Dear Friends, maybe some of you remember that I was implementing a chi^2 mapping algorithm using the IFEFFIT library. This is working now, and for the data analysis of the maps I would like to ask your opinion on the following problem: I want to check with chi^2 maps if I can separate 2 shells, e.g. if a peak is comprising two O shells or only one. For this I take the one-shell fit, half the coordination number and then vary the distance of the shells against each other (all other values taking from the best single shell fit). If I find two contributions, I should see 2 mirror-symmetric minima centered at the corresponding DR values, e.g. (0,-0.1) and (-0.1,0) if I work with the same reference. These minima are separated by a saddle point which is more or less pronounced. In the case of some Fe complexes I simulated, it is clearly visible, while for my gold oxides, it is hardly seen but still there. If I take a system where I know there IS only one shell (e.g. Au metal) and do the map, I find no saddle point and correspondingly no 2 minima. Does anyone know a relation between the height of a saddle point and the physical significance of the map? I would be glad to learn more about, but the statistic books I know don't deal with such a problem (and I am chemist and not a mathematician)... If you need an example, I can send you two slides of a recent talk which explain this problem more in detail - just drop me a mail. Cheers, Norbert -- Dr. rer. nat. Norbert Weiher (weiher@chem.ethz.ch) Institute for Chemical and Bioengineering - ETH Hönggerberg HCI E 117 - 8093 Zürich - Phone: +41 1 63 3 48 32