where the ω 0 is the reactant well frequency along the reaction coordinate, and the rate depends on the potential barrier height ∆E ‡ = E g (R ‡ ) -E g (R 0 ) along the reaction coordinate. Further, in Eq. 197, the approximate expression is obtained in the classical limit for the partition function Z 0 , and it is assumed that solvent friction is nearly zero while thermal equilibrium in the reactant well persists at all times. The entropic contribution of the environment to the free energy barrier is set to zero as a result of this crude approximation. 374 Therefore, for this simple one-dimensional transition state theory, cavity modification to chemical reactivity can occur only due to the modification of the barrier height ∆E ‡ . When considering the cavity-molecule hybrid system, it is reasonable to examine the two-dimensional potential, so-called the cavity Born-Oppenheimer surface 146