Photochemical reactions, which involve both the ground and excited electronic states of a molecule, can promote processes otherwise inaccessible by normal reactions. In general, photochemical reactions may be classified as adiabatic or nonadiabatic depending on whether the reaction takes place on the same adiabatic potential energy surface or not. From research over the last two decades, we now understand that many processes in nature turn out to be nonadiabatic { including charge transfer, electronic excitation quenching, and spin-forbidden transitions. The efficiency of such processes depends critically on the electron-nuclear interaction, which is quantified by the derivative coupling between the two involved states. The first part of the work (chapters 3-6) presented here mainly focuses on understanding the electron-nuclear interaction using the electronic structure theory. Two approaches are developed calculating the derivative couplings between the excited states within the time-dependent density functional theory. The behavior of the derivative couplings around a conical intersection is analyzed for two real molecules: benzaldehyde and protonated formaldamine. The second part of this work (chapters 7-8) focuses on understanding the electron-electron interaction in the framework of Green's function. Detailed working equations are derived for the GW approximation, which is used to calculate the electron attachment/detachment energy, and the Bethe-Salpeter equation, which is used to obtain the electron excitation energies of a system.