The latter effect, which arises due to the coupling of the angular momentum of an electron and its spin, is of special importance. Although small in magnitude for molecules composed of light atoms, spin-orbit coupling (SOC) leads to the mixing of otherwise non-interacting states, e.g., singlets and triplets, and splits electronic degeneracies, e.g., between degenerate Π or ∆ states or between different M s components of spin multiplets. Consequently, SOCs open new reaction channels (via inter-system crossing, ISC) and change the spectroscopic behavior by lighting up dark states via intensity borrowing. SOCs lead to noticeable changes in the wave functions and electronic properties, and are particularly important in open-shell systems.