Photomagnetic switches form a class of compounds that upon irradiation drastically change their magnetic properties. Although these compounds incite large interest in the materials field and several technological applications are foreseen, 2,3 computational simulations of these compounds lag behind. Only a hand full of data regarding the electronic structure of these systems can be found in the literature. Up to our knowledge, besides some experimental/theoretical hybrid works that use density functional theory, there are only three works that employ theoretical methods considered highly-accurate, conducted by Graaf et al. using a model system of the magnetic photoswitch synthesized by Herrera et al. One of the limitations when describing such series of compounds comes from the large system size. This is particularly the case when wave function based methods are necessary, due to their unfavorable scaling with the number of electrons and basis set. However, these methods are often necessary to model the mechanisms where magnetic photoswitching processes take place. They can occur via two main routes: a spin-flipping transition (also known as Light-Induced Excited Spin-State Trapping, LIESST ) and/or a metal-metal charge transfer transition (MMCT). These transition types are known to be difficult to calculate, either using density functional theory or wave function methods. However, spin-flipping processes have received substantially more attention than charge transfer processes, due to their relevance in many controversies regarding the ground state multiplicity of metal containing compounds (e.g. for the triplet/quintet states of iron-porphyrin ). As both mechanisms, LIESST and MMCT, are relevant for some photomagnetic switching properties, they need to be modelled with the same accuracy if a reliable picture of the excited electronic states is desired. Both transitions are exceptionally expensive to model due to the large number of low-lying energy states and orbitals. Moreover, they suffer, as all first-row transition metal containing compounds, from the double d -shell effect (or 3d double-shell effect), that plagues multireference wave function based methods. This effect was first described by Andersson and Roos in 1992, with the term coined in 1995. To include this effect a double d -shell active space approach was developed to describe transition metal containing compounds based in two major works, from Fischer in 1977, and Botch et al. in 1981. Influenced by these works, two distinct, but related, definitions for the double d -shell effect can be formulated.