2007-09-06 00:05:47 +08:00
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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 nve/limit 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 nve/limit xmax
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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>nve = style name of this fix command
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<LI>xmax = maximum distance an atom can move in one timestep (distance units)
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</UL>
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<P><B>Examples:</B>
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</P>
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<PRE>fix 1 all nve/limit 0.1
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</PRE>
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<P><B>Description:</B>
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</P>
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<P>Perform constant NVE updates of position and velocity for atoms in the
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group each timestep. A limit is imposed on the maximum distance an
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atom can move in one timestep. This is useful when starting a
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simulation with a configuration containing highly overlapped atoms.
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Normally this would generate huge forces which would blow atoms out of
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the simulation box, causing LAMMPS to stop with an error.
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</P>
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<P>Using this fix can overcome that problem. Forces on atoms must still
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be computable (which typically means 2 atoms must have a separation
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distance > 0.0). But large velocities generated by large forces are
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reset to a value that corresponds to a displacement of length <I>xmax</I>
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in a single timestep. <I>Xmax</I> is specified in distance units; see the
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<A HREF = "units.html">units</A> command for details. The value of <I>xmax</I> should be
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consistent with the neighbor skin distance and the frequency of
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neighbor list re-building, so that pairwise interactions are not
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missed on successive timesteps as atoms move. See the
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<A HREF = "neighbor.html">neighbor</A> and <A HREF = "neigh_modify.html">neigh_modify</A> commands
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for details.
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</P>
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<P>Note that if a velocity reset occurs the integrator will not conserve
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energy. On steps where no velocity resets occur, this integrator is
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exactly like the <A HREF = "fix_nve.html">fix nve</A> command. Since forces are
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unaltered, pressures computed by thermodynamic output will still be
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very large for overlapped configurations.
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</P>
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2013-10-30 22:34:24 +08:00
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<P>IMPORTANT NOTE: You should not use <A HREF = "fix_shake.html">fix shake</A> in
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conjunction with this fix. That is because fix shake applies
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contraint forces based on the predicted postitions of atoms after the
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next timestep. It has no way of knowing the timestep may change due
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to this fix, which will cause the constraint forces to be invalid. A
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better strategy is to turn off fix shake when performing initial
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dynamics that need this fix, then turn fix shake on when doing normal
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dynamics with a fixed-size timestep.
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</P>
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2007-10-11 06:28:11 +08:00
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<P><B>Restart, fix_modify, output, run start/stop, minimize info:</B>
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2007-09-06 00:05:47 +08:00
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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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2008-01-03 08:29:17 +08:00
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are relevant to this fix.
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</P>
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2010-04-03 00:51:06 +08:00
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<P>This fix computes a global scalar which can be accessed by various
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2011-08-26 01:01:01 +08:00
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<A HREF = "Section_howto.html#howto_15">output commands</A>. The scalar is the
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count of how many updates of atom's velocity/position were limited by
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the maximum distance criterion. This should be roughly the number of
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2010-04-03 00:51:06 +08:00
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atoms so affected, except that updates occur at both the beginning and
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end of a timestep in a velocity Verlet timestepping algorithm. This
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is a cumulative quantity for the current run, but is re-initialized to
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zero each time a run is performed. The scalar value calculated by
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this fix is "extensive".
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2008-01-03 08:29:17 +08:00
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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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2007-09-06 00:05:47 +08:00
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</P>
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<P><B>Restrictions:</B> none
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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 = "fix_nve.html">fix nve</A>, <A HREF = "fix_nve_noforce.html">fix nve/noforce</A>,
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<A HREF = "pair_soft.html">pair_style soft</A>
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</P>
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<P><B>Default:</B> none
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</P>
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</HTML>
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