(qe_interface)=

# Quantum ESPRESSO (QE) & phonopy calculation

Quantum ESPRESSO package itself has a set of the phonon calculation
system. But the document here explains how to calculate phonons using
phonopy, i.e., using the finite displacement and supercell approach.

## Supported QE-PW tags

Currently QE-PW tags that phonopy can read are shown below.  Only
`ibrav = 0` type representation of crystal structure is supported.
More tags may be supported on request.

```
nat, ntyp, celldm(1), ATOMIC_SPECIES, ATOMIC_POSITIONS, CELL_PARAMETERS
```

Chemical symbols with natural number for `ATOMIC_SPECIES` like `Xn` (`n>0`),
e.g.. `Fe1`, can be used. The formats of `X_*` and `X-*` are not supported. When
this extended symbol is used, masses of all atoms including usual chemical
symbols are read from QE structure file. Otherwise, masses of respective
chemical symbols implemented in phonopy are used. Note that when using the
extended symbol, if the unit cell of QE structure file is not a primitive cell,
and the primitive cell is defined by the transformation matrix (`PRIMITIVE_AXES`
tag or `--pa` option), atoms with the extended symbols in the unit cell have to
be mapped properly to those in the primitive cell.

## How to run

The procedure of QE-phonopy calculation is shown below using the
NaCl example found in `example/NaCl-QE` directory.

1) Read a QE-PW input file and create supercells with
   {ref}`qe_mode` option:

   ```bash
   % phonopy --qe -d --dim="2 2 2" -c NaCl.in
   ```

   In this example, 2x2x2 supercells are created. `supercell.in` and
   `supercell-xxx.in` (`xxx` are numbers) give the perfect
   supercell and supercells with displacements, respectively. In the
   case of the NaCl example, two files `supercell-001.in` and
   `supercell-002.in` are created. In these supercell files, lines
   only relevant to crystal structures are
   given. `phonopy_disp.yaml` is also generated, which contains
   information about supercell and displacements.

2) To make QE-PW input files, necessary setting information is added to
   `supercell-xxx.in` files, e.g., by:

   ```bash
   % for i in {001,002};do cat header.in supercell-$i.in >| NaCl-$i.in; done
   ```

   where `header.in` is specially made for this NaCl example and
   this file is found in `example/NaCl-QE` directory. This
   setting is of course dependent on systems and has to be written for
   each interested system. Note that supercells with displacements
   must not be relaxed in the force calculations, because atomic
   forces induced by a small atomic displacement are what we need for
   phonon calculation.

   Then QE-PW supercell calculations are executed to obtain force on
   atoms, e.g., as follows:

   ```bash
   % mpirun pw.x -i NaCl-001.in |& tee NaCl-001.out
   % mpirun pw.x -i NaCl-002.in |& tee NaCl-002.out
   ```

3) To create `FORCE_SETS`, that is used by phonopy,
   the following phonopy command is executed:

   ```bash
   % phonopy -f NaCl-001.out NaCl-002.out
   ```

   Here `.out` files are the saved text files of standard outputs of the
   QE-PW calculations. If more supercells with displacements were
   created in the step 1, all `.out` files are given in the above
   command. To run this command, `phonopy_disp.yaml` has to be located in
   the current directory because the information on atomic
   displacements stored in `phonopy_disp.yaml` are used to generate
   `FORCE_SETS`. See some more detail at
   {ref}`qe_force_sets_option`.

4) Now post-process of phonopy is ready to run. The unit cell file
   used in the step 1 has to be specified but `FORCE_SETS` is
   automatically read. Examples of post-process are shown below.

