The sharp drop in intensity that is observed in the HHG spectra of Fig. is thus in good agreement with the cutoff estimate in Eq. ( ). However, in the TDKS simulations the intensity does persist to higher harmonic orders, albeit much attenuated, and there are spikes in intensity (corresponding to fictitious harmonics) around the 45th and 60th harmonic orders. We hypothesize that this behavior results from artificial confinement of the electron due to the finite extent of the Gaussian basis set. Higher-energy parts of the outgoing wave that should be ionized are instead confined and reflected, generating spurious interference features and generally contributing to a noisy spectrum.