ON THE PREVALENCE OF POLAR AND CHIRAL SPACE-GROUPS

Previous compilations of space-group frequencies have largely ignored differences in the number of molecules or formula units in tile asymme tric unit, i.e., differences in Z' = Z/N, where N is the number of gen eral positions in the space group. This study demonstrates the importa nce of the variable Z'. Crystals with space groups that include mirror planes nearly always contain mirror-symmetric molecules located at th ese special positions; likewise, crystals with space groups that inclu de threefold rotation axes usually contain molecules with threefold sy mmetry. The relative occurrence of polar and chiral space groups is di scussed.