# compute smd/rho command

## Syntax

    compute ID group-ID smd/rho

-   ID, group-ID are documented in [compute](compute) command
-   smd/rho = style name of this compute command

## Examples

``` LAMMPS
compute 1 all smd/rho
```

## Description

Define a computation that calculates the per-particle mass density. The
mass density is the mass of a particle which is constant during the
course of a simulation, divided by its volume, which can change due to
mechanical deformation.

See [this PDF guide](PDF/MACHDYN_LAMMPS_userguide.pdf)\_ to use Smooth
Mach Dynamics in LAMMPS.

## Output info

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

The per-particle values will be in [units](units) of mass over volume.

## Restrictions

This compute is part of the MACHDYN 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 smd/vol](compute_smd_vol)

## Default

none