Tests for CH3OH + OH, C2H6 + F, and CH3NH2 + Cl showed that their method can make the following CCSD(T) energies reliably. The electronic structure changes significantly along the OH + SO → H + SO2 reaction. The explicitly correlated multi-reference configuration interaction (MRCI-F12) with the basis sets AVDZ and AVTZ revealed that for the entrance channel, the ground electronic state corresponds to OH( 2 Π) and SO( 3 Σ -) with three singly occupied orbitals, while the remaining regions corresponds to a doublet state with one singly occupied orbital. Thus, the UCCSD(T)-F12a/AVTZ calculations for a doublet state correspond to OH( 2 Π) + SO(a 1 ∆), 22.7 kcal mol -1 higher than the ground electronic state OH(X 2 Π) + SO(X 3 Σ -), which should be well described by UCCSD(T)-F12a/AVTZ for a quartet state. The correlation is depicted in Figure . Practically, for those points with the distance RHO…SO larger than 2.0 Å, UCCSD(T)-F12a/AVTZ calculation is done for a quartet and doublet state, respectively. Then the lower energy is taken as its true ground-state electronic energy. One can see that clearly, a smooth and correct potential curve is obtained.