# fix setforce command

Accelerator Variants: *setforce/kk*

# fix setforce/spin command

## Syntax

    fix ID group-ID setforce fx fy fz keyword value ...

-   ID, group-ID are documented in [fix](fix) command

-   setforce = style name of this fix command

-   fx,fy,fz = force component values

-   any of fx,fy,fz can be a variable (see below)

-   zero or more keyword/value pairs may be appended to args

-   keyword = *region*

        *region* value = region-ID
          region-ID = ID of region atoms must be in to have added force

## Examples

``` LAMMPS
fix freeze indenter setforce 0.0 0.0 0.0
fix 2 edge setforce NULL 0.0 0.0
fix 1 edge setforce/spin 0.0 0.0 0.0
fix 2 edge setforce NULL 0.0 v_oscillate
```

## Description

Set each component of force on each atom in the group to the specified
values fx,fy,fz. This erases all previously computed forces on the atom,
though additional fixes could add new forces. This command can be used
to freeze certain atoms in the simulation by zeroing their force, either
for running dynamics or performing an energy minimization. For dynamics,
this assumes their initial velocity is also zero.

Any of the fx,fy,fz values can be specified as NULL which means do not
alter the force component in that dimension.

Any of the 3 quantities defining the force components can be specified
as an equal-style or atom-style [variable](variable), namely *fx*, *fy*,
*fz*. 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 force
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 force field.

Atom-style variables can specify the same formulas as equal-style
variables but can also include per-atom values, such as atom
coordinates. Thus it is easy to specify a spatially-dependent force
field with optional time-dependence as well.

If the *region* keyword is used, the atom must also be in the specified
geometric [region](region) in order to have force added to it.

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

Style *spin* suffix sets the components of the magnetic precession
vectors instead of the mechanical forces. This also erases all
previously computed magnetic precession vectors on the atom, though
additional magnetic fixes could add new forces.

This command can be used to freeze the magnetic moment of certain atoms
in the simulation by zeroing their precession vector.

All options defined above remain valid, they just apply to the magnetic
precession vectors instead of the forces.

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

Styles with a *gpu*, *intel*, *kk*, *omp*, or *opt* suffix are
functionally the same as the corresponding style without the suffix.
They have been optimized to run faster, depending on your available
hardware, as discussed on the [Accelerator packages](Speed_packages)
page. The accelerated styles take the same arguments and should produce
the same results, except for round-off and precision issues.

These accelerated styles are part of the GPU, INTEL, KOKKOS, OPENMP, and
OPT packages, respectively. They are only enabled if LAMMPS was built
with those packages. See the [Build package](Build_package) page for
more info.

You can specify the accelerated styles explicitly in your input script
by including their suffix, or you can use the [-suffix command-line
switch](Run_options) when you invoke LAMMPS, or you can use the
[suffix](suffix) command in your input script.

See the [Accelerator packages](Speed_packages) page for more
instructions on how to use the accelerated styles effectively.

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

The region keyword is supported by Kokkos, but a Kokkos-enabled region
must be used. See the region [region](region) command for more
information.
::::

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

## 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) *respa* option is supported by this fix.
This allows to set at which level of the [r-RESPA](run_style) integrator
the fix is setting the forces to the desired values; on all other
levels, the force is set to 0.0 for the atoms in the fix group, so that
setforce values are not counted multiple times. Default is to to
override forces at the outermost level.

This fix computes a global 3-vector of forces, which can be accessed by
various [output commands](Howto_output). This is the total force on the
group of atoms before the forces on individual atoms are changed by the
fix. The 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, but you cannot set forces
to any value besides zero when performing a minimization. Use the [fix
addforce](fix_addforce) command if you want to apply a non-zero force to
atoms during a minimization.

## Restrictions

Fix *setforce/spin* is part of the SPIN 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), [fix aveforce](fix_aveforce)

## Default

none