EXC

Official Resources

• Website: http://www.bethe-salpeter.org/
• Documentation: http://www.bethe-salpeter.org/ (Manuals and Tutorials)
• License: Research/Academic License

Overview

EXC is a dedicated ab initio code for calculating the dielectric and optical properties of materials by solving the Bethe-Salpeter Equation (BSE). Developed at Ecole Polytechnique (LSI), it operates in reciprocal space and the frequency domain, utilizing a plane-wave basis. It is designed to capture electron-hole interaction effects (excitons) in absorption and energy loss spectra.

Scientific domain: Solid state physics, optical spectroscopy, core-level spectroscopy Target user community: Researchers studying optical properties of solids, surfaces, and clusters

Theoretical Methods

• Bethe-Salpeter Equation (BSE): Two-particle Green's function formalism
• GW Approximation: Uses quasiparticle energies as input
• Reciprocal Space: Solving the BSE matrix in k-space
• Haydock Method: Iterative solver for finding spectra without full diagonalization

Capabilities

• Optical Absorption: Dielectric function with excitonic effects
• EELS: Electron Energy Loss Spectroscopy
• IXS/XAS: Inelastic X-ray Scattering and X-ray Absorption Spectroscopy
• Supercells: Treatment of finite systems (atoms, clusters) via supercell approximation
• Scissor Operator: Support for simple GW corrections via energy shifts

Inputs & Outputs

• Input files:
  • input: Main configuration file (parameters like exciton, omegai, broad)
  • file.kss: Ground state Kohn-Sham structure (density/wavefunctions)
  • file.gw: Quasiparticle energy corrections
  • file.em1: Inverse dielectric matrix (screening)
• Output data types:
  • Dielectric function (real and imaginary parts)
  • Absorption spectra
  • Reflectivity and Refractive Index data
  • Exciton binding energies

Performance Characteristics

• Parallelization: MPI parallelization for matrix construction and diagonalization.
• Solvers: Haydock iterative method allows handling larger matrices than direct diagonalization.

Comparisons

• vs Yambo: Both are plane-wave BSE codes. Yambo has a larger modern community and more integrated workflow. EXC is a robust, specialized solver with a long history.
• vs OCEAN: OCEAN focuses on core-level spectroscopy (BSE), EXC covers both valence and core (check specific XAS features).

Usage & Best Practices

• Workflow: Ground State (ABINIT/others) -> GW (ABINIT/others) -> Screening -> EXC.
• Convergence: Careful convergence of k-points and empty bands is crucial for BSE.

Citations

• Primary: Reference L. Reining et al. and the http://www.bethe-salpeter.org/ website.