SIESTA

When to Use

• User explicitly requests SIESTA
• User needs linear-scaling O(N) DFT for very large systems (1000+ atoms)
• User wants numeric atomic orbital (NAO) basis sets
• User needs TDDFT or electron transport (TranSIESTA)

Prerequisites

1. SIESTA binary accessible on HPC (siesta --version)
2. Pseudopotentials available (.psf or .psml format)
3. Structure loaded in viewer — verify with catgo_view(action="get_state")

Workflow Steps

1. Verify structure

catgo_view(action="get_state")

2. Create workflow

catgo_workflow_engine(action="create", params={"name": "SIESTA relaxation"})

3. Add SIESTA task via shell

CatGo does not yet have a native SIESTA engine. Use task_type: "shell".

catgo_workflow_engine(action="add_task", params={
  "workflow_id": "wf_xxx",
  "task_type": "shell",
  "name": "siesta_relax",
  "command": "siesta < input.fdf > siesta.out 2>&1",
  "input_files": {
    "input.fdf": "<FDF input content>",
    "Si.psf": "{{pseudo_dir}}/Si.psf"
  },
  "system_name": "Si_bulk"
})

When a @register_engine("siesta") is added, use task_type: "geo_opt" with software: "siesta".

Input File Template — SCF

SystemName    TiO2_rutile
SystemLabel   tio2

NumberOfAtoms   <natoms>
NumberOfSpecies <nspecies>

%block ChemicalSpeciesLabel
  1  22  Ti
  2   8  O
%endblock ChemicalSpeciesLabel

PAO.BasisSize     DZP
PAO.EnergyShift   100 meV

LatticeConstant   1.0 Ang
%block LatticeVectors
  <a1x> <a1y> <a1z>
  <a2x> <a2y> <a2z>
  <a3x> <a3y> <a3z>
%endblock LatticeVectors

AtomicCoordinatesFormat Ang
%block AtomicCoordinatesAndAtomicSpecies
  <x> <y> <z>  <species_index>
%endblock AtomicCoordinatesAndAtomicSpecies

# Mesh and K-points
MeshCutoff        300 Ry
%block kgrid_Monkhorst_Pack
  <k1>  0  0  0.0
  0  <k2>  0  0.0
  0  0  <k3>  0.0
%endblock kgrid_Monkhorst_Pack

# SCF
MaxSCFIterations  200
DM.MixingWeight   0.1
DM.Tolerance      1.0d-4
XC.functional     GGA
XC.authors        PBE

# Electronic temperature
ElectronicTemperature  300 K

Relaxation Parameters

Add for geometry optimization:

MD.TypeOfRun      CG           # Conjugate gradient
MD.NumCGsteps     200
MD.MaxForceTol    0.02 eV/Ang
MD.VariableCell   .false.      # .true. for bulk cell optimization

For slabs, constrain atoms via %block GeometryConstraints.

Parameter Guidance

Parameter	Typical value	Notes
PAO.BasisSize	SZ / DZ / DZP / TZP	Single/double/triple-zeta + polarization
PAO.EnergyShift	50-200 meV	Basis confinement; lower = more diffuse, more accurate
MeshCutoff	200-400 Ry	Real-space grid fineness; 300 Ry usually sufficient
DM.MixingWeight	0.05-0.3	SCF mixing; lower for metals/difficult convergence
DM.Tolerance	1.0d-4	Density matrix convergence criterion
MaxSCFIterations	200	Increase for difficult systems

Common Pitfalls

1. MeshCutoff in Ry, not eV — 300 Ry = 4082 eV. Do not confuse with plane-wave cutoff.
2. Basis set quality — SZ is fast but inaccurate; DZP is the practical minimum for publishable results
3. Ghost atoms — PAO.EnergyShift too large can cause basis-set superposition error (BSSE)
4. Pseudopotential format — use .psf (Siesta native) or .psml (PSML standard). Not UPF.
5. Linear scaling — enable with SolutionMethod OrderN only for >1000 atoms with a gap. Metals need diagonalization.
6. Coordinate format — verify AtomicCoordinatesFormat matches your data (Ang vs Fractional vs Bohr)
7. Memory for diagonalization — large systems with SolutionMethod diagon need significant memory; consider OrderN or parallelization