Preliminary Comments - Symmetry


This section is intended to briefly describe how symmetry is used in DMAREL. Firstly we will discuss some of the problems that can arise if symmetry is not used.

If no iterations are carried out there will be no problems. However, if perfect lattice iterations are carried out a number of problems can arise. In principle, the perfect lattice calculation will conserve the space group symmetry of the input unit cell. In practice, rounding errors can permit the symmetry to change, although this rarely happens. Secondly, the perfect lattice relaxation is always carried out keeping the first molecule centre of mass fixed. This is to remove the translational invariance of the force constant matrix. The fixing of the first molecule centre of mass has important consequences for the symmetry. For example, this position may be related to a second by a two-fold axis. Relaxing the perfect lattice will relax these two positions keeping the first one fixed, so that the two-fold axis will move. It is possible that this two-fold axis initially passes through the point group origin.