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- Basic functionality
- General purpose electronic structure code:

finite molecular, 1D chain, 2D slab, or 3D bulk - Density functional, pseudopotential, contracted Gaussian LCAO basis
- Self-consistent total energies
- Analytic forces and stresses, with full Pulay terms

- Cartesian or lattice/fractional/direct units for atomic positions
- Cartesian representation of lattice/supercell vectors
- User-specified general symmetry
- Used to constrain geometry and cell updates, reduce BZ sample
- No automatic detection of symmetry
- No completion of geometry from symmetry-unique set. I.e., all atoms/cell parameters must be explicitly listed
- Library of symmetry definitions for many crystal symmetries

- Molecular dynamics
- Leapfrog Verlet integrators
- Equilibration and production MD phase
- Multiple MD methods: NVE; NVT: Berendsen, Hoover, temperature scaling

- Automated geometry and cell optimization
- Multiple optimization methods:

Modified Broyden, accelerated steepest descent, damped dynamics, steepest descent, ... - Full symmetry constraints enabled
- Constrained minimizations.

"freeze"/relax selected atoms

some ad hoc bond constraints, and axis-based atomic constraints

cell shape constraints - Uses results from previous step to accelerate SCF convergence

- Multiple optimization methods:
- Nudged elastic band (NEB) transition state finder
- MPI-parallel over images/beads/replicas, with parallel images
- Full suite of minimization methods
- Full symmetry constraints enabled

- Gamma-point, or complex k-point sampling
- User-specified k-vectors and weights,
*-or-* - Automatic grid generation and folding into Irreducible Zone
- Band structure (2.64)

- User-specified k-vectors and weights,
- Metallic or "closed shell" defect sampling methods
- Finite temperature (metallic) occupation method (Fermi-function)
- Closed shell (DDO-Discrete Defect Occupation), both 0K and finite temperature

- Limited post-processing tools
- Geometry history converter to MOLDEN or JMOL visualization
- Mulliken populations
- Density of states, projected density of states

- Restartable - automatic checkpoint/status/restart capability

- General purpose electronic structure code:
- Density Functional Approximation
- LDA: CAPZ - Ceperley/Alder as parameterized by Perdew/Zunger
- GGA: PBE, PW91, AM05, BLYP
- All available either with spin polarization, or without
- Fixed net spin, or automated optimization of spin polarization
- van der Waals via C6-corrections (Grimme-D2, and Kim/Goddard-ULG)
- No exact exchange (HSE in progress)
- No anti-ferromagnetism
- No time-dependent DFT (planned), GW, meta-GGA etc.

- Supercell Approximation
- Proper finite vs. periodic basis
- Basis explicitly finite in non-periodic directions

- Net charge -- choice of boundary conditions
- Jellium neutralization of charged supercells - or -
- More rigorous Local Moment CounterCharge (LMCC) treatment

- Finite Defect Supercall Approximation, rigorous localized charge defects in bulk
- LMCC method (mixed boundary conditions Poisson solver) for local charges
- Chemical potential leveling, common electron reservoir for charge
- Discrete Defect Occupation (DDO) for defect occupations
- Jost model bulk polarization for charge bulk defects

- Automatic LMCC treatment for molecular charge/dipole
- Automatic correction of slab dipole potential via LMCC
- External applied electric field (non-periodic "sawtooth")
- Double-layer (charged slab) electrostatic boundary conditions (2.63)
- No solvation fields (planned)
- No Berry Phase treatment

- Proper finite vs. periodic basis
- Atomic potentials and basis sets
- Deployed in libraries, and invoked in input files
- Extensive LDA atom library (70+ elements + terminators)
- Extensive GGA/PBE library (70+ elements + terminators)
- GGA/PW91: Only Pt, Al (no plans for more)
- AM05 : 23 atoms (as of 17-Jan-2014) and growing
- BLYP : H, and O,N,C, and Al,Si,K (Jun07) (no current plans for more)

- Norm-conserving pseudopotentials (optimized)
- Hamann/GNCPP for most atoms
- "Improved" Troullier-Martins for very "hard" atoms
- "Semi-local", i.e., non-separable form of potential used
- Highest angular projector used as local potential
- Non-linear core corrections (Louie,Froyen&Cohen) where needed
- No spin-polarized pseudopotentials
- No spin-orbit coupling (soon?)
- No projector operators, e.g., for hyperfine calculations

- Optimized high-quality contracted Gaussian basis sets
- Atom-centered and "floating" orbitals
- Double-zeta plus polarization (DZP) quality
- two radial degrees of freedom for strongly occupied shells
- single degree of freedom for polarization functions

- Developed and refined pseudo-variationally
- Pseudopotential/functional-specific basis sets
- Refinement in quantum calculations (e.g. C in diamond)
- Transferability checked (e.g. Ba in Ba metal and in BaO)

- No
*f*-orbitals

- Deployed in libraries, and invoked in input files
- User-friendly input files
- Self-documenting: user-friendly,-readable,-modifiable
- Text-driven, with mostly free-format data input
- Modular by function (setup,relaxation,cell-opt, etc.)
- Extensive web-based documentation, with Tutorials
- Intelligent defaults - not necessary to define all inputs
- GUI completed for basic functionality.

- Efficient, portable, compact implementation
- Sequential (serial) workstation version
- MPI task-parallel version (multi-processor - faster, but not bigger)

Note: data-parallel development in progress, for bigger and faster - MPI parallel-parallel-NEB version (multi-processor per image/bead/replica)
- k-parallel memory distribution, and fully k-parallel-parallel code (2.64)
- Simple makefile, typically few minutes to build executable

(usually, just type: "make*platform*") - Vanilla f77 implementation (g77-friendly)
- Highly-structured, disciplined style
- Very limited (safe and portable) extensions only
- Static memory (with internal allocation via emulated pointers)
- Passed arguments, no global data (with some emulated "objects")

- Uses BLAS/LAPACK for performance (MPI and SCALAPACK+BLACS for parallel eigensolves), no other external dependencies
- Portable: Linux, Mac, Compaq alphas, SGI, IBM, PC's (just because we could), ...

- Performance
- Compact memory usage, which enables >500 atoms on 2GB, >300 atoms on 1GB, ~160 atoms on 256MB workstation
- Linear scaling Hamiltonian (beyond second nearest neighbor)
- Turnover from O(N) Hamiltonian- to O(N^3) eigensolver-dominated computation at ~200 bulk atoms
- Very fast for systems 50-300 atoms, even with "hard" atoms.

- Local exchange (e.g. HSE), inprogress
- add new functionality (e.g., time-dependent DFT, anti-ferromagnetism, projectors, f-orbitals) to get better accuracy with improved DFT approximations and to be able to explore new physics
- add and organize post-processing to better analyze the results
- further improve computational performance (parallel and serial)
- enhance usability through building graphical user interfaces
- integrate with other (semi-)empirical simulation methods (molecular dynamics, force fields, solvation) in a "multiscale" approach

SeqQuest is NOT available under GPL, but is only available in a
limited fashion via no-fee licenses from Sandia National Labs.

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