This is an example of the use of the DFTB+ interface. An input is
given in the dftb_in.hsd file along with the conventional unit cell in
geo.gen. This example also requires the pbc-0-3 Slater-Koster
parameteters for carbon
(https://www.dftb.org/parameters/download/pbc/pbc-0-3-cc/).

1) Create the (one) displaced supercell structure(s):
   phonopy -d --dim="4 4 4" --dftb+ 

2) Run the supercell input with dftb+. This has been pre-calculated
   with DFTB+ version 18.2, with the required data stored in the file
   results.tag.

3) Collect the forces:
   phonopy -f results.tag --dftb+  ...

4) Calculate phonon dispersion data into band.yaml and save band.pdf:
   phonopy --dftb+ --dim="4 4 4" -p -s band.conf
