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 always be the same. Bob
________________________________ From: Peter Zavalij [mailto:[EMAIL PROTECTED] Sent: Wed 3/31/2004 10:37 AM To: [EMAIL PROTECTED] 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 Zavalij -----Original Message----- From: Von Dreele, Robert B. [mailto:[EMAIL PROTECTED] Sent: Wednesday, March 31, 2004 10:17 AM To: [EMAIL PROTECTED] 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" handle this by forcing the first and second atoms to have the opposite shifts for each of the arbitrary axes. So in Pn the shifts for atom #1 will be sx,sy,sz and for atom #2 they will be -sx,sy',-sz, i.e. the sx & sz shifts will be opposite but identical for these two atoms while each will have it's own sy. Beware, different damping applied to these atoms will make it drift as the applied shifts won't be equal & opposite. If you do not want this to happen then put in your own "hold" on the appropriate coordinates for one atom or else put in your own constraint. Bob Von Dreele ________________________________ From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Sent: Wed 3/31/2004 12:02 AM To: [EMAIL PROTECTED] 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, stephen
