, where ω ‡ is the barrier frequency and ∆E ‡ = E g (R ‡ ) -E g (R 0 ) is the potential energy barrier. Most of the works reviewed here consider E g (R) to be a simple double-well potential or obtain it from a Shin-Metiu model. The Ĥsb term in Eq. 192 describes the system-bath (vibration-photon) coupling