# compute inertia/chunk command

## Syntax

``` LAMMPS
compute ID group-ID inertia/chunk chunkID
```

-   ID, group-ID are documented in [compute](compute) command
-   inertia/chunk = style name of this compute command
-   chunkID = ID of [compute chunk/atom](compute_chunk_atom) command

## Examples

``` LAMMPS
compute 1 fluid inertia/chunk molchunk
```

## Description

Define a computation that calculates the inertia tensor 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.

This compute calculates the six components of the symmetric inertia
tensor for each chunk, ordered
$I_{xx},I_{yy},I_{zz},I_{xy},I_{yz},I_{xz}$. The calculation includes
all effects due to atoms passing through periodic boundaries.

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 inertia tensor 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 inertia/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 inertia/chunk cc1
fix 1 all ave/time 100 1 100 c_myChunk[*] file tmp.out mode vector
```

## 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) command. The number of columns is 6, one
for each of the 6 components of the inertia tensor for each chunk,
ordered as listed above. 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
mass\*distance$^2$ [units](units).

## Restrictions

> none

## Related commands

[variable inertia() function](variable)

## Default

none