We present a hybrid approach for GW/Bethe-Salpeter Equation (BSE) calculations of core excitation spectra, including x-ray absorption (XAS), electron energy loss spectra (EELS), and nonresonant inelastic x-ray scattering (NRIXS). The method is based on ab initio wavefunctions from the plane-wave pseudopotential code ABINIT; atomic core-level states and projector augmented wave (PAW) transition matrix elements; the NIST core-level BSE solver; and a many-pole GW self-energy model to account for final-state broadening and self-energy shifts. Multiplet effects are also accounted for. The approach is implemented using an interface dubbed OCEAN (Obtaining Core Excitations using ABINIT and NBSE). To demonstrate the utility of the code we present results for the K-edges in LiF as probed by XAS and NRIXS, the K-edges of KCl as probed by XAS, the Ti L 2,3 -edge in SrTiO 3 as probed by XAS, and the Mg L 2,3 -edge in MgO as probed by XAS.We compare the results to experiments and results obtained using other theoretical approaches. PACS numbers: 78.70.Dm, 78.20.Bh, 71.15.Qe 1 arXiv:1010.0025v1 [cond-mat.mtrl-sci] 30 Sep 2010Recently there has been considerable progress in the theory of optical response beyond the independent-particle approximation. 1 For example, methods based on time-dependent density-functional theory (TDDFT) and the GW/Bethe-Salpeter Equation (GW/BSE) approach have been extensively studied. 1-4 While computationally simpler than the BSE, TDDFT is currently limited by approximations to the exchange-correlation functional. On the other hand, the GW/BSE approach includes an explicit treatment of quasi-particle effects within Hedin's GW self-energy approximation 5 and particle-hole interactions, both of which are often crucial to a quantitative treatment. In the GW approximation the electron self-energy is related to the product of the one-electron Green's function and screenedCoulomb interaction, which are respectively denoted by symbols G and W . A number of codes based on these approaches have been developed both for periodic materials 6-8 and other systems. 9,10Calculations of core-level spectra, on the other hand, pose additional theoretical challenges. Core-hole effects, energy-dependent damping, self-energy shifts, and atomic multiplet effects all complicate the theory. Consequently relatively few GW/BSE treatments presently exist. [11][12][13] To address these challenges, we present here a hybrid GW/BSE approach for periodic systems encompassing x-ray absorption spectra (XAS) and related core-excitation spectra. Our BSE Hamiltonian also accounts for atomic-multiplet effects in the spectra.Since our implementation includes self-consistent potentials for a given system, it improves on multiplet approximations that rely on crystal-field parameters. Also, although our approach is designed for periodic systems, aperiodic systems can be modeled using supercells.However, the method is limited to a range of order 10 2 eV above a given core threshold.Thus the method is complementary to the real-space Green's fu...

We present an efficient implementation of the Bethe-Salpeter equation (BSE) method for obtaining core-level spectra including x-ray absorption (XAS), x-ray emission (XES), and both resonant and non-resonant inelastic x-ray scattering spectra (N/RIXS). Calculations are based on density functional theory (DFT) electronic structures generated either by abinit or Quantumespresso, both plane-wave basis, pseudopotential codes. This electronic structure is improved through the inclusion of a GW self energy. The projector augmented wave technique is used to evaluate transition matrix elements between core-level and band states. Final two-particle scattering states are obtained with the NIST core-level BSE solver (NBSE). We have previously reported this implementation, which we refer to as ocean (Obtaining Core Excitations from Ab initio electronic structure and NBSE) [Phys. Rev. B 83, 115106 (2011)]. Here, we present additional efficiencies that enable us to evaluate spectra for systems ten times larger than previously possible; containing up to a few thousand electrons. These improvements include the implementation of optimal basis functions that reduce the cost of the initial DFT calculations, more complete parallelization of the screening calculation and of the action of the BSE Hamiltonian, and various memory reductions. Scaling is demonstrated on supercells of SrTiO 3 and example spectra for the organic light emitting molecule Tris-(8-hydroxyquinoline)aluminum (Alq 3 ) are presented. The ability to perform large-scale spectral calculations is particularly advantageous for investigating dilute or non-periodic systems such as doped materials, amorphous systems, or complex nano-structures.

Recent debates on the oxygen redox behaviors in battery electrodes have triggered a pressing demand for the reliable detection and understanding of non-divalent oxygen states beyond conventional absorption spectroscopy. Here, enabled by high-efficiency mapping of resonant inelastic X-ray scattering (mRIXS) coupled with first-principles calculations, we report distinct mRIXS features of the oxygen states in Li2O, Li2CO3, and especially, Li2O2, which are successfully reproduced and interpreted theoretically. mRIXS signals are dominated by valence-band decays in Li2O and Li2CO3. However, the oxidized oxygen in Li2O2 leads to partially unoccupied O-2p states that yield a specific intra-band excitonic feature in mRIXS. Such a feature displays a specific emission energy in mRIXS, which disentangles the oxidized oxygen states from the dominating transition-metal/oxygen hybridization features in absorption spectroscopy, thus providing critical hints for both detecting and understanding the oxygen redox reactions in transition-metal oxide based battery materials.

Constrained-occupancy delta-self-consistent-field (ΔSCF) methods and many-body perturbation theories (MBPT) are two strategies for obtaining electronic excitations from first-principles. Using the two distinct approaches, we study the O 1s core excitations that have become increasingly important for characterizing transition-metal oxides and understanding strong electronic correlation. The ΔSCF approach, in its current single-particle form, systematically underestimates the pre-edge intensity for chosen oxides, despite its success in weakly correlated systems. By contrast, the Bethe-Salpeter equation within MBPT predicts much better lineshapes. This motivates one to reexamine the many-electron dynamics of X-ray excitations. We find that the single-particle ΔSCF approach can be rectified by explicitly calculating many-electron transition amplitudes, producing X-ray spectra in excellent agreement with experiments. This study paves the way to accurately predict X-ray near-edge spectral fingerprints for physics and materials science beyond the Bethe-Salpether equation.