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>> 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.
True...I was making a bad approximation in a case where I don't know how
to make a good approximation (if \delta_{f'} was the same for all atoms,
it's the usual situation for the dispersive component of MAD phasing; but
when there are different \delta_{f'}'s I don't know a good way to estimate
the expected changes, short of numerical experiments). Since I was
considering it as a possible source of error, I was assuming the worse....
> 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
...which apparently isn't the best strategy when considering which
possible sources of error to worry about. Oops.
Thanks,
Pete
Pete Meyer
Fu Lab
BMCB grad student
Cornell University
