ω dω. The resulting rate constant is presented in Fig. . The transmission coefficient (black dashed line in Fig. ) shows two distinct regimes: for Λ < 5 × 10 -6 the chemical rate increases with increasing Λ (so-called energy diffusionlimited regime) and for Λ > 5 × 10 -6 the chemical rate decreases with increasing Λ (so-called spatial diffusionlimited regime). The transition from the energy to the spatial diffusion-limited regime around Λ ≈ 5 × 10 -6 is referred to as the Kramers turnover. Within the classical rate theory, the cavity photon mode is regarded as an additional environmental degree of freedom which increases the effective environmental friction. Thus, depending on whether the solvent friction is in the energy or spatial diffusion-limited regime the cavity mode is expected to enhance or suppress chemical reaction rates, respectively. However, in order to capture this cavity-modified enhancement of chemical kinetics, one must go beyond the GH theory which does not capture the energy diffusion-limited regime, as shown in Fig. where the κ GH (red solid line) diverges from the true transmission coefficient (black dashed line) at low Λ.