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Subject: Re: phasing with se-met at low resolution
From: Marc SCHILTZ marc {- dot -} schiltz {- at -} EPFL {- dot -} CH
Date: 2009-05-13
Kevin Cowtan wrote:
> Marc SCHILTZ wrote:
>
>> 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 !
>>
>
> OK, that's new to me. My understanding was that f" does not drop off
> with resolution in the stationary atom case, since the anomalous
> scattering arises from the core atoms. Can you elaborate?
>
>
Yes, this is correct. And if there are atomic displacements, we would
have to multiply f" by an overall Debye-Waller factor (t) to get an
"effective" f" which then would drop off with resolution. But the
Debye-Waller factor also affects the normal scattering factors in the
same way. So the ratio of rms Friedel differences over mean intensities
remains essentially unaffected by an overall atomic displacement parameter.
Interpreting the Flack & Shmueli (2007) paper :
D = F^2(+) - F^2(-) is the Friedel difference of a reflection and A =
0.5 * [F^2(+) + F^2(-)] is its Friedel average
Then = t^4 (static) and = t ^2 (static)
So the ratio SQRT() / is independent of t (i.e. the same as for
the static case).
Marc
--
Marc SCHILTZ http://lcr.epfl.ch
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