A database survey of molecular and crystallographic symmetry

Acta Cryst. (2003). B59, 634–640

Histogram showing the distribution of the molecules of C_2h (2/m) symmetry over space group and Wyckoff position. The colours of the bars indicate the symmetry of the Wyckoff position, yellow: C_i (1) pink: C_2h (2/m).

The point of contact between molecular and crystallographic symmetries is that of the Wyckoff position, the position at which a molecule resides in a crystal structure. The Wyckoff position may have the same symmetry as the molecule, may have some symmetry in common with the molecule or may have no symmetry at all. Analysis of thousands of crystal structures, using the relational database CSDSymmetry, has led to the tabulation of the occupancies of Wyckoff positions in crystal structures, the collation of the most favoured locations for a molecule of a particular point-group symmetry in terms of Wyckoff position and space group, and the observation of a hierarchy in the degree of retention of molecular symmetry.