To obtain accurate reaction rates with multidimensional tunneling corrections, it is crucial to correctly map the potential energy surface of the reaction. Hence, to select a reliable level of theory in terms of balance between accuracy and computational cost, we performed several DFT benchmark calculations on the activation energy (ΔE ‡ ) for the forward and reverse reaction for the two-water bridge tautomerization in 1H,H and 1H,OH using CCSD(T) with the cc-pVQZ-F12 basis set. (It has recently been shown that this basis set, while strictly speaking developed for explicitly correlated calculations, is also advantageous for orbital-only correlated wavefunction calculations.) In total, we assessed twelve different DFT functionals, namely, the GGA PBE; the meta-GGA TPSS; the three hybrid-GGAs PBE0, TPPSh and B3LYP, with and without D3 correction; five hybrid-meta-GGAs -B1B95, PW6B95, BMK with and without D3 correction, MN15 and M06-2X-, and finally the long-range corrected hybrids wB97X and wB97X-D, all of these in conjunction with four triple-z quality basis sets (Def2-TZVP, 6-311+G(d,p), aug-cc-pVTZ and cc-pVTZ). From the benchmark analysis, the BMK/Def2-TZVP combination emerged as the best performer, with a calculated barrier height ΔE ‡ within 1.0 kcal mol-1 of the CCSD(T)/cc-pVQZ-F12 reference (see Table and S2 in the Supporting Information). Therefore, all molecular geometries, electronic energies, and their derivatives were computed at the BMK / Def2-TZVP level using the Gaussian 16 program system. The CCSD(T) computations were performed using the MOLPRO2021 package. Semiclassical (without tunneling) reaction rates were computed using canonical variational transition state theory (CVT), with tunneling contributions added within the small curvature tunneling (SCT) approximation. In this study, we will refer to the CVT and SCT rate constants as kSC and kT. Polyrate17 was used to compute all the rate constants, with Gaussrate17 serving as the interface between Polyrate and Gaussian 16. A step size of 0.002 Bohr (with a scaling mass of 1 amu) was employed to map the reaction energy paths for all the reactions. For hydrogen tunneling reactions with extremely narrow barrier width, the tunneling path may deviate far from the minimum energy path potential (VMEP), following a straight line from reactant to product. In such scenarios, it is recommended to use the more accurate and computationally expensive large-curvature tunneling (LCT) method, which follows a straight-line tunneling path, or a microcanonical optimized multidimensional tunneling (μOMT) approximation, which estimates the optimal transmission probability as the greater of the SCT and LCT transmission probabilities at a given energy. Our benchmark analysis of SCT shows rates that are similar to those obtained using μOMT and slightly larger than LCT, indicating that the reaction path curvatures were not very large and thus safely confirming the suitability of our cost-effective SCT computations for our studied systems (see Supporting Information).