Dirk Kostrewa wrote:
Am 10.01.2008 um 01:53 schrieb Dale Tronrud:
...
Your quote bring up another matter. An "initial" map, i.e. before
refinement, is unbiased if it is an omit map. If you have done no
refinement and you leave the interesting part out of the calculation
of Fc there can not be any bias in either the Fo-Fc or the 2Fo-Fc map.
This is why an initial map is more reliable for proving binding of
a compound, for example, than a bias reduced map after refinement.
...
But isn't this only true, if the model that is put in was not refined
against a related data set before? If the new Fobs are related to the
old Fobs (against which the model was refined before) then you carry any
model bias over to the new data, because a simple omit-map using the old
data will have model bias.
Right you are, and it's an important point too. My only defense
is that I said "If you have done no refinement...". I don't think
of the refinement of several models to data from isomorphous crystals
as separate refinements, but sometimes forget to mention this when
talking to others. I should be more careful.
Dale Tronrud
