International Tables for Crystallography

General considerations when defining a CIF data item
B. McMahon. International Tables for Crystallography (2006). Vol. G, ch. 3.1, pp. 73-91  [ doi:10.1107/97809553602060000733 ]


CIF dictionaries provide a formal taxonomy of crystallographic terms and ideas. Dictionary entries are constructed in a structured machine-readable manner that facilitates validation and structuring of data. New entries may be devised for public or private dictionaries. A candidate data-name definition should fulfil the following conditions: (i) describe a specific and well defined concept: precision of definition is essential for an effective interchange mechanism; (ii) have appropriate granularity: data names can define a very small piece of information (a standard uncertainty on a particular physical measurable) or a very large amount (the text of a scientific paper). An appropriate choice should be made (and for DDL2 formalized through membership of subcategories, category and category groups, as appropriate); (iii) have well defined relationships with other data items (through its assigned category membership and parent/child links); for DDL2 the prior construction of a formal entity/relationship schema may be helpful ; (iv) constraints on the data type and permissible values should be provided where applicable; (v) the name chosen should be globally unique; this is achieved through monitoring of names in public dictionaries by a regulatory committee (COMCIFS) and by registering of prefix strings for exclusive use in local dictionaries. Some thought may need to be applied to the choice of DDL appropriate for a candidate dictionary. Rules are described for the structuring of dictionaries and the protocol outlined for merging separate dictionary files to provide access to a global distributed dictionary.

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About International Tables for Crystallography

International Tables for Crystallography is the definitive resource and reference work for crystallography. The series consists of nine volumes and comprises articles and tables of data relevant to crystallographic research and to applications of crystallographic methods in all sciences concerned with the structure and properties of materials.