# fix oneway command

## Syntax

    fix ID group-ID oneway N region-ID direction

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

``` LAMMPS
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](restart). None of the [fix_modify](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](Howto_output). No
parameter of this fix can be used with the *start/stop* keywords of the
[run](run) command. This fix is not invoked during [energy
minimization](minimize).

## 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](Build_package) page
for more info.

## Related commands

[fix wall/reflect](fix_wall_reflect) command

## Default

none