BLAF ‐ a robust program for tracking out admittable Bravais lattice(s) from the experimental unit‐cell data

BLAF represents an original computer program to devise the Bravais lattice symmetry or possible pseudo‐symmetries (with allowance for large axial and angular distortions) of an experimental unit cell. The matrix approach to symmetry formulated by Himes & Mighell [ Acta Cryst. (1987). A 43 , 375–384] is further developed and employed to analyse admittable mappings of a lattice onto itself. Solutions of the matrix equations G = M t GM , where G is the metric tensor of the Buerger reduced lattice, are integral matrices M with det( M ) = + 1 and −1 < tr( M ) ≤ 3, composing the seven axial hemihedral point groups 432, 622, 422, 32, 222, 2, 1. For non‐triclinic symmetries these matrices carry information about important symmetry directions in the lattice, subsequently used in building up an overall transformation matrix to find a conventional (symmetry‐conditioned) unit cell. The average of the generated G tensors in accordance with the particular point‐group rules is a tensor G av bearing information about the symmetry‐constrained lattice parameters. Gruber's [ Acta Cryst. (1989), A 45 , 123–131] algorithms have been used to evaluate both Buerger cells and the Niggli cell of a triclinic lattice. BLAF is realised as a separate module suitable for incorporation in the commonly used crystallographic program packages and in the form of two subroutines: enBLAF – to tackle the lattice symmetry problem by automated single‐crystal diffractometers; and rBLAF – to be used for lattice symmetry analysis in, for example, programs for autoindexing of powder data. Applications of the three modules are demonstrated in several test examples.