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, p. 83

Section Definitions of single quantities

B. McMahona*

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

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While it is important to ensure the referential integrity of the data in a CIF through proper book-keeping of links between tables, the crystallographer who wishes to create or extend a CIF dictionary will be more interested in the definitions of data items that refer to real physical quantities, the properties of a crystal or the details of the experiment. The DDL2 formalism makes it easy to create a detailed machine-readable listing of the attributes of such data.

Example[link] parallels the example chosen for DDL1 dictionaries of the ambient temperature during the experiment.

Example DDL2 definition of a physical quantity.

[Scheme scheme18]

In the definition save frame, the category is specifically listed (although it is deducible from the DDL2 convention of separating the category name from the rest of the name by a full stop in the data name). The data type is specified as a floating-point number. (In the core dictionary there are fewer data types and the fact that the value may be a real rather than integer number must be inferred from the declared range.) The range of values is also specified with separate maximum and minimum values (unlike in DDL1 dictionaries, which give a single character string that must be parsed into its component minimum and maximum values). The assignment of the same value to a maximum and a minimum means that the absolute value is permitted; without the repeated `0.0' line the range in this example would be constrained to be positive definite; the equal value of 0.0 for maximum and minimum means that it may be identically zero.

The _item_units.code value must be one of the entries in the units table for the dictionary and can thus be converted into other units as specified in the units conversion table.

The aliases entries identify the corresponding quantity defined in the DDL1 core dictionary.

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