R(F^2) is a "gratis" parameter which is not refined. It also depends much on the actual Fobs extraction details which may get tricky if there are many overlapping peaks (subgroup refinement, phase mixtures etc). Try to get R(F^2) from RCALC after a (GSAS) powpref-genles run with almost zero shift/esd. If R(F^2) is still >7.3, my guess is that the subgroup structure serves just to refine the profile while structural parameters are actually getting worse.
best miguel On 21 Jan 2005 at 10:27, Jon Wright wrote: > Hello everyone, > > Has anyone else noticed that sometimes when comparing two models Rwp > is better for one and R(F^2) is better for the other? The case I am > looking at is spacegroups P3 versus P-3, with the non-centrosymmetric > structure having more degrees of freedom and therefore very slightly > better Rwp (7.6% versus 7.7%). I don't understand why it has a much > worse R(F^2), 8.5% versus 7.3%. These numbers are coming from GSAS, > which gives the same number of F_obs reflections for each of the two > fits. I haven't looked into the details much, but shouldn't the two > statistics be better correlated than that? > > Thanks for any hints! > > Jon > -- Miguel Gregorkiewitz Dip Scienze della Terra, Università via Laterino 8, I-53100 Siena, Europe fon +39'0577'233810 fax 233938 email [EMAIL PROTECTED]
