Supercell CAS-DMET NEVPT2-DMET CAS-DMET NEVPT2-DMET To get a meaningful understanding in the context of solid-state systems, calculations must either be performed in or extrapolated to the thermodynamic limit (TDL), which is, for instance, the limit of an infinite number of atoms. There is, therefore, a critical need to get an insight into how the DMET excitation energies approach the thermodynamic limit with increasing system size. In our case, the 1 A 1g → 1 T 1u , 1 A 1g → 3 T 1u and E em DM ET defect localized transition energies decrease monotonically with supercell size and thus a linear extrapolation to the TDL is conveniently possible. The corresponding TDL plots for the first two transitions are shown in Figure (a). The obtained values are summarized in Table . The NEVPT2-DMET value of 5.24 eV for the defect-localized singlet excitation energy is within 0.2-0.3 eV of the experimental value. The corresponding CAS-DMET value is 6.26