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. 78

Section Looped data

B. McMahona*

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

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Many of the attributes of looped data items, such as their physical units or valid numerical values, may be defined in exactly the same way as for non-looped data. However, more care needs to be taken to describe the relationships between different looped data items.

Consider the following example listing of some three-dimensional atom-site coordinates and displacement parameters. [Scheme scheme6]

These loops, or tables of values, are properties of atom sites, each identified by a label such as O1. The definition of a data name such as _atom_site_U_iso_or_equiv expresses this by using the DDL1 _list_reference attribute (Example[link]).

Example Definition relating a looped data item to the item used to identify a `loop packet', or row of entries in a table.

[Scheme scheme8]

For an entry in the table to make sense, the site identifier must be present, so the definition for _atom_site_label declares it a mandatory item within its list (Example[link]).

Example Definition of a mandatory item within a loop.

[Scheme scheme9]

It is common for an atom-site identifier to be used in several related tabulations in a particular crystal structure description, and in a CIF description this means that it may occur in several different looped lists. The dictionary definition gives a formal account of this by listing the data names in other looped lists which are just different manifestations of this same item. This is done using the _list_link_child attribute, which identifies the data names to which the one being currently defined is `parent'. In Example[link] (which is a subset of the full list in the core dictionary), _atom_site_aniso_label, _geom_bond_atom_site_label_1 and _geom_bond_atom_site_label_2 are identified as children of _atom_site_label.

It can been seen immediately that _atom_site_aniso_label is the atom-site identification label appearing in the second table in the example listing above, and the _geom_bond_ items are clearly atom-site labels in a table of bonding properties between specified sites. There is, however, a difference between the two secondary tables: the bond-properties table is described by data items in the GEOM_BOND category, but the table of anisotropic displacement parameters includes data names that have the same _category attribute as the coordinate data items, namely ATOM_SITE. The latter is an example of multiple lists or tables belonging to the same category, a feature permitted only in DDL1-based data files.

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