[Paper Review] Advanced calculations of x-ray spectroscopies with FEFF10 and Corvus

J. J. Kas, Fernando D. Vila+3 more|arXiv (Cornell University)|Jun 24, 2021

Machine Learning in Materials Science4 citations

TL;DR

This paper presents FEFF10, an advanced real-space Green's function code for x-ray spectroscopy, enhanced by integration with the Corvus workflow manager to enable automated, multi-code simulations of complex systems. The framework supports finite-temperature effects, vibrational dynamics, inelastic losses, superheavy elements (up to Z=138), and spectral fitting, significantly extending FEFF's capabilities for modern x-ray experiments including ultrafast pump-probe studies.

ABSTRACT

The real-space Green's function code FEFF has been extensively developed and used for calculations of x-ray and related spectra, including x-ray absorption (XAS), x-ray emission (XES), inelastic x-ray scattering, and electron energy loss spectra (EELS). The code is particularly useful for the analysis and interpretation of the XAS fine-structure (EXAFS) and the near-edge structure (XANES) in materials throughout the periodic table. Nevertheless, many applications, such as non-equilibrium systems, and the analysis of ultra-fast pump-probe experiments, require extensions of the code including finite-temperature and auxiliary calculations of structure and vibrational properties. To enable these extensions, we have developed in tandem, a new version FEFF10, and new FEFF based workflows for the Corvus workflow manager, which allow users to easily augment the capabilities of FEFF10 via auxiliary codes. This coupling facilitates simplified input and automated calculations of spectra based on advanced theoretical techniques. The approach is illustrated with examples of high temperature behavior, vibrational properties, many-body excitations in XAS, super-heavy materials, and fits of calculated spectra to experiment.

Motivation & Objective

• To extend FEFF's capabilities beyond standard XAS and XANES to include finite-temperature effects, vibrational dynamics, and inelastic losses.
• To address the limitations of monolithic codes like FEFF9 by enabling modular, multi-code workflows through integration with Corvus.
• To support high-accuracy simulations of complex systems, including superheavy elements (up to Z=138) and non-equilibrium dynamics in time-resolved experiments.
• To streamline spectral fitting and full optical constant calculations by automating input generation and multi-edge summation.
• To provide a scalable, extensible framework for combining FEFF10 with DFT, Monte Carlo, and many-body codes for advanced x-ray spectroscopy.

Proposed method

• FEFF10 implements a real-space Green’s function (RSGF) formalism based on relativistic atomic wave functions and muffin-tin potentials, enabling accurate XAS, XES, and EELS calculations.
• The RSGF approach avoids eigenstate summation via Fermi’s golden rule, instead computing spectral functions through the imaginary part of the Green’s function: $\rho_{L,L'}(E) = -\frac{1}{\pi} \mathrm{Im}\, G_{L,L'}(E)$.
• Finite-temperature effects are incorporated via the Fermi-Dirac distribution in the spectral function, with thermal structure and vibrational effects computed using auxiliary codes linked via Corvus.
• Corvus automates workflows by orchestrating FEFF10 with external codes (e.g., DFT, Monte Carlo, OCEAN) for structure, vibrational, and many-body corrections.
• Core-level contributions are calculated using relativistic dipole matrix elements $M_L$ from an extended Dirac-Fock code, now supporting elements up to Z=138.
• Optical constants and dielectric functions are computed via $\epsilon(\omega) = \epsilon_1 + i\epsilon_2$, with $\epsilon_2$ split into valence and core contributions, and $\kappa(\omega)$ derived from $\mu(\omega) = \frac{2\omega}{c}\kappa(\omega)$.

Experimental results

Research questions

• RQ1 How can FEFF10 and Corvus be used to simulate x-ray spectra under finite-temperature conditions with accurate vibrational and structural dynamics?
• RQ2 What is the impact of inelastic losses and multi-electron excitations on XAS and XANES in transition metal and light-element systems?
• RQ3 Can the FEFF10/Corvus framework accurately model X-ray spectra of superheavy elements (up to Z=138) in molecular environments like Sg(CO)6?
• RQ4 How does the automated workflow in Corvus improve the reliability and efficiency of spectral fitting and multi-edge analysis compared to manual workflows?
• RQ5 To what extent can the framework simulate time-resolved pump-probe x-ray spectroscopy in non-equilibrium systems using coupled DFT and FEFF10 calculations?

Key findings

• FEFF10 successfully computes XANES and EXAFS for superheavy elements up to Z=138 using an automated single-configuration Dirac-Fock code, enabling theoretical studies of elements like seaborgium.
• Finite-temperature XAS simulations are enabled through coupling with auxiliary codes for lattice dynamics and thermal structure, with results validated against experimental data.
• Corvus workflows automate the generation of energy grids, multi-edge summation (e.g., L2,3 or M4,5), and spectral fitting, reducing user error and increasing reproducibility.
• The framework accurately models resonant inelastic x-ray scattering (RIXS) and optical constants by combining core and valence contributions to $\epsilon_2(\omega)$, with improved accuracy via Bethe-Salpeter equation corrections when needed.
• Calculations of Sg(CO)6 and Sg(CO)5 show distinct XANES and EXAFS features reflecting changes in coordination number, demonstrating the method’s sensitivity to local structure.
• The hybrid FEFF10/Corvus approach enables high-fidelity simulations of ultrafast pump-probe experiments by integrating time-dependent structural and electronic responses from external codes.