We tested whether it was possible to estimate the electrostatic hydration free energy of the amino acids by solving the linearized Poisson-Boltzmann (PB) equation via a multigrid, finite difference, method 26,27 , as implemented in the APBS 20 software. For the aqueous phase, default options for the APBS package were used, namely a multigrid level of four, with 129 grid points per processor in each direction (x, y, z; 0.1 Å grid spacing) and a multiple Debye-Hückel boundary condition. The solute dielectric constant was set to one, the solvent dielectric constant to 63 (corresponding to the TIP4P-Ew water model at 298 K 1 ), with a cubic B-spline discretization of the charge mapping. The solventexcluded volume was defined from the molecular surface, with atomic radii adopted from ref. 28; the solvent probe radius was taken as 1.4 Å (corresponding to water). The temperature was set at 298.15 K. Calculations in vacuum were performed by setting the solvent dielectric constant to one and the hydration free energy was calculated as the difference in free energy from the gas state to the solvated state. For each solvation free energy calculation, 25 configurations were extracted from a 25 ns simulation of the solvated amino acid. SI Table holds the results of the PB calculations. SI Figure A makes the comparison between PB and FEP hydration free energies. This figure clearly demonstrates that the PB approach does not capture the relative differences in the electrostatic hydration free energy between the amino acids, and does not give insight into these systems.