# compute ke command

## Syntax

``` LAMMPS
compute ID group-ID ke
```

-   ID, group-ID are documented in [compute](compute) command
-   ke = style name of this compute command

## Examples

``` LAMMPS
compute 1 all ke
```

## Description

Define a computation that calculates the translational kinetic energy of
a group of particles.

The kinetic energy of each particle is computed as $\frac{1}{2} m
v^2$, where *m* and *v* are the mass and velocity of the particle,
respectively.

There is a subtle difference between the quantity calculated by this
compute and the kinetic energy calculated by the *ke* or *etotal*
keyword used in thermodynamic output, as specified by the
[thermo_style](thermo_style) command. For this compute, kinetic energy
is \"translational\" kinetic energy, calculated by the simple formula
above. For thermodynamic output, the *ke* keyword infers kinetic energy
from the temperature of the system with $\frac{1}{2} k_B T$ of energy
for each degree of freedom. For the default temperature computation via
the [compute temp](compute_temp) command, these are the same. However,
different computes that calculate temperature can subtract out different
non-thermal components of velocity and/or include different degrees of
freedom (translational, rotational, etc.).

## Output info

This compute calculates a global scalar (the summed KE). 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 erotate/sphere](compute_erotate_sphere)

## Default

none