Section 3.1.10.1. Categories and category groups

The only formal unit of classification common to all CIF dictionaries is the category. For example, in the core CIF dictionary information about the chemical and physical properties of the different atomic species in a crystal cell is collected in a few data names such as _atom_type_oxidation_number which belong to the same category, in this case the ATOM_TYPE category. As described in Section 3.1.5.3, it is conventional (although not mandatory) that CIF data names begin with components corresponding to the name of the category to which they belong.

The term category as used in CIF dictionaries has a technical meaning which constrains its normal use in grouping items that are understood to have a `natural' relationship. In a CIF, only items belonging to the same category may appear together in the same looped list. This means, for example, that data items describing collective properties of the atom sites in the lattice (such as the number of atoms of each atomic species in the unit cell) must be assigned to a different category from the data items that describe the properties of the individual sites. Hence the properties of individual sites (such as the positional coordinates defined by _atom_site_fract_x etc.) belong to the ATOM_SITE category, while the transformation matrix between Cartesian and fractional components (expressed by a collection of data names such as _atom_sites_fract_tran_matrix_11) belong to the ATOM_SITES category. Clearly, the category names have been chosen to be similar to reflect their close relationship, while the EXPTL category containing data names such as _exptl_crystal_colour is named quite differently. It is natural to wish to describe related categories in a common higher level of classification, and indeed category groups exist as formal components of DDL2-structured dictionaries. We shall, however, refer informally to `category groups' in discussions of DDL1 dictionaries as collections of categories with a close relationship that is usually implicit in their names.