Before revision: Its derivation from the Kramers-Kronig relations requires a few assumptions about the material, from which any deviations will affect the model's accuracy: All optical resonances are at small wavelengths (or high frequencies), far lower than the wavelengths of interest where the model is applied.
After revision: Its derivation from the Kramers-Kronig relations requires a few assumptions about the material, from which any deviations will affect the model's accuracy: