Is the heuristic quantum model we present here a more suitable approach to determine the spectra in such coupled core-shell particles? The well established expression in eq 3 for the permittivity is more precise than a pure Lorentz approximation, and we gave convincing arguments for using the collective vacuum Rabi frequency Ω 0 for Ω. In the following, we want to test the model's validity by comparing it to experimental data of core-shell nanorods. We turned to gold nanorods, because their aspect ratio can be used to tune the longitudinal plasmon resonance (i.e. electron oscillations along the long axis) to about 600 nm, where the excitonic absorption resonance of the shell is formed by J-aggregates of the dye molecules TDBC (5,5',6,6'-tetrachloro-1,1'-diethyl-3,3'-di(4sulfobutyl)benzimidazolocarbocyanine). See the Supplementary Information for details on the aqueous solution of the system. Using our HQM, we are able to reproduce the measured extinction quite accurately while we did not succeed with a purely classical model.