Starting with the atomic basins, we see a response like that of the total charge density in ethylene. Inter-atomic valance charge transfer is never more than a twentieth of an electron, and the prevalent motion is opposite the field. Overall, charge shifts from the butadiene central C2,3 and H1,2 atoms to the ethylene C and H atoms, and the ∆ values for every atom are nearly equal and opposite with respect to field direction. From a pairwise perspective, there is also a lesser transfer between the ethylene C atoms and the butadiene C1,4 atoms. Regarding the charge density behavior responsible for the shift in the reaction barrier, it is difficult to say whether the apparent inter-atomic C1-C5 charge transfer should positively or negatively alter the barrier. Yet, when bond bundle properties are considered, this difficulty vanishes.