The high-harmonic cutoff energy can be calculated using U p = 5.98 eV, which comes from Eq. ( ) with the field parameters I = 10 14 W/cm 2 and λ = 800 nm. The IE ranges from 14.3-16.3 eV, depending on whether the ∆SCF value is used or the Koopmans' value (IE = (a.u.)