In previous work, 26 core orbitals were correlated, which was not the case in the calculations presented above. In Fig. , the effect of the frozen core approximation on the performance of pairselected multi-level DLPNO-CCSD(T 0 ) with a target accuracy of TIGHTPNO and TIGHTPNO DLPNO-CCSD(T 0 ) is illustrated for the proton transfer reactions. With respect to TIGHTPNO DLPNO-CCSD(T 0 ) calculated without frozen-core approximation, its frozen-core variant saves on average 16% of the computation time alongside an MAD of 0.48 kJ • mol -1 for reaction energies and 0.63 kJ • mol -1 for barrier heights. However, including the core orbitals into the correlation treatment of multi-level DLPNO-CCSD(T 0 ) results in an MAD of only 0.13 kJ • mol -1 for reaction energies and 0.23 kJ • mol -1 for barrier heights while even 30% of the computation time can be saved on average. In contrast to this, the MAD increases to up to 0.55 kJ • mol -1 for reaction energies and 0.80 kJ • mol -1 for barrier heights using the frozen-core approximation while the mean relative time decreases by only 5%. Due to the prescreening of relative pair correlation energies, only those orbital pairs including at least one core orbital are treated at a high accuracy which show significant contribution. Consequently, the inclusion of core orbitals for pair-selected multi-level DLPNO-CCSD(T 0 ) only requires a small amount of additional time.