At every time step, the total energy of the system is minimized w.r.t. the electron populations. We invoke an additional requirement that the sum of all charge movement in a single nucleobase is retained on the individual molecule. That is, for each molecule M , we apply the constraint i∈M ∆n i = 0. To to so, we minimize L w.r.t. ∆n and the Lagrange multipliers λ,