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. 78

Section 3.1.5.4.2. Looped data

B. McMahona*

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

3.1.5.4.2. 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 3.1.5.5[link]).

Example 3.1.5.5. 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 3.1.5.6[link]).

Example 3.1.5.6. 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 3.1.5.6[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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