"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 com/chunk command :h3

[Syntax:]

compute ID group-ID com/chunk chunkID :pre

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

[Examples:]

compute 1 fluid com/chunk molchunk :pre

[Description:]

Define a computation that calculates the center-of-mass 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 coordinates of the center-of-mass
for each chunk, which includes all effects due to atoms passing thru
periodic boundaries.

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.

IMPORTANT NOTE: The coordinates of an atom contribute to the chunk's
center-of-mass in "unwrapped" form, by using the image flags
associated with each atom.  See the "dump custom"_dump.html command
for a discussion of "unwrapped" coordinates.  See the Atoms section of
the "read_data"_read_data.html command for a discussion of image flags
and how they are set for each atom.  You can reset the image flags
(e.g. to 0) before invoking this compute by using the "set
image"_set.html command.

The simplest way to output the results of the compute com/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 com/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 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
distance "units"_units.html.

[Restrictions:] none

[Related commands:]

"compute com"_compute_com.html

[Default:] none
