Section 3.2.4.4. Symmetry and space-group information

The data items in these categories are as follows:

(a) SYMMETRY

(b) SYMMETRY_EQUIV

The bullet () indicates a category key. In practice _symmetry_equiv_pos_site_id is often absent from older CIFs. The dagger () indicates a deprecated item, which should not be used in the creation of new CIFs.

The data items in the SYMMETRY category (now superseded by SPACE_GROUP) were used to record the space group. The Hermann–Mauguin (H-M) symbol was given by _symmetry_space_group_name_H-M. The dictionary definition recommended the use of the `full' H-M symbol as listed in International Tables for Crystallography Volume A , but was not explicit about the meaning of `full'. The dictionary examples showed short-form symbols expanded to a complete representation of individual symmetry elements; thus Pnnn would be given as 'P 2/n 2/n 2/n', and the monoclinic space group would be given as 'P 1 21/m 1' for the b-axis unique setting or 'P 1 1 21/m' for the c-axis unique setting.

In practice, abbreviated symbols were often used, following conventions established over many years; thus 'P 21/m' was often given as the Hermann–Mauguin symbol when the `usual' b setting of a monoclinic cell had been chosen. It is recommended that these conventions should continue to be followed when the new data item _space_group_name_H-M_alt is used instead.

The dictionary examples also suggested concise ways of indicating the origin choice within the _symmetry_space_group_name_H-M field; since there is no formal description of how to do this, different authors used different wording. Hence, _symmetry_space_group_name_H-M was always best considered as a container for the representation of the space group that would appear in a published article, and not as a machine-readable source of information about the crystallographic symmetry.

The two mechanisms for conveying the symmetry transformations in a fully machine-readable form were the Hall symbol _symmetry_space_group_name_Hall (Hall, 1981a,b; Hall & Grosse-Kunstleve, 2001) and a complete listing of the symmetry operations using data items in the SYMMETRY_EQUIV category.

The data item _symmetry_cell_setting indicates the crystal system, not (as suggested by its name) the setting used.

The SYMMETRY_EQUIV category, now superseded by SPACE_GROUP_SYMOP, provided a list of symmetry-equivalent positions in algebraic notation. Formally, _symmetry_equiv_pos_site_id acted as a category key, with any arbitrary numeric value that uniquely identifies each operator. Historically, the earliest versions of the core dictionary did not have such an identifier at all and the separate equivalent positions were indexed by their position in the _symmetry_equiv_pos_as_xyz list. This interpretation was vulnerable to inadvertent re-ordering of the list of equivalent positions, and for this reason, as well as to satisfy the formal need for a category key, _symmetry_equiv_pos_site_id was added (Example 3.2.4.14). For compatibility with software that was written to handle the earlier arrangement, it is recommended that _symmetry_equiv_pos_site_id gives sequential integer labels, starting with 1, to the equivalent positions in the sequence in which they appear in the CIF.

Note that the _symmetry_equiv_pos_as_xyz list must contain all symmetry-equivalent positions of the space group, including those generated by lattice centring and a centre of symmetry, if present.

Data items in these categories are as follows:

(a) SPACE_GROUP

(b) SPACE_GROUP_SYMOP

The bullet () indicates a category key.

The data items in the SPACE_GROUP category record the space group and crystal system. They recognize the common practice of supplying the space group in Hermann–Mauguin notation, though the H-M symbol does not contain complete information about the symmetry and the space-group origin. _space_group_name_H-M_alt allows the use of any legitimate H-M symbol as listed in International Tables for Crystallography Volume A or derived by similar principles. It does not give rigorous direction on how the symbols should be presented. It is recommended that the use of this symbol in CIFs containing articles for publication should follow the guidelines for _symmetry_space_group_name_H-M (Section 3.2.4.4.1).

Because a given space-group type may be described by more than one Hermann–Mauguin symbol, the space-group type should be specified by the use of _space_group_IT_number.

Two mechanisms exist for conveying fully machine-readable descriptions of the symmetry transformations relevant to the space group and setting. The first is the Hall symbol (Hall, 1981a,b; Hall & Grosse-Kunstleve, 2001), which uniquely defines the space group and its reference to a particular coordinate system; it is specified in the data item _space_group_name_Hall. Alternatively, the symmetry operations may be listed in full using data items in the SYMMETRY_EQUIV category.

The SPACE_GROUP_SYMOP category provides a list of the symmetry operators for a space group in algebraic notation. It replaces the category SYMMETRY_EQUIV_POS. Unlike the older category, where in practice the category key could be omitted from listings (and must therefore be generated implicitly by parsing software), the category key _space_group_symop_id must be given. See Example 3.2.4.15, which may be compared with Example 3.2.4.14.