<p>Define a computation that calculates the dipole vector and total dipole
for multiple chunks of atoms.</p>
<p>In LAMMPS, chunks are collections of atoms defined by a <aclass="reference internal"href="compute_chunk_atom.html"><spanclass="doc">compute chunk/atom</span></a> 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 <aclass="reference internal"href="compute_chunk_atom.html"><spanclass="doc">compute chunk/atom</span></a> doc page and “<aclass="reference internal"href="Section_howto.html#howto-23"><spanclass="std std-ref">Section_howto 23</span></a> for details of how chunks can be
defined and examples of how they can be used to measure properties of
a system.</p>
<p>This compute calculates the x,y,z coordinates of the dipole vector
and the total dipole moment for each chunk, which includes all effects
due to atoms passing thru periodic boundaries. For chunks with a net
charge the resulting dipole is made position independent by subtracting
the position vector of the center of mass or geometric center times the
net charge from the computed dipole vector.</p>
<p>Note that only atoms in the specified group contribute to the
calculation. The <aclass="reference internal"href="compute_chunk_atom.html"><spanclass="doc">compute chunk/atom</span></a> 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.</p>
<divclass="admonition note">
<pclass="first admonition-title">Note</p>
<pclass="last">The coordinates of an atom contribute to the chunk’s
dipole in “unwrapped” form, by using the image flags
associated with each atom. See the <aclass="reference internal"href="dump.html"><spanclass="doc">dump custom</span></a> command
for a discussion of “unwrapped” coordinates. See the Atoms section of
the <aclass="reference internal"href="read_data.html"><spanclass="doc">read_data</span></a> 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 <aclass="reference internal"href="set.html"><spanclass="doc">set image</span></a> command.</p>
</div>
<p>The simplest way to output the results of the compute com/chunk
calculation to a file is to use the <aclass="reference internal"href="fix_ave_time.html"><spanclass="doc">fix ave/time</span></a>
<p>This compute calculates a global array where the number of rows = the
number of chunks <em>Nchunk</em> as calculated by the specified <aclass="reference internal"href="compute_chunk_atom.html"><spanclass="doc">compute chunk/atom</span></a> command. The number of columns =
4 for the x,y,z dipole vector components and the total dipole of each
chunk. These values can be accessed by any command that uses global
array values from a compute as input. See <aclass="reference internal"href="Section_howto.html#howto-15"><spanclass="std std-ref">Section_howto 15</span></a> for an overview of LAMMPS output
options.</p>
<p>The array values are “intensive”. The array values will be in
dipole units, i.e. charge units times distance <aclass="reference internal"href="units.html"><spanclass="doc">units</span></a>.</p>
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