Figures S1-S18 in the Supporting Information show the top 10 parameter value changes for each parameter type. While the equilibrium bond lengths and angles from the AMBER-FB15 optimization procedure changed by no more than 5%, the largest parameter changes noted for FB18 are slightly greater than 10%. The parameters describing the different phosphorous-oxygen bond and angle terms were, in general, changed the most as can be seen in Figures . The equilibrium bond lengths were mainly increased for these atom types, with the exception of the OV-P bond length (oxygen linking to the phosphate group in PTR), which experienced a small reduction compared to the initial value we assigned from QM energy minimizations. The force constants for the bond and angle terms associated with the phosphate group were also largely modified, with a close to 25% reduction for the hydrogen-oxygen-phosphorous angle for the monoanionic species being the largest parameter change. The other large parameter changes are associated with the new beta carbon atom types that have been introduced. These parameters had greater changes for the dianionic residues compared to the monoanions. Figure above displays an example potential energy surface for monoanionic phosphorylated tyrosine dipeptide (Y1P) with the (χ 1 , χ 2 ) angles constrained, comparing the QM reference data with the ff99SB and FB18 force field predictions. As can be seen, ff99SB predicts a broad low energy region around the (χ 1 ) dihedral angle of -100 • which is shown not to exist in the QM data. FB18 corrects this region, albeit with a slight overestimation of the energy at the higher energy peaks. It also manages to broaden low energy regions that ff99 predicts to be too high, such as at the regions of (50, 100) and (50, -100). We expect the better agreement with the QM potential energy surface to lead to a more accurate sampling of equilibrium structures in simulation and improved temperature-dependent properties as is observed in AMBER-FB15. Heat maps for all residues/dihedral angle combinations are shown in Figures in the Supporting Information. Similar agreement as shown in Fig- 2 is observed, although the (ψ, φ) grids for dianionic phosphorylated serine and threonine appear to significantly overestimate the energy in some regions.