The zwitterionic form of glycine was parametrized using the GAFF forcefield. For consistency with the Amber forcefield, the ligand partial charges were calculated at the Hartree-Fock level with a 6-31G* basis set and a RESP fitting procedure on a geometry-optimised structure obtained at the density functional theory level with the B3LYP exchange and correlation functional and the same basis set with Gaussian 09. As the geometry optimisation was carried out in vacuo, to avoid proton transfer, the bonds containing hydrogen of the ammonium moiety were constrained.