The fitting of E pol and E ct involved the fitting of site-specific density exponents ζ pol , typewise hardness extension parameters η, polarization-charge transfer scaling s pol-ct , as well as the polarization dampening parameters s nad and τ nad . All site parameters are included in the supporting material, with average atomic hardness and exponents summarizing our findings in Table . As a result of the fitting process, hydrogen atoms were found to be the least diffusive atoms as compared to the heavy atoms, giving them the largest mean hardness of 1.64 a -1 0 with the other type-specific mean hardness' ranging from 0.45a -1 0 for oxygens to 0.68a -1 0 for carbons. As expected, the most electronegative elements (nitrogen and oxygen) as well as their lone-pair partners, were the most polarizable. Regardless of the difference between hydrogen atoms and heavy atom hardness, hydrogens still contribute an appreciable amount of charge movement (see Figure ). These parameters are also consistent with previous works of QEq and PQEq, and others, 81,82 albeit some deviation is expected due to the diffusivity of the charge sites as well as other differences in the model being presented here. Given that the additional hardness parameters are rather small for heavy atoms, with LP oxygen sites getting the largest value of only 0.029 a -1 0 , a reasonable fit to the QM energies was still achieved when neglecting these parameters, including hydrogen ∆η's. However, this exclusion led to nucleobases with extremely high polarizabilities, and overall charge movement was more akin to their QM counterparts when including the extra parameters. Overall, polarization energies generated with a fluctuating density model provide a very good fit to the QM energies. Table displays the relative mean error in all 4 energy combinations involved in the fitting process. We define the error between the QM and the AmberFD