# pair_style pod command

## Syntax

``` LAMMPS
pair_style pod
```

## Examples

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

## Description

::: versionadded
22Dec2022
:::

Pair style *pod* defines the proper orthogonal descriptor (POD)
potential [(Nguyen)](Nguyen20221). The mathematical definition of the
POD potential is described from [fitpod](fitpod_command), 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](fitpod_command).

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](fitpod_command) 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:

``` LAMMPS
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](Build_package) page
for more info.

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

## Related commands

[fitpod](fitpod_command),

## Default

none

------------------------------------------------------------------------

::: {#Nguyen20221}
**(Nguyen)** Nguyen and Rohskopf, arXiv preprint arXiv:2209.02362
(2022).
:::