\(\renewcommand{\AA}{\text{Å}}\)

# fix oneway command

## Syntax

    fix ID group-ID oneway N region-ID direction

- ID, group-ID are documented in fix command

- oneway = style name of this fix command

- N = apply this fix every this many timesteps

- region-ID = ID of region where fix is active

- direction = x or -x or y or -y or z or -z = coordinate and direction of the oneway constraint

## Examples

    fix 1 ions oneway 10 semi -x
    fix 2 all oneway 1 left -z
    fix 3 all oneway 1 right z

## Description

Enforce that particles in the group and in a given region can only move in one direction. This is done by reversing a particle’s velocity component, if it has the wrong sign in the specified dimension. The effect is that the particle moves in one direction only.

This can be used, for example, as a simple model of a semi-permeable membrane, or as an implementation of Maxwell’s demon.

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

No information about this fix is written to binary restart files. None of the fix_modify options are relevant to this fix. No global or per-atom quantities are stored by this fix for access by various output commands. 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 wall/reflect command

## Default

none