Comment on "Origin of symmetry-forbidden high-order harmonic generation in the time-dependent Kohn-Sham formulation"

Abstract

In their recent paper [Phys. Rev. A 103, 043106 (2021)10.1103/PhysRevA.103.043106], Zang et al. theoretically investigated high harmonic generation (HHG) in benchmark two-electron systems that are inversion symmetric with time-dependent density functional theory (TDDFT) in the Kohn-Sham formulation. They found that the theory wrongly predicted the emission of symmetry-forbidden even harmonics and concluded that this error originates from an inherent problem of TDDFT that unphysically populates one- and two-electron excited states. They further claimed that this effect results in an incorrect HHG cutoff energy. We reproduced their main results, but found that the unphysical even harmonics that they observed originated from numerical errors introduced by the boundary conditions. We show that contrary to their claims, the HHG cutoff energy calculated within TDDFT agrees perfectly with the standard and well-established models of HHG.