# VASP-DFPT & phonopy calculation#

## How to run#

VASP can calculate force constants in real space using DFPT. The procedure to calculate phonon properties may be as follows:

1. Prepare unit cell structure named, e.g., `POSCAR-unitcell`. The following structure is a conventional unit cell of NaCl.

```Na Cl
1.00000000000000
5.6903014761756712    0.0000000000000000    0.0000000000000000
0.0000000000000000    5.6903014761756712    0.0000000000000000
0.0000000000000000    0.0000000000000000    5.6903014761756712
4   4
Direct
0.0000000000000000  0.0000000000000000  0.0000000000000000
0.0000000000000000  0.5000000000000000  0.5000000000000000
0.5000000000000000  0.0000000000000000  0.5000000000000000
0.5000000000000000  0.5000000000000000  0.0000000000000000
0.5000000000000000  0.5000000000000000  0.5000000000000000
0.5000000000000000  0.0000000000000000  0.0000000000000000
0.0000000000000000  0.5000000000000000  0.0000000000000000
0.0000000000000000  0.0000000000000000  0.5000000000000000
```
2. Prepare a perfect supercell structure from `POSCAR-unitcell`, e.g.,

```% phonopy -d --dim="2 2 2" -c POSCAR-unitcell
```
3. Rename `SPOSCAR` created in (2) to `POSCAR` (`POSCAR-{number}` and `phonopy_disp.yaml` files will never be used.)

```% mv SPOSCAR POSCAR
```
4. Calculate force constants of the perfect supercell by running VASP with `IBRION = 8` and `NSW = 1`. An example of `INCAR` for insulator may be such like (just an example!):

```   PREC = Accurate
ENCUT = 500
IBRION = 8
EDIFF = 1.0e-08
IALGO = 38
ISMEAR = 0; SIGMA = 0.1
LREAL = .FALSE.
LWAVE = .FALSE.
LCHARG = .FALSE.
```
5. After finishing the VASP calculation, confirm `vasprun.xml` contains `hessian` elements, and then create `FORCE_CONSTANTS`:

```% phonopy --fc vasprun.xml
```
6. Run phonopy with the original unit cell `POSCAR-unitcell` and setting tag `FORCE_CONSTANTS = READ` or `--readfc` option, e.g., as found in `example/NaCl-VASPdfpt`

```% phonopy --dim="2 2 2" -c POSCAR-unitcell band.conf
_
_ __ | |__   ___  _ __   ___   _ __  _   _
| '_ \| '_ \ / _ \| '_ \ / _ \ | '_ \| | | |
| |_) | | | | (_) | | | | (_) || |_) | |_| |
| .__/|_| |_|\___/|_| |_|\___(_) .__/ \__, |
|_|                            |_|    |___/

1.1

Band structure mode
Settings: