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
