Symmetry constraints on magnetic parameters

As with other crystallographic parameters such as site multiplicities, anisotropic temperature factors, etc, CCSL takes care of the constraints imposed by symmetry on the magnetic parameters. Such constraints may apply to the propagation vector, the magnetic moment directions and the magnetic moments themselves. The propagation vector must be invariant under the symmetry operations of the reciprocal lattice, if it is not, either the cell symmetry is reduced, leading to configuration domains, or the structure is a multi-q one. Propagation vectors corresponding to special points in the Brillouin zone are fixed, those on symmetry axes or planes are constrained so as to continue to lie on them. The magnetic moments on atoms which are at special positions are constrained to have the point group symmetries of those positions. For instance, the magnetic moment of an atom which lies on an axis of order higher than 2 is constrained to be parallel to the axis. For an atom on a mirror plane the moment is parallel to the plane if the reflection operation is combined with time-reversal, and perpendicular if it is not. Atoms on centres of symmetry which are combined with time-reversal can carry no moment. The subroutine MAGCON determines the symmetry constraints on the magnetic moments and prints them on the listing file. If the value of the magnetic moment is determined to be fixed by symmetry it must be zero.