Susceptibility objects of this type contain a complete KKR dielectric function model which is explained below. The reason to have KKR dielectric function models also to appear as susceptibilities is the following: Often KKR dielectric function models must be computed on a very large spectral range which prohibits a high spectral resolution if one has to keep the number of data points low. If you use, for example, 2048 data points between 0 and 200 000 1/cm the spectral resolution is approximately 100 1/cm. If you have to describe infrared vibrational modes with the same model you run into severe difficulties with this resolution.

In these cases you can use KKR susceptibilities to describe e.g. interband transitions in the UV as in this example (1024 data points covering the range between 0 and 100 000 1/cm):

You can then use this susceptibility in a dielectric function model describing the visible and infrared properties with higher spectral resolution (where linear interpolation is used to get the required intermediate values for the real and imaginary part):

The parameters (and possible fit parameters) of KKR susceptibilities are exactly the same as those of the underlying KKR dielectric function model.