We set ∆t = 0.02 a.u. for all calculations at the oxygen, nitrogen, and carbon K-edges, which should be adequate even by very conservative convergence estimates that are stricter than the Nyquist frequency. Tests using time steps as small as ∆t = 0.001 a.u. demonstrate that oxygen K-edge spectra are converged (Fig. ). The method of Padé approximants is used to post-process all of the dipole moment data from TDKS simulations. Exponential damping is applied to the time series µ λ (t), with a damping constant corresponding to spectral broadening of 0.7 eV. As discussed in Section 2 A 3, most of the TDKS spectra reported below are based on t max = 600 a.u. (≈ 14 fs) of propagation time. Whether shorter propagation times can be used in practical calculations is considered in Section 4 C 1.