As shown, the external correlation energy, EMRCI+Q -EDW-CAS, is adjusted by the constant α. No external correlation energy corresponds to α = 0 and full MRCI+Qrot correlation energy corresponds to α = 1. For α > 1, additional correlation energy can be recovered, which enlarged the active space to some extent. The value of α was then empirically determined by some known experimental and benchmark theoretical values. For F + H2O, α was determined to be 1.135 by reproducing the FPA barrier height. As shown in Figure , the potential energies along the minimum energy path at the 2-state-MRCI+Qrot/AVTZ (labeled as ab initio) were smoothly scaled and the scaled ab initio and the scaled PES are in good agreement. Overall, the MRCI calculations are expensive, limited to small-size systems, and not as accurate as the gold standard CCSD(T). Even challenging, it is difficult to select a suitable active space that is appropriate for all dynamical regions.