To develop a high-fidelity full-dimensional PES, the first key is to choose a suitable electronic structure method, which should be sufficiently accurate, time-consuming affordably, and provide reliable descriptions for all dynamically relevant regions, including reactants, products, transition states and intermediates, and regions far from the equilibrium of these stationary points. The UCCSD(T)-F12a/AVTZ method has succeeded in developing a series of PESs for reactive and non-reactive systems, and was suggested for effective construction of full-dimensional accurate PESs. The energies, geometries and harmonic frequencies of the stationary points along R1 and R2 were determined at the level of UCCSD(T)-F12a/AVTZ as implemented in the Molpro program package. The energies and harmonic frequencies are summarized in Table , and the structural parameters are given in Figure . The corresponding reaction profile is illustrated in Figure . Note that all energies are relative the asymptotic reactants, if not specified.