International
Tables for
Crystallography
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 3.1.5.3. Category descriptions

B. McMahona*

aInternational Union of Crystallography, 5 Abbey Square, Chester CH1 2HU, England
Correspondence e-mail: bm@iucr.org

3.1.5.3. Category descriptions

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As discussed above, categories in DDL1 are intended as `natural groupings' of data items. To document the purpose of a category within a dictionary, `pseudo' data names are used. All pseudo data names are assigned a _category attribute of category_overview and have an associated _type value of `null'. They are also named by convention as _category_name_[dictionary code], for example _pd_data_[pd] for the description of the PD_DATA category in the powder dictionary (indicated by the code `pd' in square brackets). For the core dictionary, dictionarycode is not given, resulting in names like _exptl_[] to describe the EXPTL category.

Example 3.1.5.2[link] is a slightly edited extract from the core dictionary showing how a data block for a category description is composed, including the presence of an example.

Example 3.1.5.2. A category description in a DDL1 dictionary.

[Scheme scheme4]

Note that the dictionarycode extension allows a dictionary to include comments on items that it defines in a category already established in the core dictionary. For example, the modulated structures dictionary includes the category overview item _audit_link_[ms]. This describes the convention adopted to express the relationship between data blocks in a modulated structures data file using the _audit_link_ data names already defined in the core dictionary.








































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