# fix lb/rigid/pc/sphere command

## Syntax

fix ID group-ID lb/rigid/pc/sphere bodystyle args keyword values ...

• ID, group-ID are documented in fix command

• lb/rigid/pc/sphere = style name of this fix command

• bodystyle = single or molecule or group

single args = none
molecule args = none
group args = N groupID1 groupID2 ...
N = # of groups

• zero or more keyword/value pairs may be appended

• keyword = force or torque or innerNodes

force values = M xflag yflag zflag
M = which rigid body from 1-Nbody (see asterisk form below)
xflag,yflag,zflag = off/on if component of center-of-mass force is active
torque values = M xflag yflag zflag
M = which rigid body from 1-Nbody (see asterisk form below)
xflag,yflag,zflag = off/on if component of center-of-mass torque is active
innerNodes values = innergroup-ID
innergroup-ID = ID of the atom group which does not experience a hydrodynamic force from the lattice-Boltzmann fluid


## Examples

fix 1 spheres lb/rigid/pc/sphere
fix 1 all lb/rigid/pc/sphere force 1 0 0 innerNodes ForceAtoms


## Description

This fix is based on the fix rigid command, and was created to be used in place of that fix, to integrate the equations of motion of spherical rigid bodies when a lattice-Boltzmann fluid is present with a user-specified value of the force-coupling constant. The fix uses the integration algorithm described in Mackay et al. to update the positions, velocities, and orientations of a set of spherical rigid bodies experiencing velocity dependent hydrodynamic forces. The spherical bodies are assumed to rotate as solid, uniform density spheres, with moments of inertia calculated using the combined sum of the masses of all the constituent particles (which are assumed to be point particles).

By default, all of the atoms that this fix acts on experience a hydrodynamic force due to the presence of the lattice-Boltzmann fluid. However, the innerNodes keyword allows the user to specify atoms belonging to a rigid object which do not interact with the lattice-Boltzmann fluid (i.e. these atoms do not feel a hydrodynamic force from the lattice-Boltzmann fluid). This can be used to distinguish between atoms on the surface of a non-porous object, and those on the inside.

This feature can be used, for example, when implementing a hard sphere interaction between two spherical objects. Instead of interactions occurring between the particles on the surfaces of the two spheres, it is desirable simply to place an atom at the center of each sphere, which does not contribute to the hydrodynamic force, and have these central atoms interact with one another.

Apart from the features described above, this fix is very similar to the rigid fix (although it includes fewer optional arguments, and assumes the constituent atoms are point particles); see fix rigid for a complete documentation.

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

No information about the rigid and rigid/nve fixes are written to binary restart files.

Similar to the fix rigid command: The rigid fix computes a global scalar which can be accessed by various output commands. The scalar value calculated by these fixes is “intensive”. The scalar is the current temperature of the collection of rigid bodies. This is averaged over all rigid bodies and their translational and rotational degrees of freedom. The translational energy of a rigid body is 1/2 m v^2, where m = total mass of the body and v = the velocity of its center of mass. The rotational energy of a rigid body is 1/2 I w^2, where I = the moment of inertia tensor of the body and w = its angular velocity. Degrees of freedom constrained by the force and torque keywords are removed from this calculation.

All of these fixes compute a global array of values which can be accessed by various output commands. The number of rows in the array is equal to the number of rigid bodies. The number of columns is 15. Thus for each rigid body, 15 values are stored: the xyz coords of the center of mass (COM), the xyz components of the COM velocity, the xyz components of the force acting on the COM, the xyz components of the torque acting on the COM, and the xyz image flags of the COM, which have the same meaning as image flags for atom positions (see the “dump” command). The force and torque values in the array are not affected by the force and torque keywords in the fix rigid command; they reflect values before any changes are made by those keywords.

The ordering of the rigid bodies (by row in the array) is as follows. For the single keyword there is just one rigid body. For the molecule keyword, the bodies are ordered by ascending molecule ID. For the group keyword, the list of group IDs determines the ordering of bodies.

The array values calculated by these fixes are “intensive”, meaning they are independent of the number of atoms in the simulation.

No parameter of these fixes can be used with the start/stop keywords of the run command. These fixes are not invoked during energy minimization.

## Restrictions

This fix is part of the USER-LB package. It is only enabled if LAMMPS was built with that package. See the Making LAMMPS section for more info.

Can only be used if a lattice-Boltzmann fluid has been created via the fix lb/fluid command, and must come after this command. Should only be used if the force coupling constant used in fix lb/fluid has been set by the user; this integration fix cannot be used if the force coupling constant is set by default.

## Default

The defaults are force * on on on, and torque * on on on.

(Mackay et al.) Mackay, F. E., Ollila, S.T.T., and Denniston, C., Hydrodynamic Forces Implemented into LAMMPS through a lattice-Boltzmann fluid, Computer Physics Communications 184 (2013) 2021-2031.