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. 77

Section Irreducible sets of data items

B. McMahona*

aInternational Union of Crystallography, 5 Abbey Square, Chester CH1 2HU, England
Correspondence e-mail: Irreducible sets of data items

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In general, a dictionary data block defines a single data item. However, there are instances where several related data names are defined in the same data block. Sometimes this has been done for convenience, to produce a compact listing of similar data names that have common attributes and whose small differences in meaning can best be expressed by a single definition. Such groupings are discouraged, except where they represent components of a larger entity that has no sensible meaning in the absence of any of the components. For example, the data block data_refln_index_ defines the three data items _refln_index_h, _refln_index_k and _refln_index_l that represent the Miller indices of a reflection. All three indices must have a value in order to specify a reflection and so each has no meaning in isolation.

Note that there is no formal method of expressing this close relationship within DDL1 except by grouping the definitions in the same data block in this way. In DDL2 dictionaries, it is common to assign the components of an irreducible set to a specific subcategory.

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