Re: Defining the origin of P2/n

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 sim

Re: Defining the origin of P2/n

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

Re: Defining the origin of P2/n

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

Defining the origin of P2/n

Dear Peter (& others), Actually it doesn't matter how you remove the arbitrary coordinate - the error propagation gives the same result. try it sometime on some polar space group structure. Fix the offending coordiinate in any way you choose & tehn compare esds on bond lengths. They should alway

Re: Defining the origin of P2/n

innelig 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 points, and in order to keep the whole structure from shifting along the c-axis, one of the atoms has to be ar

Re: Defining the origin of P2/n

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 Messag

Re: Defining the origin of P2/n

ular 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 Stephen, I think that P2

Defining the origin of P2/n

P2/n has a center of symmetry? Please ignore my last post. - Kurt --- Kurt Leinenweber Department of Chemistry Arizona State University Tempe, AZ 85287-1604 Phone: (480)-965-8853 Fax: (480)-965-2747 --- -Original Message- From: Von Dreele, Robert B. [mailto:[EMAIL PROTECTE

Re: Defining the origin of P2/n

and Kurt, P2/n 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] >

Re: Defining the origin of P2/n

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 a

Re: Defining the origin of P2/n

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

Re: Defining the origin of P2/n

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 th

Defining the origin of P2/n

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 Zava

Re: Defining the origin of P2/n

[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 GSA

Defining the origin of P2/n

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"

Re: Defining the origin of P2/n

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 refini

Re: Defining the origin of P2/n

CTED] > 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 the first atom > z-p

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 the first atom z-parameter (was your atom on the 2-fold symmetry axis?). I remember this from refining t

Defining the origin of P2/n

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, step