[Ifeffit] Problems with fitting in ATHENA for XANES

[Matthew Marcus]

OK, I'll weigh in:
>
> Peak fitting is often useful in a situation where the physical structure 
> is
> known only partially.  If some feature of the spectrum changes in response 
> to
> changes in some extrinsic parameter (polarization, temperature, or some 
> such)
> then peak fitting can be a good way to quantify that response.

I think that PCA is a better way of quantifying the change with respect to 
an independent variable.  For one thing,
it tells you whether you can describe the change as a shift between two 
end-members, as in the reduction example,
or whether you would need something more complicated.  The obvious 'more 
complicated' is a combination of
more species, for instance an initial form, a final species, and an 
intermediate for a reaction.  However, another
possibility, depending on the system, is some continous change in the 
structure, for instance, a bond angle which
moves, so that one always has a single species, but that species changes. 
That could look enough like a combination
to fool you.  I don't know how to distinguish this sort of thing from a 
variable combination without knowing the science
behind the system.

For polarization, symmetry arguments would tell you that the data should be 
describable as a combination of two
or three contributions, depending on what angle is being varied and what the 
symmetry of the system is.

    mam