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:
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-zIt 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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