SEQQUEST

Official Resources

• Homepage: https://dft.sandia.gov/quest/
• Documentation: https://dft.sandia.gov/quest/SeqsDocumentationPage.html
• Developer: Sandia National Laboratories
• License: Available to friendly users (contact required)

Overview

SEQQUEST (Sequential Quantum Electronic Structure Tool) is a general-purpose electronic structure code developed at Sandia National Laboratories for computing energies and forces of molecules, periodic surfaces (slabs), and bulk solids. It uses the LCAO approach with contracted-Gaussian basis sets and norm-conserving pseudopotentials, with linear-scaling algorithms for Hamiltonian generation.

Scientific domain: Surfaces, solids, molecules, defects, interfaces
Target user community: Materials scientists studying surface chemistry, catalysis, and solid-state materials

Theoretical Methods

• Density Functional Theory (DFT)
• Linear Combination of Atomic Orbitals (LCAO)
• Contracted-Gaussian basis sets
• Norm-conserving pseudopotentials
• LDA and GGA exchange-correlation functionals
• Spin-polarized calculations
• Linear-scaling Hamiltonian construction
• Self-consistent field (SCF)

Capabilities (CRITICAL)

• Ground-state electronic structure
• Total energy calculations
• Force calculations
• Geometry optimization
• Periodic systems (1D, 2D, 3D)
• Surface slabs
• Point defects
• Molecular calculations
• Spin-polarized magnetism
• Band structure and DOS
• Charge density analysis
• Linear-scaling Hamiltonian generation

Sources: Sandia National Laboratories, DOE Office of Science

Key Strengths

Linear-Scaling Hamiltonian:

• O(N) Hamiltonian matrix construction
• Efficient for large systems
• Exploits sparsity
• Parallel implementation

LCAO Efficiency:

• Compact Gaussian basis sets
• Localized description
• Efficient matrix elements
• Reduced computational cost

Surface Science Focus:

• Optimized for slabs
• Adsorption studies
• Surface reconstructions
• Adsorbate-surface interactions

Sandia Quality:

• Extensively tested
• Production-ready
• DOE-funded development
• Reliable results

Inputs & Outputs

• Input formats:

  • Native SEQQUEST input format
  • Atomic coordinates
  • Basis set files
  • Pseudopotential files

• Output data types:

  • Total energies
  • Forces
  • Optimized structures
  • Band structure
  • DOS
  • Charge densities

Interfaces & Ecosystem

• Web portals:

  • NSF nanoHUB access
  • memsHUB integration
  • Web-based job submission

• Preprocessing:

  • Structure generation tools
  • Slab builders
  • Defect constructors

Advanced Features

Surface Calculations:

• Slab geometry optimization
• Adsorption energies
• Surface energy calculations
• Work function determination
• Surface reconstructions

Defect Studies:

• Point defects in solids
• Vacancy formation energies
• Dopant calculations
• Defect levels

Magnetic Systems:

• Spin-polarized DFT
• Magnetic moment calculations
• Ferromagnetic ordering
• Antiferromagnetic states

Performance Characteristics

• Speed: Efficient LCAO implementation
• Accuracy: Reliable DFT results
• System size: Hundreds to thousand+ atoms
• Memory: Moderate requirements
• Parallelization: MPI parallel

Computational Cost

• Linear scaling: Hamiltonian generation O(N)
• SCF: Cubic diagonalization
• Typical: Workstation to cluster
• Large systems: Efficient on modest resources

Limitations & Known Constraints

• Availability: Not freely distributed
• Hybrid functionals: Limited support
• All-electron: Pseudopotential only
• Documentation: Internal focus
• Community: Sandia-centered
• Learning curve: Requires training

Comparison with Other Codes

• vs SIESTA: Similar LCAO approach
• vs VASP: SEQQUEST LCAO vs VASP plane-wave
• vs Gaussian: SEQQUEST materials focus
• Unique strength: Sandia-quality, surface science, linear-scaling Hamiltonian

Application Areas

Catalysis:

• Surface reactions
• Adsorbate binding
• Reaction barriers
• Catalyst design

Semiconductor Defects:

• Dopant energetics
• Native defects
• Defect complexes
• Carrier concentrations

Surface Chemistry:

• Oxidation
• Corrosion initiation
• Surface passivation
• Interface formation

Energy Materials:

• Battery materials
• Solar cell interfaces
• Fuel cell catalysts
• Hydrogen storage

Best Practices

Basis Set Optimization:

• Use appropriate contraction
• Test convergence
• Balance accuracy and cost

Surface Models:

• Sufficient slab thickness
• Vacuum gap size
• Symmetric vs asymmetric slabs
• k-point convergence

Community and Support

• Sandia National Laboratories
• DOE funding
• nanoHUB/memsHUB access
• Internal documentation

Verification & Sources

Primary sources:

1. Sandia DFT: https://dft.sandia.gov/quest/
2. DOE Office of Science: Sandia computational materials

Confidence: VERIFIED - Sandia National Laboratories official code

Verification status: ✅ VERIFIED

• Source code: Restricted (Sandia)
• Academic use: Collaborator access
• Documentation: Internal
• Production use: DOE projects
• Specialty: Surface science, linear-scaling Hamiltonian