# compute dilatation/atom command

## Syntax

``` LAMMPS
compute ID group-ID dilatation/atom
```

-   ID, group-ID are documented in compute command
-   dilatation/atom = style name of this compute command

## Examples

``` LAMMPS
compute 1 all dilatation/atom
```

## Description

Define a computation that calculates the per-atom dilatation for each
atom in a group. This is a quantity relevant for [Peridynamics
models](pair_peri). See [this document](PDF/PDLammps_overview.pdf)\_ for
an overview of LAMMPS commands for Peridynamics modeling.

For small deformation, dilatation of is the measure of the volumetric
strain.

The dilatation $\theta$ for each peridynamic particle $i$ is calculated
as a sum over its neighbors with unbroken bonds, where the contribution
of the $ij$ pair is a function of the change in bond length (versus the
initial length in the reference state), the volume fraction of the
particles and an influence function. See the [Peridynamics
Howto](Howto_peri) for a formal definition of dilatation.

This command can only be used with a subset of the Peridynamic [pair
styles](pair_peri): *peri/lps*, *peri/ves*, and *peri/eps*.

The dilatation value will be 0.0 for atoms not in the specified compute
group.

## Output info

This compute calculates a per-atom vector, which can be accessed by any
command that uses per-atom values from a compute as input. See the
[Howto output](Howto_output) page for an overview of LAMMPS output
options.

The per-atom vector values are unitless numbers $(\theta \ge 0.0)$.

## Restrictions

This compute is part of the PERI package. It is only enabled if LAMMPS
was built with that package. See the [Build package](Build_package) page
for more info.

## Related commands

[compute damage/atom](compute_damage_atom), [compute
plasticity/atom](compute_plasticity_atom)

## Default

none