Symmetry of the lattice-energy functional of a molecular crystal

In calculating the lattice-energy hypersurface by the systematic variation of the molecular rigid-body parameters and the lattice constants, the ranges to be scanned depend on the molecular symmetry and on the space group. A generalization of Hirshfeld's approach [Hirshfeld (1968). Acta Cryst. A24, 301-311] applicable to the case of variable lattice constants is suggested. The symmetry of the multidimensional parameter space is defined by the direct product of the molecular point group and a normalizer N_A(F) of the space group F. The normalizer N_A (F) is a group of affine transformations of the crystal axes that leave invariant the coordinates of equivalent positions. An asymmetric unit of the parameter space is obtained through keeping the lattice constants within such ranges that satisfy the Niggli reduced-cell conditions.