Definition for Z' (number of molecules in the asymmetric unit)

Dear Core DMG,

Please consider the following draft definitions relating to Z', notionally the number of molecules in the asymmetric unit. Preliminary discussion of this proposal can be found at https://github.com/COMCIFS/cif_core/issues/558.

If there have been no objections in the next two weeks these definitions will be added to the current draft version of the core dictionary.

best wishes,

James.

===

save_cell.formula_units_z_prime

    _definition.id                '_cell.formula_units_Z_prime'
    _definition.update            2025-10-21
    _description.text
;
    Z' specifies the number of molecules in the asymmetric unit.
    It is defined as the number of formula units in the unit cell
    (_cell.formula_units_Z) divided by the multiplicity of the
    general position of the corresponding space group. Z' can be
    a fraction or a positive integer.

    Note that correct assignment of Z' may be difficult for some
    complex structures (Flack, 2002; Steed et al., 2014). For such
    cases a justification of the assigned value should be given in
    _cell.formula_units_Z_prime_details.

    Ref: Flack, H. D. (2002).
         https://www.iucr.org/__data/iucr/lists/coredmg/msg00196.html
         Steed, J. et al. (2014). https://zprime.co.uk/limitations
;
    _name.category_id             cell
    _name.object_id               formula_units_Z_prime
    _type.purpose                 Number
    _type.source                  Assigned
    _type.container               Single
    _type.contents                Real
    _enumeration.range            0:
    _units.code                   none

    loop_
      _description_example.case
      _description_example.detail
         5
;
         Crystal structure contains five molecules in the asymmetric unit.
         One such crystal structure (tetraphenylphosphonium octahydrotriborate)
         is described  by Beckett, M. A. et al. (2010). Acta Cryst. E66, o833.
;
         1.5
;
         Crystal structure contains one and a half instances of a molecule
         in the asymmetric unit. One such crystal structure
         [9-(methoxymethyl)anthracene] is described by Liu, J. et al.
         (2007). Acta Cryst. E63, o4442.
;
         0.5
;
         Crystal structure contains half of a molecule in the asymmetric unit.
         One such crystal structure [(E)-1,1,4,4-tetraphenylbut-2-yne-1,4-diol]
         is described by Sundar, J. K. et al. (2010). Acta Cryst. E66, o679.
;

save_

save_cell.formula_units_z_prime_details

    _definition.id                '_cell.formula_units_Z_prime_details'
    _definition.update            2025-10-21
    _description.text
;
    An account of the assigned value of Z', the number of molecules in
    the asymmetric unit, where there is difficulty in deciding the
    appropriate value for reasons of symmetry, chirality, disorder etc.
    (Flack, 2002; Steed et al., 2014).

    Ref: Flack, H. D. (2002).
         https://www.iucr.org/__data/iucr/lists/coredmg/msg00196.html
         Steed, J. et al. (2014). https://zprime.co.uk/limitations
;
    _name.category_id             cell
    _name.object_id               formula_units_Z_prime_details
    _type.purpose                 Describe
    _type.source                  Recorded
    _type.container               Single
    _type.contents                Text
    _description_example.case
;
         Z' has been assigned the value 1 as the catenane is considered
         a single complex of two interlocked molecules that cannot be
         chemically separated, in preference to a crystallographic
         interpretation that considers it as two interlinked independent
         molecules that would suggest setting Z' = 2.
;
    _description_example.detail
;
         Based on a discussion in https://zprime.co.uk/limitations of a
         structure (QINJIG) reported by Watanabe, N. et al. (1999).
         Chem. Lett. 28, 915-916.
;

save_