In this section, we explore the population dynamics and rates of our PMET model system in the collective coupling regime and compare to the dynamics and rates outside the cavity. In all cases inside the cavity, we focus on cavities in resonance with the donor excited state in the FC configuration (hω c = ∆E D + λ D = 3 eV) and reactions initiated in, or coherently pumped to, the UP state. This regime of focus allows us to examine the extent that PMET reaction rates can be significantly enhanced, particularly in the presence of dark state transfer and cavity loss. For N > 2500, the UP energy continues to increase with N but the reaction is now in the inverted Marcus regime which decreases the rate as ∆G +A j becomes more negative. For 100 < N < 8100, the reaction inside the cavity is enhanced by up to a factor of 4 relative to outside the cavity due to the more favorable driving force ∆G +A j compared to the bare ∆G = 0 outside the cavity. Although the purpose of this panel is to validate our theory and the accuracy of our quantum dynamics simulations, it is nevertheless interesting to show one can selectively achieve the Marcus normal, activationless, and inverted regime with the same type of molecules and only varying N. Note that due to the large transition dipole of the CdSe NPL, we estimate that N = 10 3 -10 4 NPLs couples to the cavity can generate Ω R ∼ 10 2 meV Rabi splitting. It should be emphasized that the mechanism of enhancement in PMET demonstrated in Fig. based on modifying the driving force relies on initiating the system in the UP state, which is a quantum state whose particular combination of energy and donor character does not exist outside the cavity near the FC region. A different mechanism for collective cavity-enhanced photochemistry was described in Ref. 20 and Ref. 57 where the cavity filtered an initial FC cavity excitation into donor states with higher energy vibrational states during a series of early-time Rabi oscillations, where the higher energy vibrations allowed faster transfer to the acceptor state. This filtering mechanism, described for T = 0, relied on a non-zero λ D and the observed enhancement vanished for λ D = 0. In contrast, the enhancement mechanism in Fig. based on increasing the energy of the electronic-photonic UP state using collective coupling does not fundamentally depend on filtering the donor vibrational states and does not diminish at Figure presents the influence of dark state transfer and cavity loss on PMET population dynamics for an uphill reaction with N = 1000. The reaction inside the cavity is initiated as a FC excitation to the resonant |+⟩ state. The ET parameters are λ A = 100 meV, V DA = 10 meV, and ∆Gλ D = 300 meV uphill at room temperature T = 300 K. In this parameter regime, the forward rate constant outside the cavity is very small (k outside f ≈ 10 -6 ps -1 ) due to the uphill ∆G that is more than 10 times larger than k B T . As a result, a population that resides in the dark states (or |G, 0⟩) will not appreciably transfer to the acceptor states on the plotted timescales (up to 1000 ps). Any visible accumulation of acceptor population must come directly from the UP state due to its more favorable ∆G +A j . Similarly, the backward rate from the acceptor to the donor states outside the cavity is also very small (for nuclear distributions in the FC region). Notably, in contrast to the forward transfer direction, the total backward rate from the acceptor states to the donor manifold is directly influenced by the density of states in the donor manifold. Consequentially, the contribution of the acceptor-to-dark transfer to the total backward rate is (N -1)/(N +1) ≈ 1 which means the population transferred to the acceptor states will not appreciably transfer backward to the donor manifold on the plotted timescales. The acceptor population dynamics in Fig. can thus be understood as a result of two competing pathways: the UP to acceptor transfer which appreciably accumulates acceptor population, and the UP to dark or UP to |G, 0⟩ transfer which becomes effectively "trapped" in the dark or |G, 0⟩ states and does not lead to appreciable acceptor state population. bath that transfers the population between the UP and the dark states. Another perspective is that increasing λ D increases the spectral linewidth of the donor states (i.e. adds disorder to the donor energies) which increases the spectral overlap between the UP and dark states resulting in increased transfer between these states. For a Gaussian sampling of nuclear configurations in the FC region, this spectral disorder has a standard deviation of 2λ D k B T . While the dark states can also transfer to the UP (and LP), this rate scales as 1/N which becomes negligible in the collective limit. The acceptor population dynamics in Fig. do not show a strong dependence on λ D ≤ 50 meV. For a collective coupling strength of √ Ng c = 300 meV, the reaction is almost barrier-less due to the favorable driving force ∆G +A j = 0. The initial UP population can thus fully transfer to the acceptor states in only a few ps while the transfer rate from the UP to the dark states