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International
Tables for Crystallography Volume G Definition and exchange of crystallographic data Edited by S. R. Hall and B. McMahon © International Union of Crystallography 2006 |
International Tables for Crystallography (2006). Vol. G. ch. 3.2, pp. 93-94
Section 3.2.2.2.1. General description
a
School of Biomedical and Chemical Sciences, University of Western Australia, Crawley, 6009, Australia,bMerck Research Laboratories, Rahway, New Jersey, USA, and cInternational Union of Crystallography, 5 Abbey Square, Chester CH1 2HU, England |
The data items in this category are as follows:
![[Scheme scheme3]](/schemes/Gach3o2scheme3.gif)
These data items give an overview of the diffraction experiment. They are intended to be independent of the instrument, techniques or methodology of the experiment.
The items describing the ambient environmental conditions are reasonably self-explanatory. They are often absent from a CIF, because an author has not thought it necessary to provide information for experiments conducted under `normal' conditions of room temperature and pressure, and in a standard atmosphere. However, `normal room temperature' may span a range of many degrees Kelvin and might have a non-negligible effect upon cell dimension measurements, so the temperature should be given. As there is significant variability in the ambient temperature at which laboratory experiments may be carried out, it is not appropriate to assign a default value for _diffrn_ambient_temperature, since any numeric value chosen as a default could be misconstrued as an experimentally determined value. If the ambient temperature has not been measured, an author may supply a best estimate of the ambient temperature with a suitable standard uncertainty. Alternatively, known upper and lower limits for the temperature may be given using _diffrn_ambient_temperature_lt and *_gt. The same considerations hold true for ambient pressure.
The default for _diffrn_ambient_environment may be understood as `air', although formally it is impossible in the dictionary to specify a default for a free-text field.
The _diffrn_measured_fraction_theta_* items are provided in this category as an indication of the completeness of a set of reflection measurements. They are not as general as the other items in this category, as they apply only to monochromatic X-ray diffraction experiments, and they do not reflect the way macromolecular crystallographers tend to analyse the completeness of a data set as a function of resolution. When used, they must be accompanied by the value of the monochromatic radiation wavelength _diffrn_radiation_wavelength and relate to the maximum ![[\theta]](/teximages/bach1o1/bach1o1fi82.svg)
angle for which the measured reflection count is considered as complete ( _diffrn_reflns_theta_full).
The other textual data items are provided for comment on other aspects of the handling of the crystal prior to the intensity measurement ( _diffrn_crystal_treatment), observations on the diffraction point symmetry, systematic absences and inferred space group or superspace group relationships ( _diffrn_symmetry_description) and any other comment on the intensity measurement process as a whole that cannot be accommodated elsewhere ( _diffrn_special_details).
