<p>In all cases, <em>r</em> is the distance from the particle to the region
surface, and Rc is the <em>cutoff</em> distance at which the particle and
surface no longer interact. The energy of the wall potential is
shifted so that the wall-particle interaction energy is 0.0 at the
cutoff distance.</p>
<p>For the <em>lj93</em> and <em>lj126</em> styles, <em>epsilon</em> and <em>sigma</em> are the usual
Lennard-Jones parameters, which determine the strength and size of the
particle as it interacts with the wall. Epsilon has energy units.
Note that this <em>epsilon</em> and <em>sigma</em> may be different than any
<em>epsilon</em> or <em>sigma</em> values defined for a pair style that computes
particle-particle interactions.</p>
<p>The <em>lj93</em> interaction is derived by integrating over a 3d
half-lattice of Lennard-Jones 12/6 particles. The <em>lj126</em> interaction
is effectively a harder, more repulsive wall interaction.</p>
<p>For the <em>colloid</em> style, <em>epsilon</em> is effectively a Hamaker constant
with energy units for the colloid-wall interaction, <em>R</em> is the radius
of the colloid particle, <em>D</em> is the distance from the surface of the
colloid particle to the wall (r-R), and <em>sigma</em> is the size of a
constituent LJ particle inside the colloid particle. Note that the
cutoff distance Rc in this case is the distance from the colloid
particle center to the wall.</p>
<p>The <em>colloid</em> interaction is derived by integrating over constituent
LJ particles of size <em>sigma</em> within the colloid particle and a 3d
half-lattice of Lennard-Jones 12/6 particles of size <em>sigma</em> in the
wall.</p>
<p>For the <em>wall/harmonic</em> style, <em>epsilon</em> is effectively the spring
constant K, and has units (energy/distance^2). The input parameter
<em>sigma</em> is ignored. The minimum energy position of the harmonic
spring is at the <em>cutoff</em>. This is a repulsive-only spring since the
interaction is truncated at the <em>cutoff</em></p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">For all of the styles, you must insure that r is always > 0 for
all particles in the group, or LAMMPS will generate an error. This
means you cannot start your simulation with particles on the region
surface (r = 0) or with particles on the wrong side of the region
surface (r < 0). For the <em>wall/lj93</em> and <em>wall/lj126</em> styles, the
energy of the wall/particle interaction (and hence the force on the
particle) blows up as r -> 0. The <em>wall/colloid</em> style is even more
restrictive, since the energy blows up as D = r-R -> 0. This means
the finite-size particles of radius R must be a distance larger than R
from the region surface. The <em>harmonic</em> style is a softer potential
and does not blow up as r -> 0, but you must use a large enough
<em>epsilon</em> that particles always reamin on the correct side of the
region surface (r > 0).</p>
</div>
<p><strong>Restart, fix_modify, output, run start/stop, minimize info:</strong></p>
<p>No information about this fix is written to <a class="reference internal" href="restart.html"><span class="doc">binary restart files</span></a>.</p>
<p>The <a class="reference internal" href="fix_modify.html"><span class="doc">fix_modify</span></a> <em>energy</em> 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 <a class="reference internal" href="thermo_style.html"><span class="doc">thermodynamic output</span></a>.</p>
<p>The <a class="reference internal" href="fix_modify.html"><span class="doc">fix_modify</span></a> <em>respa</em> option is supported by this
fix. This allows to set at which level of the <a class="reference internal" href="run_style.html"><span class="doc">r-RESPA</span></a>
integrator the fix is adding its forces. Default is the outermost level.</p>
<p>This fix computes a global scalar energy and a global 3-length vector
of forces, which can be accessed by various <a class="reference internal" href="Section_howto.html#howto-15"><span class="std std-ref">output commands</span></a>. The scalar energy is the sum
of energy interactions for all particles interacting with the wall
represented by the region surface. The 3 vector quantities are the
x,y,z components of the total force acting on the wall due to the
particles. The scalar and vector values calculated by this fix are
“extensive”.</p>
<p>No parameter of this fix can be used with the <em>start/stop</em> keywords of
the <a class="reference internal" href="run.html"><span class="doc">run</span></a> command.</p>
<p>The forces due to this fix are imposed during an energy minimization,
invoked by the <a class="reference internal" href="minimize.html"><span class="doc">minimize</span></a> command.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">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 <a class="reference internal" href="fix_modify.html"><span class="doc">fix_modify</span></a> <em>energy</em>
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