Tim Gruene wrote:
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On 10/11/2011 09:58 PM, Ethan Merritt wrote:
On Tuesday, October 11, 2011 12:33:09 pm Garib N Murshudov wrote:
In the limit yes. however limit is when we do not have solution, i.e. when model errors are very large. In
the limit map coefficients will be 0 even for 2mFo-DFc maps. In refinement we have some model. At the moment
we have choice between 0 and DFc. 0 is not the best estimate as Ed rightly points out. We replace (I am sorry
for self promotion, nevertheless: Murshudov et al, 1997) "absent" reflection with DFc, but it
introduces bias. Bias becomes stronger as the number of "absent" reflections become larger. We need
better way of estimating "unobserved" reflections. In statistics there are few appraoches. None of
them is full proof, all of them are computationally expensive. One of the techniques is called multiple
imputation.
I don't quite follow how one would generate multiple imputations in this case.
Would this be equivalent to generating a map from (Nobs - N) refls, then
filling in F_estimate for those N refls by back-transforming the map?
Sort of like phase extension, except generating new Fs rather than new phases?
Some people call this the "free-lunch-algorithm" ;-)
Tim
Doesn't work- the Fourier transform is invertable. As someone already said in
this
thread, if the map was made with coefficients of zero for certain reflections
(which is equivalent to omitting those reflections) The back-transform will
give zero for those reflections. Unless you do some density modification first.
So free-lunch is a good name- there aint no such thing!
