# compute ke command

## Syntax

    compute ID group-ID ke general_keyword general_values halfstep

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

## Examples

    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 1/2 m v^2, where m and v are the mass and velocity of the particle. By default the velocity is taken at the full time step (v^i). If the halfstep keyword is present in the definition of this compute, then the velocity is taken at the next half time step (v^(i+1/2)).

There is a subtle difference between the quantity calculated by this compute and the kinetic energy calculated by the ke keyword used in thermodynamic output, as specified by the 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 1/2 Kb T of energy for each degree of freedom. For the default temperature computation via the `compute temp` command, these are the same. But 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 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

> none

## Related commands

compute erotate/sphere

Default: none