The most widely employed reference results for reaction barriers are collected in the DBH24 compilation containing results mostly obtained at the CCSDTQ5/CBS level by means of the W4 composite method 73 for a statistically representative set including 3 prototypes for each of the following classes of reactions: heavy atom transfer, nucleophilic substitution, unimolecular and association reactions, and hydrogen-transfer reactions. variants of the CCSD-F12 method provide comparable results, so that only F12a values will be discussed in detail in the following. As shown in Table , the CCSD(T)-F12/j3Z