-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 I am glad the structures that have been solved using the free-lunch-algorithm as implemented in shelxe did not know they were not allowed to be solved. Of course there is DM involved, as has been pointed out ;-)
On 10/12/2011 10:12 PM, Edward A. Berry wrote: > Tim Gruene wrote: >> -----BEGIN PGP SIGNED MESSAGE----- >> Hash: SHA1 >> >> >> 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! > - -- - -- Dr Tim Gruene Institut fuer anorganische Chemie Tammannstr. 4 D-37077 Goettingen GPG Key ID = A46BEE1A -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.10 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/ iD8DBQFOlqZiUxlJ7aRr7hoRAgYqAKD1vthQQ3WJmHXxklWZiroRYvdFHgCeO0MP FSF50BnydKjR7ajI3XshBqE= =F0JM -----END PGP SIGNATURE-----
