openmaterials.ai / codes / phonopy
The finite-displacement supercell code behind most first-principles phonon work: symmetry-reduced displacements, force constants fitted to DFT forces, bands, DOS and free energies.
phonopy displaces atoms in a supercell by 0.01 A, a DFT code returns the forces, and crystal symmetry recovers the full second-order force constants. The Al example is a 3x3x3 conventional cell of 108 atoms (Togo 2015).
The cell is standardized to a crystallographic convention. Site symmetry picks the atoms to displace, and the space group recovers the other force constants (Togo 2023).
Dynamical matrix at any q, band structure, total and projected DOS, irreducible representations, group velocities, Cv, free energy and entropy versus T (Togo 2015).
Free energies at several volumes give G(T,p), equilibrium volume, thermal expansion, Cp and mode Gruneisen parameters. The Al Cp agrees with experiment (Togo 2015).
Born effective charges and the dielectric tensor add the long-range Coulomb part to the force constants, which the supercell approach cannot hold (Togo 2023).
phonopy builds supercells with random displacements drawn from harmonic oscillators at T and averages the properties computed on each to mimic temperature (Togo 2023).
Suspended monolayer hBN measured 751 W/m/K near room temperature and an experimental TEC of -3.58e-6/K. Phonopy gave the computed TECs of 1 to 3 layers for comparison (Cai 2019). Cai 2019
PHONOPY on DFT force constants gives the phonons of the phases that change at 165 and 327 K. The cubic phase has soft zone-boundary modes from PbI6 octahedral tilting (Brivio 2015). Brivio 2015
Bring the unit cell to the crystallographic convention and relax it until forces vanish.
Create supercells with symmetry-reduced finite displacements.
Run the DFT code on each supercell and collect the forces (FORCE_SETS).
Phonopy.force_constants from displacements and forces, Phonopy.nac_params if polar.
Phonopy.run_thermal_properties, run_total_dos, band paths, PhonopyQHA over volumes.