Re: [ccp4bb]: refine using merged MAD data of all 3 wavelength?

["C. Behnke" <cbehnke@u.washington.edu>]

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I'd be very careful about over-interpreting the increased redundancy you'd
get out of this. You maybe get a little reduction in random error, but
since you're collecting the same range of data three times, you won't get
any help on your systematic errors. On top of that you'll blow any chance
of getting a goo refinement of your anomalous scatterers, which can be
important if you've got something like a Hg derivative of a small protein.

If you really want to do this right, SHELXL using HKLF 2 and LAUE
instructions can deal with differing wavelengths and f'-f'' values. But
to simply average the three wavelength sets and report 3x the redundancy
seems a little dangerous to me.

						-Craig

-- 
Craig Behnke
Biomolecular Structure & Design Program, Univ. of Washington
cbehnke@u.washington.edu

On Wed, 11 Oct 2000, Phraenquex VD wrote:

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> 
> I'd say this is a very good idea:  if your crystal did not decay, and if
> you don't have a higher-resolution (native) dataset you'd prefer to use,
> merging all datasets for super-high redundancy can only improve matters.
> Not sure about the averaging f'/f" - do any programs actually use this?
> 
> Cheers
> Phraenquex  
> 
> 
> On Wed, 11 Oct 2000, Yong Xiong wrote:
> 
> > Date: Wed, 11 Oct 2000 17:32:36 -0400 (EDT)
> > From: Yong Xiong <yxiong@chemistry.ohio-state.edu>
> > To: ccp4bb@dl.ac.uk
> > Subject: [ccp4bb]: refine using merged MAD data of all 3 wavelength?
> > 
> > ***  For details on how to be removed from this list visit the  ***
> > ***    CCP4 home page http://www.dl.ac.uk/CCP/CCP4/main.html    ***
> > 
> > As title, I would like to know whether any one tried to use all 3 wavelength data merged/scaled
> > together for the refinement. I tried this on one structure and it appeared to improve the
> > refinement. I am not sure the theoretical/practical soundness of doing this. If the
> > crystal is stable, it should give a lot more redudency to the data and help the high resolution
> > bins. The anomalous/dispersion signals would be messed up but mostly they are not used in
> > refinement any way. We could even give an average f' and f" for the merged data, or could we? Any
> > comments?
> > 
> 
>