distance. The double proton transfer occurs in two single proton transfer stages, initiated by a proton donation from thymine to adenine. This first stage concerns the transfer of the proton in bond B2 from the nitrogen of thymine in B2 to the nitrogen of adenine in B2 and has a higher energy barrier across all increments of the strand separation. Initially, there is no response of a second proton transfer for the first two increments of strand separation and the single proton transfer is all that occurs. Since there is no second proton transfer up until 0.444 Å, the first two data points for both 'B1 Forward' and 'B1 Reverse' are 0 eV. However, the energy barriers of the second proton transfer grow in response to the gradual creation of a stable minimum for the double proton transfer tautomeric state. Since this second proton transfer is instigated by the first, it has a lower energy barrier and grows at a much slower rate across the strand separation compared to the steep linear increase of the initial single proton transfer.