# fix smd/adjust_dt command

## Syntax

``` LAMMPS
fix ID group-ID smd/adjust_dt arg
```

-   ID, group-ID are documented in [fix](fix) command

-   smd/adjust_dt = style name of this fix command

-   arg = *s_fact*

        *s_fact* = safety factor

## Examples

``` LAMMPS
fix 1 all smd/adjust_dt 0.1
```

## Description

The fix calculates a new stable time increment for use with the SMD time
integrators.

The stable time increment is based on multiple conditions. For the SPH
pair styles, a CFL criterion (Courant, Friedrichs & Lewy, 1928) is
evaluated, which determines the speed of sound cannot propagate further
than a typical spacing between particles within a single time step to
ensure no information is lost. For the contact pair styles, a linear
analysis of the pair potential determines a stable maximum time step.

This fix inquires the minimum stable time increment across all particles
contained in the group for which this fix is defined. An additional
safety factor *s_fact* is applied to the time increment.

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

## Restart, fix_modify, output, run start/stop, minimize info

Currently, no part of MACHDYN supports restarting nor minimization.

## Restrictions

This fix 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

[smd/tlsph_dt](compute_smd_tlsph_dt)

## Default

none