However, due to the steep scaling of the computational effort with the system size, the application of canonical CCSD(T) is restricted to relatively small systems. This problem was overcome to a certain extent through developments in the field of local correlation approaches by making efficiently use of the locality of dynamic electron correlation. In this context, Pulay and Saebø introduced the idea of representing the virtual orbital space with local domains of non-orthogonal projected atomic orbitals (PAOs) and approximating orbital-pair contributions that are expected to be insignificant, e.g., due to a sufficient separation of the two orbitals. These two approximations are generally referred to as domain approximation and pair approximation, respectively. Further developments in this area led to a variety of approaches, some of which are based on pair natural orbitals (PNOs) such as the widely used domain-based local pair natural orbital (DLPNO) approach by Neese and co-workers. For the latter, the accuracy is mainly determined by two truncation thresholds: one is a cutoff for the PNO eigenvalues after the diagonalization of the pair density and controls the size of the PNO domains for each orbital pair; and one controls the number of orbital pairs included in the CCSD iterations according to semi-canonical MP2 (SC-MP2) pair correlation energies. Despite the linear scaling of DLPNO-CCSD(T), an accurate prediction of relative correlation energies remains computationally expensive for large molecular systems. In those cases, the locality of chemical processes may be exploited through multi-level approaches which are able to concentrate on the part of the molecule that is directly involved into the process. For chemical reactions, the redistribution of electrons is confined to the reaction center, by definition, whereas for intermolecular interaction energies, the contact region between the interacting molecules is of par-ticular importance. Only the region of interest may be described with the local CCSD(T) method whilst the other part can be approximated using local coupled-cluster with less accurate settings, local Møller-Plesset perturbation theory (MP2) or density-functional theory (DFT) as in the context of projection-based embedding. Since these methods often rely on a manual selection of atoms, targeting a definitive accuracy seems impossible in those cases. While the atoms involved in a chemical process can often be identified based on chemical intuition, important contributions of orbitals seem less intuitive, but it is straightforward to analyze them. In previous work from our lab, the direct orbital selection (DOS) scheme was introduced for partitioning orbitals automatically within projection-based embedding along reaction pathways.