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### compute pair/gran/local command

### compute wall/gran/local command

Syntax:

    compute ID group-ID pair/gran/local keywords ...
    compute ID group-ID wall/gran/local keywords ...

- ID, group-ID are documented in compute command
- pair/gran/local or wall/gran/local = style name of this compute command
- zero or more keywords may be appended

      keyword = pos or vel or id or force or torque or history or contactArea or delta:l
        pos = positions of particles in contact (6 values)
        vel = velocities of particles in contact (6 values)
        id = IDs of particles in contact and a periodicity flag (3 values) or IDs of the mesh, the triangle and the particle (3 values)
        force = contact force (3 values)
        force_normal = normal component of contact force (3 values)
        force_tangential = tangential component of contact force (3 values)
        torque = torque divided by particle diameter (3 values)
        history = contact history (# depends on pair style, e.g. 3 shear history values)
        contactArea = area of the contact (1 value)
        delta = overlap of the contact (1 value)
        heatFlux = conductive heat flux of the contact (1 value)

Examples:

    compute 1 all pair/gran/local
    compute 1 all pair/gran/local pos force
    compute 1 all wall/gran/local

Description:

Define a computation that calculates properties of individual pairwise or particle-wall interactions of a granular pair style. The number of datums generated, aggregated across all processors, equals the number of pairwise interactions or particle-wall interactions in the system.

The local data stored by this command is generated by looping over the pairwise neighbor list. Info about an individual pairwise interaction will only be included if both atoms in the pair are in the specified compute group, and if the current pairwise distance is less than the force cutoff distance for that interaction, as defined by the pair_style and pair_coeff commands.

IMPORTANT NOTE: For accessing particle-wall contact data, only mesh walls (see fix mesh) can be used. For computing particle-wall (compute wall/gran/local), the code will automatically look for a fix wall/gran command that uses mesh walls. The order of the meshes in the fix wall/gran command is called the mesh id (starting with 0), and the triangle id reflects the order of the triangles in the STL/VTK file read via the dedicated fix mesh command. For how to output the trangle id, see "dump mesh/gran/VTK command"dump.html.

The output pos is the particle positions (6 values) in distance units. Keyword vel will do the same for velocities. For computing pairwise data, the output id will be the two particle IDs (using this option requires to use an atom map) and a flag that is 1 for interaction over a periodic boundary and 0 otherwise. For computing particle-wall data, the output id will be the mesh id, the triangle id and the particle id. The output force, force_normal, force_tangential and torque are the total contact force, the normal and tangential components of the contact force, and the torque divided by the particle radius, both in force units. Note that the torque does NOT contain any rolling friction torque. The output history will depend on what this history represents, according to the granular pair style used. The output contactArea will output the contact area, in distance^2 units. Note that contactArea is based on an analytic geometric calculation of sphere-sphere or sphere-plane intersection rather than a calculation based on mechanics. This is to ensure that contactArea works with all types of contact models.

The output delta will output the overlap (sum of radii - distance between particle centers) in distance units .The output heatFlux (available only if a fix heat/gran is used to compute heat fluxes) will output the per-contact conductive heat flux area, in energy/time units.

IMPORTANT NOTE: The data associated to the different keywords is output in the following order: pos, vel, id, force, force_normal, force_tangential, torque, history, contactArea, heatFlux. This is independant of the order in which the keywords are specified.

Note that as atoms migrate from processor to processor, there will be no consistent ordering of the entries within the local vector or array from one timestep to the next. The only consistency that is guaranteed is that the ordering on a particular timestep will be the same for local vectors or arrays generated by other compute commands. For example, pair output from the compute property/local command can be combined with data from this command and output by the dump local command in a consistent way.

Output info:

This compute calculates a local vector or local array depending on the number of keywords. The length of the vector or number of rows in the array is the number of pairs. If a single keyword is specified, a local vector is produced. If two or more keywords are specified, a local array is produced where the number of columns = the number of keywords. The vector or array can be accessed by any command that uses local values from a compute as input. See this section for an overview of LIGGGHTS(R)-PUBLIC output options.

For information on the units of the output, see above.

Restrictions:

Can only be used together with a granular pair style. For accessing particle-wall contact data, only mesh walls can be used.

Related commands:

dump local, compute property/local

Default:

By default, all of the outputs keywords (except force_normal, force_tangential, heat flux and delta) are activated, i.e. when no keyword is used, positions velocities, ids, forces, torques, history and contact area are output.