fix aveforce command — LAMMPS documentation

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# fix aveforce command

## Syntax

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

- ID, group-ID are documented in fix command

- aveforce = style name of this fix command

- fx,fy,fz = force component values (force units)

      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 pressdown topwall aveforce 0.0 -1.0 0.0
    fix 2 bottomwall aveforce NULL -1.0 0.0 region top
    fix 2 bottomwall aveforce NULL -1.0 v_oscillate region top

## Description

Apply an additional external force to a group of atoms in such a way that every atom experiences the same force. This is useful for pushing on wall or boundary atoms so that the structure of the wall does not change over time.

The existing force is averaged for the group of atoms, component by component. The actual force on each atom is then set to the average value plus the component specified in this command. This means each atom in the group receives the same force.

Any of the fx, fy, or fz values can be specified as `NULL`, which means the force in that dimension is not changed. Note that this is not the same as specifying a 0.0 value, since that sets all forces to the same average value without adding in any additional force.

Any of the three quantities defining the force components, namely fx, fy, and fz, can be specified as an equal-style variable. 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 average force.

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 average force.

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

## 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 adding its forces. Default is the outermost level.

This fix computes a global three-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. 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

> none

## Related commands

fix setforce, fix addforce

## Default

none