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<div class="section" id="fix-wall-reflect-command">
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<span id="index-0"></span><h1>fix wall/reflect command<a class="headerlink" href="#fix-wall-reflect-command" title="Permalink to this headline">¶</a></h1>
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</div>
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<div class="section" id="fix-wall-reflect-kk-command">
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<h1>fix wall/reflect/kk command<a class="headerlink" href="#fix-wall-reflect-kk-command" title="Permalink to this headline">¶</a></h1>
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<div class="section" id="syntax">
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<h2>Syntax<a class="headerlink" href="#syntax" title="Permalink to this headline">¶</a></h2>
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<div class="highlight-python"><div class="highlight"><pre>fix ID group-ID wall/reflect face arg ... keyword value ...
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</pre></div>
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</div>
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<ul class="simple">
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<li>ID, group-ID are documented in <a class="reference internal" href="fix.html"><em>fix</em></a> command</li>
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<li>wall/reflect = style name of this fix command</li>
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<li>one or more face/arg pairs may be appended</li>
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<li>face = <em>xlo</em> or <em>xhi</em> or <em>ylo</em> or <em>yhi</em> or <em>zlo</em> or <em>zhi</em></li>
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</ul>
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<pre class="literal-block">
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<em>xlo</em>,*ylo*,*zlo* arg = EDGE or constant or variable
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EDGE = current lo edge of simulation box
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constant = number like 0.0 or -30.0 (distance units)
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variable = <a class="reference internal" href="variable.html"><em>equal-style variable</em></a> like v_x or v_wiggle
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<em>xhi</em>,*yhi*,*zhi* arg = EDGE or constant or variable
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EDGE = current hi edge of simulation box
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constant = number like 50.0 or 100.3 (distance units)
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variable = <a class="reference internal" href="variable.html"><em>equal-style variable</em></a> like v_x or v_wiggle
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</pre>
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<ul class="simple">
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<li>zero or more keyword/value pairs may be appended</li>
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<li>keyword = <em>units</em></li>
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</ul>
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<pre class="literal-block">
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<em>units</em> value = <em>lattice</em> or <em>box</em>
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<em>lattice</em> = the wall position is defined in lattice units
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<em>box</em> = the wall position is defined in simulation box units
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</pre>
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</div>
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<div class="section" id="examples">
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<h2>Examples<a class="headerlink" href="#examples" title="Permalink to this headline">¶</a></h2>
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<div class="highlight-python"><div class="highlight"><pre>fix xwalls all wall/reflect xlo EDGE xhi EDGE
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fix walls all wall/reflect xlo 0.0 ylo 10.0 units box
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fix top all wall/reflect zhi v_pressdown
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</pre></div>
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</div>
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</div>
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<div class="section" id="description">
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<h2>Description<a class="headerlink" href="#description" title="Permalink to this headline">¶</a></h2>
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<p>Bound the simulation with one or more walls which reflect particles
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in the specified group when they attempt to move thru them.</p>
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<p>Reflection means that if an atom moves outside the wall on a timestep
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by a distance delta (e.g. due to <a class="reference internal" href="fix_nve.html"><em>fix nve</em></a>), then it is
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put back inside the face by the same delta, and the sign of the
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corresponding component of its velocity is flipped.</p>
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<p>When used in conjunction with <a class="reference internal" href="fix_nve.html"><em>fix nve</em></a> and <a class="reference internal" href="run_style.html"><em>run_style verlet</em></a>, the resultant time-integration algorithm is
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equivalent to the primitive splitting algorithm (PSA) described by
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<a class="reference internal" href="#bond"><span>Bond</span></a>. Because each reflection event divides
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the corresponding timestep asymmetrically, energy conservation is only
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satisfied to O(dt), rather than to O(dt^2) as it would be for
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velocity-Verlet integration without reflective walls.</p>
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<p>Up to 6 walls or faces can be specified in a single command: <em>xlo</em>,
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<em>xhi</em>, <em>ylo</em>, <em>yhi</em>, <em>zlo</em>, <em>zhi</em>. A <em>lo</em> face reflects particles
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that move to a coordinate less than the wall position, back in the
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<em>hi</em> direction. A <em>hi</em> face reflects particles that move to a
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coordinate higher than the wall position, back in the <em>lo</em> direction.</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 class="reference internal" href="variable.html"><em>equal-style variable</em></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 class="reference internal" href="thermo_style.html"><em>thermo_style</em></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.</p>
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<p>The <em>units</em> keyword determines the meaning of the distance units used
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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 <em>units</em> setting you specify.</p>
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<p>A <em>box</em> value selects standard distance units as defined by the
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<a class="reference internal" href="units.html"><em>units</em></a> command, e.g. Angstroms for units = real or metal.
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A <em>lattice</em> value means the distance units are in lattice spacings.
