Stephen,
Whether the space group is centrosymmetric or not isn't the issue. The question is
whether it is polar or not. P2/n is not polar (i.e. origin defined relative to some
symmetry element) but P2 is polar (i.e y coordinate not chosen relative to a symmetry
element).
Bob Von Dreele
Stephen and Bob,
The issue very simply is once you define a site as 'x,y,z', are -x, -y, and -z
all used in the definition of other sites in the group? If so the origin has
been defined. If not the origin is arbitrary in those directions lacking a minus
sign on their operator. This may be over
