Phonon

Workflow

Step 1. Start from a relaxed structure

If the user has not yet relaxed the geometry under the same potential, do that first in the gpumd/md subskill using a small NPT segment followed by structure extraction, or call an external relaxation tool.

Step 2. Write run.in

Annotated minimal example for silicon (see assets/examples/phonon/run.in):

potential       Si_Fan_2019.txt
replicate       8 8 8
compute_phonon  5 0.005

• potential Si_Fan_2019.txt
  • Loads the stable potential. Use the exact filename shipped with the user's tutorial or NEP fit.
• replicate 8 8 8
  • Builds the supercell from the primitive cell stored in model.xyz. The supercell must be large enough to contain the physically relevant range of the interatomic force constants. Increase until the dispersion stops changing.
• compute_phonon 5 0.005
  • 5 = cutoff in Å used when collecting pairwise force-constant contributions.
  • 0.005 = finite-displacement amplitude in Å. 0.005-0.01 Å is the conservative starting range.

If a band-structure path is requested, also supply kpoints.in (see below).

Step 3. Write kpoints.in

Each non-blank line contains three fractional coordinates and one label:

0.000 0.000 0.000 G
0.500 0.000 0.500 X
0.375 0.375 0.750 K
0.000 0.000 0.000 G
0.500 0.500 0.500 L

Use blank lines between path segments that should not be joined.

Step 4. Run and inspect

gpumd < run.in | tee gpumd.log

Primary outputs:

• D.out — dynamical matrix
• omega2.out — squared frequencies along kpoints.in

Interpretation:

• negative omega^2 corresponds to imaginary frequencies
• persistent imaginary branches can indicate a true structural instability OR poor relaxation / convergence — never report without cross-checking

Step 5. Converge

Do at least one convergence pass on:

• supercell size (replicate)
• displacement amplitude (compute_phonon second argument)
• force cutoff (compute_phonon first argument)

Phonon DOS via NVE trajectory

For a vibrational density of states from direct velocity autocorrelation (no finite displacements), use compute_dos during an NVE run:

potential   nep.txt
velocity    300
time_step   1
ensemble    npt_ber 300 300 100 0 0 0 100 100 100 1000
run         20000

ensemble    nve
compute_dos 5 200 400.0
dump_thermo 100
run         40000

The compute_dos arguments are the VAC sampling interval, the number of time correlation points, and the maximum frequency in THz. See the GPUMD docs for the exact current meaning.

Convergence checklist

• structure was relaxed under the same potential
• at least one larger supercell tested for the same dispersion
• displacement amplitude tested in the 0.005-0.01 Å range
• Γ-point acoustic modes approach zero (not a large imaginary value)
• potential stability confirmed in a short NVE sanity run

What this subskill does NOT own

• lattice thermal conductivity → gpumd/transport
• fitted IFC (fc2, fc3) and anharmonic transport → outside bare compute_phonon; point the user to references/phonon-workflow.md and the external 25_lattice_dynamics_kappa tutorial.

Cross-skill pointers

• For downstream phonon analysis (phonon DOS, thermal properties, band-structure plotting) beyond what GPUMD's compute_phonon provides directly, consider the phonopy skill which handles finite-displacement phonon workflows with force-constant post-processing.
• GPUMD's force-constant output can serve as input for phonopy-based analysis tools.

Read first

• references/phonon-workflow.md

Read when needed:

• references/core-files-and-ensembles.md
• references/gpumd-keyword-cheatsheet.md
• references/tutorial-map.md

Bundled templates

• assets/examples/phonon/model.xyz
• assets/examples/phonon/run.in
• assets/examples/phonon/kpoints.in

Expected output

1. a phonon-ready GPUMD input set
2. a convergence checklist for supercell size and displacement amplitude
3. the expected output files and how they will be checked

References

• compute_phonon: https://gpumd.org/gpumd/input_parameters/compute_phonon.html
• compute_dos: https://gpumd.org/gpumd/input_parameters/compute_dos.html
• GPUMD-Tutorials example 06_Silicon_phonon_dispersion
• GPUMD-Tutorials example 02_Carbon_density_of_states