is not efficient. One reason is that the computational cost of evaluating a single derivative coupling d α JK is equivalent to the cost of evaluating a single gradient of the excited state energy surface; thus the cost of constructing the entire derivative coupling matrix between all pairs of states would be exorbitant, equivalent to a quadratic number of expensive gradient calls at each time step. Another reason is that, when the trajectory encounters a so-called trivial crossing (a sharp crossing between two electronic states where the diabatic coupling is effectively zero and a non-adiabatic electronic transition must occur), very small time steps are required to capture the electronic transition.