In the following, the atom maps for the closed-shell BH9 reactions are discussed to briefly evaluate the proposed atom mapping algorithm. We assume that for a large number of formed and broken bonds, the atom map is more likely to be erroneous. This seems to be the case for reaction 117 from the subset of pericyclic reactions as there are four formed bonds and two broken bonds for the atom map between transition state and product. However, by visual inspection, it can be shown that the large number of formed and broken bonds is due to a mistake in the product structure containing an acetate ester in place of a methyl ester as for the reactant and transition state, as visualized in the Supporting Information. Reaction 117 is therefore excluded from the analysis below. In conclusion, the algorithm may also be suitable as an analysis tool for the verification of geometries since the construction of large benchmark sets such as the BH9 set is inevitably prone to human error. For all 233 reactions, i.e. 466 atom maps, the main criterion of finding the minimum number of formed and broken bonds is fulfilled within the given bond definition [Eq. ( )], provable by visual inspection. On the other hand, the fulfillment of the criterion for dihedral angles can neither be visually examined nor be proven in practice for most of the cases as the number of solutions with the minimum number of formed and broken bonds may become very large when, e.g., many methyl and methylene groups are involved. Yet, it can be visually confirmed whether the atom map is meaningful with respect to the orientation of certain atoms. E.g., for reaction 140 from the subset of pericyclic reactions, the hydrogen atoms of a methylene group are misassigned between the reactant and transition state in terms of the chirality not being maintained. Often, this is due to bond rotations during the reaction. Such misassignments may lead to errors in the orbital maps which are expected to be compensated by the multi-level prescreening because pair energies of orbital pairs with at least one misassigned orbital are more likely to exceed a certain pair selection threshold. Consequently, the multi-level calculation is expected to be computationally less efficient, but a high accuracy can still be ensured. For reaction 140, the barrier height calculated with pair-selected multi-level DLPNO-CCSD(T 0 ) and a target accuracy of TIGHTPNO has an error of only 0.12 kJ • mol -1 but requires 94% of the time (for the calculation of the reaction energy and barrier height) with respect to TIGHTPNO DLPNO-CCSD(T 0 ). To preserve chirality despite any bond rotations, an alternative criterion is required to compare all those pairs of atoms that are indistinguishable based on connectivity alone and bonded, e.g., to an aliphatic carbon as for the hydrogen atoms of a methylene group. Instead of dihedral angles, one could use out-of-plane distortions, also referred to as improper dihedral angles in the context of force fields. Table : Mean absolute deviations (MADs) and standard deviations (SDs) in kJ • mol -1 for reaction energies and barrier heights of the closed-shell reactions from the BH9 set with respect to TIGHTPNO DLPNO-CCSD(T 0 ). Pair-selected multi-level DLPNO-CCSD(T 0 ) with TIGHTPNO and NORMALPNO as target accuracies is compared to NORMALPNO DLPNO-CCSD(T 0 ). The mean relative time t/t TIGHT considers the time taken for the calculation of all correlation energies, i.e. for reactant, transition state, and product. the calculation due to the orbital mapping and the multi-level prescreening. The same tendency of computation time is observed for the comparison of NORMALPNO DLPNO-CCSD(T 0 ) with the corresponding multi-level variant such that it is not discussed in more detail.