Santarsiero, Bernard D. wrote:
As for the 2Fo-Fc (2mFo-DFc, or something like that) electron density map,
it again assumes that the phases are in good shape, and you essentially
lose any new information you could gain from the addition of new Fobs
terms, but the map isn't distorted since the terms are zero.
Fc is certainly not zero. D will go towards zero as the correlation
between Fobs and Fcalc decreases but may still be substantial for the
higher resolution shells. I expect that data sets that still have a
relatively high I/SigI in the outer shell will also have quite high D
values in the outer shell whereas datasets that push the limits of the
true resolution will start seeing D decrease towards zero (haven't
actually checked this though).
The most significant problem are data sets collected on square detectors
with relatively strong reflections still visible in the corners. In
these cases high resolution completeness is low and many missing
reflections will be replaced by DFc, with both D and Fc not being close
to zero.
I must admit that I've never understood the rationale for including dFc
for missing terms, although it has been discussed "lively" on a few
occassions. Yes, dFc is a better estimate for the true structure factor
than leaving out the term (equivalent to setting the amplitude to zero).
But dFc does not provide any information on how reliable the model is or
where the model may need to be changed. Since the latter is the main
function of electron density maps in my opinion I'm not confinced that
substituting dFc is a good idea at any time.
Bart
On Fri, March 23, 2007 4:02 am, Eleanor Dodson wrote:
This is a good point - I had thought that D would be very low for an
incomplete shell, but that doesnt seem to be true..
Garib - what do you think?
Eleanor
Petrus H Zwart wrote:
I typically process my data to a maximum I/sig near 1, and
completeness in
the highest resolution shell to 50% or greater. It
What about maps computed of very incomplete datasets at high resolution?
Don't you get a false sense of details when the missing reflections are
filled in with DFc when computing a 2MFo-DFc map?
P
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Dept. of Medical Microbiology & Immunology
University of Alberta
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