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
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