The rate constant expression k 2D VSC (Eq. 22) is the third key result of this paper. A careful exam of k 2D VSC (ω c ) in Eq. 22 reveals that the maximum suppression of k/k 0 indeed happenes at the normal incidence. Note that F(ω) in Eq. 24 takes a sharp maximum at k ∥ = 0 and decays quickly when k ∥ increases, because τ -1 ∥ increases quickly as k ∥ increases . This means that for a 2D cavity, which is used in most VSC experiments, the magnitude of the VSC modification maximizes around ω k (k ∥ = 0) = ω c = ω Q , leading to the largest suppression of k/k 0 . Eq. 22 thus explains the normal incidence condition for rate constant suppression. Our theory, k 2D VSC in Eq. 22 thus successfully explains (1) resonance condition, (2) normal incidence condition and operates under the (3) collective coupling regime and (4) thermally activated condition.