[Ifeffit] Using more than one E0

[Matt Newville]

Scott said:
> It is my secret, completely untested belief (which I am now 
> revealing to everyone on this mailing list!), that some of the 
> cases  of "successful" fits using multiple E0's are masking 
> problems caused by not considering a third cumulant.

Hmm, really??? I'm not sure of that.  Even disregarding the
different k-dependences of E0 and C3, I'm not sure how using
different E0's for different paths (especially with the notion that
they are to be applied to different coordination species) could mask
a third cumulant for a particular path. Do you have an example?

As discussed earlier, using different E0's for different shells does
have some physical interpretation: that the single, flat energy
origin from the muffin tin approximation is incomplete.  That's
about as physical as the first E0 and S02.  My guess is that a
second E0 is about as likely to be needed as non-zero C3 unless you
have purposely disordered (that is, hot) samples.

It's certainly easy enough to add fudge factors to a fit. The hope
is that the model has some physical meaning or that a reviewer would
catch abuses!

--Matt