The first is the most applied and utilitarian definition that follows the general premise: a system has a double d -shell effect if it requires two sets of d -orbitals in the active space to be correctly modeled. Such definition has a strong resemblance to Fischer's work, where relative energies with good agreement with experimental values can be obtained by writting the atomic states wave functions as a linear combination of 3d to 4d excitations. Extending this idea to multireference methods, it has been shown that for a better accuracy these 3d to 4d excitations are also necessary in the reference wave function. A complete explanation on the active space choice based on the ligand interaction with the metal orbitals can be found in Chapter III.B of reference. A large drawback of this approach is that the minimum active space required for transition metal containing compounds must include at least 10 active orbitals, named by Pierloot in reference as the "basic ten". In our point of view, however, this definition for the double d -shell effect has two problems. First, following this definition, systems with the double d -shell effect would always need a reference wave function based on large active spaces. Although such definition implies in a multireference character for the system, there are examples of systems that suffer from the double d -shell effect that can be better described by the canonical CCSD(T) method than by a single d -shell active space. That way, a single-reference method can recover the double d -shell effect. Second, it suggests a single specific solution to account for this effect, without providing an explanation for its existence, what limits the development of alternative approaches.