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Re: F(000)

  • To: Multiple recipients of list <coredmg@iucr.org>
  • Subject: Re: F(000)
  • From: Howard Flack <Howard.Flack@cryst.unige.ch>
  • Date: Thu, 19 Feb 1998 11:00:22 GMT
> > 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.


Howard Flack        http://www.unige.ch/crystal/ahdf/Howard.Flack.html
Laboratoire de Cristallographie               Phone:(+41 22) 702 62 49
24 quai Ernest-Ansermet             mailto:Howard.Flack@cryst.unige.ch
CH-1211 Geneva 4, Switzerland                   Fax:(+41 22) 781 21 92

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