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Re: [Query re: cif_core 2.1 definitions]

  • To: Multiple recipients of list <coredmg@iucr.org>
  • Subject: Re: [Query re: cif_core 2.1 definitions]
  • From: a.l.spek@chem.uu.nl (a.l.spek)
  • Date: Thu, 16 Nov 2000 11:43:07 GMT
Brian,

	Firstly, SHELXL & Sheldrick are right !!
	
	This is an old discussion/misconception.
	
	An atom on a two-fold axis will in general have full occupancy = 1.0

          However, such an atom occours only 1/2 the number of times in the 
          unit cell as it would on a general position.
          
          In order to take this atom correctly into the structure factor 
          calculations it should be given population = 1/2 * 1.0 = 1/2
          which is the number used in the '.res' file for this atom.
          In the CIF it still should have occupancy 1.0.
          
          The whole issue would be less confusing when structure factors
          were calculated for all atoms (thus symmetry expanded) in the
          triclinic unit cell. In such a protocol, occupation and population
          will become identical because atoms on a twofold axis will occur
          automatically only half of the time.
          
        Hopefully this clarifies somewhat.
        
       Ton Spek
       
Co-Editor Acta Cryst C

          
> Content-Length: 1115
> 
> Dear Colleagues
> 
> I have received the following inquiry from a user. Do others see there to be
> a real problem here? Have you any suggestions for a clean resolution?
> 
> Regards
> Brian
> 
> ----- Forwarded message -----
> 
> I'd like to clarify two definitions that are in the 2.1 dictionary. The two
> datanames are _atom_site_occupancy and _atom_site_multiplicity. There is no
> mention of any interaction between the two nor what interpretation should
> occur if the two values are at odds, e.g. _atom_site_occupancy is 1 and
> _atom_site_multiplicity is 2. I mentioned this to George Sheldrick (since it
> is from a CIF written by SHELXL that came to my notice) and he stated that he
> is correct. My view differs. Since the refinement programs explicitly have an
> occupancy set to 0.5 for a atom site of 2-fold symmetry should not the
> _atom_site_occupancy value be *required* to be 0.5 if that is what the
> program is set to (or refined to)? Should this not be independent of the
> _atom_site_multiplicity value which describes the nature of that position
> regardless of how much of an atom is put there?
> 
> ----- End forwarded message -----
>

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