Parameter-free ionization model for atomic and molecular high-harmonic generation spectra using Gaussian basis sets

: High-Harmonic Generation (HHG) has become crucial in attosecond science and time-resolved spectroscopy, providing insights into ultrafast electron dynamics. One approach to calculating HHG spectra for complex systems is to use Gaussian basis sets in conjunction with (correlated) wave function methods. While previous studies have shown that Gaussian functions can be effectively used to describe electron dynamics in the continuum, the treatment of basis set incompleteness is equally critical. Properly addressing this issue is essential to avoid artificial reflections and accurately incorporate ionization, which is key to obtaining realistic results. In this work, we present a comparative analysis of the ab initio lifetime model (AbILM) [Coccia et al., J. Chem. Phys. 147, 014106 (2017)] and the heuristic lifetime models (HLMs) [Klinkusch et al., J. Chem. Phys. 131, 114304 (2009); Coccia et al., Int. J. Quant. Chem. 116, 1120-1131 (2016); and Woźniak et al., J. Chem. Phys. 156, 204107 (2022)]. Our investigation focuses on hydrogen and helium atoms, as well as the hydrogen molecule, using time-dependent configuration interaction singles (TD-CIS) and doubles (TD-CISD). In addition, we extend our study to the nitrogen molecule, though only within the TD-CIS framework. Our findings assess the impact of these lifetime correction approaches on HHG spectra, highlighting how different treatments of basis set incompleteness influence the accuracy of HHG predictions. This analysis provides valuable insights into the reliability and potential biases of each method.