In other words, the following two couplings in the Hamiltonian shown in Eq. 1 cause the polariton ground states modifications. First, the off-resonance light-matter term ( Ĥel-ph ) couples through the ground state permanent dipole and transition dipoles between the ground and excited states. One simple example for the first case is the coupling between |ψ g , 0⟩ and |ψ g , 1⟩, which is proportional to ⟨ψ g , 0|μ(â † +â)|ψ g , 1⟩ = µ gg ⟨0|(â † + â)|1⟩ = µ gg , and |ψ g , 1⟩ will further couple to |ψ e , 0⟩ through ⟨ψ e , 0|μ(â † + â)|ψ g , 1⟩ = µ ge ⟨0|(â † + â)|1⟩, where µ gg and µ ge are the permenant and transition dipoles among the ground and excited states, each projected along the cavity polariation direction ê. Excited polariton states Φ e are formed when the |ψ g , 1⟩ and |ψ e , 0⟩ states become close in energy and hybridize. The second contribution is from dipole selfenergy ( ĤDSE ), which does not couple states of varying photon number but which does provide non-trivial electronic couplings between ground and excited states. The DSE terms that cou-ple to the ground state are proportional to ⟨ψ g |μ 2 |ψ α ⟩ = γ µ gγ µ γα , where α and γ include the ground and all excited electronic states. Overall, the direct coupling term Ĥel-ph and ĤDSE both contribute to modifications to the ground state. Through these non-resonant lightmatter couplings, the cavity induces modifications to the reactions that are beyond the prediction of the simple Jaynes-Cummings model. In this work, we demonstrate that the strong coupling between molecule and cavity can fundamentally change a ground state DA reaction. We apply the recently developed ab initio polariton chemistry approach, and theoretically demonstrate that one can fundamentally change the selectivity of this reaction from non-selective endo/exo products to highly selective endo/exo products by coupling this reaction inside an optical cavity. We chose ω c = 1.5 eV and the coupling strength A 0 = 0.3 a.u. that is equivalent to a mode volume V ∼ 0.19 nm 3 and field intensity of E ∼ 8.50 V/nm, which the cavity frequency and field strength are experimentally achievable for plasmonic nano-cavity parameters. We provide a direct comparison to an alternative formulation of ab initio QED 4 for the same reaction. We also provide additional chemical insight, beyond that which was provided in Ref. 4, into the cavityinduced changes to the ground state electron density and relate the changes in density to the interplay between inter-and intra-molecular bonding orbitals (see Fig. ), which are commonly used in the description of bond formation. Furthermore, we extend the previous results by computing all possible orientations of the molecule with respect to the cavity field polarization directions and show that the maximum selectivity is not along the previously explored polarization directions. Hence, we show that strong couplings between molecules inside the cavity offer a promising tool that comes with direct chemical intuition via cavity-induced ground state electron density changes to fundamentally modify the outcome of known chemical reactions, making otherwise non-selective reactions selective.