2007-10-11 06:39:11 +08:00
|
|
|
<HTML>
|
|
|
|
<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>
|
|
|
|
</CENTER>
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
<HR>
|
|
|
|
|
|
|
|
<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
|
2009-06-06 05:47:11 +08:00
|
|
|
<LI>Tmin = minimum dt allowed (can be NULL) (time units)
|
|
|
|
<LI>Tmax = maximum dt allowed (can be NULL) (time units)
|
2007-10-11 06:39:11 +08:00
|
|
|
<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>
|
2009-06-06 05:47:11 +08:00
|
|
|
<P>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.
|
2007-10-11 06:39:11 +08:00
|
|
|
</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
|
2009-06-06 05:47:11 +08:00
|
|
|
computed in the following manner.
|
2007-10-11 06:39:11 +08:00
|
|
|
</P>
|
2009-06-06 05:47:11 +08:00
|
|
|
<P>For each atom, the timestep is computed that would cause it to
|
|
|
|
displace <I>Xmax</I> 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 <I>Xmax</I>, though it may be
|
|
|
|
smaller.
|
|
|
|
</P>
|
|
|
|
<P>Given this putative timestep for each atom, the minimum timestep value
|
|
|
|
across all atoms is computed. Then the <I>Tmin</I> and <I>Tmax</I> bounds are
|
|
|
|
applied, if specified. If one (or both) is specified as NULL, it is
|
|
|
|
not applied.
|
2007-10-11 06:39:11 +08:00
|
|
|
</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.
|
2008-03-01 09:13:20 +08:00
|
|
|
The cumulative simulation time (in time units) is stored as the first
|
2007-10-11 06:39:11 +08:00
|
|
|
element of a vector. Both these quantities can be accessed by various
|
2008-01-03 03:25:15 +08:00
|
|
|
<A HREF = "Section_howto.html#4_15">output commands</A>. The scalar and vector
|
2010-03-03 05:59:33 +08:00
|
|
|
values calculated by this fix are "intensive".
|
2007-10-11 06:39:11 +08:00
|
|
|
</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>
|
2008-03-01 09:13:20 +08:00
|
|
|
<P>The cumulative time is zeroed when the fix is created and
|
2007-10-11 06:39:11 +08:00
|
|
|
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>
|
|
|
|
</HTML>
|