The goal of this work is to create AMBER-FB15 compatible parameters for six phosphorylated amino acids: phosphoserine, phosphothreonine and phosphotyrosine in their monoanionic and dianonic protonation states. We first build a reference data set composed of accurate MP2-level QM data as was done for AMBER-FB15. ForceBalance is used to optimize bonded parameters starting from those created by Homeyer et al. for the phosphorylated side chains in order to keep consistent parameters for the backbone atoms, with the nonbonded parameters unmodified from those implemented by Homeyer et al. and Steinbrecher et al. Detailed potential energy surface comparisons are made between parameters from AMBER ff99SB, which we use as a starting point for the force field optimization, the "FB18" optimized parameters presented in this work and the QM reference data for validation of the energies predicted by the new parameters. Additionally, we compare the new force field to experimental data for blocked dipeptide forms of each of the amino acids parameterized here, similarly to the procedure performed by Vymětal et al. These are essentially the simplest phosphorylated systems possible, for which a multitude of data exists, such as 3 J NMR couplings and chemical shifts, intramolecular hydrogen bond propensities, and backbone conformational preferences. The simple nature of these systems allows for relatively straightforward comparison between MD simulation results and experiment.