This level of accuracy exceeded that achieved using FEP+ (i.e., 0.39 pK) on the same 22 pK a dataset (Figure ). Our results suggest that NES can resolve histidine pK a values as accurately as FEP+ and further supports our previous suggestion that free energy calculations, in particular pK a calculations, with Amber14SB should employ the more recent, Best et al. partial charges. We note that this partial charge suggestion may also apply to Amber19SB, which utilizes the same backbone charges as Amber14SB.