Traditional MM forcefields do not explicitly account for electronic polarizability; however, charge scaling has been employed to mimic this phenomenon. Sulfur is significantly more polarizable relative to oxygen and nitrogen, and we hypothesized that the markedly reduced pK a prediction performance for cysteine, as compared to aspartate and lysine, may be due to erroneously modelled electrostatic interactions between cysteine thiolate and its protein-residue neighbourhood.