<p>This study arises from the attempt to answer the following question: how different descriptions
of electronic exchange and correlation affect the high-harmonic generation (HHG) spectroscopy
of H2, N2 and CO2 molecules? We compare HHG spectra for H2, N2 and CO2 with different ab
initio electronic structures methods: real-time time-dependent configuration interaction (RT-TDCIS) and real-time time-dependent density functional theory (RT-TDDFT) using truncated basis
sets composed of correlated wave functions expanded on Gaussian basis sets. In the framework
of RT-TDDFT, we employ PBE and LC-ωPBE functionals. We study HHG spectroscopy by
disentangling the effect of electronic exchange and correlation. We first analyse the electronic
exchange alone and in the case of RT-TDDFT with LC-ωPBE, we use ω = 0.3 and ω = 0.4 to
tune the percentage of long-range Hartree-Fock exchange and of short-range exchange PBE. Then,
we added the correlation as described by PBE functional. All the methods give very similar HHG
spectra and they seem not to be particularly sensitive to the different description of exchange and
correlation or to the correct asymptotic behaviour of the Coulomb potential. Despite this general
trend, some differences are found in the region connecting the cutoff and the background. Here,
the harmonics can be resolved with different accuracy depending on the theoretical schemes used.
We believe that the investigation of the molecular continuum and its coupling with strong fields
merits further theoretical investigations in the next future.
</p>
Role of Exchange and Correlation in High-Harmonic Generation Spectra of H2, N2 and CO2: Real-Time Time-Dependent Electronic-Structure Approaches
