# compute contact/atom command

## Syntax

``` LAMMPS
compute ID group-ID contact/atom group2-ID
```

-   ID, group-ID are documented in [compute](compute) command
-   contact/atom = style name of this compute command
-   group2-ID = optional argument to restrict which atoms to consider
    for contacts (see below)

## Examples

``` LAMMPS
compute 1 all contact/atom
compute 1 all contact/atom mygroup
```

## Description

Define a computation that calculates the number of contacts for each
atom in a group.

The contact number is defined for finite-size spherical particles as the
number of neighbor atoms which overlap the central particle, meaning
that their distance of separation is less than or equal to the sum of
the radii of the two particles.

The value of the contact number will be 0.0 for atoms not in the
specified compute group.

## Output info

This compute calculates a per-atom vector, whose values 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 will be a number $\ge 0.0$, as explained
above.

The optional *group2-ID* argument allows to specify from which group
atoms contribute to the coordination number. Default setting is group
\'all.\'

## Restrictions

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

This compute requires that atoms store a radius as defined by the
[atom_style sphere](atom_style) command.

## Related commands

[compute coord/atom](compute_coord_atom)

## Default

*group2-ID* = all

none