2006-09-22 00:22:34 +08:00
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<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 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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2010-06-01 23:06:27 +08:00
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<PRE>fix ID group-ID wall/reflect face arg ... keyword value ...
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2006-09-22 00:22:34 +08:00
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</PRE>
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2010-06-01 23:06:27 +08:00
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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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<LI>one or more face/arg pairs may be appended
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<LI>face = <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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<PRE> <I>xlo</I>,<I>ylo</I>,<I>zlo</I> arg = EDGE or constant or variable
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EDGE = current lo edge of simulation box
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constant = number like 0.0 or -30.0 (distance units)
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variable = <A HREF = "variable.html">equal-style variable</A> like v_x or v_wiggle
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<I>xhi</I>,<I>yhi</I>,<I>zhi</I> arg = EDGE or constant or variable
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EDGE = current hi edge of simulation box
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constant = number like 50.0 or 100.3 (distance units)
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variable = <A HREF = "variable.html">equal-style variable</A> like v_x or v_wiggle
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</PRE>
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2010-10-13 01:58:24 +08:00
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<LI>zero or more keyword/value pairs may be appended
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2010-06-01 23:06:27 +08:00
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<LI>keyword = <I>units</I>
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<PRE> <I>units</I> value = <I>lattice</I> or <I>box</I>
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2010-10-13 01:58:24 +08:00
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<I>lattice</I> = the wall position is defined in lattice units
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<I>box</I> = the wall position is defined in simulation box units
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2010-06-01 23:06:27 +08:00
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</PRE>
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2006-09-22 00:22:34 +08:00
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</UL>
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<P><B>Examples:</B>
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</P>
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2010-06-01 23:06:27 +08:00
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<PRE>fix xwalls all wall/reflect xlo EDGE xhi EDGE
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fix walls all wall/reflect xlo 0.0 ylo 10.0 units box
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fix top all wall/reflect zhi v_pressdown
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2006-09-22 00:22:34 +08:00
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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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2010-06-01 23:06:27 +08:00
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in the specified group 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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2010-06-01 23:06:27 +08:00
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<P>Reflection means that if an atom moves outside the wall on a timestep
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2006-09-22 00:22:34 +08:00
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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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2010-06-01 23:06:27 +08:00
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put back inside the face by the same delta, and the sign of the
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2006-09-22 00:22:34 +08:00
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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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2010-06-01 23:06:27 +08:00
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<P>Up to 6 walls or faces can be specified in a single command: <I>xlo</I>,
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2010-06-02 23:42:55 +08:00
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<I>xhi</I>, <I>ylo</I>, <I>yhi</I>, <I>zlo</I>, <I>zhi</I>. A <I>lo</I> face reflects particles
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2010-10-13 01:58:24 +08:00
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that move to a coordinate less than the wall position, back in the
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2010-06-02 23:42:55 +08:00
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<I>hi</I> direction. A <I>hi</I> face reflects particles that move to a
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2010-10-13 01:58:24 +08:00
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coordinate higher than the wall position, back in the <I>lo</I> direction.
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2010-06-01 23:06:27 +08:00
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</P>
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2010-10-13 01:58:24 +08:00
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<P>The position of each wall can be specified in one of 3 ways: as the
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2010-06-01 23:06:27 +08:00
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EDGE of the simulation box, as a constant value, or as a variable. If
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EDGE is used, then the corresponding boundary of the current
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simulation box is used. If a numeric constant is specified then the
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wall is placed at that position in the appropriate dimension (x, y, or
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z). In both the EDGE and constant cases, the wall will never move.
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2010-10-13 01:58:24 +08:00
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If the wall position is a variable, it should be specified as v_name,
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2010-08-13 07:10:38 +08:00
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where name is an <A HREF = "variable.html">equal-style variable</A> name. In this
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case the variable is evaluated each timestep and the result becomes
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the current position of the reflecting wall. Equal-style variables
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can specify formulas with various mathematical functions, and include
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<A HREF = "thermo_style.html">thermo_style</A> command keywords for the simulation
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box parameters and timestep and elapsed time. Thus it is easy to
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specify a time-dependent wall position.
