The AMBER-FB15 force field parameters for the backbone of each amino acid along with the AMBER ff99SB parameters (with some modifications detailed below) for the phospho- rylated side chains was used as the starting point of the optimization. As the purpose of this work is to add onto the existing AMBER-FB15 model, only bonded parameters pertaining to the side chain for each protonation state were optimized in order to maintain compatibility. Residue and protonation state specific β carbon atom types (and γ carbon atom types for monoanionic and dianionic phosphorylated threonine) were added in order