2009-11-18 08:30:50 +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/lj93 command
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</H3>
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<H3>fix wall/lj126 command
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</H3>
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<H3>fix wall/colloid command
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</H3>
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2010-01-09 07:03:13 +08:00
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<H3>fix wall/harmonic command
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</H3>
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2009-11-18 08:30:50 +08:00
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<P><B>Syntax:</B>
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</P>
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2010-10-13 01:58:24 +08:00
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<PRE>fix ID group-ID style face args ... keyword value ...
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2009-11-18 08:30:50 +08:00
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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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2010-01-09 06:54:29 +08:00
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<LI>style = <I>wall/lj93</I> or <I>wall/lj126</I> or <I>wall/colloid</I> or <I>wall/harmonic</I>
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2009-11-18 08:30:50 +08:00
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2010-10-13 01:58:24 +08:00
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<LI>one or more face/arg pairs may be appended
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2009-11-18 08:30:50 +08:00
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2010-10-13 01:58:24 +08:00
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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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2009-11-18 08:30:50 +08:00
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2010-10-13 01:58:24 +08:00
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<PRE> args = coord epsilon sigma cutoff
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coord = position of wall = EDGE or constant or variable
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EDGE = current lo or hi 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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2010-01-09 06:54:29 +08:00
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epsilon = strength factor for wall-particle interaction (energy or energy/distance^2 units)
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2009-11-18 08:30:50 +08:00
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sigma = size factor for wall-particle interaction (distance units)
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2010-10-13 01:58:24 +08:00
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cutoff = distance from wall at which wall-particle interaction is cut off (distance units)
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</PRE>
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<LI>zero or more keyword/value pairs may be appended
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2011-10-25 01:46:47 +08:00
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<LI>keyword = <I>units</I> or <I>fld</I>
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2010-10-13 01:58:24 +08:00
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<PRE> <I>units</I> value = <I>lattice</I> or <I>box</I>
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<I>lattice</I> = the wall position is defined in lattice units
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2011-10-25 01:46:47 +08:00
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<I>box</I> = the wall position is defined in simulation box units
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<I>fld</I> value = <I>yes</I> or <I>no</I>
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<I>yes</I> = invoke the wall constraint to be compatible with implicit FLD
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2012-10-10 00:01:51 +08:00
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<I>no</I> = invoke the wall constraint in the normal way
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</PRE>
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<PRE> <I>pbc</I> value = <I>yes</I> or <I>no</I>
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<I>yes</I> = allow periodic boundary in a wall dimension
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<I>no</I> = require non-perioidic boundaries in any wall dimension
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2009-11-18 08:30:50 +08:00
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</PRE>
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</UL>
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<P><B>Examples:</B>
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</P>
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2010-10-13 01:58:24 +08:00
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<PRE>fix wallhi all wall/lj93 xlo -1.0 1.0 1.0 2.5 units box
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fix wallhi all wall/lj93 xhi EDGE 1.0 1.0 2.5
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fix wallhi all wall/lj126 v_wiggle 23.2 1.0 1.0 2.5
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2009-11-18 08:30:50 +08:00
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fix zwalls all wall/colloid zlo 0.0 1.0 1.0 0.858 zhi 40.0 1.0 1.0 0.858
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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 domain on one or more of its faces with a flat
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wall that interacts with the atoms in the group by generating a force
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2009-11-18 09:05:23 +08:00
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on the atom in a direction perpendicular to the wall. The energy of
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2009-11-18 08:30:50 +08:00
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wall-particle interactions depends on the style.
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</P>
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2009-11-18 09:05:23 +08:00
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<P>For style <I>wall/lj93</I>, the energy E is given by the 9/3 potential:
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2009-11-18 08:30:50 +08:00
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</P>
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<CENTER><IMG SRC = "Eqs/fix_wall_lj93.jpg">
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</CENTER>
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2009-11-18 09:05:23 +08:00
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<P>For style <I>wall/lj126</I>, the energy E is given by the 12/6 potential:
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2009-11-18 08:30:50 +08:00
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</P>
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<CENTER><IMG SRC = "Eqs/pair_lj.jpg">
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</CENTER>
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2009-11-18 09:05:23 +08:00
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<P>For style <I>wall/colloid</I>, the energy E is given by an integrated form of
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2009-11-18 08:30:50 +08:00
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the <A HREF = "pair_colloid.html">pair_style colloid</A> potential:
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</P>
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<CENTER><IMG SRC = "Eqs/fix_wall_colloid.jpg">
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</CENTER>
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2010-01-09 06:54:29 +08:00
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<P>For style <I>wall/harmonic</I>, the energy E is given by a harmonic spring
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potential:
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</P>
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<CENTER><IMG SRC = "Eqs/fix_wall_harmonic.jpg">
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</CENTER>
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2009-11-18 09:05:23 +08:00
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<P>In all cases, <I>r</I> is the distance from the particle to the wall at
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position <I>coord</I>, and Rc is the <I>cutoff</I> distance at which the
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particle and wall no longer interact. The energy of the wall
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potential is shifted so that the wall-particle interaction energy is
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0.0 at the cutoff distance.
