Identification File Research Articles

Revised space-group frequencies for organic compounds

In a recent paper Mighell, Himes & Rodgers [Acta Cryst. (1983), A39, 737-740] reported the space-group frequency for nearly 30 000 organic compounds in the NBS Crystal Data Identification File [(1982). JCPDS, Swarthmore, PA]. When the frequencies tabulated by Mighell et al. for those space groups that do not contain any operations of the second kind are revised somewhat different results are obtained. (Numbers given by Mighell et al. are given in parentheses.): 75% (75%) of compounds fall into only five space groups: P21/c, 29.2% (36.0%), P212121, 18.8% (11.6%), P\bar 1, 11.1% (13.7%), P21, 10.9% (6.7%) and C2/c, 5.4% (6.6%); 12(16) space groups account for 87% (90%) of the compounds.

Space-group frequencies for organic compounds

The space-group frequency for approximately 30 000 organic compounds in the NBS Crystal Data Identification File has been calculated for each of the 230 space groups. 75% of the compounds have been reported in only five space groups: P21/c (36.0%), P{\bar 1} (13.7%), P212121 (11.6%), P21 (6.7%) and C2/c (6.6%). In contrast, there are 29 space groups with only one entry and 35 space groups with none at all. Although the space-group frequencies should be fairly representative of their distribution in nature, certain frequencies are over- or under-estimated. An analysis of the metric symmetry for about 30 000 lattices using a matrix technique has revealed that it is not uncommon for the metric symmetry to exceed the reported crystal symmetry. In many of these cases, the structures have been described in space groups of unnecessarily low symmetry. By explicitly checking for the highest possible metric symmetry during the space-group-determination procedure, errors of this type can be prevented.