Scheme 1b shows the transition states (TS) of this reaction that lead to the endo (top) and exo (bottom) products. The red dashed lines between the molecules show the bonds that will form upon the creation of the product for clarity. Furthermore, we indicate that the endo pathway becomes preferred inside the cavity under experimentally feasible cavity conditions and under and orientational average of electric field polarization directions. As suggested in Ref. 4 (and reproduced in the current work), the selectivity shifts to 99.9% for the endo product and only 0.1% for the exo. As an example of the modifications to the potential energy surface, we show the ground state potential energy surface in Scheme 1c, where the reactant (R) and TS geometries of the endo (blue) and exo (orange) isomers are placed inside the cavity with the cavity polarization along the Y-direction of the molecule. In tihs case, there is a significant shift in selectivity toward the endo species through a reduction of the TS barrier height by ∼5 kcal/mol for the endo and ∼1 kcal/mol for the exo compared to outside the cavity. This shifts the expected yields of the reaction to 99.9% and 0.1% for the endo and exo isomers, respectively, consistent with those data reported in Ref. 4. In this case, our QED-time-dependent density functional theory (QED-TD-DFT) calculations for the align closely with the QED-coupled cluster singles and doubles (QED-CCSD) approach, with more details and comparisons to be discussed in Fig. . This shift in selectivity can be understood as cavity-induced electronic redistribution under the influence of the cavity (Scheme 1d). More specificallty, coupling to the cavity induces electron density to be taken from populated/occupied intramolecular π-bonding orbitals (i.e., single-particle orbitals) to virtual/unoccupied intermolecular orbitals, thus facilitating a reduction in energy of the TS barrier height.