where V is the effective mode volume of the cavity, ε is the permittivity inside the cavity, and ω c is the cavity frequency. Through the light-matter interactions, various photon-dressed electronic states will be coupled to each other. For example, the ground electronic state with 1 photon in the cavity and an excited electronic state with 0 photons in the cavity will couple through the light-matter interaction. When the energy of these two basis states become close, they interact (mathematically similar to the interaction of atomic orbitals to form molecular orbitals), leading to the formation of excited polariton states. These (and all similar) interactions represent the result of resonant, strong light-matter coupling, causing the formation of new eigenstates, i,e, polaritons. Through non-resonant light-matter interactions, the cavity can directly modify the ground state of a molecule coupled to the cavity. In order to understand this effect, which is important for understanding how cavities can modify ground state chemical reactivity, one has to go beyond the pre-