Figure represents the previous conclusion -the energy barrier between the canonical and tautomeric state grows as the distance between the DNA strands increases. However, while there is no energy minimum at the maximum coordinate on the normalised reaction path axis for the first two separation increments, an energy minimum becomes visible for larger separation distances, representing a stable state for the A*-T* tautomer. Also, in Figure , one can observe the development of a central energy minimum for the third separation increment onward. This energy minimum is created by the intermediate zwitterionic state. We observe that the energetically preferable double proton transfer mechanism is asynchronous (a step-wise transfer mechanism) and consists of a single proton transfer from the thymine base to create the zwitterionic intermediate A + -T -followed by an induced, second single proton transfer from the adenine base. The height of the second energy barrier is much lower than the first (across all strand separations and increasingly so as strand separation increases), showing that it is much more energetically favourable for the zwitterionic product to undergo single proton transfer again to produce the A*-T* tautomeric state, rather than a single proton transfer which reverts to the canonical state.