where ψ SOMO is the singly-occupied molecular orbital, occupied by the excess electron. If the sum of these values exceeded 6.0 Å, corresponding to the QM/MM boundary region, the trajectory was discarded. Fig. shows an example of an accepted trajectory (black curve), in which the radial extent of the electron avoids the QM/MM interface. An example of a rejected trajectory (red curve) is also shown, in which the radial extent exceeded 6 Å at ∼200 fs. Discarding trajectories this way would likely lead to an underestimate of the diffusion constant of e - (aq) and also limits the length of our simulations to no more than several ps; however, this is sufficiently long to study representative equilibrium dynamics of e - (aq) on sub-picosecond timescales, which governs its vibrational spectroscopy.