fix multisphere command — LIGGGHTS v3.X documentation

# fix multisphere command

## Syntax

    fix ID group-ID multisphere

- ID, group-ID are documented in fix command
- multisphere = style name of this fix command
- zero or more keyword/value pairs may be appended
- keyword = allow_group_and_set or allow_heatsource or CAddRhoFluid

    allow_group_and_set value = 'yes' or 'no'
      yes, no = determines if group and set command are allowed in the current simulation.
    allow_heatsource value = 'yes' or 'no'
      yes, no = determines if heatsources are allowed in the current simulation.
    CAddRhoFluid value = CAdd RhoFluid
      CAdd = Coefficient for additional mass term. (Usually 0.5)
      RhoFluid = Fluid density

## Examples

    fix ms all multisphere

## Description

Treat one or more sets of atoms as independent rigid bodies. This means that each timestep the total force and torque on each rigid body is computed as the sum of the forces and torques on its constituent particles and the coordinates, velocities, and orientations of the atoms in each body are updated so that the body moves and rotates as a single entity. The integration that is performed by this command is euqivalent to fix_rigid. However, the following implementation details are different:

(1) Body data held by this fix is distributed across all processes, yielding better parallel scalabilty.

(2) Bodies (particle clumps) can be added/inserted via fix_insert_stream, fix_insert_rate_region or fix_insert_pack and are automatically added to this fix.

(3) A body is removed from the simulation domain if any of it’s particles is removed from the simulation according to the boundary settings or by any command that deleted particles (e.g. delete_atoms).

(4) By using this fix, gravity (fix_gravity) will be handled correctly for for overlapping particle clumps.

(5) This fix internally performs a neigh_modify exclude command so that particles belonging to the same rigid body are excluded from the neighborlist build.

Warning

You should not update the atoms in rigid bodies via other time-integration fixes (e.g. nve, nvt, npt), or you will be integrating their motion more than once each timestep.

The option CAddRhoFluid enables additional mass terms for the integration of multisphere particles. These model the influence of displaced fluid on the particle motion.

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

Information about this fix is written to binary restart files. This means you can restart a simulation using multisphere particles.

fix_modify can be used to manipulate the fflag and tflag switches
  of multispheres. They can be initially defined in the

fix_particletemplate_multisphere command. The fix_modify manipulates all multispheres, not only the initially defined f/tflags from the template.

    E.g. fix_modify ms fflag on on off tflag on on off

No global scalar or vector 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

Fix multisphere does not work together with fix deform

Only one fix multisphere at a time is allowed. Heat transfer simulations are not possible when using this fix (e.g. fix heat/gran/conduction)

Warning

All fixes or computes gathering statistical output (such as e.g. compute com or fix ave/time operate on a per-sphere rather than on a per-body basis.

Currently, using fix multisphere requires newton = off and dimension = 3.

## Related commands

fix_rigid fix_particletemplate_multisphere neigh_modify

## Default

allow_group_and_set = ‘no’ allow_heatsource = ‘no’