Discrimination of polymorphs and redeterminations

In an effort to develop a fully automated procedure that can be used to screen the entire CSD, van de Streek and Motherwell considered that some CSD entries are likely to contain errors, which are more likely to be in the atomic coordinates than in the unit-cell parameters. To eliminate the contribution of the unit-cell contents, the structure factor of each reflection in the simulated PXRD profile was set to an arbitrary constant value, thereby producing a normalised reduced-cell PXRD profile that was used for comparison using the WCC procedure. Applying this methodology to the CSD prior to November 2004 produced a list of 2862 unique refcode families which contain reliable pairs of polymorphic structures, amounting to ca. 1 % of all unique chemical compounds in the CSD. Numerous strict criteria were applied in the process, so that the list represents a minimum number of reliable observations rather than an exhaustive list. 

In a manner reminiscent of the self-organising maps, the methodology has been applied to produce a subset of the database that represents the best representative of each unique crystal stracture [53]. Thus, a compound with 10 CSD entries, comprising one polymorph determined seven times and a second polymorph determined three times will be reduced to two entries, which are considered to represent the two unique structure types. The details of the applied quality tests are extensive [53], but the result is a list of 231918 structures (derived from 353 666 structures in the November 2005 release) that are considered to be the best representative examples of all unique high-quality stmctures in the CSD [54], In this way, the complete contents of the CSD are reduced to a set of representative structures that contain an equivalent amount of structural information, but without any redundancy. This dataset forms an especially convenient basis for structural searches, since it is free of any duplication. 

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