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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
          +x,+y,+z  
          1/2-x,-y,1/2+z
          1/2+x,1/2-y,-z
          -x,1/2+y,1/2-z
It is also possible to group a series of values together, in this case the coordinates and atom label for a set of atoms:

    loop_
    _atom_site_label
    _atom_site_fract_x
    _atom_site_fract_y
    _atom_site_fract_z
      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:

    _publ_author_address
;     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.


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