2006-09-22 00:22:34 +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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<H3>fix wall/reflect 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 wall/reflect keyword ...
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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>wall/reflect = style name of this fix command
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2007-07-03 04:04:44 +08:00
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<LI>one or more keyword/value pairs may be appended
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2006-09-22 00:22:34 +08:00
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<LI>keyword = <I>xlo</I> or <I>xhi</I> or <I>ylo</I> or <I>yhi</I> or <I>zlo</I> or <I>zhi</I>
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</UL>
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<P><B>Examples:</B>
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</P>
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<PRE>fix xwalls all wall/reflect xlo xhi
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fix walls all wall/reflect xlo ylo zlo xhi yhi zhi
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</PRE>
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<P><B>Description:</B>
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</P>
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<P>Bound the simulation with one or more walls which reflect particles
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2007-03-08 09:01:08 +08:00
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when they attempt to move thru them.
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2006-09-22 00:22:34 +08:00
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</P>
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<P>Reflection means that if an atom moves outside the box on a timestep
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by a distance delta (e.g. due to <A HREF = "fix_nve.html">fix nve</A>), then it is
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put back inside the box by the same delta and the sign of the
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corresponding component of its velocity is flipped.
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</P>
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2008-05-14 05:49:52 +08:00
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<P>When used in conjunction with <A HREF = "fix_nve.html">fix nve</A> and <A HREF = "run_style.html">run_style
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verlet</A>, the resultant time-integration algorithm is
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equivalent to the primitive splitting algorithm (PSA) described by
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<A HREF = "#Bond">Bond</A>. Because each reflection event divides
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the corresponding timestep asymmetrically, energy conservation is only
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satisfied to O(dt), rather than to O(dt^2) as it would be for
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velocity-Verlet integration without reflective walls.
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</P>
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2007-02-20 02:03:15 +08:00
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<P>IMPORTANT NOTE: This fix performs its operations at the same point in
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the timestep as other time integration fixes, such as <A HREF = "fix_nve.html">fix
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nve</A>, <A HREF = "fix_nvt.html">fix nvt</A>, or <A HREF = "fix_npt.html">fix npt</A>.
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Thus fix wall/reflect should normally be the last such fix specified
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in the input script, since the adjustments it makes to atom
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coordinates should come after the changes made by time integration.
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LAMMPS will warn you if your fixes are not ordered this way.
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2007-02-17 05:06:13 +08:00
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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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2007-06-26 08:03:39 +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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2007-10-11 06:28:11 +08:00
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are relevant to this fix. No global scalar or vector or per-atom
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quantities are stored by this fix for access by various <A HREF = "Section_howto.html#4_15">output
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commands</A>. No parameter of this fix can be
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used with the <I>start/stop</I> keywords of the <A HREF = "run.html">run</A> command.
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This fix is not invoked during <A HREF = "minimize.html">energy minimization</A>.
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2007-06-26 08:03:39 +08:00
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</P>
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2007-03-08 09:01:08 +08:00
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<P><B>Restrictions:</B>
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</P>
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<P>Any dimension (xyz) that has a reflecting wall must be non-periodic.
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2006-09-22 00:22:34 +08:00
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</P>
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2007-03-09 23:20:25 +08:00
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<P>A reflecting wall cannot be used with rigid bodies such as those
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defined by a "fix rigid" command. This is because the wall/reflect
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displaces atoms directly rather than exerts a force on them. For
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rigid bodies, use a soft wall instead, such as <A HREF = "fix_wall_lj93.html">fix
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wall/lj93</A>.
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</P>
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2006-09-22 00:22:34 +08:00
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<P><B>Related commands:</B>
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</P>
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<P><A HREF = "fix_wall_lj93.html">fix wall/lj93</A> command
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</P>
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<P><B>Default:</B> none
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</P>
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2008-05-14 05:49:52 +08:00
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<A NAME = "Bond"></A>
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<P><B>(Bond)</B> Bond and Leimkuhler, SIAM J Sci Comput, 30, p 134 (2007).
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</P>
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2006-09-22 00:22:34 +08:00
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</HTML>
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