The electronic energies of stepwise double proton transfer (DPT), concerted DPT, and single proton transfer (SPT) in GC calculated using the B3LYP+XDM/aug-cc-pvdz/AMBER QM/MM method. The electronic energy for the first and second transition states (TS1 and TS2) and the reaction energy for each process are given in kcal mol⁻¹ relative to the canonical GC base pair. The mean energies are calculated from the QM/MM ensemble and σ is the standard deviation (where σ = ‘—’, the sample size consists of a single replica). The electric field strengths are given in V m⁻¹ and applied in the +x-direction. In the case of no field, as denoted by ‘0’, are those reported in earlier work⁶ and were calculated using the same QM/MM method.

Electric field (V m−1)	Stepwise DPT						Concerted DPT				SPT
	TS1		TS2		Reaction		TS1		Reaction		TS1		Reaction
	Mean	σ	Mean	σ	Mean	σ	Mean	σ	Mean	σ	Mean	σ	Mean	σ
0.00	14.26	1.19	15.33	1.29	13.76	1.11	16.26	2.13	12.06	0.58	11.47	—	9.18	—
1.00 × 104	14.33	1.22	15.40	1.31	13.76	1.11	16.45	2.38	12.05	0.59	11.45	—	9.17	—
1.00 × 107	14.12	1.19	15.21	1.42	13.39	1.92	16.34	2.08	12.05	0.59	11.26	—	9.13	—