The additional increase in the rate for a homodimer can be explained by an additional inter-molecular reaction channel, as illustrated in Fig. . When the first molecule is vibrationally excited and tunnels into the right well, the subsequent vibrational relaxation can induce the creation of one photon in the cavity mode, due to the light-matter interaction. Then, the loss of the photon is coupled to the vibrational heating of the second molecule in the reactant region. The reverse process can also occur, where the first molecule in the reaction region is vibrationally heated and the second molecule in the product region is cooled down, through the intermediation of the cavity mode. Overall, the reaction rates of both molecules are increased. This intermolecular reaction channel involves four energy exchange processes between two molecules and the cavity photon mode. In other words, this rate enhancement is quartic to the light-matter interaction and scales like η 4 c (µ 1 (x 1 )⋅ ⃗ e) 2 (µ 2 (x 2 )⋅ ⃗ e) . Therefore, the rate enhancement effect is not obvious when η c is small, as shown in Fig. . But after increasing the light-matter coupling to η c = 0.005 a.u. , we can observe a more distinct rate increase of a homodimer, compared to the single molecule inside the cavity, as shown in Fig. . A further comparison of the reaction rates over a range of η c between a monomer and a homodimer inside the cavity is provided in the SI. It is important to note that, in experiments, a truly macroscopic number of molecules are collectively coupled to multiple cavity modes. As such, numerous intermolecular reaction pathways could exist between any pair of molecules and their commonly coupled cavity mode. This situation could potentially result in a remarkable increase in the reaction rate. Furthermore, the above results also suggest that the rate enhancement induced by this collective effect is quadratic to ⃗ µ(x)⋅ ⃗ e, which indicates that it can survive in an isotropically disordered system.