However, AmberFD consistently underestimate the planar components of α ij for all 4 nucleobases, yet has a much higher polarization energy than either Drude or AMOEBA. While multipoles appear to help with creating an accurate potential that polarizes each nucleobase over Drude, charge penetration effects may also play just as an important role in creating an field that most resembles a QM molecule. Of course, the "correct" amount polarization energy will always be dependent on the QM decomposition scheme used. Still, no matter how one defines the quantity, the act of creating an external field that accurately perturbs each molecule clearly is just as critical as assigning it's tendency to respond to such fields. Differences in electron density can also be visualized with AmberFD and compared against DFT to observe how polarization occurs in real space for each model. Figure shows the difference in electron density of an A:U base pair rotating about the Uracil O4 -Adenine N6 hydrogen bond. Density difference is defined as ∆ρ(r) = ρ AB (r) -ρ A (r) -ρ B (r), with ρ AB being the fully SCF optimized base pair (including charge transfer components for DFT) and ρ A/B being the frozen densities of each nucleobase. Overall, AmberFD polarization reproduces the majority of electron movement seen in the DFT computations near the hydrogen bonding sites, with hydrogen's loosing electron populations and donating it to nearby electronegative oxygen and nitrogen atoms. While more detailed features can be seen in the DFT computations, such as induced dipoles over the Adenine N1 or Uracil O4 atoms in frame 1, AmberFD instead produces what amounts to an averaged density change about each atomic site. Lone pair (LP) sites are also well populated, such as the increase in density that is off-centered from the Adenine N1 atom or the Uracil O4 atom. It should be noted that LP positions in AmberFD (like in Drude) are fixed place, and some angles (such as the third frame) show that polarization is restricted as a result. Still, the overall dipole structure due to LPs across the hydrogen bond site is mostly recovered at some angles, such as in the second and last frame in Figure . Additionally, the integrated absolute electron change |∆N | upon polarization is also closely matched to the DFT values. This was computed by approximating the integral as a Riemann sum, |∆N | = |∆ρ(r)|dr ≈ i |∆ρ(r i )|∆V . We note that while DFT allows for charge transfer to occur between the two nucleobases, for example by adding density between an H-bond donor and proton acceptor, AmberFD in contrast restricts all differences to occur within a single fragment.