The explicitly correlated CCSD(T), CCSD(T)-F12, converges faster with respect to the size of the basis set than the conventional CCSD(T) calculations. CCSD(T)-F12/AVTZ has been widely used to obtain accurate electronic energies of numerous configurations for constructing PESs of reactive and non-reactive systems due to its efficiency and accuracy. However, many issues may encounter, as dynamical simulations often reach regions where different electronic states have small gaps in energy. For these multi-reference regions, the CCSD(T) and the HF calculation are very likely to converge to a wrong electronic state or the HF calculation can't successfully converge. If an incorrect high-state HF solution was obtained, the calculated properties are not reliable. Many useful options have been suggested and tested. The Molpro manual provides some useful solutions, such as using a small but reasonable basis set (VDZ, def2-SVP, AVDZ, etc.) firstly, adding a level shift in the HF calculation step, using initial guesses from its corresponding close-shell cation or anion, using initial guesses from DFT calculations with a small basis set, using initial guessed from its neighboring configuration, etc. If all fail, the robust MCSCF method can be launched. Dawes and Ndengué have discussed various options and gave some suggestions in their review. Vaucher and Reiher proposed a fast and automated approach to detect and cure incorrect orbital convergence by the randomized orbital perturbation method. The reaction OH + HO2 → H2O + O2 is of multi-reference in nature. Using initial guesses of the neutral, anion and cation species as the spin-restricted Hartree-Fock calculations, the following CCSD(T)-F12a/AVTZ calculations always predict some strange points of high energies along the MEP of this reaction, as shown in Figure (b). To make the PES smooth and reliable, for each configuration, three individual HF calculations were calculated with initial guesses of cationic, neutral, and anionic species, respectively. The lowest HF solution for the neutral species was then employed in the CCSD(T)-F12a calculation. As shown in Figure (b), the potential along the MEP is smooth and the resulting exothermicity is also in good agreement with experiment. For calculating ample points of reactive PESs, Gyori and Czakó recently reported a pragmatic method, called ManyHF, which can automatically get better, lower energy HF solutions by carrying out an empirically determined set of preliminary SCF calculations to generate multiple initial guesses.