# fix addtorque command

## Syntax

``` LAMMPS
fix ID group-ID addtorque Tx Ty Tz
```

-   ID, group-ID are documented in [fix](fix) command
-   addtorque = style name of this fix command
-   Tx,Ty,Tz = torque component values (torque units)
-   any of Tx,Ty,Tz can be a variable (see below)

## Examples

``` LAMMPS
fix kick bead addtorque 2.0 3.0 5.0
fix kick bead addtorque 0.0 0.0 v_oscillate
```

## Description

Add a set of forces to each atom in the group such that:

-   the components of the total torque applied on the group (around its
    center of mass) are $T_x$, $T_y$, and $T_z$
-   the group would move as a rigid body in the absence of other forces.

This command can be used to drive a group of atoms into rotation.

Any of the three quantities defining the torque components can be
specified as an equal-style [variable](variable), namely *Tx*, *Ty*,
*Tz*. If the value is a variable, it should be specified as v_name,
where name is the variable name. In this case, the variable will be
evaluated each timestep, and its value used to determine the torque
component.

Equal-style variables can specify formulas with various mathematical
functions, and include [thermo_style](thermo_style) command keywords for
the simulation box parameters and timestep and elapsed time. Thus it is
easy to specify a time-dependent torque.

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

## Restart, fix_modify, output, run start/stop, minimize info

No information about this fix is written to [binary restart
files](restart).

The [fix_modify](fix_modify) *energy* option is supported by this fix to
add the potential \"energy\" inferred by the added torques to the global
potential energy of the system as part of [thermodynamic
output](thermo_style). The default setting for this fix is [fix_modify
energy no](fix_modify). Note that this is a fictitious quantity but is
needed so that the [minimize](minimize) command can include the forces
added by this fix in a consistent manner (i.e., there is a decrease in
potential energy when atoms move in the direction of the added forces).

The [fix_modify](fix_modify) *respa* option is supported by this fix.
This allows to set at which level of the [r-RESPA](run_style) integrator
the fix is adding its torque. Default is the outermost level.

This fix computes a global scalar and a global 3-vector, which can be
accessed by various [output commands](Howto_output). The scalar is the
potential energy discussed above. The vector is the total torque on the
group of atoms before the forces on individual atoms are changed by the
fix. The scalar and vector values calculated by this fix are
\"extensive\".

No parameter of this fix can be used with the *start/stop* keywords of
the [run](run) command.

The forces due to this fix are imposed during an energy minimization,
invoked by the [minimize](minimize) command.

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

If you want the fictitious potential energy associated with the added
forces to be included in the total potential energy of the system (the
quantity being minimized), you MUST enable the [fix_modify](fix_modify)
*energy* option for this fix.
::::

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

You should not specify force components with a variable that has
time-dependence for use with a minimizer, since the minimizer increments
the timestep as the iteration count during the minimization.
::::

## Restrictions

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

## Related commands

[fix addforce](fix_addforce)

## Default

none