[Ifeffit] Debye model on artemis

[Matt Newville]

Hi Kleber,

> I'd like to use the third cumulant option in Artemis, but considering the
> Debye model. Artemis has a Debye function for the second cumulant
> (Debye-Waller factor) but not to the third. How could this be done in
> Artemis?

The debye and eins functions in Artemis/Ifeffit calculates sigma2
only.  One could use the debye (or eins) function for sigma2 and
either allow the third cumulant to vary freely or constrain it to be
based on the value of sigma2.  There are a few approaches to this in
the literature, but I'm not aware of a general solution.   Would that
approach be sufficient, or do you have a particular model relating the
third cumulant to sigma2 in mind?

--Matt