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 simplifying things, but it might clear up some of this discussion.
For P2/n the 2-fold axis defines the x and z origin and the n-glide defines the y origin. Allen C. Larson "Von Dreele, Robert B." wrote: > > 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 > > ________________________________ > > From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] > Sent: Wed 3/31/2004 9:23 PM > To: [EMAIL PROTECTED] > > Dear Robert B.Von Dreele > > Thank you for your explanation. > Therefore GSAS could do the origin fixing for us when the space group is not > centrosymmetric. Can I boardly say that? > > regards, > stephen
