? You
need to know the ratio. The twin fraction is one way, but once it gets
close to 0.5 the "de-twinning" gets very noisy. I believe what the
refinement programs do is use the ratio of the two Fcalc^2 values in
order to "split" the Iobs value. Doesn't that introduce "bias"? Yes, of
course it does. That's one of the many reasons why twinning sucks.
Now, of course, there are probably fancy weighting schemes that make
things a bit better, but rather than speculate I'd ask the program
authors themselves. The best thing, of course, is to refine until Icalc
becomes more accurate than Iobs, which is the typical result of
small-molecule refinements. I expect that is why they are not nearly as
afraid of twinning as we are.
-James Holton
MAD Scientist
On 4/28/2020 2:19 PM, Frank von Delft wrote:
Hi all - feel free to point me to the docs if it's already clear
somewhere:
When refmac generates 2mFo-DFc maps after twinned refinement, are they
the "untwinned" view of the electron density? I.e. with the twinnig
convoluted out by his statistical magic?
I remember Garib mentioning the ambition a few years ago; but I've
not been paying enough attention to remember if it's actually
implemented, or still in progress (because it's hard).
Thanks
Frank
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