First, we demonstrate that a lack of sampling of the relevant dihedral degrees of freedom in amino acid side chains with the CHARMM36m force field reduces the accuracy of the correction potentials for λ-dynamics. To overcome these convergence problems, we modify the force field by reducing the barriers in the torsion potential and show that this significantly improves the accuracy of the correction potentials and hence the results of constant pH simulations, including pK a estimates, without affecting the protein conformational dynamics.