Additionally, phbuilder allows the user to set up the parameterization of a new titratable group. The results of constant pH simulations depend on the accuracy of the correction potential V MM , which compensates for the quantum mechanical contributions to proton affinity that are missing from the force field description. These correction potentials are specific for each titratable group and are obtained as polynomial fits to the deprotonation free energy profiles, as we discussed in detail in our previous work. Obtaining a sufficiently accurate polynomial fit can sometimes be difficult, which is why we introduce a new approach to the parameterization, including two steps: (i) fast parameterization to obtain an approximate polynomial fit, followed by (ii) a reweighting of this approximate fit using inverse-Boltzmann statistics. phbuilder automates the setting up of both steps, analyzing the trajectories, and calculating the optimal polynomial coefficients for V MM . We demonstrate this feature of phbuilder by parameterizing the two-state correction potential for arginine.