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New version (2.3) of core CIF dictionary available

Dear colleagues

The core CIF dictionary (coreCIF) version 2.3 has been posted.

Among the significant changes are the following:

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.

==

Further information is on the coreCIF page of the IUCr web site:
    http://www.iucr.org/iucr-top/cif/cif_core/index.html

The canonical version of the dictionary is available from
    ftp://ftp.iucr.org/pub/cif_core_2.3.dic

This dictionary was formally approved by COMCIFS on 19 August 2003.


Brian McMahon
Coordinating Secretary, COMCIFS

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