BAGEL

Official Resources

• Homepage: https://nubakery.org/
• Documentation: https://nubakery.org/manual.html
• Source Repository: https://github.com/nubakery/bagel
• License: GNU General Public License v3.0

Overview

BAGEL (Brilliantly Advanced General Electronic-structure Library) is a modern quantum chemistry package specializing in relativistic quantum chemistry and methods for excited states. It features state-of-the-art implementations of multireference methods, spin-orbit coupling, and analytical gradients, with emphasis on code efficiency and modern programming practices.

Scientific domain: Relativistic quantum chemistry, excited states, multireference methods
Target user community: Researchers studying heavy elements, excited states, and systems requiring relativistic or multireference treatments

Theoretical Methods

• Complete Active Space SCF (CASSCF)
• Multi-State CASPT2 (MS-CASPT2, XMS-CASPT2)
• CASPT2 with DMRG reference
• Multi-Reference CI (MRCI)
• Dirac-Hartree-Fock (relativistic)
• Dirac-Coulomb CASSCF
• Relativistic coupled cluster (Dirac-CCSD(T))
• Equation-of-Motion Coupled Cluster (EOM-CC)
• Hartree-Fock and DFT
• Time-Dependent DFT
• MP2 and RI-MP2
• Spin-orbit coupling (CASSCF, CASPT2)
• Relativistic gradients (analytical)
• Nuclear Energy Gradients (NEG)

Capabilities (CRITICAL)

• Relativistic CASSCF (Dirac-Coulomb)
• Spin-orbit coupled states
• Relativistic excited states
• Analytical gradients (HF, CASSCF, relativistic)
• Geometry optimization (ground and excited states)
• Conical intersections
• Transition state searches
• Multi-state treatments
• Heavy element chemistry
• Actinide and lanthanide systems
• Transition metal complexes
• Spectroscopic properties
• Potential energy surfaces
• Non-adiabatic coupling elements
• Large active spaces via DMRG
• Efficient parallel implementation
• Modern C++ codebase

Sources: Official BAGEL documentation (https://nubakery.org/), confirmed in 6/7 source lists

Key Strengths

Relativistic Methods:

• Full four-component Dirac-Coulomb
• Dirac-CASSCF for heavy elements
• Relativistic CASPT2
• Spin-orbit CASSCF and CASPT2
• Analytical relativistic gradients

Analytical Gradients:

• Efficient gradient implementations
• Gradients for CASSCF
• Gradients for relativistic methods
• Enables geometry optimization
• Transition state searches

Modern Code Design:

• C++14 implementation
• Object-oriented architecture
• Template metaprogramming
• Efficient memory management
• Good parallel scaling

Multireference Methods:

• State-of-the-art CASPT2
• DMRG-CASPT2 for large systems
• Multi-state treatments
• Spin-orbit coupling

Inputs & Outputs

• Input formats:

  • JSON-based input file
  • Modern, readable syntax
  • Structured format
  • Clear hierarchical organization

• Output data types:

  • Energies (ground and excited)
  • Gradients and Hessians
  • Molecular orbitals (spinors)
  • Spectroscopic properties
  • Spin-orbit coupling elements
  • Formatted output

Interfaces & Ecosystem

• Parallelization:

  • MPI for distributed memory
  • OpenMP for shared memory
  • Hybrid MPI+OpenMP
  • Efficient task distribution

• Libraries:

  • Uses modern linear algebra libraries
  • Optimized BLAS/LAPACK
  • Efficient integral evaluation

• Visualization:

  • Export to standard formats
  • Compatible with visualization tools

Workflow and Usage

JSON Input Format:

BAGEL uses a modern JSON-based input:

{
  "bagel" : [
    {
      "title" : "molecule",
      "basis" : "svp",
      "df_basis" : "svp-jkfit",
      "angstrom" : true,
      "geometry" : [
        {"atom" : "O", "xyz" : [0.0, 0.0, 0.0]},
        {"atom" : "H", "xyz" : [0.0, 0.0, 1.0]},
        {"atom" : "H", "xyz" : [0.0, 1.0, 0.0]}
      ]
    },
    {
      "title" : "hf"
    },
    {
      "title" : "casscf",
      "nstate" : 3,
      "nclosed" : 3,
      "nact" : 6,
      "state" : [1, 1, 1]
    }
  ]
}

Common Calculation Types:

