Lattice symmetry determination

Transformation matrices required to obtain a conventional cell (Crystal Data cell) from the reduced cell have been applied to 47 000 crystalline compounds in the Crystal Data file. For 97% of the compounds, the calculations from the reduced cells yield conventional cells (lattice parameters, lattice type, and crystal system) that are entirely consistent with those reported in the original literature. In a few instances in which the reduced-cell matrix indicated a higher symmetry, the author has often noted that the crystal was unusual in some way or there was an error in the reported symmetry. Some implications of the results of this survey are: (1) metric symmetry as determined from the reduced cell is usually identical to the crystal-lattice symmetry; (2) determination of precise cell parameters defining any primitive cell of the lattice is valuable because from them one can conveniently determine the crystal system with a high degree of confidence (the results, however, should still be verified by checking equivalent intensities and systematic extinctions); (3) if the metric symmetry obtained from the reduced cell and the symmetry determined by other techniques do not agree, the reason should be sought as there are often important structural implications; (4) the sequence of steps in an automatic procedure for the determination of space groups could be: primitive cell, reduced cell, lattice metric symmetry, crystal-lattice symmetry, extinction conditions.