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fix settorque/atom command

Syntax

fix ID group-ID settorque/atom tx ty tz keyword value ...

  • ID, group-ID are documented in fix command

  • settorque/atom = style name of this fix command

  • tx,ty,tz = torque component values

  • any of tx,ty,tz can be a variable (see below)

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

  • keyword = region

    region value = region-ID
  region-ID = ID of region atoms must be in to have added torque

Examples

fix freeze indenter settorque/atom 0.0 0.0 0.0
fix 2 edge settorque/atom NULL 0.0 0.0
fix 2 edge settorque/atom NULL 0.0 v_oscillate

Description

Added in version 10Dec2025.

This fix is intended to set the peratom torque of individual finite-size atoms in the fix group to the specified values. Unlike fix addtorque/group, it does not set a collective torque to a set of point particles.

Each component of torque on each atom in the group to the specified values tx, ty, tz. This erases all previously computed torques on the atom, though additional fixes could add new torques. This command can be used to freeze the rotation of certain atoms in the simulation by zeroing their torque, assuming their initial angular velocities are also zero.

Freezing both rotational and translational degrees of freedom can also be accomplished using fix freeze.

Any of the tx, ty, tz values can be specified as NULL which means do not alter the torque component in that dimension.

Any of the 3 quantities defining the torque components can be specified as an equal-style or atom-style variable, namely tx, ty, tz. If the value is a variable, it should be specified as v_name, where name is the variable name. In this case, the variable will be evaluated each timestep, and its value used to determine the torque component.

Equal-style variables can specify formulas with various mathematical functions, and include thermo_style command keywords for the simulation box parameters and timestep and elapsed time. Thus it is easy to specify a time-dependent torque field.

Atom-style variables can specify the same formulas as equal-style variables but can also include per-atom values, such as atom coordinates. Thus it is easy to specify a spatially-dependent torque field with optional time-dependence as well.

If the region keyword is used, the atom must also be in the specified geometric region in order to have torque added to it.

Note

This fix is not (currently) designed to be used with rigid fixes. While it will set the torque of all of the atoms in a rigid body as described above, there is not always an easy mapping between these peratom torques and the torque experienced by the body. The exception may be when it is used in conjunction with fix setforce to simultaneously zero forces and torques to freeze a rigid body.

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 setting the torques to the desired values; on all other levels, the torque is set to 0.0 for the atoms in the fix group, so that settorque/atom values are not counted multiple times. Default is to to override torques at the outermost level.

This fix computes a global 3-vector of torques, which can be accessed by various output commands. This is the total torque on the group of atoms before the torques on individual atoms are changed by the fix. 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.

Restrictions

Fix settorque/atom is part of the GRANULAR package. It is only enabled if LAMMPS was built with that package. See the Build package page for more info.

Default

none