We find, for two-electron closed-shell systems (H2 molecule, He 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.
Bibliographical noteKAUST Repository Item: Exported on 2021-04-30
Acknowledgements: We acknowledge financial support from King Abdullah University of Science and Technology (KAUST). For computer time, this research used the resources of the Supercomputing Laboratory at KAUST. We thank Nicolas Tancogne-Dejean and Aurélien Manchon for helpful discussions.