[Ifeffit] a question about the white line

[Matt Newville]

Hi John,

>    Thus we have attempted to use the theoretical mu_0 from FEFF as
> an a priori, and spline corrections on top of that. Thus the form
> of mu_0 would be:
> 
>      mu_0 = mu^thy_0(E,E_0,Gamma)[1 + lambda(E)]
> 
> where the theoretical mu^thy_0  has an adjustable edge position
> (E_0) and broadening (Gamma), and lambda(E) is the spline correction
> which includes both instrumental variations with E and theoretical
> errors.
> 
>   In our experience, the FEFF8 mu^thy_0 can often give a good
> approximation to mu_0, even near the edge where there are large
> white lines.

That sounds interesting.  But if you include a spline with the
calculated mu0(E), how important is the mu^thy_0(E)?  Like, how
much work do you need to put into mu^thy_0 if you have lambda to
pick up the slack?

On the one hand, since a spline is needed, it might imply that you
don't really gain much.  On the other hand, it might also imply
that you could calculate and tabulate a reasonable 'universal
background function' for any given absorber-scatterern pair as a
starting background curve.  Otherwise, it seems to me that the
prior information that goes into getting mu0(E) might be roughly
equivalent to using Feff to generate a standard chi(k) for autobk.

>   We'd be interested in comments/suggestions on this approach.
> For example, how best to represent small corrections to the
> broadened edge step (e.g., arctangent corrections).

I think the experimental broadening should be adjustable, probably
defaulting to a Lorenztian with Delta E/E = 1.e-4.

Anyway, I'd be interested in comparisons of this with autobk
with/without a standard chi(k).  And, of course, code donations
for improved algorithms are always welcome. ;).

--Matt