Fig. presents the FGR rates under different 𝜂 c values. Fig. is the same as Fig. , which corresponds to the single-mode case (or the many-mode case inside a 1D FP cavity). Fig. presents the estimated value of 𝑘/𝑘 0 using 𝑘 VSC expression in Eq. 35, corresponding to the case of many modes inside a 2D FP cavity. Here, we choose 𝒟/𝑐 = 1 fs, which corresponds to 𝒟 = 0.3 𝜇m. The numerical integration scheme is the same as the calculation of 𝒜(𝜔), and the convergence is carefully checked. One can observe that the resonance peak is still centered around 𝜔 c = 𝜔 0 , which demonstrates the normal incidence condition. The resonance peak is asymmetric due to the asymmetry of 𝒜(𝜔) (see Figs. and). Moreover, the rate profile tails toward the lower energy regions, which is the opposite of the trend in 𝒜(𝜔) (see Fig. ). Compared to the single mode version of the theory, considering many modes in a 2D FP cavity predicts that the "action spectrum" of the VSC-modified rate constant has an asymmetric behavior around 𝜔 c = 𝜔 0 , with a longer tail when 𝜔 c < 𝜔 0 . This is an interesting prediction from the current theory in Eq. 35. In recent VSC experiments by Simpkins , it does seem that the 𝜔 c < 𝜔 0 side has a longer tail than the 𝜔 c > 𝜔 0 side of the action spectrum (𝑘 VSC vs 𝜔 c plot, see Fig. of Ref. 8). However, this could be due to the lack of more experimental data points for a blue-tuned cavity (𝜔 c > 𝜔 0 ) due to the experimental difficulty. More experimental data are required to definitively test this trend.