Dear Donita, I'm sure you know that the vanadyl (V=0) bond length is short - you could put in a restraint on its length as one possible control on your refinement. If their sites are partially occupied then make sure you have a constraint to make FRAC(V)=FRAC(O). I don't particularly understand your comment about "special position" - unless the V=O lies on some axis (2-fold?). In GSAS you don't have to worry about adjusting the atom occupancy for the effect of the special position. FRAC is defined as the fraction of an atom that is on the site irrespective of symmetry concerns. Thus, you may have mistakenly set FRAC=0.5 when it should be 1.0. Bob Von Dreele
________________________________ From: Doinita E Neiner [mailto:[EMAIL PROTECTED] Sent: Fri 11/5/2004 4:52 PM To: [EMAIL PROTECTED] Dear list, I am working with a tetragonal cell that has a vanadyl unit. I think that both of the atoms, V and O, are in the same special position. The problem is that both have partial occupancies on the site, .5 and .5, and GSAS does not handle them very good (or I am doing something wrong). It keeps on putting them to form very unlikely bonds with each other. Knowing that the participants at this list are very knowledgeable in the matter, I would appreciate very much any kind of suggestions. Thank you in advance, Doinita
