forked from lijiext/lammps
176 lines
7.6 KiB
HTML
176 lines
7.6 KiB
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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<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>
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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</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 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</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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</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 10.0 1.0 1.0 2.5 vel 1.0
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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 E 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 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 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 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>In all cases, <I>r</I> is the distance from the particle to the wall
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<I>coord</I>, and Rc is the <I>cutoff</I> distance at which the particle and
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wall no longer interact, as specified in the command. The energy of
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the wall potential is shifted so that the energy of the wall-particle
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interaction is 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. Note that this
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<I>sigma</I> may be different than any <I>sigma</I> values defined for a pair
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style that 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 for the colloid-wall interaction, <I>R</I> is the radius of the
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colloid particle, <I>D</I> is the distance from the surface of the colloid
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particle to the wall (r-R), and <I>sigma</I> is the size of the constituent
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LJ particle inside the colloid particle. Note that the cutoff
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distance Rc in this case is the distance from the colloid particle
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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 with the same
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<I>sigma</I> in the wall.
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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 position (coord), the specified velocity (vel), and the time
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elapsed since the beginning of the simulation. See the note below
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about making a wall move continuously across multiple runs. A
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positive velocity means each wall moves inward, towards the center of
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the box. I.e. an <I>xlo</I> wall will move in the +x direction; an <I>xhi</I>
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wall will move in the -x direction. A negative velocity means each
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wall moves outward, away from the center of the box. If you want
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different walls to move with different velocities, then you need to
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use multiple fix wall commands.
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</P>
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<P>If the <I>wiggle</I> keyword is specified, the position of all walls will
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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 = pos0 + A sin (omega * delta)
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</PRE>
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<P>where <I>pos0</I> is the position at the time the beginning of the
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simulation, <I>A</I> is the <I>amplitude</I>, <I>omega</I> is 2 PI / <I>period</I>, and
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<I>delta</I> is the time elapsed since the beginning of the simulation.
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See the note below about making a wall oscillate continuously across
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multiple runs. A positive amplitude means each wall initially moves
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inward, towards the center of the box. I.e. an <I>xlo</I> wall will move
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initially in the +x direction; an <I>xhi</I> wall will move in the -x
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direction. A negative velocity means each wall moves initially
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outward, away from the center of the box. If you want different walls
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to oscillate with different amplitudes or periods, then you need to
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use multiple fix wall commands.
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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 3-vector of forces (on the
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walls), 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 if you define multiple
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walls, then the energy and force will be summed over all the walls.
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If you want the energy/force on a per-wall basis, you need to use
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multiple fix wall commands.
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</P>
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<P>This fix can change the position of each wall, due to the <I>vel</I> or
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<I>wiggle</I> keywords, continuously over multiple runs, using the <I>start</I>
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and <I>stop</I> keywords of the <A HREF = "run.html">run</A> command. If you do not do
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this, the wall position will be reset to <I>coord</I> at the beginning of
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each run.
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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 <I>wiggle</I> keywords 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<A HREF = "fix_wall_gran.html">>fix wall/reflect</A> wall/gran</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 and no wiggle.
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</P>
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</HTML>
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