Fig. (i) presents the TS barrier height ∆E ‡ = E 0 (R TS ) -E 0 (R Reac ) (polariton energy difference between transition state and reactant nuclear configuration) on the polariton ground state |Φ 0 (R)⟩ as a function of the light-matter coupling A 0 a.u. with the cavity frequency set to ω c = 3.0 eV. The optimized reactant, transition state (TS), and product geometries were calculated outside the cavity, as was done in Ref. 50, which will be used for direct comparison. Note that the mechanism for modifying the group state potential energy surface in this case due to the coupling to the excited states, which is different than the recent studies of vibrational strong coupling. The X-polarization (solid black line) demonstrates a larger effects on barrier height, because there exists a strong transition dipole between the ground state and second electronic excited state (µ x 01 ∼ 1.47/1.26 a.u.) as well as strong ground state permanent dipole (µ x 00 ∼ 1.37/0.90 a.u.) in the reactant/transitionstate geometries (see Fig. in the Supporting Information for dipole matrices for aminopropenal at the reactant, transition state, and product geometries, including the 20 lowest-energy adiabtic states). The magnitude of the ground-to-excited transition dipole in the Z-direction is almost negligible in comparison showing only a slight decrease in the barrier height (solid, red line).