Moreover, we compare the quantum-classical approximation to the reaction rate k and C flux (t) at t = 10ps as a function of the cavity frequency in Fig. ). We observe that while the former exhibits a monotonic increase with the optical frequency ω c , the latter displays a dip centered at ω c = 1185cm -1 . This dip is not an indication of a cavitymodified rate, but instead arises due to the lingering influence of the initial transient dynamics. To elaborate on this, in Fig. (a), we illustrate the reactant dynamics for reactions outside and inside the cavity with three distinct optical frequencies. The solid lines correspond to the converged fully quantum-mechanical results, while the dotted lines represent the mixed quantum-classical results. In both cases, compared to the reaction outside the cavity, the reactant dynamics inside the cavity exhibit a more pronounced discontinuity in slopes around t = 400 fs. Notably, the reactant population experiences the most significant short-time drop when on or near resonance, i.e., the optical frequency is close to the transition energies ∆ε mol 0 + ↔1 -= 1140cm -1 for the dipoleallowed molecular vibrational transition |v mol 0 + ⟩ → |v mol 1 -⟩ and ∆ε mol 0 -↔1 + = 1238cm -1 for |v mol 0 -⟩ → |v mol 1 + ⟩, and thus the photon energy can be absorbed. This short-time behavior may stem from the initial establishment of molecule-cavity correlation, which is factorized at t = 0. Further experiments, compatible with real-time spectroscopic monitoring, may be able to directly track the polariton wavepacket on subpicosecond scales and probe the impact of the formation of this correlation. Moreover, the mixed quantum-classical treatment tends to overestimate this initial reactant reduction, as shown in Fig. , which compares the fully quantum-mechanical and mixed quantum-classical results for the short-time reactant population (at t = 1ps) as a function of ω c . It is worth noting that directly extracting reaction rates from fitting the population dynamics can potentially lead to a spurious resonant peak, with the values being orders of magnitude larger than the exact rate constant. This peak completely disappears when the rate is calculated correctly [k in Fig. ]. This finding may explain why previous studies claimed to see resonant peaks from mixed quantum-classical trajectory simulations. Further analysis is provided in the SI.