Following Ian's excellent comment, do I understand correctly that
2mFo-DFc is the maximum likelihood estimate of the full model map (i.e.
the best map possible) or it's simply modification of 2Fo-Fc map where
plain Fo/Fc are replaced by their maximum likelihood estimates? Other
words, is k=2 the maximum likelihood estimate of the best approximation
of the true map in the following form
DFc + k*(mFo-DFc)
Ed.
On Wed, 2010-09-01 at 10:49 +0100, Ian Tickle wrote:
> 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
--
"I'd jump in myself, if I weren't so good at whistling."
Julian, King of Lemurs
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