2006-09-22 00:22:34 +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 wall/reflect command
|
|
|
|
|
</H3>
|
|
|
|
|
<P><B>Syntax:</B>
|
|
|
|
|
</P>
|
2010-06-01 23:06:27 +08:00
|
|
|
<PRE>fix ID group-ID wall/reflect face arg ... keyword value ...
|
2006-09-22 00:22:34 +08:00
|
|
|
</PRE>
|
2010-06-01 23:06:27 +08:00
|
|
|
<UL><LI>ID, group-ID are documented in <A HREF = "fix.html">fix</A> command
|
|
|
|
|
|
|
|
|
|
<LI>wall/reflect = style name of this fix command
|
|
|
|
|
|
|
|
|
|
<LI>one or more face/arg pairs may be appended
|
|
|
|
|
|
|
|
|
|
<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>
|
|
|
|
|
|
|
|
|
|
<PRE> <I>xlo</I>,<I>ylo</I>,<I>zlo</I> arg = EDGE or constant or variable
|
|
|
|
|
EDGE = current lo edge of simulation box
|
|
|
|
|
constant = number like 0.0 or -30.0 (distance units)
|
|
|
|
|
variable = <A HREF = "variable.html">equal-style variable</A> like v_x or v_wiggle
|
|
|
|
|
<I>xhi</I>,<I>yhi</I>,<I>zhi</I> arg = EDGE or constant or variable
|
|
|
|
|
EDGE = current hi edge of simulation box
|
|
|
|
|
constant = number like 50.0 or 100.3 (distance units)
|
|
|
|
|
variable = <A HREF = "variable.html">equal-style variable</A> like v_x or v_wiggle
|
|
|
|
|
</PRE>
|
2010-10-13 01:58:24 +08:00
|
|
|
<LI>zero or more keyword/value pairs may be appended
|
2010-06-01 23:06:27 +08:00
|
|
|
|
|
|
|
|
<LI>keyword = <I>units</I>
|
|
|
|
|
|
|
|
|
|
<PRE> <I>units</I> value = <I>lattice</I> or <I>box</I>
|
2010-10-13 01:58:24 +08:00
|
|
|
<I>lattice</I> = the wall position is defined in lattice units
|
|
|
|
|
<I>box</I> = the wall position is defined in simulation box units
|
2010-06-01 23:06:27 +08:00
|
|
|
</PRE>
|
|
|
|
|
|
2006-09-22 00:22:34 +08:00
|
|
|
</UL>
|
|
|
|
|
<P><B>Examples:</B>
|
|
|
|
|
</P>
|
2010-06-01 23:06:27 +08:00
|
|
|
<PRE>fix xwalls all wall/reflect xlo EDGE xhi EDGE
|
|
|
|
|
fix walls all wall/reflect xlo 0.0 ylo 10.0 units box
|
|
|
|
|
fix top all wall/reflect zhi v_pressdown
|
2006-09-22 00:22:34 +08:00
|
|
|
</PRE>
|
|
|
|
|
<P><B>Description:</B>
|
|
|
|
|
</P>
|
|
|
|
|
<P>Bound the simulation with one or more walls which reflect particles
|
2010-06-01 23:06:27 +08:00
|
|
|
in the specified group when they attempt to move thru them.
|
2006-09-22 00:22:34 +08:00
|
|
|
</P>
|
2010-06-01 23:06:27 +08:00
|
|
|
<P>Reflection means that if an atom moves outside the wall on a timestep
|
2006-09-22 00:22:34 +08:00
|
|
|
by a distance delta (e.g. due to <A HREF = "fix_nve.html">fix nve</A>), then it is
|
2010-06-01 23:06:27 +08:00
|
|
|
put back inside the face by the same delta, and the sign of the
|
2006-09-22 00:22:34 +08:00
|
|
|
corresponding component of its velocity is flipped.
|
|
|
|
|
</P>
|
2008-05-14 05:49:52 +08:00
|
|
|
<P>When used in conjunction with <A HREF = "fix_nve.html">fix nve</A> and <A HREF = "run_style.html">run_style
|
|
|
|
|
verlet</A>, the resultant time-integration algorithm is
|
|
|
|
|
equivalent to the primitive splitting algorithm (PSA) described by
|
|
|
|
|
<A HREF = "#Bond">Bond</A>. Because each reflection event divides
|
|
|
|
|
the corresponding timestep asymmetrically, energy conservation is only
|
|
|
|
|
satisfied to O(dt), rather than to O(dt^2) as it would be for
|
|
|
|
|
velocity-Verlet integration without reflective walls.
