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
