Re: [ccp4bb]: unmeasured data in map-calculations

["Eleanor J. Dodson" <ccp4@ysbl.york.ac.uk>]

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Jan Abendroth wrote:
> 
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> Dear all,
> with many apologizes (because it's mostly a a cns-related question) I
> like to submit a question to the ccp4bb. But it's soon x-mass and you
> might forgive this.
> 
> My data set is quite incomplete in the highest resolution shell. When I
> calculate maps with the cns script model_map the sigmaA-weighted
> fofc-map (mFoDFc) has many "unreasonable" peaks at 3sigma-level. When I
> make cns to take Fc for the unmeasured Fo's the sigmaA-weighted fofc-map
> shows much fewer and much more reasonable peaks, simply this map becomes
> significanly better.
> 
> This seems to make sense, but what is the physical/mathematical reason
> for this?
> How treats cns unmeasured hkl's in the "normal" mode in difference maps?
> Is there a related ccp4-option in refmac/fft?
> 
> Thanks for any reply, kind regards and merry christmas
> 
> Jan


 Obv. if the cns solution is take unmeasured Fobs as = 0.0, then that
will mean you are adding in terms -Fc, which would be bad..

But most programs I know leave out any term with Fobs unmeasured, which
should be exactly the same as setting Fobs = Fc thus generating a
difference of 0.00000

 Refmac certainly does set unmeasured Fobs = Fc - the diff maps thus
have terms ~ = 0.00
and the 2mFobs-DFc have terms = DFc

It gets rid of ripples in the 2mFoDfc at the expense of introducing
bias..

No perfect solution except to measure data..

Elranor> --
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> Jan Abendroth
> Institut fuer Biochemie
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Eleanor J.Dodson, Chemistry Department, University of York, U.K.
Tel: Home +44 (1904) 42 44 49, work:  +44 (1904) 43 25 65
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