This is the most flexible KKR susceptibility since you can define an arbitrary function for the imaginary part, specified by a user-defined formula.

The SCOUT dialog is this:

The formula string represents the formula, of course, in which you can use up to 6 constants (C1, ..., C6) which all appear as possible fit parameters. You will need to refer to the wavenumber in the formula which can be done by the symbol X.

The formula in the dialog above is taken from literature [Campi 1988] and leads to the following dielectric function:

Note that this model (Campi-Coriasso) is also available as a built-in susceptibility.