Traditional multireference methods such as the complete active space self consistent field (CASSCF) and multireference configuration interaction (MRCI) have also been tested on molecular cluster models of the NV -center. However, finite-size models do not include bulk long-range effects, and they may introduce spurious boundary effects. Furthermore, defect excitation energies, especially those involving higher energy states, are highly sensitive to the shape and size of the cluster. On the other hand, for supercell models based on periodic boundary conditions, that are generally more appropriate to describe such solid state systems, direct application of multireference methods is often unfeasible due to their high computational cost. As a middle ground, quantum embedding theories offer an attractive solution, providing a trade-off between computational cost and accuracy.