# compute smd/tlsph/dt command

## Syntax

    compute ID group-ID smd/tlsph/dt

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

## Examples

``` LAMMPS
compute 1 all smd/tlsph/dt
```

## Description

Define a computation that outputs the CFL-stable time increment per
particle. This time increment is essentially given by the speed of
sound, divided by the SPH smoothing length. Because both the speed of
sound and the smoothing length typically change during the course of a
simulation, the stable time increment needs to be re-computed every time
step. This calculation is performed automatically in the relevant SPH
pair styles and this compute only serves to make the stable time
increment accessible for output purposes.

See [this PDF guide](PDF/MACHDYN_LAMMPS_userguide.pdf)\_ to using 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 given in [units](units) of time.

## 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.

This compute can only be used for particles interacting with the
Total-Lagrangian SPH pair style.

## Related commands

[smd/adjust/dt](fix_smd_adjust_dt)

## Default

none