Tables for
Volume G
Definition and exchange of crystallographic data
Edited by S. R. Hall and B. McMahon

International Tables for Crystallography (2006). Vol. G, ch. 3.1, pp. 78-79

Section Units

B. McMahona*

aInternational Union of Crystallography, 5 Abbey Square, Chester CH1 2HU, England
Correspondence e-mail: Units

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The physical units in which a quantitative data item must be expressed are identified by the DDL1 attributes _units and _units_detail. The latter is a character field describing the units; the _units attribute is a code that may be interpreted by machine. In DDL1-based dictionaries, type codes are purely conventional, and there is no mechanism for converting units or relating quantities in different units. Table[link] lists the units codes used in the DDL1-based dictionaries described in this volume. There can be some inconsistencies: two codes (`s' and `sec') are already in use to indicate the time unit of seconds.

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Units codes and their interpretation in DDL1-based dictionaries

Unit code ( _units)Meaning ( _units_detail)
A Ångströms
A^-1^ Reciprocal ångströms
A^2^ Ångströms squared
A^3^ Ångströms cubed
Da Daltons
K Kelvins
Kmin^-1^ Kelvins/minute
Mgm^-3^ Megagrams per cubic metre
\ms Microseconds
deg Degrees
deg/min Degrees per minute
eV Electronvolts
e_A^-3^ Electrons per cubic ångström
fm Femtometres
kPa Kilopascals
kV Kilovolts
kW Kilowatts
mA Milliamperes
min Minutes
mm Millimetres
mm^-1^ Reciprocal millimetres
s Seconds
sec Seconds

The original CIF paper (Hall et al., 1991[link]) described a convention allowing physical quantities to be listed in a CIF in units other than those specified in the dictionary. Under this convention, a data name representing a value expressed in different units could be constructed by appending one of a series of known `units extension codes' to the standard data name. Thus _cell_length_a_pm would represent a cell length expressed in picometres instead of the default ångströms. This approach is now deprecated, and all quantities must be expressed in the single unit permitted in their definition block. However, to allow the formal validation of old CIFs, a `compatibility dictionary' is available which defines all data names that could have been constructed under this convention in a properly DDL1.4-compliant form. This dictionary should only be used for validating old CIFs, and must not be used to construct new data files. The dictionary is called cif_compat.dic in the IUCr CIF dictionary register (see Section[link]).


Hall, S. R., Allen, F. H. & Brown, I. D. (1991). The Crystallographic Information File (CIF): a new standard archive file for crystallography. Acta Cryst. A47, 655–685.

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