   ```
   % phonopy --qe -c NaCl.in -p band.conf
           _
     _ __ | |__   ___  _ __   ___   _ __  _   _
    | '_ \| '_ \ / _ \| '_ \ / _ \ | '_ \| | | |
    | |_) | | | | (_) | | | | (_) || |_) | |_| |
    | .__/|_| |_|\___/|_| |_|\___(_) .__/ \__, |
    |_|                            |_|    |___/
                                         1.13.0

   Python version 2.7.14
   Spglib version 1.10.3
   Calculator interface: qe
   Band structure mode
   Settings:
     Supercell: [2 2 2]
     Primitive axis:
       [ 0.   0.5  0.5]
       [ 0.5  0.   0.5]
       [ 0.5  0.5  0. ]
   Spacegroup: Fm-3m (225)
   Computing force constants...
   max drift of force constants: -0.001194 (zz) -0.000000 (zz)
   Reciprocal space paths in reduced coordinates:
   [ 0.00  0.00  0.00] --> [ 0.50  0.00  0.00]
   [ 0.50  0.00  0.00] --> [ 0.50  0.50  0.00]
   [ 0.50  0.50  0.00] --> [-0.00 -0.00  0.00]
   [ 0.00  0.00  0.00] --> [ 0.50  0.50  0.50]
   ...
   ```

   ```{image} NaCl-pwscf-band.png
   :width: 50%
   ```

   `--qe -c NaCl.in` is specific for the QE-phonopy
   calculation but the other settings are totally common among calculator
   interfaces such as

   ```
   % phonopy --qe -c NaCl.in --dim="2 2 2" [other-OPTIONS] [setting-file]
   ```

   For settings and command options, see
   {ref}`setting_tags` and {ref}`command_options`, respectively, and
   for examples, see {ref}`examples_link`.

### Non-analytical term correction (Optional)

To activate non-analytical term correction, {ref}`born_file` is
required. This file contains the information of Born effective charge
and dielectric constant. These physical values are also obtained from
the PW (`pw.x`) & PH (`ph.x`) codes in Quantum ESPRESSO
package. There are two steps. The first step is usual self-consistent
field (SCF) calculation
by and the second step is running its response function calculations
under DFPT.

For the SCF calculation, the input file `NaCl.in` looks like:

```
 &control
    calculation = 'scf'
    tprnfor = .true.
    tstress = .true.
    pseudo_dir = '/home/togo/espresso/pseudo/'
 /
 &system
    ibrav = 0
    nat = 8
    ntyp = 2
    ecutwfc = 70.0
 /
 &electrons
    diagonalization = 'david'
    conv_thr = 1.0d-9
 /
ATOMIC_SPECIES
 Na  22.98976928 Na.pbe-spn-kjpaw_psl.0.2.UPF
 Cl  35.453      Cl.pbe-n-kjpaw_psl.0.1.UPF
ATOMIC_POSITIONS crystal
 Na   0.0000000000000000  0.0000000000000000  0.0000000000000000
 Na   0.0000000000000000  0.5000000000000000  0.5000000000000000
 Na   0.5000000000000000  0.0000000000000000  0.5000000000000000
 Na   0.5000000000000000  0.5000000000000000  0.0000000000000000
 Cl   0.5000000000000000  0.5000000000000000  0.5000000000000000
 Cl   0.5000000000000000  0.0000000000000000  0.0000000000000000
 Cl   0.0000000000000000  0.5000000000000000  0.0000000000000000
 Cl   0.0000000000000000  0.0000000000000000  0.5000000000000000
CELL_PARAMETERS angstrom
 5.6903014761756712 0 0
 0 5.6903014761756712 0
 0 0 5.6903014761756712
K_POINTS automatic
 8 8 8 1 1 1
```

where more the k-point mesh numbers are specified. This may be exectued as:

```bash
% mpirun ~/espresso/bin/pw.x -i NaCl.in |& tee NaCl.out
```

Many files whose names stating with `pwscf` should be created. These
are used for the next calculation. The input file for the response
function calculations, `NaCl.ph.in`, is
created as follows:

```
 &inputph
  tr2_ph = 1.0d-14,
  epsil = .true.
 /
0 0 0
```

Similary `ph.x` is executed:

```bash
% mpirun ~/espresso/bin/ph.x -i NaCl.ph.in |& tee NaCl.ph.out
```

