[Ifeffit] Re: Athena Suggestions

[Matt Newville]

Hi,

The data in question also had energies that were not in strictly
increasing order and had many visible spikes and dropouts in the
mu(E) data that could certainly confuse the 'find the maximum of the
first derivative' algorithm.

But that being said, I could not reproduce Dave's problem, even with
the data out of order and with dropouts.  Exporting the xmu data, I
got e0's that were very close to one another:

 > read_data(gadowee.xmu, group=gad, type=xmu)
 > pre_edge(gad.energy, gad.xmu, find_e0=1)
     pre_edge: energy data appears out of order
 > print e0
      10374.0000

 > read_data(gamo.xmu, group=gam, type=xmu)
 > pre_edge(gam.energy, gam.xmu, find_e0=1)
     pre_edge: energy data appears out of order
 > print e0
      10374.0900

I don't know why athena got to a different answer, or how to force
it to re-determine e0.

It would not be difficult to interpolate the data onto a very fine
energy grid (say, 0.05eV) and do a multi-point derivative
measurement to get a slightly more robust measure of E0.  I'd stress
the 'slightly' not because of the issue of stating that the max in
the 1st derivative is the Fermi level, but because of the problem of
dropouts and glitches, as the data here contains.

--Matt

PS: Whether the max of the 1st derivative is a measure of the Fermi 
energy is a whole other story.