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2009-11-18 08:30:50 +08:00
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</P>
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2010-10-13 01:58:24 +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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<I>xhi</I>, <I>ylo</I>, <I>yhi</I>, <I>zlo</I>, <I>zhi</I>. A <I>lo</I> face interacts with
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particles near the lower side of the simulation box in that dimension.
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A <I>hi</I> face interacts with particles near the upper side of the
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simulation box in that dimension.
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</P>
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<P>The position of each wall can be specified in one of 3 ways: as the
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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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If the wall position is a variable, it should be specified as v_name,
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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. See examples below.
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</P>
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2009-11-18 08:30:50 +08:00
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<P>For the <I>wall/lj93</I> and <I>wall/lj126</I> styles, <I>epsilon</I> and <I>sigma</I> are
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the usual Lennard-Jones parameters, which determine the strength and
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2010-01-09 06:54:29 +08:00
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size of the particle as it interacts with the wall. Epsilon has
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energy units. Note that this <I>epsilon</I> and <I>sigma</I> may be different
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than any <I>epsilon</I> or <I>sigma</I> values defined for a pair style that
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computes particle-particle interactions.
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2009-11-18 08:30:50 +08:00
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</P>
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<P>The <I>wall/lj93</I> interaction is derived by integrating over a 3d
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half-lattice of Lennard-Jones 12/6 particles. The <I>wall/lj126</I>
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interaction is effectively a harder, more repulsive wall interaction.
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</P>
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<P>For the <I>wall/colloid</I> style, <I>epsilon</I> is effectively a Hamaker
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2010-01-09 06:54:29 +08:00
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constant with energy units for the colloid-wall interaction, <I>R</I> is
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the radius of the colloid particle, <I>D</I> is the distance from the
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surface of the colloid particle to the wall (r-R), and <I>sigma</I> is the
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size of a constituent LJ particle inside the colloid particle. Note
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that the cutoff distance Rc in this case is the distance from the
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colloid particle center to the wall.
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2009-11-18 08:30:50 +08:00
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</P>
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<P>The <I>wall/colloid</I> interaction is derived by integrating over
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constituent LJ particles of size <I>sigma</I> within the colloid particle
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2009-11-18 09:05:23 +08:00
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and a 3d half-lattice of Lennard-Jones 12/6 particles of size <I>sigma</I>
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in the wall.
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2009-11-18 08:30:50 +08:00
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</P>
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2010-01-09 06:54:29 +08:00
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<P>For the <I>wall/harmonic</I> style, <I>epsilon</I> is effectively the spring
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constant K, and has units (energy/distance^2). The input parameter
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<I>sigma</I> is ignored. The minimum energy position of the harmonic
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spring is at the <I>cutoff</I>. This is a repulsive-only spring since the
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interaction is truncated at the <I>cutoff</I>
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</P>
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<P>IMPORTANT NOTE: For all of the styles, you must insure that r is
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always > 0 for all particles in the group, or LAMMPS will generate an
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error. This means you cannot start your simulation with particles at
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the wall position <I>coord</I> (r = 0) or with particles on the wrong side
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of the wall (r < 0). For the <I>wall/lj93</I> and <I>wall/lj126</I> styles, the
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energy of the wall/particle interaction (and hence the force on the
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particle) blows up as r -> 0. The <I>wall/colloid</I> style is even more
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restrictive, since the energy blows up as D = r-R -> 0. This means
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the finite-size particles of radius R must be a distance larger than R
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from the wall position <I>coord</I>. The <I>harmonic</I> style is a softer
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potential and does not blow up as r -> 0, but you must use a large
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enough <I>epsilon</I> that particles always reamin on the correct side of
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the wall (r > 0).
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</P>
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2010-10-13 01:58:24 +08:00
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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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2010-10-13 01:58:24 +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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2011-10-25 01:46:47 +08:00
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<P>The <I>fld</I> keyword can be used with a <I>yes</I> setting to invoke the wall
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constraint before pairwise interactions are computed. This allows an
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implicit FLD model using <A HREF = "pair_lubricateU.html">pair_style lubricateU</A>
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to include the wall force in its calculations. If the setting is
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<I>no</I>, wall forces are imposed after pairwise interactions, in the
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usual manner.
