git-svn-id: svn://svn.icms.temple.edu/lammps-ro/trunk@14950 f3b2605a-c512-4ea7-a41b-209d697bcdaa

doc/html/_sources/fix_dt_reset.txt

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+.. index:: fix dt/reset
+
+fix dt/reset command
+====================
+
+Syntax
+""""""
+
+.. parsed-literal::
+
+   fix ID group-ID dt/reset N Tmin Tmax Xmax keyword values ...
+
+* ID, group-ID are documented in :doc:`fix <fix>` 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*
+
+.. parsed-literal::
+
+     *units* value = *lattice* or *box*
+       lattice = Xmax is defined in lattice units
+       box = Xmax is defined in simulation box units
+
+Examples
+""""""""
+
+.. parsed-literal::
+
+   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
+
+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
+:doc:`timestep <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.
+
+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 :doc:`run style <run_style>` is *respa*\ , this fix resets the
+outer loop (largest) timestep, which is the same timestep that the
+:doc:`timestep <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 :doc:`thermo_style time <thermo_style>` keyword.
+
+Restart, fix_modify, output, run start/stop, minimize info
+""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
+
+No information about this fix is written to :doc:`binary restart files <restart>`.  None of the :doc:`fix_modify <fix_modify>` options
+are relevant to this fix.
+
+This fix computes a global scalar which can be accessed by various
+:ref:`output commands <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 :doc:`run <run>` command.  This fix is not invoked during :doc:`energy minimization <minimize>`.
+
+Restrictions
+""""""""""""
+ none
+
+Related commands
+""""""""""""""""
+
+:doc:`timestep <timestep>`
+
+Default
+"""""""
+
+The option defaults is units = lattice.
+
+
+.. _lws: http://lammps.sandia.gov
+.. _ld: Manual.html
+.. _lc: Section_commands.html#comm