When the coupling is weak (g c = 0.001), the polaritonic PESs (panel (a)) have nearly identical curvatures of the photon dressed states |g1 and |e0 . The lightinduced mixing only occurs near R = 0 between |e0 and |g1 . The population dynamics presented in Fig. and Fig. suggest that the population transition mainly occurs between |g1 and |e0 states. Both Ehrenfest and FSSH provide accurate description of the population dynamics.