Special-point ordering in general crystal structures

The classification of families of ordered structures, in terms of positions in reciprocal space where two or more symmetry elements of the pair potential intersect at a point, is extended to include general crystal structures. Only a relatively small number of space groups, 24 in all, are shown to be relevant to special-point ordering. The method is illustrated with the study of special-point families for a hexagonal crystal with two atoms per unit cell.