In order to understand how cavity-induced CIs can affect photochemical reactivity, a characteristic of CIs called the Berry phase 332 (also known as the geometrical phase) should be discussed. For a diatomic system with a stretching coordinate R and a rotation angle, θ, relative to the cavity polarization vector (not be confused with the incident angle of photon used in Fig. ), the coordinates in the configuration space can be denoted as X ≡ {R, θ}. Based on the JC model (Eq. 4), the upper and lower polariton states can be expressed as