Oh good ! We are going to have one of those philosophical debates about the fundamentals of Rietveld related refinement, where we can all safely defend different opinions based on our limited personal experience.
PDF refinement is great, but is not new. It was known as the "Patterson function" in crystallography well before any of us were born (even me) and van Hove extended it to space-time correlations in ~1956 and produced a classic Physics textbook. PDF has been essential for diffraction from liquids and amorphous materials (because you can't do any better :-) and for inelastic scattering, but it is not much used today in its original form for crystallography with single crystals. This even though PDF is in principle more powerful when used with single crystals than with powders. The reason is that PDF as its name implies only gives a list of interatomic distances and scattering strengths, without even directions for powders. And you don't want to throw away the information that you do have about the atoms that are ordered, if only the "heavy" atoms. So Patterson methods for single crystals have been superseded by difference Fourier techniques. Still, if you can apply "heavy atom" or other resonance scattering methods to help sort out the pairs, PDF like other Fourier techniques remains important. Potential disadvantages of PDF/Fourier techniques, such as calculation efficiency, truncation errors etc are not really relevant with today's powerful computers, hard X-rays, maximum entropy and other fancy tricks. The problem with Rietveld refinement :-) is that you only get out a refinement of the model you put in. If your model can't actually describe the data, even the calculated errors will be wrong. But you may still obtain useful information about where the model is wrong eg from unreasonable anisotropic temperature factors that may indicate disorder or lower symmetry. A simple PDF function, unlike Rietveld refinement, does not assume anything about the structure. Of course it may not help much either, since in any but very simple structures there are a lot of different inter-atomic pairs, and with a powder they are all collapsed to 1D, just like the powder pattern itself. So you end up having to input a model after all. The advantage of Rietveld refinement :-) is that you only get out a refinement of what you put in. You only refine a few physical parameters such as lattice constants, atom positions etc, and so you reduce correlations between these parameters, which is a major problem with peak fitting methods due mainly to peak overlap, even for Pawley (or le Bail) peak fitting. OK, so you can also input a physically reasonable model for PDF refinement, but then I suspect that the potential advantages become less obvious. It does perhaps come down to a choice between fitting the actual data or its PDF function. In the case of Rietveld refinement, you might use a split-atom model guided by Fourier difference maps, so is there really some model that can only be applied with PDF refinement and not with Rietveld refinement ? Perhaps a "big box" model hasn't yet been tried for refinement in real space? Otherwise the methods must give much the same result, and it may become a matter of personal choice. For my part, given a choice, I would prefer to use the original data (with short wavelengths, combined neutron/xray data, resonance scattering or whatever else is available and not specific to PDF). Alan. ______________________________________________ Dr Alan Hewat, NeutronOptics, Grenoble, FRANCE <[EMAIL PROTECTED]> +33.476.98.41.68 http://www.NeutronOptics.com/hewat ______________________________________________