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# pair_style pod command¶

## Syntax¶

pair_style pod


## Examples¶

pair_style pod
pair_coeff * * Ta_param.pod Ta_coefficients.pod Ta


## Description¶

New in version 22Dec2022.

Pair style pod defines the proper orthogonal descriptor (POD) potential (Nguyen). The mathematical definition of the POD potential is described from fitpod, which is used to fit the POD potential to ab initio energy and force data.

Only a single pair_coeff command is used with the pod style which specifies a POD parameter file followed by a coefficient file.

The coefficient file (Ta_coefficients.pod) contains coefficients for the POD potential. The top of the coefficient file can contain any number of blank and comment lines (start with #), but follows a strict format after that. The first non-blank non-comment line must contain:

• POD_coefficients: ncoeff

This is followed by ncoeff coefficients, one per line. The coefficient file is generated after training the POD potential using fitpod.

The POD parameter file (Ta_param.pod) can contain blank and comment lines (start with #) anywhere. Each non-blank non-comment line must contain one keyword/value pair. See fitpod for the description of all the keywords that can be assigned in the parameter file.

As an example, if a LAMMPS indium phosphide simulation has 4 atoms types, with the first two being indium and the third and fourth being phophorous, the pair_coeff command would look like this:

pair_coeff * * pod InP_param.pod InP_coefficients.pod In In P P


The first 2 arguments must be * * so as to span all LAMMPS atom types. The two filenames are for the parameter and coefficient files, respectively. The two trailing ‘In’ arguments map LAMMPS atom types 1 and 2 to the POD ‘In’ element. The two trailing ‘P’ arguments map LAMMPS atom types 3 and 4 to the POD ‘P’ element.

If a POD mapping value is specified as NULL, the mapping is not performed. This can be used when a pod potential is used as part of the hybrid pair style. The NULL values are placeholders for atom types that will be used with other potentials.

Examples about training and using POD potentials are found in the directory lammps/examples/PACKAGES/pod.

## Restrictions¶

This style is part of the ML-POD package. It is only enabled if LAMMPS was built with that package. See the Build package page for more info.

This pair style does not compute per-atom energies and per-atom stresses.

## Default¶

none

(Nguyen) Nguyen and Rohskopf, arXiv preprint arXiv:2209.02362 (2022).