# compute erotate/sphere command

## Syntax

    compute ID group-ID erotate/sphere general_keywords general_values halfstep

- ID, group-ID are documented in compute command
- erotate/sphere = style name of this compute command
- general_keywords general_values are documented in compute
- halfstep = optional keyword to compute energy at half step

## Examples

    compute 1 all erotate/sphere

## Description

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

The rotational energy is computed as 1/2 I w^2, where I is the moment of inertia for a sphere and w is the particle’s angular velocity. By default the angular velocity is taken at the full time step (w^i). If the halfstep keyword is present in the definition of this compute, then the angular velocity is taken at the next half time step (w^(i+1/2)).

Warning

For 2d models, particles are treated as spheres, not disks, meaning their moment of inertia will be the same as in 3d.

## Output info

This compute calculates a global scalar (the KE). This value can be used by any command that uses a global scalar value from a compute as input. See Section_howto 15 for an overview of LIGGGHTS(R)-PUBLIC output options.

The scalar value calculated by this compute is “extensive”. The scalar value will be in energy units.

## Restrictions

This compute requires that atoms store a radius and angular velocity (omega) as defined by the atom_style sphere command.

All particles in the group must be finite-size spheres or point particles. They cannot be aspherical. Point particles will not contribute to the rotational energy.

## Related commands

compute erotate/asphere

Default: none