Because of the periodicity of crystals, every object is regularly repeated in three-dimensional space, however in the case of disorder, the orientations of some atoms differ in different unit cells. The structure determined from the diffraction pattern is the spatial average over the whole crystal. In by far most cases, disorder only affects small parts of molecules like organic side chains or SiMe3-groups and the order predominates, especially when only two different conformations are present in the crystal. By means of five examples, this chapter introduces the various forms of disorder such as static and dynamic positional disorder as well substitutional disorder and bulk solvent. It describes in detail how to parameterize disorder in SHELXL, including the use of constraints, restraints, and free variables.
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