# compute erotate/rigid command

## Syntax

``` LAMMPS
compute ID group-ID erotate/rigid fix-ID
```

-   ID, group-ID are documented in [compute](compute) command
-   erotate/rigid = style name of this compute command
-   fix-ID = ID of rigid body fix

## Examples

``` LAMMPS
compute 1 all erotate/rigid myRigid
```

## Description

Define a computation that calculates the rotational kinetic energy of a
collection of rigid bodies, as defined by one of the [fix
rigid](fix_rigid) command variants.

The rotational energy of each rigid body is computed as
$\frac12 I \omega_\text{body}^2$, where $I$ is the inertia tensor for
the rigid body and $\omega_\text{body}$ is its angular velocity vector.
Both $I$ and $\omega_\text{body}$ are in the frame of reference of the
rigid body (i.e., $I$ is diagonal).

The *fix-ID* should be the ID of one of the [fix rigid](fix_rigid)
commands which defines the rigid bodies. The group specified in the
compute command is ignored. The rotational energy of all the rigid
bodies defined by the fix rigid command in included in the calculation.

## Output info

This compute calculates a global scalar (the summed rotational energy of
all the rigid bodies). 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) 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

This compute is part of the RIGID package. It is only enabled if LAMMPS
was built with that package. See the [Build package](Build_package) page
for more info.

## Related commands

[compute ke/rigid](compute_ke_rigid)

## Default

none