[ccp4bb]: 3Fo-2Fc maps

[Stefano Trapani <stefano@if.sc.usp.br>]

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A question about electron density maps.

Does it make any sense to use a weighted (3mFo-2DFc) difference map
(with phases from a MR solution) during model building and refinement,
where m and D are the Sigma-A-style weights of Reads (Acta Cryst
A42,1986)?

The use of these weights in (2Fo-Fc) maps is known to reduce map bias by
the model. 
However, maps of the type [nFo-(n-1)Fc] should show missing pieces of your
model as higher peaks with increasing n (am I correct?. Does anybody have
a reference for this?)

So, can I say (very crudely) that a (3mFo-2DFc) map is better than a
(2mFo-DFc) map, in that 
- it shows missing parts of the model better than a (2mFo-DFc) map
  does; and 
- it is less biased by errors in the model than an unweighted (3Fo-2Fc)
  map is?


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Stefano Trapani
Cristalografia de Proteinas e Biologia Estrutural
Instituto de Fisica de São Carlos (IFSC) 
Universidade de São Paulo (USP)

Avenida Trabalhador Sancarlense, 400, Centro 
Caixa postal 369 
13566-590 São Carlos - SP
Brasil

E-mail:	stefano@if.sc.usp.br
Phone ++55 16 274.9868
FAX: ++55 16 273.9881
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