Through a combination of the existing AMBER-FB15 protein force field and a systematic optimization of the intramolecular parameters for the side chains of phosphorylated serine, threonine, and tyrosine, we have built FB18, a new set of parameters for the simulation of phosphorylated peptides and proteins. We demonstrated that it was possible to generate a substantially improved fit to the QM data in both the low-energy and high-energy regions.