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Acta Cryst. (1991). A47, 655-685

CIF Dictionary

Each data item in a data block is identified by a unique data name. The currently accepted CIF data names are listed and defined in Appendix I as the CIF Dictionary (Core Version 1991). These are the IUCr `standard' data items currently accepted for the submission of machine-readable documents to the IUCr and to the crystallographic databases. The data items in the Core Dictionary are intended primarily for use in the description of most small-molecule and inorganic structures. Future extensions to this Dictionary will define data items used in more specialized areas of crystallography, such as powder diffraction and macromolecular studies. Appendix II contains an example of a CIF employing data names from the Core Dictionary in the submission of a manuscript and data to Acta Crystallographica Section C (Willis, Beckwith & Tozer, 1991).

The Dictionary (Core Version 1991) is also available [2] as an electronic file cifdic.C91. This file, which has been constructed using the STAR Dictionary Definition Language (DDL) proposal of Cook (1991), contains more information about the data items than the printed Dictionary in Appendix I. It represents the current master reference file for checking the application of CIF data items. The CIF applications programs CIFER (Allen & Edgington, 1992) and CYCLOPS (Hall, 1991b), which are described later, employ this Dictionary for the validation and manipulation of standard data names.

It must be emphasized that the CIF Dictionary only contains the definitions of data items recognized by the IUCr as standard crystallographic data. In the future, data items will be added to the Dictionary but the definitions of existing standard items must never be changed. This ensures perpetual access in long-term archive files. In the passage of time it can be expected that some data items will fall into disuse and be replaced by more useful data names, but the connection between a standard data name and its definition, once it has been entered into the CIF Dictionary, must remain sacrosanct.

The contents of a CIF are not restricted to data items defined in the standard Dictionary. The presence of non-standard data does not affect the logical integrity of a CIF nor the access to the standard data. A fundamental property of a STAR File is that its logical integrity is quite independent of the order or nature of the data items. This means that local and standard data can be mixed in the same file. The only potential problem that could arise is if a local data name is subsequently adopted as a standard data name. Careful choice of local data names, perhaps containing a unique code, should avoid this possibility.

A CIF need not contain every data item listed in the Dictionary. All data items are optional and should be included only if required. A list of data items that are essential for submissions to IUCr journals is included in a new Notes for Authors [Acta Cryst. (1991), C47, 2266-2275] which includes rules for machine-readable manuscripts. Although the CIF allows data items to be in any order, it is good practice to group the items according to the data categories described below. Thus, all _exptl_ items would be entered together to simplify the task of visual searching and editing.

[2] From the Technical Editor, International Union of Crystallography, 5 Abbey Square, Chester CH1 2HU, England. Fax: (44) 244 314888. Email: teched@iucr.ac.uk

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