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
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