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*** CCP4 home page http://www.ccp4.ac.uk ***
Hi Peter, the signs arise because these are structure amplitudes with
phases, i.e. structure factors. Obviously the definition given by
Drenth (and also Blundell & Johnson) only works for centrics where the -
sign is for amplitudes with exactly equal phases and the + sign for
those with phases differing by 180 deg. There is a more useful
definition of isomorphous R_Cullis used by e.g. MLPHARE that is defined
for both centrics and acentrics:
Sum Lack_of_closure_at_best_phase / Sum Isomorphous_difference
(there are variations on this, e.g. probability-weighted and integrated
around the phase circle)
or Sum ||FPHobs| - |FP + FHcalc|| / Sum ||FPHobs| - |FPobs||
For centrics this reduces (well almost) to the original definition.
HTH
-- Ian
> -----Original Message-----
> From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On
> Behalf Of Peter Adrian Meyer
> Sent: Thursday, May 18, 2006 2:19 PM
> To: [EMAIL PROTECTED]
> Cc: [email protected]
> Subject: Re: [ccp4bb]: Anomalous Cullis Rfactor
>
> *** For details on how to be removed from this list visit the ***
> *** CCP4 home page http://www.ccp4.ac.uk ***
>
>
> Hi,
>
> It might be using isomorphous Cullis R factor for the
> anomalous dispersion
> change (using a difference between wavelengths instead of
> between F+ and
> F-). Otherwise, it wouldn't be very meaningful (R factors
> that are always
> infinite probably aren't useful to assess data quality).
>
> The defination I've got for isomorphous Cullis is (
> \sum_{hkl}{ ( |F_{PH}
> (+/) F_{P}| - | F_{H,calc} | } ) / ( \sum_{hkl}{ | F_{PH}
> (+/-) F_{P} | }
> ) ; where the (+/-) is + if the signs of F_{PH} and F_{P} are
> opposite,
> and - if the signs are the same (from Dreth 2nd edition...I'm
> unclear on
> how the signs could be opposite, as that would imply a
> negative amplitude
> for either F_{PH} or F_{P}); substitute inflection/remote/peak as
> necessary for F_{PH} and F_{P}.
>
> Pete
>
> > *** For details on how to be removed from this list visit
> the *** ***
> CCP4 home page http://www.ccp4.ac.uk ***
> >
> >
> > Hi everyone:
> >
> > How is an anomalous Cullis R defined for centric
> reflections? (If it is
> the F+ - F-, how can that be meaningful?)
> >
> > Thanks.
> >
> > Bill Scott
> >
> >
> >
>
>
> Pete Meyer
> Fu Lab
> BMCB grad student
> Cornell University
>
>
>
>
>
>
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