Let us consider the energy landscape of a reactant and product system. A system refers to the collection of all configurations an ensemble of nuclei can be in, by maintaining the same electron orbital configuration. Every change of the electron orbital configuration will be considered a chemical reaction and is a (adiabatic) transition event (e.g. electron quantum tunneling between different electron orbitals). The surface drawn in Fig. corresponds to a particular configuration 𝑣 ⃗ 𝑖 with minimal energy. States with the same configuration, but with excited vibrational modes, rotational modes, and deformed electron orbitals would exhibit a higher energy leading to the energy landscape to be more accurately characterized as a hypervolume rather than a hypersurface. For convenience, I consider a configuration to mean both the spatial orientation of the nuclei and their rotational and vibrational modes. Obviously, different configurations with different modes may exhibit the same energy 𝜀(𝑞) and would then belong to the same energy state 𝑞. The number of configurations per state is the degeneracy of that state 𝑔(𝑞). A configuration can transition to another configuration with a certain probability. A transition to a configuration of a lower energy state is accompanied by the emission of a photon, while the transition to a configuration of a higher energy level is caused by the adsorption of a photon. In both cases, the photon energy corresponds to the energy difference of that transition.