Tutorial using VASP as calculator

Pre-process

The input stureture of POSCAR is supposed to be this.

In the pre-process, supercell structures with (or without) displacements are created from a unit cell fully consiering crystal symmetry.

To obtain supercells () with displacements, run phonopy:

phonopy -d --dim="2 2 3"

You should find the files, SPOSCAR, disp.yaml, and POSCAR-{number} as follows:

% ls
disp.yaml  POSCAR  POSCAR-001  POSCAR-002  POSCAR-003  SPOSCAR

SPOSCAR is the perfect supercell structure, disp.yaml contains the information on displacements, and POSCAR-{number} are the supercells with atomic displacements. POSCAR-{number} corresponds to the different atomic displacements written in disp.yaml.

Calculation of sets of forces

Force constants are calculated using the structure files POSCAR-{number} (from forces on atoms) or using the SPOSCAR file (direct calculation of force constants) by your favorite calculator. See the details.

In the case of VASP, the calculations for the finite displacement method can be proceeded just using the POSCAR-{number} files as POSCAR of VASP calculations. An example of the INCAR is as follows:

   PREC = Accurate
 IBRION = -1
  ENCUT = 500
  EDIFF = 1.0e-08
 ISMEAR = 0; SIGMA = 0.01
  IALGO = 38
  LREAL = .FALSE.
ADDGRID = .TRUE.
  LWAVE = .FALSE.
 LCHARG = .FALSE.

Be careful not to relax the structures. Then create FORCE_SETS file using VASP interface:

% phonopy -f disp-001/vasprun.xml disp-002/vasprun.xml disp-003/vasprun.xml

or

% phonopy -f disp-{001..003}/vasprun.xml

If you want to calculate force constants by VASP-DFPT directory, see VASP-DFPT & phonopy calculation.

Post-process

In the post-process,

1. Force constants are calculated from the sets of forces
2. A part of dynamical matrix is built from the force constants
3. Phonon frequencies and eigenvetors are calculated from the dynamical matrices with the specified q-points.

For mesh sampling calculation, prepare the following setting file named, e.g., mesh.conf:

ATOM_NAME = Si O
DIM = 2 2 3
MP = 8 8 8

The density of states (DOS) is plotted by:

% phonopy -p mesh.conf

Thermal properties are calculated with the sampling mesh by:

% phonopy -t mesh.conf

You should check the convergece with respect to the mesh numbers. Thermal properties can be plotted by:

% phonopy -t -p mesh.conf

Projected DOS is calcualted by the following setting file named, e.g., pdos.conf:

ATOM_NAME = Si O
DIM = 2 2 3
MP = 8 8 8
PDOS = 1 2, 3 4 5 6

and plotted by:

% phonopy -p pdos.conf

Band structure is calculated with the following setting file named, e.g., band.conf by:

ATOM_NAME = Si O
DIM =  2 2 3
BAND = 0.5 0.5 0.5  0.0 0.0 0.0  0.5 0.5 0.0  0.0 0.5 0.0

The band structure is plotted by:

% phonopy -p band.conf

In either case, by setting the -s option, the plot is going to be saved in the PDF format. If you don’t need to plot DOS, the (partial) DOS is just calculated using the --dos option.

Details

Following files are required in your working directory.

• POSCAR, and FORCE_SETS or FORCE_CONSTANTS
• disp.yaml is required to create FORCE_SETS.

In the case of finite difference approach, there are three steps.

1. Create supercells and introduce atomic displacements. Each supercell contains one atomic displacement. It is done by using -d option with --dim option that specifies supercell dimension. The files of supercells with atomic displacements like as POSCAR-001, POSCAR-002, ..., are created in current directory (the file format and names are different in WIEN2k mode.) by running phonopy. The files disp.yaml and SPOSCAR are also created. The file SPOSCAR is the perfect supercell that contains no atomic displacement. This file is not usually used.
2. Calculate forces on atoms of the supercells with atomic displacements. Currently phonopy has VASP and WIEN2k interfaces to create FORCE_SETS. After obtaining forces on atoms that calculated by some calculator (it’s out of phonopy), the forces are summarized in FORCE_SETS file following the format.
3. Calculate phonon related properties. See Features.

If you already have force constants, the first and second steps can be omitted. However your force constants have to be converted to the format that phonopy can read. The VASP interface to convert force constants is prepared in phonopy.