Figure : The mean absolute deviations (MADs) of pair-selected multi-level DLPNO-CCSD(T 0 ) and DLPNO-CCSD(T 0 ) with and without frozen-core approximation are plotted against mean relative timings t/t TIGHT for the (a) reaction energies and (b) barrier heights of the proton transfer reactions from BH9 with respect to TIGHTPNO DLPNO-CCSD(T 0 ) without frozen core. The error bars illustrate standard deviations.