I have a program which computes the atomic electron density profile (attached) as you would see it in a map, using accurate scattering factors and taking the resolution limit into account. I wouldn't call the profile for a C atom with B=100 at 2.5 Ang resolution 'flat', maybe 'flatter'. 'Flat' would imply that it's lost in the noise of other atoms with B=100.
My point is that it's relative. Since my average B is 85 Ang.^2, an individual B of 100 or even 120 doesn't seem out of the ordinary at all. If the average B were 10 then I would agree that anything over say 50 would appear flat and insignificant. The reason I think is simply that atoms with low B factor have series termination ripples around them which can swamp the density of other atoms with high B factor (for example the ripples from a B=10 C atom are half the height of the peak of a B=150 atom). So the net 'noise' level in a map with low average B is much higher than in one with high average B, so that any atoms with high individual B just get lost in the noise. Cheers -- Ian On Thu, Dec 23, 2010 at 8:05 PM, Ronald E Stenkamp <stenk...@u.washington.edu> wrote: > Something related to the results in the 1984 paper, but never published, is > that the calculated electron density for an atom with a B of 100 > Angstroms**2 is so flat that you wonder how those atoms can be seen in > electron density maps. > > Ron > > On Thu, 23 Dec 2010, Bernhard Rupp (Hofkristallrat a.D.) wrote: > >>> can anyone point me to a more exact theory of distance accuracy compared >> >> to >>> >>> optical resolution, preferably one that would apply to microscopy as >>> well. >> >> Stenkamp RE, & Jensen LH (1984) Resolution revisited: limit of detail in >> electron density maps. Acta Crystallogr. A40(3), 251-254. >> >> MX, BR >> >
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