For more accurate electronic excitation descriptions, particularly in materials and extended molecular systems, the GW approximation and Bethe-Salpeter Equation (BSE) provide powerful alternatives. The GW method improves quasiparticle energy predictions by Describing the electronic structure may require more involved methods when dealing with complex situations, such as dissociation, bond formation, multielectronic excited states, and conical intersections. Complete active space perturbation theory to the second order (CASPT2) is one of the primary choices in such cases. It combines complex wave function initial guesses, advanced coefficient optimizations, and perturbative corrections to recover electron correlations. It's used in Section 3.8.3.