Weak-field asymptotic theory – an efficient and accurate alternative to the time-dependent DFT for ionization calculations

Strong-field ionization (SFI) is a process that is sensitive to the instantaneous field strength, making SFI a promising tool for time-resolving the study of ultrafast dynamics in molecules. The common method by which one calculates ionization is by propagating a given initial state and analyzing the portion of the wave function that ventures far enough from the nuclei. This method, however, becomes very expensive as basis requirements increase (e.g. as electron energy increases). Using Gaussian basis sets remedies this problem, but the so-called leakage flux issue now becomes relevant. Here, we implement the weak-field asymptotic theory (WFAT) using orbitals obtained through density functional theory. To validate the use of WFAT for representing ionization from time-dependent simulations, we compare angle-resolved ionizations of both methods for some molecules. We also develop a many-electron extension of WFAT that is essential for analyzing the effect of neutral excited states on the ionization.