# fix pair command

## Syntax

    fix ID group-ID pair N pstyle name flag ...

-   ID, group-ID are documented in [fix](fix) command
-   pair = style name of this fix command
-   N = invoke this fix once every N timesteps
-   pstyle = name of pair style to extract info from (e.g. eam)
-   one or more name/flag pairs can be listed
-   name = name of quantity the pair style allows extraction of
-   flag = 1 if pair style needs to be triggered to produce data for
    name, 0 if not

## Examples

``` LAMMPS
fix request all pair 100 eam rho 0
fix request all pair 100 amoeba uind 0 uinp 0
```

## Description

::: versionadded
15Sep2022
:::

Extract per-atom quantities from a pair style and store them in this fix
so they can be accessed by other LAMMPS commands, e.g. by a [dump](dump)
command or by another [fix](fix), [compute](compute), or
[variable](variable) command.

These are example use cases:

-   extract per-atom density from [pair_style eam](pair_eam) to a dump
    file
-   extract induced dipoles from [pair_style amoeba](pair_amoeba) to a
    dump file
-   extract accuracy metrics from a machine-learned potential to trigger
    output when a condition is met (see the [dump_modify
    skip](dump_modify) command)

The *N* argument determines how often the fix is invoked.

The *pstyle* argument is the name of the pair style. It can be a
sub-style used in a [pair_style hybrid](pair_hybrid) command. If there
are multiple sub-styles using the same pair style, then *pstyle* should
be specified as \"style:N\", where *N* is the number of the instance of
the pair style you wish monitor (e.g., the first or second). For
example, *pstyle* could be specified as \"pace/extrapolation\" or
\"amoeba\" or \"eam:1\" or \"eam:2\".

One or more *name/flag* pairs of arguments follow. Each *name* is a
per-atom quantity which the pair style must recognize as an extraction
request. See the doc pages for individual [pair_styles](pair_style) to
see what fix pair requests (if any) they support.

The *flag* setting determines whether this fix will also trigger the
pair style to compute the named quantity so it can be extracted. If the
quantity is always computed by the pair style, no trigger is needed;
specify *flag* = 0. If the quantity is not always computed (e.g. because
it is expensive to calculate), then specify *flag* = 1. This will
trigger the quantity to be calculated only on timesteps it is needed.
Again, see the doc pages for individual [pair_styles](pair_style) to
determine which fix pair requests (if any) need to be triggered with a
*flag* = 1 setting.

The per-atom data extracted from the pair style is stored by this fix as
either a per-atom vector or array. If there is only one *name* argument
specified and the pair style computes a single value for each atom, then
this fix stores it as a per-atom vector. Otherwise a per-atom array is
created, with its data in the order of the *name* arguments.

For example, [pair_style amoeba](pair_amoeba) allows extraction of two
named quantities: \"uind\" and \"uinp\", both of which are 3-vectors for
each atom, i.e. dipole moments. In the example below a 6-column per-atom
array will be created. Columns 1-3 will store the \"uind\" values;
columns 4-6 will store the \"uinp\" values.

``` LAMMPS
pair_style amoeba
fix ex all pair 10 amoeba uind 0 uinp 0
```

## Restart, fix_modify, output, run start/stop, minimize info

No information about this fix is written to [binary restart
files](restart). None of the [fix_modify](fix_modify) options are
relevant to this fix.

As explained above, this fix produces a per-atom vector or array which
can be accessed by various [output commands](Howto_output). If an array
is produced, the number of columns is the sum of the number of per-atom
quantities produced by each *name* argument requested from the pair
style.

## Restrictions

none

## Related commands

[compute pair](compute_pair)

## Default

none