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On Tuesday 01 August 2006 10:07 am, Peter Adrian Meyer wrote: > > Sorry, I guess I wasn't very clear. I was thinking in terms of the > change in f' from the "non-anomalous" atoms (C,N,O) which aren't > being used for phasing, or anomalous difference fourier; as opposed > to the non-anomalous component of the Se/Mn scattering. Far from the > absorbtion edge, this is very small (using the values from atomsf.lib > for carbon at Cu and Mo, the difference is 0.015; for oxygen and > nitrogen it's larger), but for large numbers of non-anomalous atoms > this would accumulate (for 1000 carbons, there would be a cumulative > difference of 15). It doesn't work that way. You can't just sum the scattering factor over N atoms. See the usual diagram (Argand diagram) for summing the vector contribution of multiple scattering sites to the overall F. http://xrayweb.msg.ku.edu/notes/crystallography.html#calc In any event, if you are going to start worrying about a change of 0.015e/6e = 0.2% in the scattering power of C, then perhaps you should first worry about the difference in scattering power of charged atoms vs. neutral atoms. See, for example, the tabulated values for O- vs. O in tables 6.1.1.1 and 6.1.1.3 of International Tables Vol C. -- Ethan A Merritt Biomolecular Structure Center University of Washington, Seattle WA
