Also for carboxyl groups in the side chains of Glu, and in the C-terminus, a polynomial fit to ∂V /∂λ λ of at least seventh order is needed to provide a sufficiently accurate correction potential (Fig. , S11, and S14 in SI). For the imidazole ring of His with three coupled titratable sites, a seventh order fit suffices as well (Fig. ), while for the amino bases in the side chain of Lys and the N-terminus, at least an eight order fit is required (Fig. and S15 in SI). We speculate that the higher-order fit is needed for the latter sites due to the larger change in the charge on the central nitrogen atom from -0.3 e to -0.96 e upon deprotonation. The change in the charge of the carboxylic oxygen from 0.55 e to -0.76 e is smaller, as are the changes on the nitrogen atoms of the imidazole ring of His (-0.36 e to -0.7 e).