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fix_dt_reset.html
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<CENTER><A HREF = "http://lammps.sandia.gov">LAMMPS WWW Site</A> - <A HREF = "Manual.html">LAMMPS Documentation</A> - <A HREF = "Section_commands.html#comm">LAMMPS Commands</A>
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<H3>fix dt/reset command
</H3>
<P><B>Syntax:</B>
</P>
<PRE>fix ID group-ID dt/reset N Tmin Tmax Xmax keyword values ...
</PRE>
<UL><LI>ID, group-ID are documented in <A HREF = "fix.html">fix</A> command
<LI>dt/reset = style name of this fix command
<LI>N = recompute dt every N timesteps
<LI>Tmin = minimum dt allowed (can be INF) (time units)
<LI>Tmax = maximum dt allowed (can be INF) (time units)
<LI>Xmax = maximum distance for an atom to move in one timestep (distance units)
<LI>zero or more keyword/value pairs may be appended
<LI>keyword = <I>units</I>
</UL>
<PRE> <I>units</I> value = <I>lattice</I> or <I>box</I>
lattice = Xmax is defined in lattice units
box = Xmax is defined in simulation box units
</PRE>
<P><B>Examples:</B>
</P>
<PRE>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>
<P><B>Description:</B>
</P>
<P>Reset the timestep size every N steps during a run, 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.
</P>
<P>This fix overrides the timestep size setting made by the
<A HREF = "timestep.html">timestep</A> command. The new timestep size <I>dt</I> is
computed in the following way.
</P>
<UL><LI>compute Vmax of any atom in group
<LI>compute Amax of any atom in group
<LI>dt1 = Xmax/Vmax
<LI>dt2 = sqrt(2 Xmax/Amax)
<LI>new dt = MIN(dt1,dt2)
<LI>if dt < Tmin, dt = Tmin
<LI>if dt > Tmax, dt = Tmax
</UL>
<P>Vmax is the maximum velocity; Amax is the maximum acceleration =
force/mass. Note that Tmin or Tmax can be specified as INF, in which
case one or both of the last 2 checks will not be performed.
</P>
<P>When the <A HREF = "run_style.html">run style</A> is <I>respa</I>, this fix resets the
outer loop (largest) timestep, which is the same timestep that the
<A HREF = "timestep.html">timestep</A> command sets.
</P>
<P><B>Restart, fix_modify, output, run start/stop, minimize info:</B>
</P>
<P>No information about this fix is written to <A HREF = "restart.html">binary restart
files</A>. None of the <A HREF = "fix_modify.html">fix_modify</A> options
are relevant to this fix.
</P>
<P>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
<A HREF = "Section_howto.html#4_15">output commands</A>. The scalar and vector
values calculated by this fix are "intensive", meaning they are
independent of the number of atoms in the simulation.
</P>
<P>No parameter of this fix can be used with the <I>start/stop</I> keywords of
the <A HREF = "run.html">run</A> command. This fix is not invoked during <A HREF = "minimize.html">energy
minimization</A>.
</P>
<P><B>Restrictions:</B>
</P>
<P>The cumulative time is zeroed when the fix is created and
continuously accrues thereafter. Using the
<A HREF = "reset_timestep.html">reset_timestep</A> command while this fix is defined
will mess up the time accumulation.
</P>
<P><B>Related commands:</B>
</P>
<P><A HREF = "timestep.html">timestep</A>
</P>
<P><B>Default:</B>
</P>
<P>The option defaults is units = lattice.
</P>
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