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(qlm_interface)=
# Questaal & phonopy calculation
This is a tutorial for the interface between phonopy and Questaal
[https://www.questaal.org/](https://www.questaal.org/).
The interface uses the site files. Such files have only a small amount of
information within them relating to the calculation. Extention to support
ctrl files is planned.
## How to run
The following is a walkthrough for a phonopy calculation with LM:
1. Setup an `lmf` calculation with external site file instead of a `site`
section in the ctrl file. Both relative and cartesian coordinates are
supported. Remember to set `forces=1` in section `ham` of the ctrl file.
2. Read the LM site file and create supercells with a command of the form,
```bash
phonopy --qlm -d --dim='2 2 2' -c site.lm
```
In this example, 2x2x2 supercells are created. `supercell.lm` and
`supercell-{id}.lm` correspond to the perfect supercell and supercells
with displacements, respectively. These supercell files are LM site files
ready to be used in `ctrl.lm`. The original `alat` present in site.lm is
preserved. A file named `phonopy_disp.yaml` is also created in the current
directory. This file contains information pertaining to displacements.
3. Next, run the calculations for the generated displacements and be sure to
use the `--wforce=force` flag. This will write a file containing forces.
Phonopy will require this file to be read in the next step.
4. Create `FORCE_SETS` by:
```bash
phonopy --qlm -f supercell-001/force.lm supercell-002/force.lm ...
```
To run this command, `phonopy_disp.yaml` has to be located in the current
directory because the atomic displacements are written into the
`FORCE_SETS` file.
5. Run post-process of phonopy with the LM site file for the unit cell used in
the first step
```bash
phonopy --qlm -c site.lm -p band.conf
```
or
```bash
phonopy --qlm -c site.lm --dim="2 2 2" [other-OPTIONS] [setting-file]
```
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