To ensure tight convergence of R (t), we used a total of 10 6 trajectories, although typically a total of 10 3 -5 × 10 3 trajectories will provide visually converged results. The propagation time step for the Chebyshev propagation was chosen as ∆t = 20 a.u. (≈ 0.5 fs). Once having a converged R (t), we perform the numerical Fourier transform