In this Diels-Alder reaction, the numbers of states we used to solve the Eq. 3 are N F = 10 and N el = 50. We use the light-matter coupling strength A 0 = 0.3 a.u. and coupling frequency ω c = 1.5 eV to perform the reaction. Further details regarding the pQED approach and higher coupling frequency results are provided in the Supporting Information.