The whole cluster consists of M molecules for which indices m, n, . . . are used. Every cluster molecule consists of N atoms which are indexed as A, B, . . . . Atomic positions are provided in r. While crystal symmetry is not used explicitly, the symmetry relation between molecules in the unit cell is enforced by adequate projection of the gradients. For each molecule m a single rotation matrix U is determined which aligns the latter with the "central molecule", while neglecting the translational component t for the complete rotranslation between the molecules coordinates r: