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
Kurt
I think that P2/n has no fixed points, and in order to keep the whole
structure from shifting along the c-axis, one of the atoms has to be
arbitrarily fixed. GSAS automatically fixes the first atom z-parameter (was
your atom on the 2-fold symmetry axis?). I remember this from refining
Dear Stephen ( others),
I know there have been a number of replies to this since P2/n does have an inversion
center which is positioned at the unit cell origin. However, if the space group is Pn
(or P2) then the location of the origin is arbitrary on one or more axes. GSAS does
automatically
[EMAIL PROTECTED] wrote:
Dear Rietvelders,
I used GSAS to refine my structure in P2/n, however the program automatically
fixed the the XYZ positions of the first atom during refinement. To my
surprise, this atom is not seen in the fixed atom list in the atom parameters
menu of GSAS.
Bob,
Thank you for the clarification. I never had doubts that GSAS handles fixed origin
issue properly but also never understand how it
is done. This way has one big advantage over user fixed origin -- it yields standard
uncertainties for all atoms and therefore for
all distances, etc.
Peter
Dear Peter,
the most correct constraint would be to fix a sum of coordinates
of all atoms along polar axis, but not only of the first two.
It gives the best estimation of the standard uncertainties.
Yaroslav Filinchuk
PZ Bob,
PZ Thank you for the clarification. I never had doubts
Peter Zavalij wrote:
Bob,
Thank you for the clarification. I never had doubts that GSAS handles fixed origin
issue properly but also never understand how it
is done. This way has one big advantage over user fixed origin -- it yields standard
uncertainties for all atoms and therefore for
Obviously. I was talking about fixing the origin when it is not fixed by symmetry.
By the way, GSAS when generate symmetry from space group symbol provide information
about the origin.
If the origin is not fixed by symmetry is it clearly stated:
For example for P2 The location of the origin is
has a 2-fold axis perpendicular to a glide plane, so it does have fixed
points (unlike e.g. R3c).
Best regards,
Magnus H. Sørby
-Opprinnelig melding-
Fra: Kurt Leinenweber [mailto:[EMAIL PROTECTED]
Sendt: 31. mars 2004 08:55
Til: [EMAIL PROTECTED]
Emne: Defining the origin of P2/n
, so it does have fixed
points (unlike e.g. R3c).
Best regards,
Magnus H. Srby
-Opprinnelig melding-
Fra: Kurt Leinenweber [mailto:[EMAIL PROTECTED]]
Sendt: 31. mars 2004 08:55
Til: [EMAIL PROTECTED]
Emne: Defining the origin of P2/n
Stephen,
I think that P2/n has no fixed
Sorry Allen - I guess this new generation of crystallographers really is as
bad as you think!
- Kurt
---
Kurt Leinenweber
Department of Chemistry
Arizona State University
Tempe, AZ 85287-1604
Phone: (480)-965-8853
Fax: (480)-965-2747
---
-Original
PROTECTED]]
Sendt: 31. mars 2004 08:55
Til: [EMAIL PROTECTED]
Emne: Defining the origin of P2/n
Stephen,
I think that P2/n has no fixed points, and in order to keep the whole
structure from shifting along the c-axis, one of the atoms has to be
arbitrarily fixed. GSAS automatically fixes
Dear Rietvelders,
I used GSAS to refine my structure in P2/n, however the program automatically
fixed the the XYZ positions of the first atom during refinement. To my
surprise, this atom is not seen in the fixed atom list in the atom parameters
menu of GSAS.
Please advise and many thanks,
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