CIF(5)                                                                  CIF(5)



NAME
       CIF - Crystallographic Information File

DESCRIPTION
       The  Crystallographic  Information  File is a new standard archive file
       for crystallography, adopted by the International Union of  Crystallog-
       raphy  as  the  recommended  medium  for the electronic transmission of
       crystallographic data between  individual  laboratories,  journals  and
       databases.  It is based on the Self-Defining Text Archive and Retrieval
       format STAR(5).

       The CIF syntax

       The archival facilities provided by the STAR process  are  general  and
       open-ended.  There  is no restriction on the number of loop levels, the
       length of the file records (i.e. the lines of data) or on the length of
       data names. Syntax of this generality is unlikely to be needed in crys-
       tallography. It was therefore considered reasonable to impose  restric-
       tions  on  the STAR syntax which will simplify the software required to
       generate or access a CIF.

       The CIF restrictions to the STAR File syntax are:

       1.   Lines (physical records) may not exceed 80 characters.

       2.   Data names and block codes may not exceed 32 characters.  All data
       names  and  block  codes  are  case insensitive, i.e. _ABS and _abs are
       treated identically.

       3.   In a STAR File, a data item may be of any data type.  However,  it
       simplifies  processing if data types are known in advance. The CIF Dic-
       tionary identifies whether a CIF data item is number or character.  The
       character and text fields are considered interchangeable.

       4.   A  data  item  is assumed to be a number if it starts with a digit
       "0"-"9", plus "+", minus "-" or a period "." and it is not  bounded  by
       matching  single  or double quotes or semicolons as the first character
       on a line.

       5.   A number may be supplied as an integer, as a  floating-point  num-
       ber,  or  in scientific notation.  When concatenated with an integer in
       parentheses, that integer is assumed to be the estimated standard devi-
       ation  in  the  final digit(s) of the number.  For example: 34.5 3.45E1
       34.5(12)  3.45E1(12)  are all versions of  34.5  with  and  without  an
       e.s.d. of 1.2.

       6.   A  data item is assumed to be of data type text if it extends over
       more than one line, i.e. it starts and ends with an  semicolon  as  the
       first character of a line.

       7.   A data item is assumed to be of data type character if it is not a
       number or text.

       8.   Only one level  of  loop_  is  permitted.   Additional  levels  of
       repeated data must be stored as lists within a text field.

       9.   Many  numeric  fields  contain  data  for  which the units must be
       known.  Each CIF data item has a default units code which is stated  in
       the  CIF Dictionary. If a data item is not stored in the default units,
       the units code is appended to the data name.  For example, the  default
       units  for  a crystal cell dimension are Angstroms.  If it is necessary
       to include this data item in a CIF with the units  of  picometres,  the
       data  name  of  _cell_length_a  is replaced by _cell_length_a_pm.  Only
       those units defined in the CIF Dictionary are acceptable.  The  default
       units,  except  for the Angstrom, conform to the SI Standard adopted by
       the IUCr.  These default units should be used whenever possible.

       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 the CIF
       Dictionary (Core Version 1991).  These are  the  IUCr  "standard"  data
       items  currently  accepted for the submission of machine readable docu-
       ments to the IUCr and to the crystallographic databases. The data items
       in  the  Core Dictionary are intended primarily for use in the descrip-
       tion of most small-molecule and inorganic  structures.   Future  exten-
       sions  to  this  dictionary  will  define  data items used in more spe-
       cialised areas of  crystallography,  such  as  powder  diffraction  and
       macromolecular studies.

       The  Dictionary  (Core version 1991) is also available as an electronic
       file cifdic.C91. This file has been constructed using the STAR  Dictio-
       nary  Definition  Language  (DDL)  proposal of Cook (1991).  Entries in
       this Dictionary may be examined using the online utility cman(1).   The
       CIF  application  program  cyclops  (1) employs this dictionary for the
       validation of standard data names.

AUTHORS
       Sydney R. Hall, Crystallography  Centre,  University  of  Western  Aus-
       tralia, Nedlands 6009, Australia

       Frank H. Allen, Crystallographic Data Centre, University Chemical Labo-
       ratory, Lensfield Road, Cambridge CB2 1EW, England

       I. David Brown, Institute for Materials Research, McMaster  University,
       Hamilton, Ontario L8S 4M1, Canada

SEE ALSO
       ciftex(1), cman(1), cyclops(1), quasar(1), STAR(5)



                                                                        CIF(5)