fix setforce command — LAMMPS documentation

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# 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 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

    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, 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 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 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 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 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 when you invoke LAMMPS, or you can use the suffix command in your input script.

See the Accelerator packages page for more instructions on how to use the accelerated styles effectively.

Note

The region keyword is supported by Kokkos, but a Kokkos-enabled region must be used. See the 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.

The fix_modify respa option is supported by this fix. This allows to set at which level of the r-RESPA 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. 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 command.

The forces due to this fix are imposed during an energy minimization, invoked by the minimize command, but you cannot set forces to any value besides zero when performing a minimization. Use the 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 page for more info.

## Related commands

fix addforce, fix aveforce

## Default

none