Chemical reactions mediated by localized surface plasmons have been identified as a viable pathway for efficiently converting solar energy to chemical energy. The excitation and decay of plasmons have found applications in catalysis via different mechanisms. Especially, recent reports demonstrated that the hot charge and energy carriers generated from plasmon decay in nanoparticles could transfer to attached molecules and drive photochemistry, which was previously thought to be impossible. In recent work, we have computationally explored the atomic-scale mechanism of a plasmonic hot-carrier-mediated chemical process, H2 dissociation. Numerical simulations demonstrate that, after photoexcitation, hot carriers transfer to the antibonding state of the H2 molecule from the nanoparticle, resulting in a repulsive-potential-energy surface and H2 dissociation. This process occurs favorably when the molecule is close to a single nanoparticle. However, if the molecule is located at the center of the spatial gap in a plasmonic dimer, dissociation is suppressed due to sequential charge transfer, which efficiently eliminates occupation in the antibonding state and, in turn, reduces dissociation. An asymmetric displacement of the molecule in the gap breaks the symmetry and restores dissociation when the additional charge transfer becomes significantly suppressed. Thus, these models demonstrate the possibility of structurally tunable photochemistry via plasmonic hot carriers.