is suppressed by the large energy gap between these states even for λ D = 50. On the other hand, in Fig. , there is a strong dependence on λ D due to the smaller collective coupling strength of √ Ng c = 150 meV which both reduces the UP to acceptor rate constant and allows for a significant spectral overlap between the UP and dark states. As a result, a modest λ D = 10 meV causes the most population to transfer to the dark states before it can transfer to the acceptor states and λ D = 50 meV prevents any visible accumulation of acceptor population on these timescales. Thus the magnitude of effect that dark states have on suppressing UP-to-acceptor PMET depends on the competing rates of the UP-to-acceptor pathway and the UP-to-dark pathway, the latter of which is very sensitive to the spectral gap (or lack thereof) between the UP and dark states. As a result, the presence of dark states in PMET must be seriously considered when the timescale of the UP-to-acceptor transfer is similar to or longer than the timescale of UP-to-dark transfer. Similar consequences of dark state transfer have also been described in other related works on cavity-modified photochemistry. Fig. and Fig. show the acceptor population dynamics for varying cavity loss rates with quality factors Q ≡ ω c /Γ ranging from lossless Q = ∞ (Γ = 0) down to Q = 10 (Γ = 300 meV) and for collective coupling strengths √ Ng c = 300 meV and √ Ng c = 150 meV, respectively, with λ D = V l = 0. The effect of increasing Γ in this case is an increase in the transfer rate from the UP to the |G, 0⟩ state. Due to V l = 0, once the population is transferred to |G, 0⟩, it remains trapped there as there are no transfer pathways from |G, 0⟩ to other states. For λ D = 0, the resonant UP state experiences cavity loss at a rate of Γ/2 due to the UP possessing half photonic character. In contrast to the dark state transfer rate dependence on λ D , the cavity loss rate that the UP experiences in this model is not strongly dependent on the energy of the UP or the energy gap between the UP and the dark states. As a result, the acceptor population dynamics in Fig. are visibly suppressed across the entire range of finite Q, even for Q = 100000 which corresponds to a minuscule Γ = 0.03 meV. The accumulated acceptor population steadily decreases with lower Q and higher Γ until there is no visible acceptor accumulation for Q = 10. In contrast to Fig. , the dynamics in Fig. are not protected from cavity loss by the large collective coupling of the UP. The acceptor population dynamics in Fig. are similarly impacted by lower Q factors with decreasing accumulated acceptor population as Q is decreased. Due to the lack of energy dependence on the UP cavity loss rate, the difference in the amount of suppression observed between Fig. and Fig. is not as large as that between Fig. and Fig. . Overall, the presence of cavity loss in PMET must be seriously considered when the timescale of the UP-to-acceptor transfer is similar to or longer than the timescale of cavity loss, which is the case for many realistic strongly-coupled moleculecavity designs whose Q are often less than Q = 1000, and this cavity loss effect cannot be easily protected against by increasing the collective coupling strength. Similar consequences of cavity loss have also been described in other related works on cavity-modified photochemistry. Figure presents the influence of CW laser driving and cavity loss on PMET population dynamics and rate constants for an uphill reaction with N = 1000. The ET parameters are the same as those used in Fig. and λ D = 0. The laser pumping strength was set to V l = 10 meV and the laser frequency was tuned to hω l = hω c + √ Ng c which is the energy difference between the resonant |+⟩ state and |G, 0⟩ in the FC region. Isolated from other dynamics, this CW laser will coherently drive population between |G, 0⟩ and the UP due to the photonic character of the UP. Instead of an FC excitation, the reaction inside the cavity is initiated in the |G, 0⟩ state which will coherently pump to the UP state and then experience transfer to the acceptor states. The LP state, while also possessing half photonic character, is significantly detuned from hω l in this case and does not experience any appreciable pumping from |G, 0⟩. Fig. and Fig. show the propagated and fitted acceptor population dynamics in the presence of CW laser driving to the UP for varying cavity loss rates from Q = 50 to Q = ∞ and for collective coupling strengths √ Ng c = 300 meV and √ Ng c = 150 meV, respectively. As Q is decreased in Fig. , the propagated acceptor population (solid lines) accumulates slower yet still approaches ρ {A} = 1 during the 50 ps of propagation. This stands in sharp contrast to Fig. where lower Q factors significantly reduced the plateau acceptor populations. This difference is due to the CW laser in Fig. which allows for population that was trapped in |G, 0⟩ due to cavity loss to be repopulated into the UP and continue transfer to the acceptor states. Fig. shows a similar dependence on Q where the plateau acceptor populations are much larger than those in Fig. for finite Q.