>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%.
In the original Rietveld program, R(F2) was included only as a rough guide for comparison with single crystal refinements, because people were unfamiliar with R-profile. But R(F2) is not minimised by the refinement, and you should not take much notice of it when comparing different profile refinements. If you only improve Rwp from 7.7% to 7.6% in almost doubling the number of parameters by removing the center of symmetry, then you should not do that. Structures have to be physically and chemically "reasonable", not just have the lowest R-factors :-) Alan. Alan Hewat, ILL Grenoble, FRANCE <[EMAIL PROTECTED]> fax (33) 4.76.20.76.48 (33) 4.76.20.72.13 (.26 Mme Guillermet) http://www.ill.fr/dif/AlanHewat.htm ___________________________________________________________
