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lammps/doc/src/fix_dt_reset.txt
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fix_dt_reset.txt
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"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)
:line
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 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 = {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.
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"_thermo_style.html keyword.
[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.
This fix computes a global scalar which can be accessed by various
"output commands"_Section_howto.html#howto_15. 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"_run.html command. This fix is not invoked during "energy
minimization"_minimize.html.
[Restrictions:] none
[Related commands:]
"timestep"_timestep.html
[Default:]
The option defaults is units = lattice.
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