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# compute smd/tlsph/dt command

## Syntax

    compute ID group-ID smd/tlsph/dt

- ID, group-ID are documented in compute command

- smd/tlsph/dt = style name of this compute command

## Examples

    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 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 page for an overview of LAMMPS output options.

The per-particle values will be given in 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 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

## Default

none