We analyzed the impact of the size of the DMET embedding subspace on the computed vertical excitation energies (VEE) for three supercells. To do this, we used a 16-atom impurity cluster (N C 15 ) and performed the CAS-DMET/NEVPT2-DMET calculations using different numbers of fragment + bath orbitals. A comprehensive explanation of the procedure in which this was accomplished can be found in the SI. The individual VEEs corresponding to different numbers of embedding orbitals are also reported in the SI.