Classification and use of core data

Abstract

Each data item in a CIF has a unique identity and properties, and these are described in the DDL dictionary definitions. The name or tag of a data item is its primary identifier and it serves as the unique access key to a data value. Although each data item is unique, it is often closely related to other data items. These relationships, which are specified in the dictionaries using particular DDL attributes, can require certain items to be grouped into common lists, and others, because of specific relational dependencies, to be present in a CIF when another data item is used. Such relationships lead to data being classified into groups, or categories of data. The data name is chosen to be as self-descriptive of the data item as possible and contains the category name at the front. For example, data describing atom sites in a structural model have names that start with the string _atom_site and are placed into the category group of the same name. This chapter describes the rationale for the classification of crystallographic data into the existing categories used in the core dictionary. This chapter is also available as HTML from the International Tables Online site hosted by the IUCr.