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[Query re: cif_core 2.1 definitions]



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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