Efficient core-excited state orbital perspective on calculating x-ray absorption transitions in determinant framework

X-ray absorption spectroscopy (XAS) is an explicit probe of the unoccupied electronic structure of materials and an invaluable tool for fingerprinting various electronic properties and phenomena. Computational methods capable of simulating and analyzing such spectra are therefore in high demand for complementing experimental results and for extracting valuable insights therefrom. In particular, a recently proposed first-principles approach titled many-body XAS (MBXAS) [Y. Liang et al., Phys. Rev. Lett. 118, 096402 (2017)], which approximates the final (initial) state as a Slater determinant constructed from Kohn-Sham (KS) orbitals optimized in the absence (presence) of the relevant core electron has shown promising prospects in evaluating the transition amplitudes. In this article, we show that the MBXAS approach can be rederived using a transition operator expressed entirely in the basis of core-excited state KS orbitals and that this reformulation offers substantial practical and conceptual advantages. In addition to circumventing previous issues of convergence with respect to the number of unoccupied ground-state orbitals, the aforementioned representation reduces the computational expense by rendering the calculation of such orbitals unnecessary altogether. The reformulated approach also provides a direct pathway for comparing the many-body approximation with the so-called single-particle treatment and indicates the relative importance in observed XAS intensity of the relaxation of the valence occupied subspace induced by the core excitation. Finally, using the core-excited state basis, we define auxiliary orbitals for x-ray absorption and demonstrate their utility in explaining the spectral intensity by contrasting them with single-particle approximations to the excited state.

Automated summary

X-ray absorption spectroscopy (XAS) is a valuable tool for studying the electronic structure and properties of materials. A first-principles approach called many-body XAS (MBXAS) has shown promises in evaluating the transition amplitudes in XAS. In this article, the authors propose a reformulation of the MBXAS approach using a transition operator expressed in the basis of core-excited state KS orbitals, which offers practical and conceptual advantages. The reformulated approach also allows for comparisons with single-particle approximations and provides insights into the relaxation of the valence occupied subspace induced by core excitation.