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(a)chem.ethz.ch)
Institute for Chemical and Bioengineering - ETH Hönggerberg
HCI E 117 - 8093 Zürich - Phone: +41 1 63 3 48 32
