The need to improve force field accuracy for phosphorylated residues can be met by leveraging recent methods that were used to reparameterize force fields for the canonical amino acids. One of the well-established avenues for improving these force fields is to refit the bonded parameters, in particular those describing the dihedral angle degrees of freedom, by fitting to high-quality ab initio quantum chemistry calculations. The AMBER-FB15 protein force field is one example of this approach. In the parameterization of AMBER-FB15, the parameters fit to a large data set consisting of RI-MP2/aug-cc-pVTZ level calculations on blocked dipeptide versions of the 20 canonical amino acids in standard and alternate protonation states, including relative potential energies and gradients of constrained optimized structures on dense grids of main chain and side chain dihedral angle constraints, and vibrational frequencies at optimized geometries. The parameters were optimized in automated fashion using ForceBalance, a program which has been successfully used for systematic and reproducible force field parameterization for a variety of systems. Although AMBER-FB15 produced notably improved predictions of equilibrium and temperature-dependent properties of proteins, the original work did not develop compatible parameters for PTMs such as phosphorylation.