# compute dilatation/atom command

## Syntax

compute ID group-ID dilatation/atom

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

## Examples

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. See this document 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 PDLAMMPS user guide for a formal definition of dilatation.

This command can only be used with a subset of the Peridynamic pair styles: 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 doc page for an overview of LAMMPS output options.

The per-atom vector values are unitless numbers (theta) >= 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 doc page for more info.