Whereas for Asp and Glu the individual titration replicas are consistent when the modified force field is used, the replicas for HIS4 are not converged. As shown in Figure , HIS4 interacts with TYR12 and PHE10. At pH=5.5, close to the experimental pK a of HIS4, we find three dominant conformations of HIS4-TYR12 pair in both normal and constant pH MD trajectories: A close contact (around 5 Å), a medium-range contact (around 6 Å), and a long-range contact (larger than 7 Å, Figure ). The distributions of these distances are not the same in all replicas (Figure ), and neither are the distributions of the λ-coordinates associated with the doubly-protonated state of HIS4 (Figure ). These differences suggest a lack of convergence of the conformational dynamics of the protein in constant pH MD. To test if the protonation state of HIS-4 correlates with the distance distribution, we performed standard MD of cardiotoxin V with HIS-4 in the three different protonation states (Figure . S16). Because there is no clear correlation between the protonation state and the HIS4-TYR12 distance distributions, we cannot conclude that the protonation states are coupled to the conformation of the pair, at least not directly. Instead, the differences between the replicas suggest a lack of sampling of these conformations. Because the local environment differs between the states, we speculate that this lack of conformational sampling also affects the λ-distributions. Thus, we can only conclude that the sampling of these distances would require more than 100 ns to converge. Thus, even if the corrections to the torsion potentials overcome the convergence issues associated with sampling the intrinsic dihedral degrees of freedom in single amino acids, reaching converged sampling of the protonation states in proteins may still require longer timescales if the inherent conformational dynamics is too slow.