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This is an interesting conceptual question. A reflection with an
intensity of virually zero does not contribute to your electron density
map, so can it be data? If it is can I claim to have a 0.02 Angstrom
structure although all intensities beyond 2 Angstrom are virtually zero?
The answer to question 1 is YES it can be data
The answer to question 2 is NO KEEP DREAMING
A very weak reflection does not contribute to your map so leaving it out
doesn't hurt since missing a reflection is equivalent to assuming it is
zero, not a bad approximation. However, a lot of our maps are of the
2Fo-Fc and Fo-Fc types. Similarly refinement a depends on |Fobs-Fcalc|.
Thus, if Fcalc is big and you measured it as being near zero then the
discrepancy is large and the reflection has a large contribution to
refinement and these kinds of maps. Had you not measured the reflection
then its contribution would be ignored (equivalent to Fo-Fc being zero).
The difference at the high resolution limit is that Fcalc starts to
approach zero as well as Fobs. So now both Fobs and Fo-Fc are virtually
zero and small relative to the errors in Fo and Fc. So this time there
really is no information left to speak of.
Bart
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What is "observable data"? If I observe a reflection by passing it through
the Ewald sphere I can measure its intensity
Phil
Here's what Acta Cryst has to say about resolution-
From the "Notes for authors 2006" from Acta Cryst D:
http://journals.iucr.org/d/issues/2006/02/00/me0308/index.html
"11.1. Resolution
The effective resolution should be described clearly. Values of the
internal
agreement of the data, Rmerge, together with the multiplicity, the mean
value
of I/ and the percentage completeness of the data are required for the
overall
data set and the highest resolution shell together with the limits of that
shell in Å. For high-quality data obtained with synchrotron radiation,
values
of Rmerge < 20%, completeness > 93% and observable data > 70% should
be
achievable for the highest resolution shell. A complete table listing the
above
criteria as a function of resolution should also be submitted, but will
normally
be included in the supplementary material..."
