International
Tables for Crystallography Volume F Crystallography of biological macromolecules Edited by E. Arnold, D. M. Himmel and M. G. Rossmann © International Union of Crystallography 2012 |
International Tables for Crystallography (2012). Vol. F, ch. 9.1, p. 211
Section 9.1.3. Data completeness^{a}NCI Frederick & Argonne National Laboratory, Building 202, Argonne, IL 60439, USA, and ^{b}York Structural Biology Laboratory, Department of Chemistry, University of York, York YO10 5YW, England |
The advantage of diffraction methods over spectroscopy is that they provide a full 3D view of the object. Diffraction methods are theoretically limited by the wavelength of the radiation used, but, in practice, every diffraction experiment is further limited by the aperture and quality of the lens. In the X-ray experiment, the aperture corresponds to the resolution limit and the quality of the `lens' to the completeness and accuracy of the measured Bragg reflection intensities.
In this context, completeness has two components, the first of which is geometric and hence quantitative. It is necessary to rotate the crystal so that all unique reciprocal-lattice points pass through the Ewald sphere and the associated intensities are recorded on the detector. Ideally, the intensities of 100% of the unique Bragg reflections should be measured. The second component is qualitative and statistical: for each hkl, the intensity, , should be significant, with its accuracy correctly estimated in the form of an associated standard uncertainty, . The data should be significant in terms of the ratio throughout the resolution range. This point will be returned to below, but it is especially important that the data at low resolution are complete and not overloaded on the detector, and that there is not a significant fraction of essentially zero-level intensities in the higher-resolution shells.