The resulting state energies and state populations from a single trajectory are shown in Figure . The dynamics are in excellent agreement, both in energy and population, until 30 fs, when small numerical differences in the electronic structure output leads to visible differences in their trajectories. We verified that these differences remain even when employing full double precision and tighter integral thresholds in TeraChem. Our testing revealed that, at least for ethylene, small differences in nuclear position can lead to large differences in NAC vectors. Take for example the avoided crossing that occurs around 46 fs when the S 1 and S 2 energies approach each other in Figure . At this simulation frame, the minimum RMSD 66 between the geometry produced by the GAMESS and TeraChem driven trajectories is less than 0.03 Å, whereas the magnitude of their NAC vectors differed by over 30%, thus sending the TeraChem and GAMESS simulations on slightly different trajectories that eventually diverge from each other. However, we expect that, when averaged over multiple trajectories with varying initial conditions, the average rate of population change resulting from simulations utilizing both drivers should be very similar.