Thanks Anders (and indirectly Aaron) for answering Lleahs question, I forgot to look at this forum for a while…
I could add that omega can be found even simpler if assuming harmonic oscillator (then all peak-spacings are approximated as equal), and might even be marginally more correct for (delta)r since the formulas for S and (delta)r were derived in the harmonic model. (I don't care if you use anharmonic omega_0 also here.)
As for Felix's question, I'm sorry that I don't know which face belongs to which name. If you have talked to me today, we'll leave it at that, otherwise send an e-mail or come tomorrow. You will need x_0, area, Gamma_Gaussian and Gamma_Lorentzian.