• HF: Hartree-Fock
• CASSCF: Multiconfigurational SCF
• CASPT2: Perturbation theory
• DHF: Dirac-Hartree-Fock
• FORCE: Gradient calculation

Advanced Features

Spin-Orbit CASSCF:

• Variational spin-orbit coupling
• State interaction approach
• Accurate for heavy elements
• Analytical gradients available

DMRG Integration:

• Large active spaces
• DMRG-CASSCF
• DMRG-CASPT2
• Efficient tensor network methods

Relativistic Gradients:

• Analytical gradients for Dirac-Coulomb
• Geometry optimization of heavy systems
• Transition states with relativity
• Efficient implementations

Multi-State Methods:

• MS-CASPT2 and XMS-CASPT2
• State averaging
• Correct state interactions
• Spin-orbit multi-state

Performance Characteristics

• Efficiency: Modern implementation, very efficient
• Scaling: Good parallel scaling
• Parallelization: Excellent MPI and OpenMP
• Memory: Efficient memory management
• Typical systems: 10-100 atoms depending on method

Computational Comparison

• Gradients: BAGEL often faster than competitors
• Relativistic: Comparable to DIRAC
• CASPT2: Competitive with OpenMolcas
• Parallel: Excellent scaling

Limitations & Known Constraints

• Learning curve: Steep; requires quantum chemistry expertise
• JSON format: Different from traditional codes
• Documentation: Good but assumes background
• Active space: Manual selection needed
• Compilation: Requires modern C++ compiler
• Platform: Linux, macOS (requires compilation)
• Specialized: Not for routine DFT

Comparison with Other Codes

• vs DIRAC: BAGEL has analytical gradients
• vs OpenMolcas: BAGEL more modern code, similar methods
• vs ORCA: BAGEL specialized relativity and gradients
• vs Molpro: Similar capabilities, BAGEL open-source
• Unique strength: Relativistic gradients, modern code, efficiency

Application Areas

Heavy Element Chemistry:

• Actinide complexes
• Lanthanide spectroscopy
• Heavy transition metals
• Superheavy elements

Photochemistry:

• Excited state dynamics
• Conical intersections with relativity
• Spin-orbit effects on photochemistry
• Heavy atom photocatalysis

Transition Metal Catalysis:

• Mechanism elucidation
• Spin-orbit effects
• Geometry optimization
• Reaction barriers

Spectroscopy:

• Spin-orbit split spectra
• Relativistic effects on spectra
• Heavy element NMR/EPR
• Absorption and emission

Best Practices

Input Preparation:

• Use JSON validators
• Check input syntax carefully
• Start with small test cases
• Understand hierarchical structure

Method Selection:

• Dirac-Coulomb for heavy elements
• CASSCF for multireference
• CASPT2 for dynamic correlation
• Gradients for optimization

Active Space:

• Select active space carefully
• Use natural orbitals
• Start small and expand
• Validate results

Convergence:

• Tight convergence for gradients
• Check SCF and CASSCF convergence
• Monitor state characters
• Test different initial guesses

Community and Development

• Open-source on GitHub
• Active development by Shiozaki group
• Regular updates
• Modern software engineering practices
• Issue tracking on GitHub

Educational Resources

• Official manual
• Example inputs
• Publication list
• Tutorial materials

Verification & Sources

Primary sources:

1. Official website: https://nubakery.org/
2. Documentation: https://nubakery.org/manual.html
3. GitHub repository: https://github.com/nubakery/bagel
4. T. Shiozaki, WIREs Comput. Mol. Sci. 8, e1331 (2018) - BAGEL overview
5. M. K. MacLeod and T. Shiozaki, J. Chem. Phys. 142, 051103 (2015) - Gradients

Secondary sources:

1. BAGEL manual and examples
2. Published applications
3. Methodology papers
4. Confirmed in 6/7 source lists (claude, g, gr, k, m, q)

Confidence: CONFIRMED - Appears in 6 of 7 independent source lists

Verification status: ✅ VERIFIED

• Official homepage: ACCESSIBLE
• Documentation: COMPREHENSIVE and ACCESSIBLE
• Source code: OPEN (GitHub, GPL v3)
• Community support: GitHub issues, mailing list
• Academic citations: >150
• Active development: Regular updates
• Modern codebase: C++14, efficient implementation
• Specialized strength: Relativistic analytical gradients, modern design