DFTB+ & phonopy calculation#

How to run#

DFTB+ phonon band structures are created as follows:

Create a DFTB+ input file dftb_in.hsd that is set up to perform a single-point energy and force calculation for a structure which is named geo.gen. The dftb_in.hsd file should turn on force evaluation by setting CalculateForces = Yes in its analysis block, and write the tagged results by enabling WriteResultsTag = Yes in its options.
Generate the the required set of structures and the phonopy_disp.yaml file by issuing the command
```
% phonopy -d --dim="4 4 4" --dftb+
```
This example builds 4 x 4 x 4 supercell files. The undistorted supercell is stored in geo.genS, while the required displacements are stored in files matching the pattern geo.genS-*. Note that you have to increase the supercell dimension until you reach convergence of the band structure.

For each each geo.genS-* structure perform a DFTB+ calculations, retaining the resulting results.tag file.
Create the FORCE_SETS file with the command
```
% phonopy -f disp-*/results.tag --dftb+  ...
```
Where the location of all of the results.tag files is given on the command line. To run this command, the phonopy_disp.yaml file has to be located in the current directory, because the atomic displacements are written into the FORCE_SETS file.
Create a band.conf file to specify the path in the Brillouin zone you are interested in (see the phonopy documentation). Then post-process the phonopy data, either in the settings file (DIM) or by providing the dimensions of the the supercell repeat on the command line
```
% phonopy -p band.conf --dim="4 4 4" --dftb+
```

Create a band structure in gnuplot format

% phonopy-bandplot --gnuplot band.yaml > band.dat

All major phonopy options should be available for the DFTB+ interface.