# compute dipole/chunk command

# compute dipole/tip4p/chunk command

## Syntax

``` LAMMPS
compute ID group-ID style chunkID arg
```

-   ID, group-ID are documented in [compute](compute) command
-   style = *dipole/chunk* or *dipole/tip4p/chunk*
-   chunkID = ID of [compute chunk/atom](compute_chunk_atom) command
-   arg = *mass* or *geometry* = use COM or geometric center for charged
    chunk correction (optional)

## Examples

``` LAMMPS
compute 1 fluid dipole/chunk molchunk
compute dw water dipole/chunk 1 geometry
```

## Description

Define a computation that calculates the dipole vector and total dipole
for multiple chunks of atoms.

In LAMMPS, chunks are collections of atoms defined by a [compute
chunk/atom](compute_chunk_atom) 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)
and [Howto chunk](Howto_chunk) doc pages for details of how chunks can
be defined and examples of how they can be used to measure properties of
a system.

These computes calculate 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 through 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. Both per-atom charges and
per-atom dipole moments, if present, contribute to the computed dipole.

::: versionadded
28Mar2023
:::

Compute *dipole/tip4p/chunk* includes adjustments for the charge
carrying point M in molecules with TIP4P water geometry. The
corresponding parameters are extracted from the pair style.

Note that only atoms in the specified group contribute to the
calculation. The [compute chunk/atom](compute_chunk_atom) 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.

:::: note
::: title
Note
:::

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 [dump custom](dump) command for a discussion of \"unwrapped\"
coordinates. See the Atoms section of the [read_data](read_data) 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) 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)
command, for example:

``` LAMMPS
compute cc1 all chunk/atom molecule
compute myChunk all dipole/chunk cc1
fix 1 all ave/time 100 1 100 c_myChunk[*] file tmp.out mode vector
```

## Output info

These computes calculate a global array where the number of rows = the
number of chunks *Nchunk* as calculated by the specified [compute
chunk/atom](compute_chunk_atom) command. The number of columns is 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 the [Howto
output](Howto_output) page for an overview of LAMMPS output options.

The array values are \"intensive\". The array values will be in dipole
units (i.e., charge [units](units) times distance [units](units)).

## Restrictions

Compute style *dipole/tip4p/chunk* is part of the EXTRA-COMPUTE package.
It is only enabled if LAMMPS was built with that package. See the [Build
package](Build_package) page for more info.

Compute style *dipole/tip4p/chunk* can only be used with tip4p pair
styles.

## Related commands

[compute com/chunk](compute_com_chunk), [compute dipole](compute_dipole)

## Default

Using the center of mass is the default setting for the net charge
correction.