forked from lijiext/lammps
100 lines
3.8 KiB
HTML
100 lines
3.8 KiB
HTML
<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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<HR>
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<H3>fix dt/reset command
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</H3>
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<P><B>Syntax:</B>
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</P>
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<PRE>fix ID group-ID dt/reset N Tmin Tmax Xmax keyword values ...
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</PRE>
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<UL><LI>ID, group-ID are documented in <A HREF = "fix.html">fix</A> command
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<LI>dt/reset = style name of this fix command
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<LI>N = recompute dt every N timesteps
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<LI>Tmin = minimum dt allowed which can be NULL (time units)
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<LI>Tmax = maximum dt allowed which can be NULL (time units)
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<LI>Xmax = maximum distance for an atom to move in one timestep (distance units)
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<LI>zero or more keyword/value pairs may be appended
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<LI>keyword = <I>units</I>
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</UL>
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<PRE> <I>units</I> value = <I>lattice</I> or <I>box</I>
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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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</PRE>
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<P><B>Examples:</B>
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</P>
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<PRE>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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</PRE>
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<P><B>Description:</B>
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</P>
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<P>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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</P>
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<P>This fix overrides the timestep size setting made by the
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<A HREF = "timestep.html">timestep</A> command. The new timestep size <I>dt</I> is
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computed in the following manner.
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</P>
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<P>For each atom, the timestep is computed that would cause it to
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displace <I>Xmax</I> 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 <I>Xmax</I>, though it may be
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smaller.
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</P>
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<P>Given this putative timestep for each atom, the minimum timestep value
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across all atoms is computed. Then the <I>Tmin</I> and <I>Tmax</I> 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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</P>
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<P>When the <A HREF = "run_style.html">run style</A> is <I>respa</I>, this fix resets the
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outer loop (largest) timestep, which is the same timestep that the
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<A HREF = "timestep.html">timestep</A> command sets.
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</P>
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<P><B>Restart, fix_modify, output, run start/stop, minimize info:</B>
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</P>
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<P>No information about this fix is written to <A HREF = "restart.html">binary restart
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files</A>. None of the <A HREF = "fix_modify.html">fix_modify</A> options
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are relevant to this fix.
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</P>
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<P>This fix computes a global scalar and a global vector of length 2,
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which can be accessed by various <A HREF = "Section_howto.html#howto_15">output
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commands</A>. The scalar is the current
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timestep size. The first element of the vector stores the cumulative
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simulation time (in time units). The second element of the vector
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stores the last timestep on which the timestep was reset to a new
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value. The scalar and vector values calculated by this fix are
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"intensive".
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</P>
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<P>No parameter of this fix can be used with the <I>start/stop</I> keywords of
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the <A HREF = "run.html">run</A> command. This fix is not invoked during <A HREF = "minimize.html">energy
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minimization</A>.
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</P>
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<P><B>Restrictions:</B>
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</P>
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<P>The cumulative time is zeroed when the fix is created and continuously
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accrues thereafter. Using the <A HREF = "reset_timestep.html">reset_timestep</A>
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command while this fix is defined will mess up the time accumulation.
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</P>
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<P><B>Related commands:</B>
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</P>
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<P><A HREF = "timestep.html">timestep</A>
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</P>
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<P><B>Default:</B>
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</P>
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<P>The option defaults is units = lattice.
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</P>
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</HTML>
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