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Acta Cryst. (1991). A47, 655-685

Looped and non-looped data items

The STAR File format permits any data item to be included in a loop_ list. This is also the case in a CIF. However, only data items which need to be repeated should appear in a looped list. Looped lists are composed of data items with common properties. For example, atom site information (represented by _atom_site_ items) may appear in one list and reflection data (represented by _refln_ items) in another, but they cannot be mixed together. Appendix II should be consulted for examples of looped lists.

Note that it is essential to loop repeated data items, rather than declaring them as a series of single data items. This is because the STAR format prohibits a data name from appearing more than once in a data block. The Dictionary identifies which individual data items will normally appear in looped lists. In most cases this is obvious from the nature of the data and its definition. Those data categories where individual items often appear in looped lists are summarized below:

atomic site parameters
atomic site anisotropic U values
atom species
reflection data used in the unit-cell measurement
chemical properties in connectivity table
diffraction attenuator scales
reflections that define the diffractometer orientation matrix
discrete wavelengths used in diffraction measurements
diffraction intensity data
scale factors for groups of diffraction intensities
standard reflections for scaling diffraction intensities
separate loops for crystals and crystal faces
interatomic angles
bond distances
contact distances
torsion angles
authors' addresses
authors' names
structure-factor reflection data
scale factors for groups of structure-factor data
space-group equivalent positions

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