For the subset of proton transfer reactions from the BH9 set, the error compared to canonical CCSD(T) was analyzed using the def2-TZVP basis set. The error of pair-selected multi-level DLPNO-CCSD(T 0 ) was found to be dominated by the errors already introduced through DLPNO-CCSD(T 0 ). Moreover, the influence of core orbitals and the effect of different basis sets as well as basis set extrapolation was discussed for this subset. The pair-selected multi-level DLPNO-CCSD(T 0 ) method can include core orbitals in a computationally cheap way such that it requires less computation time for the proton transfer reactions than frozen-core DLPNO-CCSD(T 0 ) while being more accurate (Fig. ). The application of basis set extrapolation techniques is demonstrated for a two-point extrapolation of def2-TZVP and def2-QZVP, cc-pVTZ and cc-pVQZ as well as for aug-cc-pVTZ and aug-cc-pVQZ. In this comparison, the MADs with respect to TIGHTPNO DLPNO-CCSD(T 0 ) are the smallest for the use of correlation-consistent basis sets without diffuse basis functions. In case of the multi-level calculations, the comparison of MADs of the extrapo-lated relative energies and the MADs of the individual basis sets indicates that the errors which occur for basis sets of different sizes may not be systematic. Here, explicitly correlated local coupled-cluster would be a promising alternative strategy that avoids basis set extrapolation techniques. The inclusion of diffuse basis functions to the quadruple-ζ basis set aug-cc-pVQZ leads to drastically deteriorated results for multi-level DLPNO-CCSD(T 0 ) as well as NORMALPNO DLPNO-CCSD(T 0 ). Here, the error is primarily due to the error of the triples correction (Tab. 5)