Added _cell.formula_units_Z_prime and _cell.formula_units_Z_prime_details #562
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This is a proposal for documenting Z' in CIF, a suggestion that has often been put forward (e.g. https://www.iucr.org/__data/iucr/lists/coredmg/msg00194.html and subsequent discussion). There are difficulties relating to the way in which a molecule is defined within the crystallographic concept of an asymmetric unit. David Brown suggested a working group to explore this further, but nothing seems to have come of it.
The current proposal suggests a definition for Z' consonant with the current Online Dictionary text at https://dictionary.iucr.org/Z_and_Z%27 that should cover most straightforward cases. The definition of
_cell.formula_units_Z_primeincludes pointers to informative web discussions by Howard Flack and by Jon Steed's group that give examples of alternative choices that might be made for Z'.The data item
_cell.formula_units_Z_prime_detailsis provided for authors to discuss such difficulties and how they resolved them, and references the same two web-page discussions.