Hi Eleanor,
On Fri, Aug 10, 2007 at 10:27:16AM +0100, Eleanor Dodson wrote:
> If you have significant anomalous scatterering and are using <F> you
> need to modify the scattering factor for that atom appropriately.
>
> Here is an extract from $CLIBD/atomsf.lib
> Se
> 34 34 2.840900
> 17.000599 5.819600 3.973100 4.354300
> 2.409800 0.272600 15.237200 43.816299
> -0.879000 1.139000 -0.178000 2.223000
>
> The values -0.879000 1.139000 are the f' and f" for Cu
> wavelength ( 1.54A)
>
> I you think you have complete substitution and are using data at some
> other wavelength for refinement you should change the f' and f"
> appropriately
>
> ( Copy $CLIBD/atomsf.lib to your area and edit it then assign refmac5
> ATOMSF my_atomsf.lib etc )
>
> But in practice this is often less useful than you might expect and you
> still see holes in your maps at the Se. I blame it on incomplete
> substitution
Are you sure this would be the way to work it out in CCP4? As far as I
can see (see top of atomsf.lib), the actual formfactor for an atom
will be calculated as
Formfactor: a1*exp(-b1*s*s) + a2*exp(-b2*s*s) + a3*exp(-b3*s*s)
+ a4*exp(-b4*s*s) + c
with the coefficients (a1-4, b1-4 and c) given in atomsf.lib as
AD This contains for each element:
AD ID
AD IWT IELEC C
AD A(4)
AD B(4)
AD CU(2) MO(2)
So I would be suprised if any program actually uses the tabulated
f'/f" values for CuKa and Mo radiation to calculate formfactors. If
you want to adjust the formfactors for elements that have an f' value
very different from CuKa you need to change the C coefficient -
according to the difference between the f'(CuKa) and
f'(observed). This will have an observable effect then.
At least that's what we do in BUSTER with good results ...
Or am I missing something here?
Cheers
Clemens
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