# compute pe command

## Syntax

``` LAMMPS
compute ID group-ID pe keyword ...
```

-   ID, group-ID are documented in [compute](compute) command
-   pe = style name of this compute command
-   zero or more keywords may be appended
-   keyword = *pair* or *bond* or *angle* or *dihedral* or *improper* or
    *kspace* or *fix*

## Examples

``` LAMMPS
compute 1 all pe
compute molPE all pe bond angle dihedral improper
```

## Description

Define a computation that calculates the potential energy of the entire
system of atoms. The specified group must be \"all\". See the [compute
pe/atom](compute_pe_atom) command if you want per-atom energies. These
per-atom values could be summed for a group of atoms via the [compute
reduce](compute_reduce) command.

The energy is calculated by the various pair, bond, etc. potentials
defined for the simulation. If no extra keywords are listed, then the
potential energy is the sum of pair, bond, angle, dihedral, improper,
$k$-space (long-range), and fix energy (i.e., it is as though all the
keywords were listed). If any extra keywords are listed, then only those
components are summed to compute the potential energy.

The $k$-space contribution requires 1 extra FFT each timestep the energy
is calculated, if using the PPPM solver via the [kspace_style
pppm](kspace_style) command. Thus it can increase the cost of the PPPM
calculation if it is needed on a large fraction of the simulation
timesteps.

Various fixes can contribute to the total potential energy of the system
if the *fix* contribution is included. See the doc pages for [individual
fixes](fix) for details of which ones compute a potential energy.

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

The [fix_modify energy yes](fix_modify) command must also be specified
if a fix is to contribute potential energy to this command.
::::

A compute of this style with the ID of \"thermo_pe\" is created when
LAMMPS starts up, as if this command were in the input script:

``` LAMMPS
compute thermo_pe all pe
```

See the \"thermo_style\" command for more details.

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

## Output info

This compute calculates a global scalar (the potential energy). This
value can be used by any command that uses a global scalar value from a
compute as input. See the [Howto output](Howto_output) doc page for an
overview of LAMMPS output options.

The scalar value calculated by this compute is \"extensive\". The scalar
value will be in energy [units](units).

## Restrictions

> none

## Related commands

[compute pe/atom](compute_pe_atom)

## Default

none