In Fig. , we present the purely absorptive 2D polariton spectra (Eq. 7) of a coupled dimer inside a lossless cavity for various coupling strengths g c , from g c = 100 cm -1 for panels (a)-(c), to g c = 200 cm -1 for panels (d)-(f), and to g c = 500 cm -1 for panels (g)-(i). The cavity loss rate is set to be Γ = 0 for clarity. The detailed model parameters for the phonon bath are provided in Sec. V A. In short, a very fast phonon bath relaxation time τ c = 50 fs combined with large reorganization energy λ = 200 cm -1 causes the bath correlation function to decay before the bath coordinates relax to the excited-state minima. This causes the system to dephase very quickly, as can also be confirmed by the coherence dynamics of the reduced density matrix (and confirmed with Fig. ). Further, the bath parameter regime (ω 0 = 200 cm -1 ) causes homogeneous broadening of the individual polariton peaks. The large reorganization of the phonon bath also causes a large spread of 2D lineshapes and a large shift of polariton peaks from the eigenvalue in Eq. 57. For g c = 500 cm -1 and Γ = 10 meV, the t 2 = 0 fs 2DES result is provided in the panel (l) of Fig. , which is similar to Fig. , except with a lower intensity for the signal. Additional 2DES results for various Γ and t 2 are provided in Fig. of the Supporting Information.