|
|
|
|
|
</P>
|
2010-06-01 23:06:27 +08:00
|
|
|
<P>Up to 6 walls or faces can be specified in a single command: <I>xlo</I>,
|
2010-06-02 23:42:55 +08:00
|
|
|
<I>xhi</I>, <I>ylo</I>, <I>yhi</I>, <I>zlo</I>, <I>zhi</I>. A <I>lo</I> face reflects particles
|
2010-10-13 01:58:24 +08:00
|
|
|
that move to a coordinate less than the wall position, back in the
|
2010-06-02 23:42:55 +08:00
|
|
|
<I>hi</I> direction. A <I>hi</I> face reflects particles that move to a
|
2010-10-13 01:58:24 +08:00
|
|
|
coordinate higher than the wall position, back in the <I>lo</I> direction.
|
2010-06-01 23:06:27 +08:00
|
|
|
</P>
|
2010-10-13 01:58:24 +08:00
|
|
|
<P>The position of each wall can be specified in one of 3 ways: as the
|
2010-06-01 23:06:27 +08:00
|
|
|
EDGE of the simulation box, as a constant value, or as a variable. If
|
|
|
|
|
EDGE is used, then the corresponding boundary of the current
|
|
|
|
|
simulation box is used. If a numeric constant is specified then the
|
|
|
|
|
wall is placed at that position in the appropriate dimension (x, y, or
|
|
|
|
|
z). In both the EDGE and constant cases, the wall will never move.
|
2010-10-13 01:58:24 +08:00
|
|
|
If the wall position is a variable, it should be specified as v_name,
|
2010-08-13 07:10:38 +08:00
|
|
|
where name is an <A HREF = "variable.html">equal-style variable</A> name. In this
|
|
|
|
|
case the variable is evaluated each timestep and the result becomes
|
|
|
|
|
the current position of the reflecting wall. Equal-style variables
|
|
|
|
|
can specify formulas with various mathematical functions, and include
|
|
|
|
|
<A HREF = "thermo_style.html">thermo_style</A> command keywords for the simulation
|
|
|
|
|
box parameters and timestep and elapsed time. Thus it is easy to
|
|
|
|
|
specify a time-dependent wall position.
|
2010-06-01 23:06:27 +08:00
|
|
|
</P>
|
|
|
|
|
<P>The <I>units</I> keyword determines the meaning of the distance units used
|
2010-10-13 01:58:24 +08:00
|
|
|
to define a wall position, but only when a numeric constant is used.
|
2010-06-01 23:06:27 +08:00
|
|
|
It is not relevant when EDGE or a variable is used to specify a face
|
|
|
|
|
position.
|
|
|
|
|
</P>
|
|
|
|
|
<P>A <I>box</I> value selects standard distance units as defined by the
|
|
|
|
|
<A HREF = "units.html">units</A> command, e.g. Angstroms for units = real or metal.
|
|
|
|
|
A <I>lattice</I> value means the distance units are in lattice spacings.
|
|
|
|
|
The <A HREF = "lattice.html">lattice</A> command must have been previously used to
|
|
|
|
|
define the lattice spacings.
|
|
|
|
|
</P>
|
|
|
|
|
<HR>
|
|
|
|
|
|
2010-10-13 01:58:24 +08:00
|
|
|
<P>Here are examples of variable definitions that move the wall position
|
|
|
|
|
in a time-dependent fashion using equal-style
|
|
|
|
|
<A HREF = "variable.html">variables</A>.
