Our results highlight three force field modifications that can improve pK a prediction accuracy for cysteine and histidine: 1) increasing the vdW of the deprotonated cysteine sulfur in Amber14SB; 2) altering the backbone partial charges of doubly protonated histidine in Amber14SB; and 3) charge scaling all unit charges in CHARMM36m and Amber14SB. Our investigation is not intended to provide definitive parameters for either force field or an absolute strategy for improving relative free energy calculations, particularly pK a prediction, instead, we aim to highlight potential avenues for further investigation and development.