However, for core excitations using the CVS scheme, the scaling is O(N 6 ) in an optimized implementation. Nevertheless, the ground-state calculation is rather expensive, even for systems where the CCSD calculation is routine. The costs can be significantly reduced using the multilevel coupled-cluster approach (MLCC) in which the higher-order excitations in the cluster operator are restricted to an active orbital space. In the MLCC3 model, the triple excitation operator is restricted, while the single and double excitation operators are unrestricted and act in the entire orbital space. The MLCC models target intensive properties, such as excitation or ionization energies. By restricting the entire cluster operator to the active space, one obtains the CC-in-HF models in which the inactive orbitals are not correlated but contribute to the energy via the Fock matrix. This approach, which can be described as a type of electronic embedding, has been demonstrated to work well for modeling solvent effects in spectroscopy. The multilevel models rely on a physically appropriate selection of the active orbital space.