Donghyuk
The combination of two things gives me cause for concern:
1. You've reindexed something that apparently scaled OK in point group
622 into point group 2, with a smaller cell. Since it's hard to fake
that sort of data agreement in 622, I assume your data is at the very
least pseudo-622.
2. You've modeled that additional symmetry using a whole array of twin
operators and some non-crystallographic symmetry.
This may in fact be the correct model, but there's a significant risk
that you're inappropriately modeling something.
Let's assume for a moment that your small-cell C2 refinement with 6 twin
ops improves the model somewhat. Now go back and generate scaled data
sets in all possible point groups suggested by Pointless for that data,
and try and find molecular replacement solutions with your
partially-refined model by testing every possible space group in every
possible point group based on the Pointless suggestions. The idea is to
try and model more of the 622 pseudo-symmetry as crystallographic
symmetry, using fewer twin operators in refinement. If you test all of
these combinations, which might be quite extensive, you might find one
or more that fit nearly as well as your current C2 solution and has
higher symmetry. You should take a hard look at that those potential
solutions. Only when you've thoroughly exhausted alternative molecular
replacement solutions would you be confident that your C2 model is in
fact the only reasonable explanation of your data.
But as it stands it is rather atypical and it warrants further investigation
Additional evidence that your C2 cell is the only reasonable model would
be identifiable electron density differences between each chain in your
(presumed) multi-chain model.
Cheers
Phil Jeffrey
Princeton
On 1/10/19 11:08 AM, Donghyuk Shin wrote:
Dear Jacob Keller and Vipul,
Thank you both very much for the reply.
Regarding the R-values, I am just wondering whether the huge gap between
refinements w/ w/o twin operator can be possible even the crystal is not
twinned?
Best wishes,
Donghyuk
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