An extension of the simple harmonic oscillator model for vibrational modes suggested by Kim et al. [Kim 1992] allows a continuous shift of the line shape between a Gaussian and a Lorentzian profile. This is achieved by the following frequency dependence of the damping constant:
The constant s is called Gauss-Lorentz-switch. Like almost all fit parameters it may vary between 0 and infinity. For σ = 0 a Gaussian line shape is achieved. Large values of σ (larger than 5) lead to a Lorentzian.
The Kim model is similar to the Brendel oscillator which also gives lineshapes between a Gaussian and a Lorentzian. However, the computational effort is much less. Thus, if you have to work with many oscillators, the use of Kim oscillators instead of Brendel oscillators can save a lot of time.
The four quantities of Kim oscillators can be set in the following dialog: