[phenixbb] unit cell discrepancies

Jacob Keller j-keller2 at fsm.northwestern.edu
Thu Apr 21 11:46:49 PDT 2011

> But I think the error needs to be much larger than you're likely to
> get from processing the data for the refinement to suffer
> significantly.  It definitely doesn't explain the discrepancy in
> R-factors, since even an excellent protein crystal diffracting to
> atomic resolution - where there is no doubt as to the unit cell
> parameters - will still have this problem.

Yes, it does seem that the size of the R difference might be small,
but one could also imagine a beta-strand, for example, aligned along
one of the cell axes, and "being told" one thing by geometric
constraints, and another thing by a cell constant that was off by,
say, 0.5%. If the putatively-imprecise cell constant information were
removed, the strand might "relax," and the R value might drop a bit.
It seems to me that cell parameter measurements are always fairly
imprecise due to having a lot of experimentally-variable parameters.
And being able to predict spots on the detector is no proof that the
parameters are exactly right, I don't think.


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