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).