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The study of isolated defects in solids is a natural target for classical orquantum embedding methods that treat the defect at a high level of theory andthe rest of the solid at a lower level of theory. Here, in the context ofactive-space-based quantum embeddings, we study the performance of threeactive-space orbital selection schemes based on canonical (energy-ordered)orbitals, local orbitals defined in the spirit of density matrix embeddingtheory, and approximate natural transition orbitals. Using equation-of-motioncoupled-cluster theory with single and double excitations (CCSD), we applythese active space selection schemes to the calculation of the vertical singletexcitation energy of a substitutional carbon dimer defect in hexagonal boronnitride, an oxygen vacancy in magnesium oxide, and a carbon vacancy in diamond.Especially when used in combination with a simple composite correction, we findthat the best performing schemes can predict the excitation energy to about0.1-0.2 eV of its converged value using only a few hundred orbitals, even whenthe full supercell has thousands of orbitals, which amounts tomany-orders-of-magnitude computational savings when using correlated electronicstructure theories. When compared to assigned experimental spectra andaccounting for vibrational corrections, we find that CCSD predicts excitationenergies that are accurate to about 0.1-0.3 eV, which is comparable to itsperformance in molecules and bulk solids.

Optical properties of defects in solids via quantum embedding with good active space orbitals