Differences between versions 2.3 and 2.2 of the Core CIF Dictionary

Contents

• Version 2.3 of the core dictionary is fully compliant with the version 1.1 CIF specification

• Two new categories, SPACE_GROUP and SPACE_GROUP_SYMOP have been introduced to provide more correct definitions of the crystallographic space group symmetry of a structure. These categories have been imported from the symmetry CIF dictionary, which is designed to provide data names required for a complete description of crystallographic symmetry. The existing categories SYMMETRY and SYMMETRY_EQUIV are formally marked as deprecated, and developers should begin the process of migrating to the new categories. (In accordance with the archival purpose of CIF, the old data names will be retained in the dictionary, but CIF reading software that needs to extract symmetry information should be designed to query both sets of possible names, either through hard-coding or by following the _related_items links in the dictionary.)
• Several new data names of the form _*_gt and _*_lt (e.g. _diffrn_ambient_pressure_gt), have been introduced to indicate upper and lower limits to the known value of an approximate physical quantity. These are not recommended for use in new studies, but provide a way formally to store such imprecise data extracted from existing databases and legacy data collections.
• Three new data items, _atom_site_refinement_flags_adp , _atom_site_refinement_flags_occupancy and _atom_site_refinement_flags_posn , replace the now-deprecated _atom_site_refinement_flags. There are two reasons for this; the existing enumeration list contained single-letter codes which the definition claimed could be combined, in breach of the normal rules governing enumeration lists; and the three new items allow a more structured cataloguing of refinement restraints or constraints.
• Data names have been introduced for reciprocal cell lengths and angles.

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