Changes in polarization dictate necessary approximations for modeling electronic deexcitation intensity: Application to x-ray emission

Accurate simulation of electronic transitions is critical for complementing spectroscopic experiments and for validating theoretical approaches. Using a generalized framework, we contrast the accuracy and validity of orbital-constrained and linear-response approaches that build upon Kohn-Sham density functional theory (DFT) to simulate emission spectra of electronic origin and propose an efficient approximation, named many-body x-ray emission spectroscopy (MBXES) [1], for simulating such processes. We show analytically and with computed examples that for electronic deexcitation leading to an appreciable change in polarization (i.e., density rearrangement), the adiabatic approximation in a response-based formalism is inadequate for the calculation of oscillator strength. Thus, the change in the electrostatic dipole moment of a finite system can be used as a metric for evaluating the applicability of the adiabatic response-based approach and can be particularly valuable in x-ray emission spectroscopy. On the other hand, MBXES, the flexible method introduced here, can compute oscillator strengths accurately at a much lower computational expense on the basis of two DFT-based self-consistent field calculations. Using illustrative examples of emission spectra, the efficacy of the MBXES method is demonstrated by comparison with its parent theory, orbital-optimized DFT, and with experiments.

[1] S. Roychoudhury, L.A. Cunha, M. Head-Gordon, D. Prendergast, Phys. Rev. B 106, 075133