|
|
|
|
|
</P>
|
|
|
|
|
<PRE>variable ramp equal ramp(0,10)
|
|
|
|
|
fix 1 all wall xlo v_ramp 1.0 1.0 2.5
|
|
|
|
|
</PRE>
|
|
|
|
|
<PRE>variable linear equal vlinear(0,20)
|
|
|
|
|
fix 1 all wall xlo v_linear 1.0 1.0 2.5
|
|
|
|
|
</PRE>
|
|
|
|
|
<PRE>variable wiggle equal swiggle(0.0,5.0,3.0)
|
|
|
|
|
fix 1 all wall xlo v_wiggle 1.0 1.0 2.5
|
|
|
|
|
</PRE>
|
|
|
|
|
<PRE>variable wiggle equal cwiggle(0.0,5.0,3.0)
|
|
|
|
|
fix 1 all wall xlo v_wiggle 1.0 1.0 2.5
|
|
|
|
|
</PRE>
|
|
|
|
|
<P>The ramp(lo,hi) function adjusts the wall position linearly from lo to
|
|
|
|
|
hi over the course of a run. The linear(c0,velocity) function does
|
|
|
|
|
something similar using the equation position = c0 + velocity*delta,
|
|
|
|
|
where delta is the elapsed time.
|
|
|
|
|
</P>
|
|
|
|
|
<P>The swiggle(c0,A,period) function causes the wall position to
|
|
|
|
|
oscillate sinusoidally according to this equation, where omega = 2 PI
|
|
|
|
|
/ period:
|
|
|
|
|
</P>
|
|
|
|
|
<PRE>position = c0 + A sin(omega*delta)
|
|
|
|
|
</PRE>
|
|
|
|
|
<P>The cwiggle(c0,A,period) function causes the wall position to
|
|
|
|
|
oscillate sinusoidally according to this equation, which will have an
|
|
|
|
|
initial wall velocity of 0.0, and thus may impose a gentler
|
|
|
|
|
perturbation on the particles:
|
|
|
|
|
</P>
|
|
|
|
|
<PRE>position = c0 + A (1 - cos(omega*delta))
|
|
|
|
|
</PRE>
|
|
|
|
|
<HR>
|
|
|
|
|
|
2007-10-11 06:28:11 +08:00
|
|
|
<P><B>Restart, fix_modify, output, run start/stop, minimize info:</B>
|
2007-06-26 08:03:39 +08:00
|
|
|
</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
|
2010-04-03 00:51:06 +08:00
|
|
|
are relevant to this fix. No global or per-atom quantities are stored
|
|
|
|
|
by this fix for access by various <A HREF = "Section_howto.html#4_15">output
|
2007-10-11 06:28:11 +08:00
|
|
|
commands</A>. 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>.
|
2007-06-26 08:03:39 +08:00
|
|
|
</P>
|
2007-03-08 09:01:08 +08:00
|
|
|
<P><B>Restrictions:</B>
|
|
|
|
|
</P>
|
|
|
|
|
<P>Any dimension (xyz) that has a reflecting wall must be non-periodic.
|
2006-09-22 00:22:34 +08:00
|
|
|
</P>
|
2009-11-13 08:04:38 +08:00
|
|
|
<P>A reflecting wall should not be used with rigid bodies such as those
|
2007-03-09 23:20:25 +08:00
|
|
|
defined by a "fix rigid" command. This is because the wall/reflect
|
|
|
|
|
displaces atoms directly rather than exerts a force on them. For
|
2010-05-07 23:11:21 +08:00
|
|
|
rigid bodies, use a soft wall instead, such as <A HREF = "fix_wall.html">fix
|
2009-11-13 08:04:38 +08:00
|
|
|
wall/lj93</A>. LAMMPS will flag the use of a rigid
|
|
|
|
|
fix with fix wall/reflect with a warning, but will not generate an
|
|
|
|
|
error.
|
2007-03-09 23:20:25 +08:00
|
|
|
</P>
|
2006-09-22 00:22:34 +08:00
|
|
|
<P><B>Related commands:</B>
|
|
|
|
|
</P>
|
2010-05-07 23:11:21 +08:00
|
|
|
<P><A HREF = "fix_wall.html">fix wall/lj93</A> command
|
2006-09-22 00:22:34 +08:00
|
|
|
</P>
|
|
|
|
|
<P><B>Default:</B> none
|
|
|
|
|
</P>
|
2010-10-28 04:54:22 +08:00
|
|
|
<HR>
|
|
|
|
|
|
2008-05-14 05:49:52 +08:00
|
|
|
<A NAME = "Bond"></A>
|
|
|
|
|
|
|
|
|
|
<P><B>(Bond)</B> Bond and Leimkuhler, SIAM J Sci Comput, 30, p 134 (2007).
|
|
|
|
|
</P>
|
2006-09-22 00:22:34 +08:00
|
|
|
</HTML>
|
