[Ifeffit] correlated Einstein model and Debye model

[Matt Newville]

Dear Renato,

Do you mean that you'd like to refine not only heta_E, but also refine
the reduced mass in the fit, with sigma2  defined as

sigma2 = [ (hbar*c)^2  / 2* k_B  ]  *  [ coth(theta_E / (2*T))  / (
theta_E * M_R ) ]  + sigma2_static

That seems doable (that is, you could just 'def'ine this formula for
sigma2).  I doubt it would be highly robust to vary all of theta_E,
M_R, and sigma2_static for a simple problem, but you might be able to
put other constraints on some of these parameters.....

Is that what you getting at, or did I miss the question entirerly?

--Matt

PS: I'd double-check the formula,  but I think it's right and that
    [ (hbar*c)^2  / 2* k_B ]  ~=  97 / 4
in the units you want (that is, theta_E and T in K, sigma2 in Ang^2,
and M_R in amu).