For our discussion on collectivity, we begin by considering the FGR expression in Eq. 14. If all the molecules' dipoles are perfectly aligned with the cavity polarization, then cos 𝜙 𝑗 = 1 for all molecules, 𝑗, and Ŝ = ∑︀ 𝑗 𝜇( R𝑗 ). For simplicity, we just focus on the 1D cavity case, since for 2D cavity there is no apparent collective effect either. Evaluating Eq. 19 using Eq. 23 leads to