Reserved prefixes for local dictionary extensions

The CIF standard permits the free incorporation of local and standard data names in any file. However, to work properly as an information interchange standard, CIF provides a facility to register certain prefixes which will guard against name clashes between arbitrarily chosen local data names.

Use of a reserved prefix

A reserved prefix as registered in the IUCr list above may be used to generate unique local data names in the applications managed by the registrant.

DDL1 applications

For use in CIF data files based on DDL1 dictionaries, the reserved prefix may form the first element of any data name.

Examples

     _foo_chemical_formula       'H6OC2'

loop_
   _geom_bond_atom_site_label_1
   _geom_bond_atom_site_label_2
   _geom_bond_distance
   _foo_geom_bond_comment
           C1      C2      1.528(4)   normal
           C1      C3      1.358(3)   short

DDL2 applications

For use in CIF data files based on DDL2 dictionaries, where the data name explicitly includes the category to which it belongs in the standard dictionaries, the reserved prefix must be used either as the first element following the point character separator between category and member components, or as the first component of the complete dataname, in which latter case it implies the formation of a local category. Only data names from the same category (standard or local) may be looped together in the same list construct.

Examples

     _foo_chemical.formula       'H6OC2'

loop_
   _geom_bond.atom_site_label_1
   _geom_bond.atom_site_label_2
   _geom_bond.distance
   _geom_bond.foo_comment
           C1      C2      1.528(4)   normal
           C1      C3      1.358(3)   short

loop_
    _foo_symmetry_Wyckoff_group.notation
    _foo_symmetry_Wyckoff_group.count
    _foo_symmetry_Wyckoff_group.point                   
       a 2 -1    b 2 -1    c 2 -1    d 2 -1    e 4 1