forked from lijiext/lammps
104 lines
3.3 KiB
Plaintext
104 lines
3.3 KiB
Plaintext
.. index:: fix dt/reset
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fix dt/reset command
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====================
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Syntax
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""""""
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.. parsed-literal::
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fix ID group-ID dt/reset N Tmin Tmax Xmax keyword values ...
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* ID, group-ID are documented in :doc:`fix <fix>` command
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* dt/reset = style name of this fix command
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* N = recompute dt every N timesteps
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* Tmin = minimum dt allowed which can be NULL (time units)
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* Tmax = maximum dt allowed which can be NULL (time units)
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* Xmax = maximum distance for an atom to move in one timestep (distance units)
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* zero or more keyword/value pairs may be appended
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* keyword = *units*
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.. parsed-literal::
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*units* value = *lattice* or *box*
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lattice = Xmax is defined in lattice units
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box = Xmax is defined in simulation box units
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Examples
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""""""""
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.. parsed-literal::
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fix 5 all dt/reset 10 1.0e-5 0.01 0.1
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fix 5 all dt/reset 10 0.01 2.0 0.2 units box
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Description
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"""""""""""
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Reset the timestep size every N steps during a run, so that no atom
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moves further than Xmax, based on current atom velocities and forces.
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This can be useful when starting from a configuration with overlapping
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atoms, where forces will be large. Or it can be useful when running
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an impact simulation where one or more high-energy atoms collide with
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a solid, causing a damage cascade.
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This fix overrides the timestep size setting made by the
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:doc:`timestep <timestep>` command. The new timestep size *dt* is
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computed in the following manner.
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For each atom, the timestep is computed that would cause it to
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displace *Xmax* on the next integration step, as a function of its
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current velocity and force. Since performing this calculation exactly
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would require the solution to a quartic equation, a cheaper estimate
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is generated. The estimate is conservative in that the atom's
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displacement is guaranteed not to exceed *Xmax*, though it may be
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smaller.
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Given this putative timestep for each atom, the minimum timestep value
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across all atoms is computed. Then the *Tmin* and *Tmax* bounds are
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applied, if specified. If one (or both) is specified as NULL, it is
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not applied.
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When the :doc:`run style <run_style>` is *respa*, this fix resets the
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outer loop (largest) timestep, which is the same timestep that the
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:doc:`timestep <timestep>` command sets.
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Note that the cumulative simulation time (in time units), which
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accounts for changes in the timestep size as a simulation proceeds,
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can be accessed by the :doc:`thermo_style time <thermo_style>` keyword.
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Restart, fix_modify, output, run start/stop, minimize info
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""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
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No information about this fix is written to :doc:`binary restart files <restart>`. None of the :doc:`fix_modify <fix_modify>` options
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are relevant to this fix.
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This fix computes a global scalar which can be accessed by various
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:ref:`output commands <howto_15>`. The scalar stores
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the last timestep on which the timestep was reset to a new value.
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The scalar value calculated by this fix is "intensive".
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No parameter of this fix can be used with the *start/stop* keywords of
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the :doc:`run <run>` command. This fix is not invoked during :doc:`energy minimization <minimize>`.
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Restrictions
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""""""""""""
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none
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Related commands
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""""""""""""""""
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:doc:`timestep <timestep>`
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Default
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"""""""
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The option defaults is units = lattice.
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.. _lws: http://lammps.sandia.gov
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.. _ld: Manual.html
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.. _lc: Section_commands.html#comm
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