# fix propel/self command

## Syntax

fix ID group-ID propel/self mode magnitude keyword values ...

• ID, group-ID are documented in fix command

• propel/self = style name of this fix command

• mode = velocity or quat

• magnitude = magnitude of the active force

• one or more keyword/value pairs may be appended to args

• keyword = types

types values = one or more atom types

## Examples

fix active_group all propel/self velocity 1.0
fix constant_velocity all viscous 1.0

fix active_group all propel/self quat 1.0

fix active all propel/self quat 1.0 types 1 2 4


## Description

Adds a force of a constant magnitude to each atom in the group. The nature in which the force is added depends on the mode.

For mode = velocity, the active force acts along the velocity vector of each atom. This can be interpreted as a velocity-dependent friction, such as proposed by (Erdmann).

For mode = quat the force is applied along the axis obtained by rotating the x-axis along the atom’s quaternion. In other words, the force is along the x-axis in the atom’s body frame. This mode requires all atoms in the group to have a quaternion, so atom_style should either be ellipsoid or body. In combination with Langevin thermostat for translation and rotation in the overdamped regime, the quaternion mode corresponds to the active Brownian particle model introduced by (Henkes), (Bialke) and (Fily).

By default, this fix is applied to all atoms in the group. You can override this behavior by specifying the atom types the fix should work on through the types keyword.

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

No information about this fix is written to binary restart files.

This fix is not imposed during minimization.

## Restrictions

In quat mode, this fix makes use of per-atom quaternions to take into account the fact that the orientation can rotate and hence the direction of the active force can change. The quat mode of this fix only works with atom_style ellipsoid.