Orbital pairs that do not fulfill Eq. ( ) for the highest τ ∆ but fulfill this prescreening condition for the second highest τ ∆ are consequently described with the settings of the second level and so on. (aug)-cc-pVQZ] were used. The def2-TZVP basis set is used for all test systems given in Fig. , whereas def2-QZVP, (aug)-cc-pVTZ, and (aug)-cc-pVQZ are also applied to proton transfer reactions from the BH9 set to analyze basis set effects and to employ basis set extrapolation techniques.