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 <[email protected]> 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