Finally the Born effective charges and dielectric constant are obtained in the
output file `NaCl.ph.out`. The `BORN` file has to be created following the
`BORN` format ({ref}`born_file`). The `BORN` file for this NaCl calculation
would be something like below:

```
default value
2.472958201 0 0 0 2.472958201 0 0 0 2.472958201
1.105385 0 0 0 1.105385 0 0 0 1.105385
-1.105385 0 0 0 -1.105385 0 0 0 -1.105385
```

This `BORN` file can be made using `phonopy-qe-born` command.

```bash
% phonopy-qe-born NaCl.in  | tee BORN
```

Once this is made, the non-analytical term correction is included
just adding the `--nac` option as follows:

```bash
% phonopy --qe --nac -c NaCl.in -p band.conf
```

```{image} NaCl-pwscf-band-NAC.png
:width: 50%
```

(qe_q2r)=

## Using `q2r.x` to create phonopy force constants file

**Experimental**

A parser of `q2r.x` output is implemented experimentally. Currently
command-line user interface is not prepared. Using the following
script, the force constants file readable by phonopy is
created. Probably thus obtained force constants are required to be
symmetrized by the translational invariance condition using
`FC_SYMMETRY = .TRUE.`.

```python
#!/usr/bin/env python

import sys
from phonopy.interface.qe import read_pwscf, PH_Q2R

primcell_filename = sys.argv[1]
q2r_filename = sys.argv[2]
cell, _ = read_pwscf(primcell_filename)
q2r = PH_Q2R(q2r_filename)
q2r.run(cell)
q2r.save()
```

Saving this script as `make_fc_q2r.py`, this is used as, e.g.,

```bash
% python make_fc_q2r.py NaCl.in NaCl.fc
```

This gives `phonopy_params_q2r.yaml` file that contains supercell force
constants.

### Non-analytical term correction

Treatment of non-analytical term correction (NAC) is different between
phonopy and QE. For insulator, QE automatically calculate dielectric
constant and Born effective charges at PH calculation when q-point
mesh sampling mode (`ldisp = .true.`), and these data are written in
the Gamma point dynamical matrix file (probably in `.dyn1`
file). When running `q2r.x`, these files are read including the
dielectric constant and Born effective charges, and the real space
force constants where QE-NAC treatment is done are written to the q2r
output file. This is not that phonopy expects. Therefore the
dielectric constant and Born effective charges data have to be removed
manually from the Gamma point dynamical matrix file before running
`q2r.x`. Alternatively Gamma point only PH calculation with 'epsil =
.false.' can generate the dynamical matrix file without the dielectric
constant and Born effective charges data. So it is possible to replace
the Gamma point file by this Gamma point only file to run `q2r.x`
for phonopy.

#### Creating BORN file

If the `q2r.x` output contains dielectric constant and Born
effective charges, the following script can generate `BORN` format
text.

```python
#!/usr/bin/env python

import sys
import numpy as np
from phonopy.structure.symmetry import elaborate_borns_and_epsilon
from phonopy.interface.qe import read_pwscf, PH_Q2R

primcell_filename = sys.argv[1]
q2r_filename = sys.argv[2]
cell, _ = read_pwscf(primcell_filename)
q2r = PH_Q2R(q2r_filename)
q2r.run(cell, parse_fc=False)
if q2r.epsilon is not None:
    borns, epsilon, _ = elaborate_borns_and_epsilon(
        cell,
        q2r.borns,
        q2r.epsilon,
        supercell_matrix=np.diag(q2r.dimension),
        symmetrize_tensors=True)
    print("default")
    print(("%13.8f" * 9) % tuple(epsilon.ravel()))
    for z in borns:
        print(("%13.8f" * 9) % tuple(z.ravel()))
```

Saving this script as `make_born_q2r.py`,

```bash
% python make_born_q2r.py NaCl.in NaCl.fc > BORN
```

#### NaCl example

NaCl example is found at
<https://github.com/phonopy/phonopy/tree/master/example/NaCl-QE-q2r>.

```bash
% phonopy-load phonopy_params_q2r.yaml --band="0 0 0  1/2 0 0  1/2 1/2 0  0 0 0  1/2 1/2 1/2" -p
```

```{image} NaCl-q2r-band-NAC.png
:width: 50%
```