Section 2.2.7.4.6. Note on handling of units

(13) The published specification for CIF version 1.0 permitted data values expressed in different units to be tagged by variant data names (Hall et al., 1991, p. 657):

… 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 ångströms. 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 ångström, conform to the SI Standard adopted by the IUCr.

This approach is deprecated and has not been supported by any official CIF dictionary published subsequent to version 1.0 of the core. All data values must be expressed in the single unit assigned in the associated dictionary.

A small number of archived CIFs exist with variant data names as permitted by the above clause. If it is necessary to validate them against versions of the core dictionary subsequent to version 1.0, the formal compatibility dictionary cif_compat.dic (ftp://ftp.iucr.org/cifdics/cif_compat.dic ) may be used for the purpose. No other use should be made of this dictionary.