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The <a class="reference internal" href="lattice.html"><em>lattice</em></a> command must have been previously used to
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define the lattice spacings.</p>
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<hr class="docutils" />
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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 class="reference internal" href="variable.html"><em>variables</em></a>.</p>
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<div class="highlight-python"><div class="highlight"><pre>variable ramp equal ramp(0,10)
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fix 1 all wall/reflect xlo v_ramp
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</pre></div>
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</div>
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<div class="highlight-python"><div class="highlight"><pre>variable linear equal vdisplace(0,20)
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fix 1 all wall/reflect xlo v_linear
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</pre></div>
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</div>
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<div class="highlight-python"><div class="highlight"><pre>variable wiggle equal swiggle(0.0,5.0,3.0)
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fix 1 all wall/reflect xlo v_wiggle
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</pre></div>
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</div>
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<div class="highlight-python"><div class="highlight"><pre>variable wiggle equal cwiggle(0.0,5.0,3.0)
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fix 1 all wall/reflect xlo v_wiggle
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</pre></div>
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</div>
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<p>The ramp(lo,hi) function adjusts the wall position linearly from lo to
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hi over the course of a run. The vdisplace(c0,velocity) function does
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something similar using the equation position = c0 + velocity*delta,
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where delta is the elapsed time.</p>
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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:</p>
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<div class="highlight-python"><div class="highlight"><pre>position = c0 + A sin(omega*delta)
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</pre></div>
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</div>
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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:</p>
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<div class="highlight-python"><div class="highlight"><pre><span class="n">position</span> <span class="o">=</span> <span class="n">c0</span> <span class="o">+</span> <span class="n">A</span> <span class="p">(</span><span class="mi">1</span> <span class="o">-</span> <span class="n">cos</span><span class="p">(</span><span class="n">omega</span><span class="o">*</span><span class="n">delta</span><span class="p">))</span>
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</pre></div>
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</div>
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<hr class="docutils" />
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<p>Styles with a <em>cuda</em>, <em>gpu</em>, <em>intel</em>, <em>kk</em>, <em>omp</em>, or <em>opt</em> suffix are
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functionally the same as the corresponding style without the suffix.
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They have been optimized to run faster, depending on your available
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hardware, as discussed in <a class="reference internal" href="Section_accelerate.html"><em>Section_accelerate</em></a>
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of the manual. The accelerated styles take the same arguments and
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should produce the same results, except for round-off and precision
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issues.</p>
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<p>These accelerated styles are part of the USER-CUDA, GPU, USER-INTEL,
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KOKKOS, USER-OMP and OPT packages, respectively. They are only
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enabled if LAMMPS was built with those packages. See the <a class="reference internal" href="Section_start.html#start-3"><span>Making LAMMPS</span></a> section for more info.</p>
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<p>You can specify the accelerated styles explicitly in your input script
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by including their suffix, or you can use the <a class="reference internal" href="Section_start.html#start-7"><span>-suffix command-line switch</span></a> when you invoke LAMMPS, or you can
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use the <a class="reference internal" href="suffix.html"><em>suffix</em></a> command in your input script.</p>
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<p>See <a class="reference internal" href="Section_accelerate.html"><em>Section_accelerate</em></a> of the manual for
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more instructions on how to use the accelerated styles effectively.</p>
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</div>
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<hr class="docutils" />
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<div class="section" id="restart-fix-modify-output-run-start-stop-minimize-info">
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<h2>Restart, fix_modify, output, run start/stop, minimize info<a class="headerlink" href="#restart-fix-modify-output-run-start-stop-minimize-info" title="Permalink to this headline">¶</a></h2>
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<p>No information about this fix is written to <a class="reference internal" href="restart.html"><em>binary restart files</em></a>. None of the <a class="reference internal" href="fix_modify.html"><em>fix_modify</em></a> options
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are relevant to this fix. No global or per-atom quantities are stored
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by this fix for access by various <a class="reference internal" href="Section_howto.html#howto-15"><span>output commands</span></a>. No parameter of this fix can
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be used with the <em>start/stop</em> keywords of the <a class="reference internal" href="run.html"><em>run</em></a> command.
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This fix is not invoked during <a class="reference internal" href="minimize.html"><em>energy minimization</em></a>.</p>
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</div>
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<div class="section" id="restrictions">
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<h2>Restrictions<a class="headerlink" href="#restrictions" title="Permalink to this headline">¶</a></h2>
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<p>Any dimension (xyz) that has a reflecting wall must be non-periodic.</p>
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<p>A reflecting wall should not be used with rigid bodies such as those
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defined by a “fix rigid” command. This is because the wall/reflect
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displaces atoms directly rather than exerts a force on them. For
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rigid bodies, use a soft wall instead, such as <a class="reference internal" href="fix_wall.html"><em>fix wall/lj93</em></a>. LAMMPS will flag the use of a rigid
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fix with fix wall/reflect with a warning, but will not generate an
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error.</p>
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</div>
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<div class="section" id="related-commands">
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<h2>Related commands<a class="headerlink" href="#related-commands" title="Permalink to this headline">¶</a></h2>
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<p><a class="reference internal" href="fix_wall.html"><em>fix wall/lj93</em></a>, <a class="reference internal" href="fix_oneway.html"><em>fix oneway</em></a></p>
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<p><strong>Default:</strong> none</p>
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<hr class="docutils" />
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<p id="bond"><strong>(Bond)</strong> Bond and Leimkuhler, SIAM J Sci Comput, 30, p 134 (2007).</p>
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