fix oneway command


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


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


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.


This fix is part of the MISC package. It is only enabled if LAMMPS was built with that package. See the Build package page for more info.