We briefly review our implementation of the real-space Green's function (RSGF) approach for calculations of X-ray spectra, focusing on recently developed parameter free models for dominant many-body effects. Although the RSGF approach has been widely used both for near edge (XANES) and extended (EXAFS) ranges, previous implementations relied on semi-phenomenological methods, e.g., the plasmon-pole model for the self-energy, the final-state rule for screened core hole effects, and the correlated Debye model for vibrational damping. Here we describe how these approximations can be replaced by efficient ab initio models including a many-pole model of the self-energy, inelastic losses and multiple-electron excitations; a linear response approach for the core hole; and a Lanczos approach for Debye-Waller effects. We also discuss the implementation of these models and software improvements within the FEFF9 code, together with a number of examples.

There has been dramatic progress in recent years both in the calculation and interpretation of various x-ray spectroscopies. However, current theoretical calculations often use a number of simplified models to account for many-body effects, in lieu of first principles calculations. In an effort to overcome these limitations we describe in this article a number of recent advances in theory and in theoretical codes which offer the prospect of parameter free calculations that include the dominant many-body effects. These advances are based on ab initio calculations of the dielectric and vibrational response of a system. Calculations of the dielectric function over a broad spectrum yield system dependent self-energies and mean-free paths, as well as intrinsic losses due to multielectron excitations. Calculations of the dynamical matrix yield vibrational damping in terms of multiple-scattering Debye-Waller factors. Our ab initio methods for determining these many-body effects have led to new, improved, and broadly applicable x-ray and electron spectroscopy codes.

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...

The experimental valence band photoemission spectrum of semiconductors exhibits multiple satellites that cannot be described by the GW approximation for the self-energy in the framework of many-body perturbation theory. Taking silicon as a prototypical example, we compare experimental high energy photoemission spectra with GW calculations and analyze the origin of the GW failure. We then propose an approximation to the functional differential equation that determines the exact one-body Green's function, whose solution has an exponential form. This yields a calculated spectrum, including cross sections, secondary electrons, and an estimate for extrinsic and interference effects, in excellent agreement with experiment. Our result can be recast as a dynamical vertex correction beyond GW, giving hints for further developments.Photoemission is a prominent tool to access information about electronic structure and excitations in materials. Modern synchrotron sources can provide detailed insight, thanks to their high intensity and broad photon energy range. But the interpretation of the experimental data is far from obvious, and theory is an essential complementary tool. However, ab initio calculations typically focus on bulk bandstructure [1, 2]; thus surface effects are ignored, and satellites are not included. The latter are a pure many-body effect due to coupling to excitations of the material. Such many-body effects are contained in approaches developed for correlated materials [3,4] however, these are usually based on models with short-range interactions, whereas satellites such as plasmons involve long-range effects. Plasmon satellites have been extensively studied in core-level experiments [5]. There they can be described by a theoretical model where a single dispersionless fermion couples to bosons. The resulting exact Green's function has an exponential form given by the so-called cumulant expansion (CE). A Taylor expansion of the exponential leads to a well defined quasi-particle (QP) peak followed by a decaying series of plasmon satellites at energy differences given by the plasmon energy, consistent with experimental observations [6][7][8][9][10]. In the valence region, plasmon satellites are much less studied, though ab initio approaches can provide a good starting point. At high photoelectron energies the photoemission spectrum is approximately proportional to the intrinsic spectral function A(ω) = −(1/π)Im G(ω), where G is the one-particle Green's function. The latter is typically calculated using the widely used GW approximation (GWA) [7,11,12]. In principle, the GWA contains correlations effects beyond the quasiparticle approximation. However, these additional features are rarely calculated due to computational complexity and, more importantly, the serious discrepancies between GWA and experiment (see e.g. [13][14][15][16]). The CE has also been used for the homogeneous electron gas [17] and simple metals [14,15], yielding an improved description of satellites over GW. Silicon [16] and graphite [18] ...