where p(λ) is the probability distribution of λ-coordinates obtained from sampling runs. After the reweighting step and performing the sampling simulations of arginine at pK a =pH and with the biasing barrier set to 0, the obtained distributions of λ-coordinates are flat (Figure , S5). phbuilder provides functionality to perform both steps and outputs the appropriate entries for the lambdagrouptypes.dat and .mdp files, thereby making parameterization of new molecules more straightforward.