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Ab Initio Phonons

The ab initio procedure of phonon calculations is usually as follows:

1. In Phonon Software the crystal and supercell are build up. For that one should know the crystal space group, approximate lattice parameters and atomic positions. The file with the above crystallographic data is exported from Phonon to the DFT software.

catio3 2. In DFT software the crystal is optimized. Thus, lattice parameters, atomic positions, and external stress components (pressure) acting on the supercell are established. Since this configuration corresponds to equilibrium, all forces acting on all atoms, so called Hellmann-Feynman forces, should be negligible, i.e. less then 0.000100 eV/Å.

3. The optimized crystallographic parameters are returned from DFT to Phonon Software in order to derive the necessary atomic displacements needed to calculate the lists of Hellmann-Feynman forces.

4. In DFT software all lists of Hellmann-Feynman forces are calculated, and collected in a single file called HFFILE. This file must be exported to Phonon Software.

5. Remaining operations are done within Phonon Software. There, the phonon dispersion relations, phonon density of states, thermodynamical functions, spectroscopic spectra, and other quantities can be obtained and plotted.