communicate command

Syntax

communicate keyword value ...

zero or more keyword/value pairs may be appended

keyword = cutoff or group or vel or mode:
  cutoff value = Rcut (distance units) = communicate atoms from this far away
  group value = group-ID = only communicate atoms in the group
  vel value = yes or no = do or do not communicate velocity info with ghost atoms
  mode value = single or multi = communicate atoms within a single or multiple distances

Examples

communicate mode multi
communicate mode multi group solvent
communicate mode single vel yes
communicate mode single cutoff 5.0 vel yes

Description

This command sets the style of inter-processor communication that occurs each timestep as atom coordinates and other properties are exchanged between neighboring processors and stored as properties of ghost atoms.

The mode keyword determines whether a single or multiple cutoff distances are used to determine which atoms to communicate.

The default mode is single which means each processor acquires information for ghost atoms that are within a single distance from its sub-domain. The distance is the maximum of the neighbor cutoff for all atom type pairs.

For many systems this is an efficient algorithm, but for systems with widely varying cutoffs for different type pairs, the multi mode can be faster. In this case, each atom type is assigned its own distance cutoff for communication purposes, and fewer atoms will be communicated. However, for granular systems optimization is automatically performed with the single mode, so multi is not necessary/available for granular systems. See the neighbor multi command for a neighbor list construction option that may also be beneficial for simulations of this kind.

The cutoff option allows you to set a ghost cutoff distance, which is the distance from the borders of a processor’s sub-domain at which ghost atoms are acquired from other processors. By default the ghost cutoff = neighbor cutoff = pairwise force cutoff + neighbor skin. See the neighbor command for more information about the skin distance. If the specified Rcut is greater than the neighbor cutoff, then extra ghost atoms will be acquired. If it is smaller, the ghost cutoff is set to the neighbor cutoff. This option exists for backward compatibility reasons, and is usually not needed.

The group option will limit communication to atoms in the specified group. This can be useful for models where no ghost atoms are needed for some kinds of particles. All atoms (not just those in the specified group) will still migrate to new processors as they move. The group specified with this option must also be specified via the atom_modify first command.

The vel option enables velocity information to be communicated with ghost particles. Depending on the atom_style, velocity info includes the translational velocity, angular velocity, and angular momentum of a particle. If the vel option is set to yes, then ghost atoms store these quantities; if no then they do not. The yes setting is needed by some pair styles which require the velocity state of both the I and J particles to compute a pairwise I,J interaction.

Note that if the fix deform command is being used with its “remap v” option enabled, then the velocities for ghost atoms (in the fix deform group) mirrored across a periodic boundary will also include components due to any velocity shift that occurs across that boundary (e.g. due to dilation or shear).

Restrictions

none

Default

The default settings are mode = single, group = all, cutoff = 0.0, vel = no. The cutoff default of 0.0 means that ghost cutoff = neighbor cutoff = pairwise force cutoff + neighbor skin.