In the second half of this review, we discussed theoretical and computational applications that use the approaches outlined in the previous sections. Experiments have shown that, by tuning the cavity photon frequency and light-matter coupling between the quantized cavity photons and electronic transitions, photochemical reactions can be controlled inside an optical cavity. In Sec. 4, we revealed how through excited state nonadiabatic polariton dynamics simulations, theorists, inspired by experiments, have discovered new ways of modifying and enabling photochemical reactivity by exploiting quantum light-matter interactions. In Sec. 4.1, we briefly outlined some of the quantum dynamics approaches used for simulating polariton dynamics exactly and approximately. Then, in Sec. 4.2, we introduced intuitive schemes and possible approaches for modifying and manipulating photochemistry with the readily available theoretical tools from cavity QED. Following this, in Sec. 4.3, we showed how ab initio on-the-fly simulations can validate these schemes toward modifying photochemical reactivity in real molecular systems, thereby revealing previously unknown basic principles of how polaritons can be used to manipulate excited state features and dynamical properties. Further, in Sec. 4.4 we show that the same ideas can be applied in the modification of photo-induced charge transfer reactions. We then reviewed works in Sec. 4.5 that demonstrate the possibility of introducing new conical intersections through light-matter interactions and their impacts on excited state processes. In Sec. 4.6, we showed that the choice of the initially prepared quantum state of the cavity photon can also be used to directly control photochemistry. We concluded this section with a discussion in Sec. 4.7 on the important role of cavity loss in these excited state processes and illustrated how the non-ideal nature of real experiments (e.g., partially transparent mirrors) can inhibit or enhance cavity control of photochemistry.