Firstly, we determine the quantum and classical rates for reaction 1 using our open quantum systems approach. Reaction 1 has a prohibitively high and wide reaction barrier (see Fig. ) resulting in a low classical and quantum reaction rate. We evaluate that the quantum-to-classical ratio is small, κ = 1.48, suggesting that tunnelling is negligible. We find that the overall reaction rate is 1.907 × 10 -3 s -1 and is therefore many orders of magnitude smaller than the dNTP unbinding rate, which is of the order 70 000 s -1 . 2 Furthermore, we determine the effect of isotopic substitutions on the reaction rate and found that the reaction rate is essentially unaffected by deuterium substitution (KIE=1.1). Due to the slow reaction rate, the dNTPs unbinding rate and subsequent base rejection compete with the proton transfer mechanism. As a result, statistically, some of the wobble mismatches will eventually diffuse from the polymerase's active site before proton transfer occurs. Since the diffusion timescale competes with the proton transfer timescale, the final population of tautomers incorporated will be reduced as accounted for by the kinetic network in Ref. If one applies classical transition state theory to the PES described in figure , the tunnelling rate would be negligible because the barrier width would be on the order of 4.0 Å. Thus, the overall (classical) reaction rate would be 1.907 × 10 -3 s -1 , much slower than experimentally measured (0.6-68 s -1 2 ). However, we note that the PES in Fig. describes three fundamentally different molecular motions, and only the inner barrier (section 2 in Fig. ) corresponds to the proton transfer between the bases. In contrast, regions 1 and 3 correspond to overall translations of the bases without significant changes in the hydrogen bond length. This PES topology is compatible with a tunnelling-ready state composed of the reactant's activated structure seen at the end of region 1. The activation process concerns the reorganisation of the heavy atoms where thermal energy is required for the reactants to reach an activated tunnelling-ready state, whereby the reactant and product states become similar in energy.