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2010-06-01 23:06:27 +08:00
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</P>
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<P>The <I>units</I> keyword determines the meaning of the distance units used
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2011-10-26 22:47:30 +08:00
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to define a wall position, but only when a numeric constant or
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variable is used. It is not relevant when EDGE is used to specify a
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face position. In the variable case, the variable is assumed to
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produce a value compatible with the <I>units</I> setting you specify.
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2010-06-01 23:06:27 +08:00
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</P>
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<P>A <I>box</I> value selects standard distance units as defined by the
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<A HREF = "units.html">units</A> command, e.g. Angstroms for units = real or metal.
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A <I>lattice</I> value means the distance units are in lattice spacings.
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The <A HREF = "lattice.html">lattice</A> command must have been previously used to
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define the lattice spacings.
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</P>
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<HR>
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2010-10-13 01:58:24 +08:00
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<P>Here are examples of variable definitions that move the wall position
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in a time-dependent fashion using equal-style
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<A HREF = "variable.html">variables</A>.
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</P>
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<PRE>variable ramp equal ramp(0,10)
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2011-10-25 23:07:42 +08:00
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fix 1 all wall/reflect xlo v_ramp
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2010-10-13 01:58:24 +08:00
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</PRE>
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<PRE>variable linear equal vlinear(0,20)
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2011-10-25 23:07:42 +08:00
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fix 1 all wall/reflect xlo v_linear
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2010-10-13 01:58:24 +08:00
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</PRE>
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<PRE>variable wiggle equal swiggle(0.0,5.0,3.0)
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2011-10-25 23:07:42 +08:00
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fix 1 all wall/reflect xlo v_wiggle
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2010-10-13 01:58:24 +08:00
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</PRE>
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<PRE>variable wiggle equal cwiggle(0.0,5.0,3.0)
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2011-10-25 23:07:42 +08:00
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fix 1 all wall/reflect xlo v_wiggle
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2010-10-13 01:58:24 +08:00
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</PRE>
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<P>The ramp(lo,hi) function adjusts the wall position linearly from lo to
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hi over the course of a run. The linear(c0,velocity) function does
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something similar using the equation position = c0 + velocity*delta,
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where delta is the elapsed time.
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</P>
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<P>The swiggle(c0,A,period) function causes the wall position to
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oscillate sinusoidally according to this equation, where omega = 2 PI
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/ period:
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</P>
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<PRE>position = c0 + A sin(omega*delta)
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</PRE>
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<P>The cwiggle(c0,A,period) function causes the wall position to
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oscillate sinusoidally according to this equation, which will have an
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initial wall velocity of 0.0, and thus may impose a gentler
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perturbation on the particles:
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</P>
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<PRE>position = c0 + A (1 - cos(omega*delta))
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</PRE>
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<HR>
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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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2010-04-03 00:51:06 +08:00
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are relevant to this fix. No global or per-atom quantities are stored
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2011-08-26 01:01:01 +08:00
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by this fix for access by various <A HREF = "Section_howto.html#howto_15">output
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commands</A>. No parameter of this fix can
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be used with the <I>start/stop</I> keywords of the <A HREF = "run.html">run</A> command.
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2007-10-11 06:28:11 +08:00
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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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2009-11-13 08:04:38 +08:00
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<P>A reflecting wall should not be used with rigid bodies such as those
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2007-03-09 23:20:25 +08:00
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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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2010-05-07 23:11:21 +08:00
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rigid bodies, use a soft wall instead, such as <A HREF = "fix_wall.html">fix
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2009-11-13 08:04:38 +08:00
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wall/lj93</A>. LAMMPS will flag the use of a rigid
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fix with fix wall/reflect with a warning, but will not generate an
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error.
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2007-03-09 23:20:25 +08:00
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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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2010-05-07 23:11:21 +08:00
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<P><A HREF = "fix_wall.html">fix wall/lj93</A> command
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2006-09-22 00:22:34 +08:00
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</P>
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<P><B>Default:</B> none
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</P>
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2010-10-28 04:54:22 +08:00
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<HR>
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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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