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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
> Ethan
> [...]
- --
- --
Dr Tim Gruene
Institut fuer anorganische Chemie
Tammannstr. 4
D-37077 Goettingen
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