OCEAN (Obtaining Core Excitations from Ab initio electronic structure and NIST)

Official Resources

• Homepage: https://www.nist.gov/services-resources/software/ocean
• Documentation: https://github.com/times-physics/ocean
• Source Repository: https://github.com/times-physics/ocean
• License: BSD 3-Clause License

Overview

OCEAN is a code for calculating core-level spectra (XAS, XES, RIXS) from first principles using the Bethe-Salpeter Equation (BSE). It combines DFT (typically using Quantum ESPRESSO or ABINIT) with a GW/BSE approach to accurately treat core-hole interactions and excitonic effects. It is designed to handle periodic systems and provides accurate spectra for K-edges and L-edges.

Scientific domain: X-ray spectroscopy, BSE, core-level excitations
Target user community: Spectroscopists, materials scientists

Theoretical Methods

• Bethe-Salpeter Equation (BSE)
• Density Functional Theory (DFT)
• GW approximation (screening)
• Core-hole interaction
• Projector Augmented Wave (PAW) / Pseudopotentials
• Multiplet effects (limited)

Capabilities (CRITICAL)

• Calculation of X-ray Absorption Spectra (XAS)
• X-ray Emission Spectra (XES)
• Resonant Inelastic X-ray Scattering (RIXS)
• Non-resonant Inelastic X-ray Scattering (NRIXS)
• Accurate treatment of core-hole screening
• Periodic systems (solids, surfaces)

Sources: OCEAN website, Comp. Phys. Comm. 182, 409 (2011)

Key Strengths

BSE Accuracy:

• Excitonic effects included
• Core-hole screening
• Many-body treatment
• Accurate near-edge

First-Principles:

• No empirical parameters
• DFT-based workflow
• Periodic systems
• Multiple edges

Open Source:

• BSD licensed
• GitHub hosted
• Active development
• NIST supported

Inputs & Outputs

• Input formats: ocean.in (main input), DFT input files
• Output data types: Spectra files (energy vs intensity), absorption cross-sections

Interfaces & Ecosystem

• Quantum ESPRESSO: Supported DFT backend
• ABINIT: Supported DFT backend
• Parallelization: MPI supported

Workflow and Usage

1. Prepare DFT input structure.
2. Configure ocean.in (edges, screening parameters).
3. Run OCEAN script (automates DFT SCF, screening calculation, BSE Hamiltonian construction, and diagonalization).
4. Analyze spectral output.

Performance Characteristics

• Computationally intensive (BSE solving)
• Scales with system size and basis set
• Parallelized for clusters

Limitations & Known Constraints

• Computational cost: BSE is expensive
• Convergence: Requires careful k-point/band testing
• Multiplet effects: Limited treatment
• Learning curve: Complex setup

Comparison with Other Tools

• vs FEFF: OCEAN BSE-based, FEFF real-space MS
• vs exciting-XS: Both BSE, different DFT backends
• vs xspectra: OCEAN more accurate, xspectra faster
• Unique strength: First-principles BSE for core levels

Application Areas

• Transition metal oxides
• Battery materials (Li K-edge)
• Organic crystals
• Surface adsorbates

Best Practices

• Converge k-points and bands
• Test screening parameters
• Validate against experiment
• Use appropriate core-hole treatment

Community and Support

• Developed at NIST and University of Washington (Rehr group connection)
• Open-source (BSD)
• GitHub repository

Verification & Sources

Primary sources:

1. Homepage: https://www.nist.gov/services-resources/software/ocean
2. GitHub: https://github.com/times-physics/ocean
3. Publication: J. Vinson et al., Phys. Rev. B 83, 115106 (2011)

Confidence: VERIFIED

Verification status: ✅ VERIFIED

• Website: ACTIVE
• Documentation: AVAILABLE
• Source: OPEN (GitHub)
• Development: ACTIVE (NIST/Times Physics)
• Applications: BSE for X-ray spectroscopy