The interband transition model suggested by Campi and Coriasso [Campi 1988A] leads to the following expression for the imaginary part of the susceptibility:

The 4 parameters of the model must be entered using wavenumber units in the following dialog:

Here the following assignments apply:

The following example shows the dielectric function obtained with this model (the parameters are those displayed in the dialog above) where a constant dielectric background of 1 has been added:

Note that the peak of the imaginary part occurs at the sum of the gap and the resonance frequency. Below the gap the imaginary part is exactly zero, i.e. there is  no absorption.For large resonance frequencies the Campi-Coriasso model approaches the harmonic oscillator model.