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:link(ld,Manual.html)
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fix wall/lj93 command :h3

[Syntax:]

fix ID group-ID wall/lj93 style coord epsilon sigma cutoff keyword values ... :pre

ID, group-ID are documented in "fix"_fix.html command :ulb,l
wall/lj93 = style name of this fix command :l
style = {xlo} or {xhi} or {ylo} or {yhi} or {zlo} or {zhi} :l
coord = position of wall :l
epsilon = Lennard-Jones epsilon for wall-particle interaction :l
sigma = Lennard-Jones sigma for wall-particle interaction :l
cutoff = distance from wall at which wall-particle interaction is cut off (distance units) :l
one or more keyword/value pairs may be appended :l
keyword = {vel} :l
  {vel} args = v
    v = velocity of wall in perpendicular direction (velocity units) :pre
:ule

[Examples:]

fix wallhi all wall/lj93 xhi 10.0 1.0 1.0 2.5
fix leftwall all wall/lj93 zlo 0.0  1.0 1.0 0.858 vel 1.0 :pre

[Description:]

Bound the simulation domain on one of its faces with a Lennard-Jones
wall that interacts with the atoms in the group.  The energy E of
wall-particle interactions is given by the 9-3 potential

:c,image(Eqs/fix_wall_lj93.jpg)

where {r} is the distance from the particle to the wall {coord}, and
epsilon and sigma are the usual LJ parameters.  Rc is the cutoff value
specified in the command.  This interaction is derived by integrating
over a 3d half-lattice of Lennard-Jones 12-6 particles.  A harder,
more repulsive wall interaction can be computed by using the "fix
wall/lj126"_fix_wall_lj126.html command.

The wall potential is shifted so that the energy of a wall-particle
interaction is 0.0 at the cutoff distance.

If the {vel} keyword is specified, the position of wall will move
during the simulation, based on its initial position (coord) and the
specified velocity (vel).  {Xlo} or {xhi} walls move in the x
direction with whatever sign you give the {v} argument.  Ditto for {y}
and {z} walls.  Note that if you do multiple runs, the initial
position of the wall does not change, so it will continue to move at
the specified velocity.

[Restart, fix_modify, output, run start/stop, minimize info:]

No information about this fix is written to "binary restart
files"_restart.html.

The "fix_modify"_fix_modify.html {energy} option is supported by this
fix to add the energy of interaction between atoms and the wall to the
system's potential energy as part of "thermodynamic
output"_thermo_style.html.

This fix computes a scalar energy and a 3-vector of forces (on the
wall), which can be accessed by various "output
commands"_Section_howto.html#4_15.  The scalar and vector values
calculated by this fix are "extensive", meaning they scale with the
number of atoms in the simulation.

No parameter of this fix can be used with the {start/stop} keywords of
the "run"_run.html command.

The forces due to this fix are imposed during an energy minimization,
invoked by the "minimize"_minimize.html command.

IMPORTANT NOTE: If you want the atom/wall interaction energy to be
included in the total potential energy of the system (the quantity
being minimized), you MUST enable the "fix_modify"_fix_modify.html
{energy} option for this fix.

[Restrictions:]

Any dimension (xyz) that has a LJ 9/3 wall must be non-periodic.

[Related commands:]

"fix wall/reflect"_fix_wall_reflect.html, "fix
wall/lj126"_fix_wall_lj126.html

[Default:]

The option defaults are vel = 0.
