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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 17:06:35 GMT
> 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.  

  Since the non-dispersive F(000) has a use, then certainly let it stay
with an improved definition.
  In line with the concurrent discussion of difference densities should
it be worded to be appropriate for x-rays, neutrons and electrons or
should it be just an x-ray F(000)?
  In all cases, non-dispersive F(000) is a structure factor and not a
structure amplitude. It has both a magnitude and a sign.
   For x-rays non-dispersive F(000) is positive definite and in many
cases an integer. It is not an integer for non-stoichiometric compounds.
It counts the number of electrons in the cell.
   For neutrons non-dispersive F(000) is either positive or negative and
counts the total nuclear scattering power in the cell. I'm not aware
that there is any simpler interpretation of its value.
   Who knows a bit about electron diffraction. What sign has the mean


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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