On Wed, Sep 1, 2010 at 4:26 AM, Ed Pozharski <[log in to unmask]> wrote:
> The
> reason you see the missing region in (2mFo-DFc) map is because it is
> effectively the sum of model map (mFo) which shows you the parts of the
> model you have already placed and difference map (mFo-DFc) which shows
> you the regions which are still missing.
This is not true. The 'model map' (i.e. the map calculated from the
model) is obviously the one with coefficient DFc. The mFo map
represents the model (i.e. the structure already placed) + *half* of
the missing structure (represented by mFo-DFc), for acentric
reflections. To get the 'minimally biased' map you have to make it up
by adding the other half of the missing structure so we have (for
acentrics):
2mFo-DFc = DFc + (mFo-DFc) + (mFo-DFc)
= DFc + 2(mFo-DFc)
For centrics mFo represents the model + *all* of the missing
structure, so in that case no further contribution is needed,
We had this discussion a while back: it seems to me that it is
precisely this confusion that is engendered by thinking in terms of
2mFo-DFc = mFo + (mFo-DFc).
Cheers
-- Ian
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