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### fix multisphere command

Syntax:

    fix ID group-ID multisphere

- ID, group-ID are documented in fix command

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.

IMPORTANT NOTE: 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.

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

No information about this fix is written to binary restart files. This means you can currently not restart a simulation using multisphere particles.

None of the fix_modify options are relevant to this fix. 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:

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)

The PUBLIC version does not support (i) parallel computation, (ii) restart of multisphere simulations

IMPORTANT NOTE: 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_sphere neigh_modify

Default: none