International
Tables for Crystallography Volume F Crystallography of biological macromolecules Edited by M. G. Rossmann and E. Arnold © International Union of Crystallography 2006 |
International Tables for Crystallography (2006). Vol. F, ch. 11.5, p. 237
Section 11.5.4. Restraints and constraints^{a}Department of Biological Sciences, Purdue University, West Lafayette, IN 47907-1392, USA |
Scale factors will depend on the variation of the incident X-ray beam intensity, crystal absorption and radiation damage. Hence, in general, scale factors can be constrained to follow an analytical function or restrained to minimize variation between successive frames. The scale factors can be restrained by adding a term to ψ, expression (11.5.1.1), where and are scale factors for the nth and ()th frame and w is a suitably chosen weight. Such procedures will increase but will also increase the accuracy of the scaled intensities as additional reasonable physical conditions have been applied.
The mis-setting angles of a single crystal should remain constant throughout the data set. Thus, in principle, the mis-setting angles should be constrained to be the same for all frames associated with a single crystal in the data set. However, in practice, independent refinement of the mis-setting angles can detect problems in the data set when there are discontinuities in these angles with respect to frame number. Cell dimensions should be the same for all crystals and might therefore be constrained. However, care should be taken, as the exact conditions of freezing may cause some variations in cell dimensions between crystals. As radiation damage proceeds, mosaicity is likely to increase. Hence, constraint between the refined mosaicities of neighbouring frames can be useful.