For both molecular CASSCF/NEVPT2 and periodic CAS-DMET/NEVPT2-DMET calculations an active space containing 10 electrons in 9 orbitals (10e, 9o) was employed. This includes symmetry-adapted orbitals that are consistently obtainable across all the different supercell sizes as well as for the finite molecular cluster model. The detailed analysis of the active space convergence is presented in the SI. Further, the triplet excitation involves the non-degenerate ground state triplet 3 A 2 and the doubly degenerate 3 E state, whereas the optically inactive singlet transition takes place from the doubly degenerate 1 E to the non-degenerate 1 A 1 state. We therefore considered two separate state-average CASSCF (SA-CASSCF) calculations, one on the three lowest-energy triplets, and one on the three lowest-energy singlets. In both calculations equal weight in the state average was attributed to all states. The triplet-singlet energy gaps ( 3 A 2 → 1 E and 3 A 2 → 1 A 1 ) were computed from the difference between the state-averaged singlet and the state-averaged triplet calculations.