Phonon

Phonon Skill

Compute phonon properties (frequencies, DOS, thermal properties) and assess dynamic stability of crystal structures using the finite-displacement method via phonopy with forces from ML interatomic potentials (UMA/fairchem).

A structure is dynamically stable if all phonon frequencies are real (no imaginary modes). Imaginary frequencies indicate the structure is at a saddle point on the potential energy surface and would spontaneously distort.

Prerequisites

• phonopy installed (pip install phonopy)
• fairchem-core installed (for UMA calculator)
• ase installed
• GPU recommended for UMA force calculations

Scripts

phonon_stability.py — Check dynamic stability of crystal structures

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)