THE PERILS OF CC - COMPARING THE FREQUENCIES OF FALSELY ASSIGNED SPACE-GROUPS WITH THEIR GENERAL-POPULATION

A compilation of 221 space-group corrections from a false low symmetry (FS) to a higher true symmetry (TS) shows that higher symmetry is often overlooked in only a few space-group types. An incorrect lattice (false crystal class) is found most often for rhombohedral space-group types, and there especially for R3mBAR (TS) and R3cBAR (TS), and for space group Fdd2 (TS). An inversion center is most often missed in space groups C2/c (TS) and Pnma (TS). The group which has the highest probability of being falsely assigned in a crystal structure determination is space group Cc (FS), both in absolute numbers and relative to its frequency among determined crystal structures. Possibly over 10% of all crystal structures reported in Cc (FS) should actually be described in a higher true symmetry (C2/c, Fdd2 or R3cBAR). For comparative purposes statistics of the frequency of space groups among inorganic crystal structures are given and compared with the corresponding distribution among organic compounds. Each space group appears to be occupied by at least one example. Inorganic substances are heavily concentrated in space groups of the most symmetric crystal classes of trigonal, tetragonal, hexagonal and cubic symmetry. Inorganic crystal structures are, compared to the organic structures, much more evenly distributed over the crystal systems. Surprisingly inversion centers are not much more common among inorganic substances when compared with organic compounds.