Computation of high-harmonic generation spectra of H2 and N2 in intense laser pulses using quantum chemistry methods and time-dependent density functional theory

In this work, we present a study of H-2 and N-2 electron dynamics in intense laser fields with a specific focus on high-harmonic generation (HHG) spectroscopy. We performed this study with different theoretical methods: time-dependent configuration interaction singles (TD-CIS), perturbative doubles (TD-CIS(D)), time-dependent equation-of-motion coupled-cluster singles and doubles (TD-EOM-EE-CCSD) and time-dependent density-functional theory (TDDFT). All methods were implemented using a finite expansion in field-free eigenstates, and additionally direct propagation of the time-dependent density was employed for TDDFT. Within the sum over states approximation, the effect of electron correlation and the character of the atomic orbital basis set on the electron dynamics associated with HHG spectra is analysed. With respect to basis set, use of multiple sets of diffuse functions was found to be essential, while with respect to electron correlation, the treatment of double excitations in EOM-CCSD significantly affected the cutoff region of the HHG spectrum. With TDDFT, we also compared direct propagation against finite eigenstate expansion, and found a dramatic effect associated with the incorrect long-range potential in TDLDA. This permitted us to discuss the importance of correct behaviour of the long-range potential in HHG spectroscopy.