Efficient nonequilibrium electron dynamics from first-principles: leveraging Koopmans spectral functionals and Wannier localization
We present an efficient first-principles approach for simulating the nonequilibrium electron dynamics in extended systems beyond the linear regime. The method combines Koopmans-compliant functionals, which provide an accurate quasiparticle band structures, with the real-time evolution of the electronic density matrix in a Wannier basis within the Hartree plus screened exchange (HSEX) approximation. The locality of the orbital basis enables physically motivated approximations that significantly reduce both the computational cost and memory requirements while preserving accuracy. The screened Coulomb interaction, the central ingredient of the HSEX self-energy, is computed efficiently using density-functional perturbation theory. We benchmark the approach in the linear regime against experimental spectra and reference Green's function calculations for systems featuring both weakly and strongly bound excitons. Moving to the nonlinear regime, we investigate high-harmonic generation (HHG) in silicon and lithium fluoride. While in silicon the HHG spectrum is largely governed by the quasiparticle band structure, in LiF, a material featuring strong excitonics effect, the harmonic emission is selectively enhanced at excitonic resonances, suggesting that HHG probes correlated electron-hole excitations rather than solely the quasiparticle band structure. The present framework enables fully \textit{ab-initio} simulations of excitonic effects in nonlinear optical spectra at a significantly reduced computational cost compared to real-time Green's function approaches, providing an efficient route to the study of ultrafast and strong-field phenomena in solids.