[Ifeffit] High SO2

[Matt Newville]

Scott,

For many cases, I think it makes very little difference if you first
fit the first shell and then fit the second shell keeping "the first
shell" fixed.  That is, for many cases, the two are nearly completely
separated.  If they are not well separated, then you pretty much have
to fit them together.

But many EXAFS problems do have well separated shells, often with the
first shell much larger, of a known atom type, and at a pretty
well-known distance.  A fairly typical class of EXAFS analysis would
have oxygen as the first shell and a metal as the second shell and the
goals are

   1) is the oxygen distance / coordination of type A or type B (say,
      tetrahedral v. octahedral).   The XANES often tells you this,
      and the EXAFS signatures (N, R) are pretty distinct.  It may
      not be important how distorted this coordination is: one just
      wants the average, gross coordination type.

   2) what is the second coordination shell?  What atom type, what
      coordination number, what distance?  There is often a much wider
      range of possibilities that are harder to distinguish, and this
      is important to study (fit) in detail.

In these cases, many people use the first shell (again, often well
isolated, large, and of a known atom type at a pretty well known
distance) to determine S02 and E0.  This can often be done in a few
minutes. With that information, higher shells can be tackled adjusting
atom type, number, distance, and disorder, and can take a long time to
do.  In many of these cases, leakage from the first shell just does
not matter much, and one does not need (or have) constraints or
restraints between parameters affecting the separate shells.

In these cases, It is generally better to tell the fit to look only at
the higher shell so that the inherent misfit in the first shell
(inevitable, and generally of little importance to the result) does
not dominate the total fit uncertainty.  In short, it speeds up the
process (both for the computer and the human) to fit only the second
shell.

I would say that this approach is not only "not wrong", it is a
completely valid and recommended.

In the end, it is a good idea to fit to the entire spectra as there
can be subtle correlations between the parameters.  If this step is
forgotten, it hardly invalidates the analysis, but it is definitely
better to do.

--Matt