Rowan Docs

Phonons

Python API · stjames models · API examples

How it works

Calculate crystal vibrations, phonon band structures, densities of states, and harmonic thermodynamic properties to assess a periodic structure's dynamical stability. Rowan first relaxes the atoms and cell, then uses small atomic displacements and the resulting forces to calculate vibrations with phonopy.

Settings

  • Input: use a structure periodic in two or three dimensions. Conventional 3D cells are reduced automatically to primitive cells. For 2D structures, supply a primitive cell in the standard band-path orientation and mark the vacuum direction as non-periodic.
  • Level of theory: choose a periodic method that supports force calculations. The same method relaxes the structure and calculates displaced-supercell forces.
  • Cell optimization: “Optimize cell?” is required. Phonons are calculated only after relaxation converges.
  • Supercell: “Supercell” sets the expansion along A, B, and C, initially (2, 2, 2). Expansion is fixed at 1 along non-periodic directions. These factors apply to the primitive cell after any automatic reduction.
  • Optimization accuracy: “Geometry optimization mode” controls convergence. “Auto” resolves to “Careful.” Constraints and transition-state optimization are unsupported.

Notes

A phonon is a collective vibration of atoms in a crystal. Larger supercells capture longer-range interactions but cost more; check that key frequencies converge with supercell size. Results describe the relaxed structure, which may have different symmetry from the input.

Results

The workflow returns phonon frequencies along standard high-symmetry paths, total and per-atom densities of states, and zone-center (Γ) mode displacement patterns. Γ modes have symmetry labels and IR/Raman selection rules when assignable; these do not provide spectral intensities.

The stability summary gives the minimum frequency and the fraction of sampled frequencies below −1 cm−1. Imaginary frequencies appear as negative values. Substantial imaginary modes can indicate instability, incomplete relaxation, or numerical error; small negative acoustic frequencies near Γ can be numerical noise. This complements the elastic tensor, which tests homogeneous cell distortions. A stable phonon spectrum does not establish stability against other crystal phases.

Thermal results include vibrational Helmholtz free energy, entropy, constant-volume heat capacity, and zero-point energy per primitive cell, from 0 to 1000 K. Imaginary modes are excluded from thermal integration, so check stability before interpreting these curves. The harmonic approximation omits thermal expansion and phonon–phonon interactions. For polar crystals, the workflow omits long-range electric-field corrections that can affect frequencies near Γ.

Further reading