In Fig. , the HHG spectra from Fig. are plotted again, along with Koopmans IEs and LR-TDHF excitation energies, both as stick spectra. For clarity, only excited states with nonzero oscillator strengths (greater than 1 × 10 -10 ) are shown, as these should dominate the transition rate and are sufficient to illustrate the fact that some spurious features in the HHG spectrum coincide with physical excitation energies. Specifically, fictitious peaks between the 60th and 80th harmonic orders are observed to coincide with Koopmans IEs, -ε k (where ε k is an occupied energy level). Particularly revealing is the spectrum computed using r 0 = 4.0 bohr (Fig. ), where the ionization is rapid. A broad feature around the 30th harmonic order coincides with a particular Koopmans IE, as does another broad feature around the 70th harmonic. (Note that the MO energy levels are much more sparse after the 30th harmonic order, as compared to the low-energy regime.) We conclude from this that these ionizable MOs do provide ionization channels if the (a.u.) (a) 0 < t < 1,000 a.u.