[phenixbb] difference density map

Dale Tronrud det102 at uoxray.uoregon.edu
Mon Jan 18 21:54:48 PST 2010


   My favorite explanation for why differences show at full height
in a 2Fo-Fc map is that the map is really a Fc + 2(Fo-Fc) map.  You
start with an Fc map (with maximal bias to the model) and add twice
a difference map to it to bring in the experimental data.  The half
height features of the difference map are doubled to full high in
the computation.

   The question of why they are only half height in a Fo-Fc map to
begin with is a more subtle question.  The seat-of-the-pants answer
is that the Fo-Fc coefficients only consider the model's failure to
match the Fo, not the additional errors in phase.  I'm sure there
is a paper in Acta Cryst from the 1950's that gives the definitive
derivation but I don't know the reference.

Dale Tronrud

丁玮 wrote:
> Dear all,
> Why the density map with coeffients |Fobs|-|Fcalc| show the density only 
> at half the height,
> and  with coeffients 2|Fobs|-|Fcalc| show the density at full height?
> Thanks!
>              Breth       
>                 dingding830106 at sina.com
>           2010-01-15
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