Hi Bruce and Shelly, thanks for your replies. The problem I am encountering is that we know virtually nothing about the crystallography, only the elements that are present. We expect the structure to be quite different to CuWO4, even though this is the stoichiometry obtained. Addressing some of your questions/comments: Shelly: 1) I will try this. In other systems I've tried the second oxygen shells don't contribute significantly, so maybe they are unnecessary and only adding to the uncertainty in the other shells. 2) & 3) As mentioned above, we don't even know this about the structure. But it is worth me having a go at restricting the delr's to relate to CuWO4. I am loathe to do the same with the amplitudes however, as I expect the co-ordination numbers to be completely different. 4) Yes, I ran a CuWO4 standard at the same time. It looks completely different ;) 5) & 6) Thanks for these tips! Bruce: I have kept E0 the same for all shells, hence 13 parameters (4 shells x amp, delr, ss; plus E0). Sorry for not making that clear enough. The data stops at 9 Ang-1 because there seems to be some Zn edge impurity :( I will play around with the constraints a bit more (starting with the CuWO4 standard) and let you know how I get on. Thanks again for your help! Bridget This electronic transmission and any documents accompanying this electronic transmission contain confidential information belonging to the sender. This information may be legally privileged. The information is intended only for the use of the individual or entity named above. If you are not the intended recipient, you are hereby notified that any disclosure, copying, distribution or the taking of any action in reliance on or regarding the contents of this electronically transmitted information is strictly prohibited. This message has been scanned for viruses and dangerous content by MailScanner, and is believed to be clean.