The Design of a Knowledge-Based System for Crystal Structure Determination
The determination of crystal structures from their diffraction data belongs to the general class of image reconstruction exercises from incomplete and/or noisy data. In the crystallographic example, recovery of a three-dimensional image requires knowledge of both the amplitude and the phase of the diffracted rays. Since it is not usually possible to measure the phases experimentally, this exercise is not straightforward. Rather it involves the retrieval of some phase information from either additional isomorphous data sets, partial structure information or, in a direct method approach, from the amplitude data themselves through the application of probability theory. In the simplest cases, the process of image recovery is one of image processing. In more complex cases and, in particular in the case of macromolecular crystal structures, pattern recognition is an important component of the image reconstruction exercise. Chemical and structural rules are used to guide the iterative structure determination process.
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