fix dt/reset command — LAMMPS documentation

\(\renewcommand{\AA}{\text{Å}}\)

# fix dt/reset command

Accelerator Variants: dt/reset/kk

## Syntax

    fix ID group-ID dt/reset N Tmin Tmax Xmax keyword values ...

- ID, group-ID are documented in fix command

- dt/reset = style name of this fix command

- N = re-compute dt every N timesteps

- Tmin = minimum dt allowed which can be NULL (time units)

- Tmax = maximum dt allowed which can be NULL (time units)

- Xmax = maximum distance for an atom to move in one timestep (distance units)

- zero or more keyword/value pairs may be appended

- keyword = emax or units

    emax value = Emax
      Emax = maximum kinetic energy change for an atom in one timestep (energy units)
    units value = lattice or box
      lattice = Xmax is defined in lattice units
      box = Xmax is defined in simulation box units

## Examples

    fix 5 all dt/reset 10 1.0e-5 0.01 0.1
    fix 5 all dt/reset 10 0.01 2.0 0.2 units box
    fix 5 all dt/reset 5 NULL 0.001 0.5 emax 30 units box

## Description

Reset the timestep size every N steps during a run, so that no atom moves further than the specified Xmax distance, based on current atom velocities and forces. Optionally an additional criterion is imposed by the emax keyword, so that no atom’s kinetic energy changes by more than the specified Emax.

This can be useful when starting from a configuration with overlapping atoms, where forces will be large. Or it can be useful when running an impact simulation where one or more high-energy atoms collide with a solid, causing a damage cascade.

This fix overrides the timestep size setting made by the timestep command. The new timestep size dt is computed in the following manner.

For each atom, the timestep is computed that would cause it to displace Xmax on the next integration step, as a function of its current velocity and force. Since performing this calculation exactly would require the solution to a quartic equation, a cheaper estimate is generated. The estimate is conservative in that the atom’s displacement is guaranteed not to exceed Xmax, though it may be smaller.

In addition if the emax keyword is used, the specified Emax value is enforced as a limit on how much an atom’s kinetic energy can change. If the timestep required is even smaller than for the Xmax displacement, then the smaller timestep is used.

Given this putative timestep for each atom, the minimum timestep value across all atoms is computed. Then the Tmin and Tmax bounds are applied, if specified. If one (or both) is specified as NULL, it is not applied.

When the run style is respa, this fix resets the outer loop (largest) timestep, which is the same timestep that the timestep command sets.

Note that the cumulative simulation time (in time units), which accounts for changes in the timestep size as a simulation proceeds, can be accessed by the thermo_style time keyword.

Also note that the dump_modify every/time option allows dump files to be written at intervals specified by simulation time, rather than by timesteps. Simulation time is in time units; see the units doc page for details.

Styles with a gpu, intel, kk, omp, or opt suffix are functionally the same as the corresponding style without the suffix. They have been optimized to run faster, depending on your available hardware, as discussed on the Accelerator packages page. The accelerated styles take the same arguments and should produce the same results, except for round-off and precision issues.

These accelerated styles are part of the GPU, INTEL, KOKKOS, OPENMP, and OPT packages, respectively. They are only enabled if LAMMPS was built with those packages. See the Build package page for more info.

You can specify the accelerated styles explicitly in your input script by including their suffix, or you can use the -suffix command-line switch when you invoke LAMMPS, or you can use the suffix command in your input script.

See the Accelerator packages page for more instructions on how to use the accelerated styles effectively.

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

No information about this fix is written to binary restart files. None of the fix_modify options are relevant to this fix.

This fix computes a global scalar which can be accessed by various output commands. The scalar stores the last timestep on which the timestep was reset to a new value.

The scalar value calculated by this fix is “intensive”.

No parameter of this fix can be used with the start/stop keywords of the run command. This fix is not invoked during energy minimization.

## Restrictions

> none

## Related commands

timestep, dump_modify every/time

## Default

The option defaults are units = lattice, and no emax kinetic energy limit.