Phonon

From a directory of CIF files:

python3 {baseDir}/scripts/phonon_stability.py \
  --structures-dir ./relaxed_structures \
  --format json

From a single CIF file:

python3 {baseDir}/scripts/phonon_stability.py \
  --structure relaxed_LaH10.cif \
  --format json

With custom supercell size:

python3 {baseDir}/scripts/phonon_stability.py \
  --structures-dir ./candidates \
  --supercell 2,2,2 \
  --model uma-s-1p2 \
  --format json

Parameters

Output (JSON)

{
  "status": "COMPLETED",
  "results": [
    {
      "label": "LaH10_relaxed",
      "formula": "LaH10",
      "dynamically_stable": true,
      "min_frequency_THz": 0.23,
      "max_frequency_THz": 45.2,
      "n_imaginary_modes": 0,
      "n_modes": 33,
      "thermal_properties": {
        "300K": {
          "free_energy_kJ_per_mol": -12.5,
          "entropy_J_per_mol_K": 45.2,
          "heat_capacity_J_per_mol_K": 38.1
        }
      }
    }
  ]
}

Dynamic Stability Criteria

A structure is flagged as dynamically unstable if:

• Any phonon frequency at the Gamma point is below the imaginary threshold (default: -0.5 THz)
• Any phonon frequency at non-Gamma q-points is below 0 THz

Small negative frequencies near Gamma (> -0.5 THz) are typically numerical artifacts from the acoustic sum rule and are ignored.

How It Works

1. Load structure from CIF file
2. Create phonopy object with appropriate supercell
3. Generate symmetry-inequivalent displaced supercells (finite displacement method)
4. Compute forces on each displaced supercell using UMA
5. Build force constant matrix, apply symmetry and acoustic sum rule
6. Compute phonon frequencies at commensurate q-points
7. Check for imaginary modes → determine stability
8. Optionally compute thermal properties (free energy, entropy, heat capacity)