forked from lijiext/lammps
234 lines
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234 lines
11 KiB
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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/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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<H3>fix wall/harmonic 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 style keyword values ...
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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>style = <I>wall/lj93</I> or <I>wall/lj126</I> or <I>wall/colloid</I> or <I>wall/harmonic</I>
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<LI>one or more keyword/value pairs may be appended
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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> or <I>vel</I> or <I>wiggle/sin</I> or <I>wiggle/cos</I> or <I>units</I>
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<PRE> <I>xlo</I>, <I>xhi</I>, <I>ylo</I>, <I>yhi</I>, <I>zlo</I>, <I>zhi</I> values = coord epsilon sigma cutoff
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coord = position of wall (distance units)
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epsilon = strength factor for wall-particle interaction (energy or energy/distance^2 units)
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sigma = size factor for wall-particle interaction (distance units)
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cutoff = distance from wall at which wall-particle interaction is cut off (distance units)
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<I>vel</I> value = v
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v = velocity of wall in perpendicular direction (velocity units)
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<I>wiggle/sin</I> values = amplitude period
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amplitude = size of oscillation (distance units)
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period = time of oscillation (time units)
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<I>wiggle/cos</I> values = amplitude period
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amplitude = size of oscillation (distance units)
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period = time of oscillation (time units)
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<I>units</I> value = <I>lattice</I> or <I>box</I>
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lattice = the wall is defined in lattice units
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box = the wall is defined in simulation box units
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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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<PRE>fix wallhi all wall/lj93 xhi 10.0 1.0 1.0 2.5
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fix wallhi all wall/lj126 xhi 23.2 1.0 1.0 2.5 vel 1.0 units box
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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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on the atom in a direction perpendicular to the wall. The energy of
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wall-particle interactions depends on the style.
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</P>
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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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</P>
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<CENTER><IMG SRC = "Eqs/fix_wall_lj93.jpg">
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</CENTER>
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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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</P>
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<CENTER><IMG SRC = "Eqs/pair_lj.jpg">
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</CENTER>
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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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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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<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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<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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</P>
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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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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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</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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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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</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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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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</P>
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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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<P>If the <I>vel</I> keyword is specified, the position of all walls will move
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during the simulation in a perpendicular direction, based on their
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initial <I>coord</I> position, the specified velocity <I>vel</I>, and the time
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elapsed since the fix was specified. A positive velocity means each
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wall moves inward, towards the center of the box. I.e. an <I>xlo</I> wall
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will move in the +x direction and an <I>xhi</I> wall will move in the -x
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direction. A negative velocity means each wall moves outward, away
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from the center of the box. If you want different walls to move with
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different velocities, then you need to use multiple fix wall commands.
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</P>
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<P>If the <I>wiggle/sin</I> keyword is specified, the position of all walls
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will oscillate sinusoidally during the simulation in the perpendicular
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direction, according to the equation:
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</P>
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<PRE>position = coord + A sin(omega*delta)
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</PRE>
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<P>If the <I>wiggle/cos</I> keyword is specified, the position of all walls
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will oscillate sinusoidally during the simulation in the perpendicular
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direction, according to the equation:
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</P>
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<PRE>position = coord + A (1 - cos(omega*delta))
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</PRE>
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<P>In both cases, <I>coord</I> is the specified initial position of the wall,
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<I>A</I> is the <I>amplitude</I>, <I>omega</I> is 2 PI / <I>period</I>, and <I>delta</I> is the
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time elapsed since the fix was specified. A positive amplitude means
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each wall initially moves inward, towards the center of the box.
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I.e. an <I>xlo</I> wall will move initially in the +x direction and an
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<I>xhi</I> wall will move initially in the -x direction. A negative
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velocity means each wall moves initially outward, away from the center
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of the box. Note that the <I>wiggle/sin</I> option oscillates with
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amplitude <I>A</I> around the pos0 position and the velocity of the wall is
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a maximum at time 0. By contrast, for the <I>wiggle/cos</I> option the
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wall moves up to <I>2A</I> away from pos0 in one direction and the velocity
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of the wall is 0 at time 0. If you want different walls to oscillate
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with different amplitudes or periods, then you need to use multiple
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fix wall commands.
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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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to define the position of the wall and its velocity and wiggle
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amplitude. A <I>box</I> value selects standard distance units as defined
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by the <A HREF = "units.html">units</A> command, e.g. Angstroms for units = real or
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metal. A <I>lattice</I> value means the distance units are in lattice
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spacings. The <A HREF = "lattice.html">lattice</A> command must have been
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previously used to define the lattice spacing. Note that with the
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<I>lattice</I> option, the wall's position is specified in lattice
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spacings, the wall's velocity is specified in lattice spacings per
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time, and the wall's oscillation amplitude is specified in lattice
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spacings. Each of these 3 quantities may be dependent on the x,y,z
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dimension, since the lattice spacings can be different in x,y,z.
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</P>
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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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<P>This fix computes a scalar energy and a 6-length vector of forces (one
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force magnitude per wall), which can be accessed by various <A HREF = "Section_howto.html#4_15">output
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commands</A>. The scalar and vector values
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calculated by this fix are "extensive", meaning they scale with the
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number of atoms in the simulation. Note that the scalar energy is the
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sum of interactions with all defined walls. If you want the energy on
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a per-wall basis, you need to use multiple fix wall commands. The 6
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vector quantities are the force on the <I>xlo</I> wall, the <I>xhi</I> wall,
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<I>ylo</I>, <I>yhi</I>, <I>zlo</I>, <I>zhi</I>. These values will only be non-zero if the
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corresponding wall is defined. Note that an outward force on a wall
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will be a negative value for <I>lo</I> walls and a positive value for <I>hi</I>
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walls.
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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.
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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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<P><B>Restrictions:</B>
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</P>
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<P>Any dimension (xyz) that has a wall must be non-periodic.
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</P>
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<P>You cannot use both the <I>vel</I> and either of the <I>wiggle</I> keywords
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together.
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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_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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</P>
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<P><B>Default:</B>
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</P>
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<P>The option defaults are no velocity, no wiggle, and units = lattice.
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</P>
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</HTML>
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