"LAMMPS WWW Site"_lws - "LAMMPS Documentation"_ld - "LAMMPS Commands"_lc :c

:link(lws,http://lammps.sandia.gov)
:link(ld,Manual.html)
:link(lc,Section_commands.html#comm)

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fix dt/reset command :h3

[Syntax:]

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

ID, group-ID are documented in "fix"_fix.html command
dt/reset = style name of this fix command
N = recompute dt every N timesteps
Tmin = minimum dt allowed (can be NULL) (time units)
Tmax = maximum dt allowed (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 = {units} :ul
  {units} value = {lattice} or {box}
    lattice = Xmax is defined in lattice units
    box = Xmax is defined in simulation box units :pre

[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 :pre

[Description:]

Reset the timestep size every N steps during a run, so that no atom
moves further than Xmax, based on current atom velocities and forces.
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"_timestep.html 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.

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"_run_style.html is {respa}, this fix resets the
outer loop (largest) timestep, which is the same timestep that the
"timestep"_timestep.html command sets.

[Restart, fix_modify, output, run start/stop, minimize info:]

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

The current timestep size is stored as a scalar quantity by this fix.
The cumulative simulation time (in time units) is stored as the first
element of a vector.  Both these quantities can be accessed by various
"output commands"_Section_howto.html#4_15.  The scalar and vector
values calculated by this fix are "intensive".

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

[Restrictions:]

The cumulative time is zeroed when the fix is created and
continuously accrues thereafter.  Using the
"reset_timestep"_reset_timestep.html command while this fix is defined
will mess up the time accumulation.

[Related commands:]

"timestep"_timestep.html

[Default:]

The option defaults is units = lattice.
