Martin gives a good example for which interesting new information was obtained from the PDF function - the fact that the Si-O bond lengths do not change during a structural transition in quartz.
But quartz is a rather special case. First of all quartz is simple enough that the Si-O distances might actually be measured directly from the PDF - this cannot often be the case. Second quartz is a material that exhibits strong librational motion of small rigid units (associated with the transition) and unless such libration is correctly modelled (using spherical harmonics in a periodic structural model) this is a textbook example of Rietveld refinement under-estimating bond lengths by trying to fit a banana-shaped distribution with an ellipsoid. With a correct model for libration you would also obtain correct bond lengths by refinement from the Bragg peaks alone. As Martin says, "the Bragg peaks describe the average periodic structure", which in this case consists of banana-shaped O-distributions at constant distance from Si. Let's not quibble over whether the PDF and Patterson functions are the same or not; in 1935 Patterson didn't have a computer and could only measure Bragg peaks. What he described is the instantaneous electron density folded with itself, which is precisely the definition of the PDF today. If you obtain this as he did by Fourier transforming the Bragg intensities (F**2) you do ignore correlations that are non-periodic with the lattice, but he had no choice. Much is made of "total scattering" implying that modulations in the background bring essential new information to PDF not available to Rietveld. "Total scattering" was in fact advocated for Rietveld refinement by Sabine 20+ years ago - see Young's book on the Rietveld method. (Sabine wrote the first neutron paper with Rietveld (Nature 1961) and coined the term Rietveld Refinement). A better example of the interest of background scattering might be anti-ferroelectric perovskite transitions (Mike:-), where atom displacements doubling the cell manifest as diffuse scattering between peaks. Of course if we *define* Rietveld refinement as Bragg peak refinement, we restrict its power. I would define Rietveld refinement as direct refinement of the structural parameters (atom positions etc) to fit the observed data (without first extracting structure factors or performing other intermediate operations, which was usual before Rietveld). I still think that Fourier transforming the total scattering, and then refining against that, cannot do more than refining against the total scattering directly. Martin almost says as much when he talks about RMC. The original question I think, was whether there is any computational advantage in Fourier transforming the data before refining. Probably not. Alan. ______________________________________________ Dr Alan Hewat, NeutronOptics, Grenoble, FRANCE <[EMAIL PROTECTED]> +33.476.98.41.68 http://www.NeutronOptics.com/hewat ______________________________________________