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</P>
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2012-10-10 00:01:51 +08:00
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<P>The <I>pbc</I> keyword can be used with a <I>yes</I> setting to allow walls to
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be specified in a periodic dimension. See the
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<A HREF = "boundary.html">boundary</A> command for options on simulation box
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boundaries. The default for <I>pbc</I> is <I>no</I>, which means the system
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must be non-periodic when using a wall. But you may wish to use a
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periodic box. E.g. to allow some particles to interact with the wall
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via the fix group-ID, and others to pass through it and wrap around a
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periodic box. In this case you should insure that the wall if
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sufficiently far enough away from the box boundary. If you do not,
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then particles may interact with both the wall and with periodic
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images on the other side of the box, which is probably not what you
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want.
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</P>
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2010-10-13 01:58:24 +08:00
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<HR>
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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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2009-11-18 23:22:46 +08:00
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</P>
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2010-10-13 01:58:24 +08:00
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<PRE>variable ramp equal ramp(0,10)
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fix 1 all wall xlo v_ramp 1.0 1.0 2.5
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2009-11-18 23:22:46 +08:00
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</PRE>
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2012-03-15 02:43:27 +08:00
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<PRE>variable linear equal vdisplace(0,20)
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2010-10-13 01:58:24 +08:00
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fix 1 all wall xlo v_linear 1.0 1.0 2.5
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</PRE>
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<PRE>variable wiggle equal swiggle(0.0,5.0,3.0)
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fix 1 all wall xlo v_wiggle 1.0 1.0 2.5
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</PRE>
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<PRE>variable wiggle equal cwiggle(0.0,5.0,3.0)
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fix 1 all wall xlo v_wiggle 1.0 1.0 2.5
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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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2012-04-06 22:28:14 +08:00
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hi over the course of a run. The vdisplace(c0,velocity) function does
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2010-10-13 01:58:24 +08:00
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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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2009-11-18 08:30:50 +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 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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2009-11-19 00:14:23 +08:00
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</P>
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2010-10-13 01:58:24 +08:00
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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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2009-11-18 08:30:50 +08:00
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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>.
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</P>
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<P>The <A HREF = "fix_modify.html">fix_modify</A> <I>energy</I> option is supported by this
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fix to add the energy of interaction between atoms and each wall to
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the system's potential energy as part of <A HREF = "thermo_style.html">thermodynamic
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output</A>.
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</P>
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2010-10-28 04:54:22 +08:00
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<P>This fix computes a global scalar energy and a global vector of
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2011-08-26 01:01:01 +08:00
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forces, which can be accessed by various <A HREF = "Section_howto.html#howto_15">output
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commands</A>. Note that the scalar energy is
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the sum of interactions with all defined walls. If you want the
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energy on a per-wall basis, you need to use multiple fix wall
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commands. The length of the vector is equal to the number of walls
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defined by the fix. Each vector value is the normal force on a
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specific wall. Note that an outward force on a wall will be a
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negative value for <I>lo</I> walls and a positive value for <I>hi</I> walls.
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The scalar and vector values calculated by this fix are "extensive".
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2009-11-18 08:30:50 +08:00
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</P>
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2009-12-04 02:37:34 +08:00
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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.
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2009-11-18 08:30:50 +08:00
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</P>
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<P>The forces due to this fix are imposed during an energy minimization,
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invoked by the <A HREF = "minimize.html">minimize</A> command.
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</P>
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<P>IMPORTANT NOTE: If you want the atom/wall interaction energy to be
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included in the total potential energy of the system (the quantity
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being minimized), you MUST enable the <A HREF = "fix_modify.html">fix_modify</A>
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<I>energy</I> option for this fix.
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</P>
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2012-10-10 00:01:51 +08:00
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<P><B>Restrictions:</B> none
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2009-11-18 08:30:50 +08:00
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</P>
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<P><B>Related commands:</B>
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</P>
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2010-01-09 06:54:29 +08:00
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<P><A HREF = "fix_wall_reflect.html">fix wall/reflect</A>,
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<A HREF = "fix_wall_gran.html">fix wall/gran</A>,
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<A HREF = "fix_wall_region.html">fix wall/region</A>
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2009-11-18 08:30:50 +08:00
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</P>
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<P><B>Default:</B>
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</P>
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2012-10-10 00:01:51 +08:00
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<P>The option defaults units = lattice, fld = no, and pbc = no.
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2009-11-18 08:30:50 +08:00
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</P>
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</HTML>
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