"LAMMPS WWW Site"_lws - "LAMMPS Documentation"_ld - "LAMMPS Commands"_lc :c

:link(lws,http://lammps.sandia.gov)
:link(ld,Manual.html)
:link(lc,Section_commands.html#comm)

:line

compute vcm/chunk command :h3

[Syntax:]

compute ID group-ID vcm/chunk chunkID :pre

ID, group-ID are documented in "compute"_compute.html command
vcm/chunk = style name of this compute command
chunkID = ID of "compute chunk/atom"_compute_chunk_atom.html command :ul

[Examples:]

compute 1 fluid vcm/chunk molchunk :pre

[Description:]

Define a computation that calculates the center-of-mass velocity for
multiple chunks of atoms.

In LAMMPS, chunks are collections of atoms defined by a "compute
chunk/atom"_compute_chunk_atom.html command, which assigns each atom
to a single chunk (or no chunk).  The ID for this command is specified
as chunkID.  For example, a single chunk could be the atoms in a
molecule or atoms in a spatial bin.  See the "compute
chunk/atom"_compute_chunk_atom.html doc page and ""Section_howto
23"_Section_howto.html#howto_23 for details of how chunks can be
defined and examples of how they can be used to measure properties of
a system.

This compute calculates the x,y,z components of the center-of-mass
velocity for each chunk.  This is done by summing mass*velocity for
each atom in the chunk and dividing the sum by the total mass of the
chunk.

Note that only atoms in the specified group contribute to the
calculation.  The "compute chunk/atom"_compute_chunk_atom.html command
defines its own group; atoms will have a chunk ID = 0 if they are not
in that group, signifying they are not assigned to a chunk, and will
thus also not contribute to this calculation.  You can specify the
"all" group for this command if you simply want to include atoms with
non-zero chunk IDs.

The simplest way to output the results of the compute vcm/chunk
calculation to a file is to use the "fix ave/time"_fix_ave_time.html
command, for example:

compute cc1 all chunk/atom molecule
compute myChunk all vcm/chunk cc1
fix 1 all ave/time 100 1 100 c_myChunk file tmp.out mode vector :pre

[Output info:]

This compute calculates a global array where the number of rows = the
number of chunks {Nchunk} as calculated by the specified "compute
chunk/atom"_compute_chunk_atom.html command.  The number of columns =
3 for the x,y,z center-of-mass velocity coordinates of each chunk.
These values can be accessed by any command that uses global array
values from a compute as input.  See "Section_howto
15"_Section_howto.html#howto_15 for an overview of LAMMPS output
options.

The array values are "intensive".  The array values will be in
velocity "units"_units.html.

[Restrictions:] none

[Related commands:] none

[Default:] none
