For R1 channel, as shown in Figure , OH and CH3OH first form a reaction complex (RC), which is -6.53 kcal/mol in energy relative to the asymptotic reactants. This RC complex is stabilized by the hydrogen bond: the distance between the hydrogen atom of OH and the oxygen atom of CH3OH is For the title reaction of multi-reference nature, particularly in the entrance channel, the singlereference methods, including UCCSD(T)-F12a/AVTZ, may fail to give the correct solutions. The multiconfiguration reference internally contracted configuration interaction (MRCI), the optimal choice, however, is extremely expensive for the title reaction. Besides, CCSD(T) outperforms MRCI for stationary points generally. In the PES construction of the OH + HO2 system, we found that the wavefunction with the lowest Hartree-Fock (HF) energy from three different initial HF guesses, neutral, anion, and cation species, can make the consequent UCCSD(T)-F12a results smooth and correct. Tests show this method also works well for the title reaction, as shown in Figure .