[Ifeffit] Athena (linear combination fitting)

[Matt Newville]

Just to re-iterate and put more bluntly some of what Bruce has said:

Doing Linear combination fits of EXAFS chi(k) *requires* that the
    1.  mu(E)->chi(k) transformation uses the same absolute value for
E0 for all spectra.

    2.  the mu(E) data are on the same energy scale to begin with (or
point 1 is impossible).  Shifting the energy scales by "align"ing with
Athena may be the right thing to do, or it might completely destroy
your data, depending on how the data were collected.

As linear combination fits (either for XANES normalized mu(E) or EXAFS
chi(k)) always give an answer, they are further complicated by the
choice of good "standards", and without these can give completely
wrong results. Linear combinations fits are more challenging for EXAFS
than for XANES, as EXAFS are complicated by disorder terms that do not
strongly effect XANES, and this disorder is a "component" of the data.
  Unless you understand the set of possible atomic environments very
well and are only interested in the fractions of known phases, I'd
recommend against using simple linear combinations of EXAFS for
anything except exploratory analysis.

That's all to say that you have to be really careful with these, and
you convince yourself that you can extract reasonable results from
spectra with known combinations before applying this to unknown data.

--Matt