# compute pair command

## Syntax

``` LAMMPS
compute ID group-ID pair pstyle [nstyle] [evalue]
```

-   ID, group-ID are documented in [compute](compute) command
-   pair = style name of this compute command
-   pstyle = style name of a pair style that calculates additional
    values
-   nsub = *n*-instance of a sub-style, if a pair style is used multiple
    times in a hybrid style
-   *evalue* = *epair* or *evdwl* or *ecoul* or blank (optional)

## Examples

``` LAMMPS
compute 1 all pair gauss
compute 1 all pair lj/cut/coul/cut ecoul
compute 1 all pair tersoff 2 epair
compute 1 all pair reaxff
```

## Description

Define a computation that extracts additional values calculated by a
pair style, and makes them accessible for output or further processing
by other commands.

:::: note
::: title
Note
:::

The group specified for this command is **ignored**.
::::

The specified *pstyle* must be a pair style used in your simulation
either by itself or as a sub-style in a [pair_style hybrid or
hybrid/overlay](pair_hybrid) command. If the sub-style is used more than
once, an additional number *nsub* has to be specified in order to choose
which instance of the sub-style will be used by the compute. Not
specifying the number in this case will cause the compute to fail.

The *evalue* setting is optional. All pair styles tally a potential
energy *epair* which may be broken into two parts: *evdwl* and *ecoul*
such that *epair* = *evdwl* + *ecoul*. If the pair style calculates
Coulombic interactions, their energy will be tallied in *ecoul*.
Everything else (whether it is a Lennard-Jones style van der Waals
interaction or not) is tallied in *evdwl*. If *evalue* is blank or
specified as *epair*, then *epair* is stored as a global scalar by this
compute. This is useful when using [pair_style hybrid](pair_hybrid) if
you want to know the portion of the total energy contributed by one
sub-style. If *evalue* is specified as *evdwl* or *ecoul*, then just
that portion of the energy is stored as a global scalar.

:::: note
::: title
Note
:::

The energy returned by the *evdwl* keyword does not include tail
corrections, even if they are enabled via the [pair_modify](pair_modify)
command.
::::

Some pair styles tally additional quantities, e.g. a breakdown of
potential energy into 14 components is tallied by the [pair_style
reaxff](pair_reaxff) command. These values (1 or more) are stored as a
global vector by this compute. See the page for [individual pair
styles](pair_style) for info on these values.

## Output info

This compute calculates a global scalar which is *epair* or *evdwl* or
*ecoul*. If the pair style supports it, it also calculates a global
vector of length $\ge$ 1, as determined by the pair style. These values
can be used by any command that uses global scalar or vector values from
a compute as input. See the [Howto output](Howto_output) doc page for an
overview of LAMMPS output options.

The scalar and vector values calculated by this compute are
\"extensive\".

The scalar value will be in energy [units](units). The vector values
will typically also be in energy [units](units), but see the page for
the pair style for details.

## Restrictions

> none

## Related commands

[compute pe](compute_pe), [compute bond](compute_bond), [fix
pair](fix_pair)

## Default

The keyword defaults are *evalue* = *epair*, nsub = 0.