\(\renewcommand{\AA}{\text{Å}}\)

# fix drag command

## Syntax

    fix ID group-ID drag x y z fmag delta

- ID, group-ID are documented in fix command

- drag = style name of this fix command

- x,y,z = coord to drag atoms towards

- fmag = magnitude of force to apply to each atom (force units)

- delta = cutoff distance inside of which force is not applied (distance units)

## Examples

    fix center small-molecule drag 0.0 10.0 0.0 5.0 2.0

## Description

Apply a force to each atom in a group to drag it towards the point (x,y,z). The magnitude of the force is specified by fmag. If an atom is closer than a distance delta to the point, then the force is not applied.

Any of the x,y,z values can be specified as NULL which means do not include that dimension in the distance calculation or force application.

This command can be used to steer one or more atoms to a new location in the simulation.

## 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 3-vector of forces, which can be accessed by various output commands. This is the total force on the group of atoms by the drag force. 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. This fix is not invoked during energy 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 page for more info.

## Related commands

fix spring, fix spring/self, fix spring/rg, fix smd

## Default

none