To demonstrate that with the modifications of the torsional barriers, a correction potential obtained by fitting at least a 7 th order polynomial to the ∂V /∂λ λ of reference simula-Figure : Titration of cardiotoxin V. Panels A and B show the results of titrations of 1CVO 1 system performed with original and modified force fields, respectively. Gray lines show the fitted Henderson-Hasselbalch curves to the data. Red lines are Henderson-Hasselbalch curves computed for the experimental pK a value of the corresponding residue. For ASP59 the exact pK a is not known. Black dots show the deprotonation ratio for the individual replica. For each curve, the standard deviation between the calculated and fitted deprotonation ratio, averaged over all pH values and replicas, is shown. tions, and buffer particles with optimized parameters, it is possible to perform accurate constant pH MD simulations, we calculated the pK a values of all four cardiotoxin V titratable residues. We performed the pH titration simulations with both the original and modified CHARMM36m force fields. In Figure we show the titration curves obtained in the simulations and compare them to the experiment. Because there is no exact experimental estimate for the pK a of ASP59, we only compare the pK a 's obtained for the other residues.