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Merge pull request #259 from phonopy/rd-doc
Better support of random displacements
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(random-displacements)= | ||
# Randan displacements | ||
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Random displacements and corresponding forces in supercells can be employed as a | ||
displacement-force dataset for computing force constants. This requires an | ||
external force constants calculator, e.g., symfc or ALM. Here, examples are | ||
presented with using symfc that can be installed via pip or conda easily. | ||
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## Related setting tags | ||
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- {ref}`random_displacements_option` (`--rd`, `--random-seed`) | ||
- {ref}`fc_calculator_option` (`--fc-calc`) | ||
- {ref}`fc_calculator_options_option` (`--fc-calc-opt`) | ||
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## Generation of random directional displacements | ||
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The option `--rd NUM` is used instead of `-d` in generating displacements as follows: | ||
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```bash | ||
% phono3py --rd 100 --dim 2 2 2 --pa auto -c POSCAR-unitcell | ||
``` | ||
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`NUM` means the number of supercells with random directional displacements. This | ||
must be specified, and the initial guess may be from around the number of | ||
supecells generated for the systematic displacements by `-d`. In the case of the | ||
`NaCl-rd` example, 146 supercells are generated with `-d`, so similar | ||
number `--rd 100` was chosen here. | ||
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If random directional displacements for fc2 are expected, `--rd-fc2` and | ||
`--dim-fc2` have to be specified: | ||
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```bash | ||
% phono3py --rd 100 --dim 2 2 2 --rd-fc2 2 --dim-fc2 4 4 4 --pa auto -c POSCAR-unitcell | ||
``` | ||
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where `--dim` is necessary but `--rd` is not. |
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# Example of using random directional displacements | ||
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## How to use symfc | ||
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This example utilizes an NaCl calculation result from A. Togo and A. Seko, J. | ||
Chem. Phys. 160, 211001 (2024). Supercells of 2x2x2 and 4x4x4 conventional unit | ||
cells are chosen for the third-order force constants (fc3) and second-order | ||
force constants (fc2), respectively. Displacement-force datasets consisting of | ||
100 supercells for fc3 and 2 supercells for fc2 are extracted and stored in | ||
`phono3py_params_NaCl.yaml.xz`. Random directional displacements of a constant | ||
0.03 Angstrom are used. | ||
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To calculate force constants, an external force constants calculator is | ||
necessary. Here, the symfc tool (available at https://github.com/symfc/symfc) is | ||
used, which can be easily installed via pip or conda. | ||
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The `fc3.hdf5` and `fc2.hdf5` are computed using the command: | ||
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``` | ||
% phono3py-load phono3py_params_NaCl.yaml.xz --symfc -v | ||
``` | ||
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Lattice thermal conductivity (LTC) is calculated with the following command: | ||
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``` | ||
% phono3py-load phono3py_params_NaCl.yaml.xz --br --ts 300 --mesh 50 | ||
``` | ||
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By this, LTC is obtained around 8.3 W/m-k. | ||
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## How to use pypolymlp | ||
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When supercell energies are included in `phono3py_params.yaml` like file, the | ||
polynomial machine learning potential (poly-MLP) by pypolymlp can be used to | ||
calculate fc3 by the following command: | ||
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```bash | ||
% phono3py-load phono3py_params_NaCl.yaml.xz --pypolymlp --symfc --rd 400 -v | ||
``` | ||
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the procedure below is performed: | ||
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1. Poly-MLPs are computed from the displacement-force dataset for fc3. This is | ||
activated by `--pypolymlp` option. | ||
2. 800=400+400 supercells for random directional displacements are generated, | ||
where 400+400 means 400 supercells with random displacements (u) and 400 | ||
supercells with opposite displacement vectors (-u). This is activated by | ||
`--rd 400` option. The default displacement distance is 0.001 Angstrom in | ||
`--pypolymlp` mode. Since random displacements are generated `--symfc` has to | ||
be specified for fc3. In this example, random displacements are used for fc2, | ||
too, `--symfc` is applied to both of fc3 and fc2. Without `--rd` option, | ||
systematic displacements are generated, for which the option `--fc-calc "symfc|"` | ||
has to be specified instead of `--symfc` (equivalent to `--fc-calc "symfc|symfc")`). | ||
3. Forces on atoms in these 800 supercells are calculated using poly-MLP. | ||
4. Force constants are calculated. | ||
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The `fc3.hdf5` and `fc2.hdf5` are obtained. Using these force constants, LTC is | ||
calculated by | ||
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```bash | ||
% phono3py-load phono3py_params_NaCl.yaml.xz --br --ts 300 --mesh 50 | ||
``` | ||
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and the LTC value of around 8.2 W/m-k is obtained. | ||
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## Generating phono3py_params.yaml from vasprun.xml's | ||
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`phono3py_params.yaml` is generated from | ||
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```bash | ||
% phono3py phono3py_disp.yaml --cf3 NaCl-vasprun/vasprun-{00001..00100}.xml --cf2 NaCl-vasprun/vasprun-ph0000{1,2}.xml --sp | ||
``` | ||
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This command reads electronic energies of supercells from `vasprun.xml`s and | ||
writes them into `phono3py_params.yaml`, too. Here, `phono3py_disp.yaml` is not | ||
included in this example, but `phono3py_params_NaCl.yaml.xz` can be used to run | ||
this example since corresponding information of displacements is included in | ||
this file, too. |
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