An extension of the harmonic oscillator model described in [Gervais 1974] has been included in the SCOUT software since it turned out to be useful for the description of infrared active phonons.

The original formula is

and it represents the dielectric function as a product of individual oscillator terms. For each term there are four parameters, the resonance frequencies ΩjTO of the TO modes (poles of the dielectric function), those of the LO modes (poles of the inverse dielectric function) ΩjLO and a damping constant for each of the modes (γjTO, γjLO).

Due to multiplicative form of the expression it is not recommended to mix this model with others. The parameters of all ten possible oscillators and the dielectric background are set in a dialog box like the following:

Note that only those parameters appear as fit parameters and are used for the computation which belong to an oscillator that was given a name. In the example above the four parameters of the 'Main phonon' (and the dielectric background) would appear as possible fit parameters. The others are completely ignored by the program.

Warning: Not all combinations of the four parameters are reasonable. Large differences of the damping constants for the TO and LO modes may lead to an unphysical negative imaginary part of the dielectric function. This may also happen if the resonance frequency of the LO mode is chosen to be smaller than that of the TO mode.