CSDsymmetry

The first requirement for development of CSDSymmetry is a robust and automated method for analysing exact and approximate molecular symmetry. The specific methodology that has been employed is a combination of checks for topological equivalence in the 2-D connectivity diagram followed by full 3-D symmetry perception [29]. The first part of the process is based on the fact that atoms related by symmetry within a molecule must have identical chemical environments - that is, symmetry-related atoms 

Examples of the types of questions that can be posed to CSDSymmetry include the following [28]  

CSDSymmetry identifies 1420 molecular crystal structures (in which the molecule contains more than five atoms) within space groups that contain both special positions of point symmetry 2 and m. Of these, 1079 molecules (76.0%) have point symmetry higher than 1, T, 2 or m. By contrast, there are 2299 structures identified in space group PI, of which only 167 molecules (7.2%) have point symmetry higher than 

2 or m. Therefore, there is a clear preference for molecules with higher point symmetry to be accommodated in space groups with higher symmetry. 

Molecular point group Hermann-Mauguin notation (Schoenflies notation) Percentage of crystal structures in noncentrosymmetric space groups Number of structures considered 

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The CSD has been used to classify the occurrence and connectivity of crystal hydrates43 44 (Section 8.6.3), symmetry and space group frequency (as in the sub-database CSDSymmetry), 45 frequencies of low symmetry packing where there is more then one molecule in the asymmetric unit (Section 8.7)46 and the occurrence of polymorphism (Section 8.5),47 CSD-based tables for bond length distributions for organic and coordination compounds have been derived48 49 and efforts are currently underway to develop an automated library of such parameters called Mogul.50... 

Figure 1.3 Topological equivalence of symmetry-related atoms used by CSDSymmetry for the first step of automated symmetry detection. A full description of the algorithm used to partition the groups is given in Cole et al. [29]...

Table 1.2 Results of a query from CSDSymmetry, showing the ten molecular point groups (with more than 30 occurrences in the CSD) that give rise most often to noncentrosymmetric crystal structures. Reproduced from [28] by permission of the International Union of Crystallography...

J. W. Yao, J. C. Cole, E. Pidcock, F. H. Allen, J. A. K. Howard and W. D. S. Motherwell, CSDSymmetry the definitive database of point-group and space-group symmetry relationships in small-molecule crystal structures, Acta Cryst., B58, 640-646 (2002). 

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