Multi-reference methods are generally much more expensive than the single-reference method, CCSD(T). Even worse, the predicted results of the stationary points by the multi-reference methods are generally less accurate than those calculated by the single-reference methods. As discussed above for the F + H2O reaction, the 2-state-MRCI+Qrot/AVTZ calculation predicted a barrier height of 3.83 kcal/mol, while the UCCSD(T)-F12b/AVTZ result was 1.93 kcal/mol, Unlike the CCSD(T) method, MRCI even with Qrot does not have the size consistency and extensivity properties and it is prohibitively expensive to capture high-order correlations within the multireference realm. The simple and efficient external scaling strategy, proposed by Brown and