Abstract
We find, for two-electron closed-shell systems ( molecule, atom), symmetry-forbidden peaks in the high-order harmonic spectra obtained by the time-dependent Kohn-Sham equations, and clarify their origin. It turns out that fixation of the number of Kohn-Sham orbitals and their occupations gives rise to unphysical transition paths and, therefore, incorrect populations of the one-electron excited states, which leads to even-order harmonics in systems with inversion symmetry. We show that the time-dependent natural Kohn-Sham and time-dependent configuration interaction equations do not suffer from this shortcoming.
- Received 26 April 2020
- Accepted 23 March 2021
DOI:https://doi.org/10.1103/PhysRevA.103.043106
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