Tables I,II, and III summarize relevant energy gaps computed with EOM-SF-CCSD/cc-pVXZ for the diradicals and triradicals used for benchmark purposes in this work. The structures of the molecules are shown in Figures and. Detailed discussion of their underlying electronic structure can be found in Refs. , and . Gaps in copper diradicas are listed in Table . For methylene series, we consider the manifold of all 4 diradical states (2 closed-shell singlets, open-shell singlet and triplet), whereas for the rest of the systems we focus on the two lowest states and consider only singlet-triplet and doublet-quartet gaps. As the first example, we consider the methylene diradical. Direct application of the original FNO scheme to the CCSD calculation of the high-spin triplet state shows errors of a similar magnitude as in the closed-shell FNO calculations , as illustrated in Figure in SI. However, errors in the EOM-SF-CCSD energies are considerably larger, as shown in Figure . Already at a small truncation of the total population (a few percent), the errors reach a magnitude of 1 electron-volt. Such a rapid growth of the errors can be explained in terms of contributions of the physically important orbitals responsible for strong correlation (i.e., those from the singly occupied subspace) to the frozen virtual orbital space. The singlet-triplet gap between the two lowest EOM states also shows large errors (Figure ).