Kevin Cowtan wrote:
> This is absolutely correct - in the analysis you present, the
> non-anomalous scattering drops with resolution, but the anomalous part
> does not. And since counting noise varies with intensity, we should
> actually be better off at high resolution, since there is less
> non-anomalous scattering to contribute to the noise! (This is somewhat
> masked by the background, however).
>
> So why don't we see this in practice?
>
> The reason is that you've missed out one important term: the atomic
> displacement parameters (B-factors), which describe a combination of
> thermal motion and positional disorder between unit cells. This motion
> and disorder applies equally to the core and outer electrons, and so
> causes a drop-off in both the anomalous and non-anomalous scattering,
> over and above that caused by the atomic scattering factors.
>
I agree with everything but would like to add the following: if we
assume an overall atomic displacement parameter, the drop-off in both
the anomalous and non-anomalous scattering is the same. Therefore, the
ratio of anomalous differences over mean intensity (which is what comes
closest to R_{ano} - in whichever way this is defined) is essentially
unaffected by atomic displacements and should still go up at high
resolution, irrespective of the values of the atomic displacement
parameter !
Things are more complicated if individual isotropic atomic displacements
are considered, because the anomalously scattering atoms (e.g. the Se
atoms) may have significantly larger or smaller displacement parameters
than the average.
All this is discussed in section 4.4. of Flack & Shmueli (2007) Acta
Cryst. A63, 257--265.
Marc
> But your reasoning was sound as far as it went, and it is a point which
> many people haven't recognised!
>
> Kevin
>
>
> Raja Dey wrote:
>
>> Dear James,
>>
>> I don't understand why measuring anomalous differences has nothing to do
>> with resolution.
>>
>> Heavy atoms
>>
>> scatter anomalously because the inner shell electrons
>>
>> of the heavy atom cannot be considered to be free anymore
>>
>> as was assumed for normal Thomson scattering. As a result
>>
>> the atomic scattering factor of the heavy atom becomes
>>
>> complex and this compex contribution to the structure
>>
>> factor leads to non-equality of Friedel pairs in non-centro
>>
>> symmetric systems(excluding centric zone). This feature is taken
>> advantage in
>>
>> phase determination. Since the inner shell electrons
>>
>> being relatively more strongly bound in heavy atoms
>>
>> contribute to anomalous scattering and its effect
>>
>> is more discernable for high angle reflections . Here
>>
>> the anomalous component of the scattering do not
>>
>> decrease much because of the effectively small atomic
>>
>> radii (only inner shell being effective). FOR HIGH
>>
>> ANGLE REFLECTIONS ANOMALOUS DATA
>>
>> BECOMES IMPORTANT.
>>
>> Raja
>>
--
Marc SCHILTZ http://lcr.epfl.ch
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