[Ifeffit] Fitting using Experimental standard

[Matthew]

>
> The very valid question whenever the topic of empirical standards
> comes up is "Why do you think you need it?"  I am unconvinced that
> empirical standards are ever needed.  Of course, I am also unconvinced
> that I am right in saying that!  So who knows...?  There is some
> evidence that empirical standards might be preferable in certain
> specific cases, such as the case in which a hydrogen atom is collinear
> or nearly collinear with an absorber and scatterer.
>
>>From a the persepective of "guy who writes programs" I suppose the
> fact that there is interest in having the capability of using
> empirical standards is itself good enough reason to include it.

One reason I can think of is when the unknown is in some sense close to a model, for instance, a glass compared with a corresponding 
crystal, a dilute solid solution compared with an appropriate compound, if one exists,
or a nanoparticle compared with bulk.  In that case, one might argue that Nature's calculation might be more accurate than FEFF, and 
that the use of a model will automatically correct for artifacts in the extraction of the (filtered) shell.
I will grant that this is now a niche technique whereas, when I started <mumble> years ago, it was mainstream.
    mam