FEFF

Official Resources

• Homepage: https://feff.uw.edu/
• Documentation: https://feff.uw.edu/documentation/
• Source Repository: Proprietary (Academic/Commercial licenses)
• License: Proprietary

Overview

FEFF is an automated program for ab initio multiple scattering calculations of X-ray Absorption Fine Structure (XAFS), X-ray Absorption Near-Edge Structure (XANES), and various other spectroscopies for clusters of atoms. Developed at the University of Washington, FEFF uses a real-space Green's function approach, making it highly effective for non-periodic systems, nanoparticles, and defects, as well as crystals.

Scientific domain: X-ray spectroscopy (XAFS, XANES, EXAFS), multiple scattering theory
Target user community: Spectroscopists, materials scientists, chemists

Theoretical Methods

• Real-space Multiple Scattering Theory (RSMS)
• Green's function formalism
• Self-consistent field (SCF) potentials
• Muff-tin approximation
• Full multiple scattering (FMS)
• Time-dependent DFT (TDDFT) for core-hole screening (in FEFF9)
• Many-pole self-energy (GW approximation)

Capabilities (CRITICAL)

• Calculation of EXAFS and XANES spectra
• Electron Energy Loss Spectroscopy (EELS)
• X-ray Emission Spectroscopy (XES)
• Compton scattering
• Local Density of States (LDOS)
• X-ray Magnetic Circular Dichroism (XMCD)
• Non-resonant Inelastic X-ray Scattering (NRIXS)
• Core-hole effects
• Debye-Waller factors via correlated Debye model

Sources: FEFF website, Rev. Mod. Phys. 72, 621 (2000)

Key Strengths

Real-Space Method:

• No periodicity required
• Clusters and nanoparticles
• Defects and interfaces
• Amorphous materials

Comprehensive Spectroscopy:

• EXAFS and XANES
• EELS, XES, XMCD
• Compton scattering
• Multiple techniques

Industry Standard:

• Widely validated
• Extensive user base
• Demeter/Larch integration
• Commercial support

Inputs & Outputs

• Input formats: feff.inp (geometry, potentials, control flags)
• Output data types: xmu.dat (absorption cross-section), chi.dat (EXAFS), ldos.dat, feff.bin

Interfaces & Ecosystem

• JFEFF: GUI for input generation
• Athena/Artemis (Demeter): Standard analysis software that uses FEFF for fitting
• Larch: Analysis library wrapping FEFF
• ASE: Interface available

Workflow and Usage

1. Create feff.inp file (atomic coordinates, potentials).
2. Run FEFF modules:
   • pot: Calculate potentials
   • xsph: Phase shifts
   • fms: Full multiple scattering
   • path: Path expansion (for EXAFS)
   • genfmt: XAFS parameters
   • ff2chi: Chi calculation
3. Analyze xmu.dat or use output for fitting in Artemis.

Performance Characteristics

• Highly efficient for high-energy EXAFS (path expansion)
• XANES (FMS) scales with cluster size (up to hundreds of atoms)
• Parallelized (MPI)

Limitations & Known Constraints

• Muffin-tin approximation: Spherical potential assumption
• Proprietary: License required
• Cluster size: XANES needs large clusters
• Parameter tuning: Some expertise required

Comparison with Other Tools

• vs FDMNES: FEFF faster, FDMNES non-muffin-tin option
• vs OCEAN: FEFF real-space, OCEAN BSE-based
• vs xspectra: FEFF standalone, xspectra QE-integrated
• Unique strength: Industry standard, path-based EXAFS

Application Areas

• Structure determination from EXAFS (coordination numbers, bond lengths)
• Catalyst characterization (nanoparticles)
• Biological metalloproteins
• Amorphous materials and liquids
• Phase identification via XANES

Best Practices

• Use adequate cluster size for XANES
• Converge path expansion for EXAFS
• Validate with known reference compounds
• Use Demeter/Larch for fitting

Community and Support

• Developed by Rehr Group (University of Washington)
• Very large user base in spectroscopy community
• Commercial support available
• Extensive workshops and training

Verification & Sources

Primary sources:

1. Homepage: https://feff.uw.edu/
2. Publication: J. J. Rehr and R. C. Albers, Rev. Mod. Phys. 72, 621 (2000)

Confidence: VERIFIED

Verification status: ✅ VERIFIED

• Website: ACTIVE
• Documentation: COMPREHENSIVE
• Source: PROPRIETARY (Widely used)
• Development: ACTIVE (Rehr Group)
• Applications: XAFS, XANES, EELS, multiple scattering