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(IUCr) A Brief Introduction to CIF A Very Brief Introduction to CIF.

The Crystallographic Information File (CIF) is a mechanism for storing crystallographic information in a computer-readable ASCII file. The format is also intended to be human-readable and editable. The syntax of CIF is a subset of the Self-defining Text Archive and Retrieval (STAR) standard.

A CIF file consists of a series of data items (entries) and corresponding values, e.g:

 _cell_length_a    5.959(1)

A data item may also have a series of values associated with it by preceding the data item with "loop_". The following example shows a list of four symmetry operators associated with a single data item:

    loop_ _symmetry_equiv_pos_as_xyz
It is also possible to group a series of values together, in this case the coordinates and atom label for a set of atoms:

      O1  .4154(4)  .5699(1)  .3026(3)
      C2  .5630(5)  .5087(2)  .3246(1)
Related data items are grouped together in a block. The beginning of a block is designated by the string "data_" prefixing the name of the block. Data items can be recognized because they always begin with an underscore (_) and values are delimited by spaces, quotes or pairs of lines beginning with a semicolon. Quotation marks are used to delimit a value that contains spaces, but do not span lines:

 _chemical_formula_sum    'C18 H25 N O3'

and semicolons are used for values that span more than one line:

;     Research School of Chemistry
      Australian National University
      GPO Box 4, Canberra, A.C.T.
      Australia    2601

Thus, CIF is largely free format. Two restrictions were implemented to allow facile transmission of CIFs by e-mail: lines may not exceed 80 characters and only printable ASCII characters may be used.

Data items used in CIF are described in a dictionary, which defines meaning and usage. For example, the dictionary entry defining _cell_length_a specifies that the value will be a number in angstroms and that an ESD is allowed. The dictionary is itself a STAR file, where the dictionary syntax is defined in a separate Dictionary Definition Language (DDL) file.

Copyright © 1997 International Union of Crystallography

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