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Re: F(000)



Howard Flack wrote:
> 
> > > Please can you advise?
> 
> > In any case it's no
> > longer a required item.
> 
>   As a calculated F(000) including dispersion is never used in any
> crystallographic calculation, I would recommend him not to include it in
> the CIF.
>   If by any chance he has found a use for the dispersive F(000), I only
> hope he did not calculate it with:
> 
> >
> >                F(000) = [ sum (f~r~^2^ + f~i~^2^) ]^1/2^
> 
>  because it should be:
> 
> 
>                F(000) = [ (sum f~r~)^2^ + (sum f~i~)^2^ ]^1/2^
>  the sum being over each atom in the unit cell.
> 
>   Dispersion-free calculated F(000) is used in electron-density
> calculation by Fourier inversion. In this case it corresponds to the
> number of electrons in the unit cell which will be integral for
> stoichiometric compounds.
> 
>   I recommend coredmg to remove _exptl_crystal_F_000 from the
> dictionary as it is useless, ill-defined and causes more bother than it
> is worth.
> 
> H.

=======================================================================

I agree entirely with Howard (indeed, I was about to make a comment
along the same lines), except that I would like to retain the item but
with an improved definition to make it the (integer) number of electrons
in the cell.  

The .fcf reflection data file written by SHELX-97 includes F(000) (as
the integer number of electrons) and certain other items precisely so
that all the information is included that is needed for calculating 
various Fourier syntheses.  This is already being used extensively for
the production of macromolecular maps for viewing with programs such as
"O", Turbo-Frodo and "XtalView", and it would be extremely inconvenient
to start messing about with the format of this file now. Most of the
programs that read this file are by no means fully general CIF readers -
a task beyond my capabilities - and might get confused.

Acta Editors and the Acta diagnostic software should simply ignore the
item.

George Sheldrick


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