Starting from a ground-state SCF calculation, converged to a threshold of 10 -8 Ha, the density is perturbed using a δ-function pulse in which the external field is only nonzero during the first two time steps. To normalize the initial perturbation across different choices of ∆t, we follow Ref. 55 and report the integrated electric field strength Ē, whose components are Ēκ = E κ ∆t (21) for κ ∈ {x, y, z}.