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<div class="section" id="fix-msst-command">
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<span id="index-0"></span><h1>fix msst command</h1>
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<div class="section" id="syntax">
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<h2>Syntax</h2>
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<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">fix</span> <span class="n">ID</span> <span class="n">group</span><span class="o">-</span><span class="n">ID</span> <span class="n">msst</span> <span class="nb">dir</span> <span class="n">shockvel</span> <span class="n">keyword</span> <span class="n">value</span> <span class="o">...</span>
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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"><span class="doc">fix</span></a> command</li>
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<li>msst = style name of this fix</li>
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<li>dir = <em>x</em> or <em>y</em> or <em>z</em></li>
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<li>shockvel = shock velocity (strictly positive, distance/time units)</li>
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<li>zero or more keyword value pairs may be appended</li>
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<li>keyword = <em>q</em> or <em>mu</em> or <em>p0</em> or <em>v0</em> or <em>e0</em> or <em>tscale</em></li>
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</ul>
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<pre class="literal-block">
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<em>q</em> value = cell mass-like parameter (mass^2/distance^4 units)
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<em>mu</em> value = artificial viscosity (mass/length/time units)
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<em>p0</em> value = initial pressure in the shock equations (pressure units)
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<em>v0</em> value = initial simulation cell volume in the shock equations (distance^3 units)
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<em>e0</em> value = initial total energy (energy units)
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<em>tscale</em> value = reduction in initial temperature (unitless fraction between 0.0 and 1.0)
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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</h2>
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<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">fix</span> <span class="mi">1</span> <span class="nb">all</span> <span class="n">msst</span> <span class="n">y</span> <span class="mf">100.0</span> <span class="n">q</span> <span class="mf">1.0e5</span> <span class="n">mu</span> <span class="mf">1.0e5</span>
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<span class="n">fix</span> <span class="mi">2</span> <span class="nb">all</span> <span class="n">msst</span> <span class="n">z</span> <span class="mf">50.0</span> <span class="n">q</span> <span class="mf">1.0e4</span> <span class="n">mu</span> <span class="mf">1.0e4</span> <span class="n">v0</span> <span class="mf">4.3419e+03</span> <span class="n">p0</span> <span class="mf">3.7797e+03</span> <span class="n">e0</span> <span class="o">-</span><span class="mf">9.72360e+02</span> <span class="n">tscale</span> <span class="mf">0.01</span>
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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</h2>
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<p>This command performs the Multi-Scale Shock Technique (MSST)
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integration to update positions and velocities each timestep to mimic
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a compressive shock wave passing over the system. See <a class="reference internal" href="#reed"><span class="std std-ref">(Reed)</span></a>
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for a detailed description of this method. The MSST varies the cell
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volume and temperature in such a way as to restrain the system to the
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shock Hugoniot and the Rayleigh line. These restraints correspond to
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the macroscopic conservation laws dictated by a shock
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front. <em>shockvel</em> determines the steady shock velocity that will be
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simulated.</p>
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<p>To perform a simulation, choose a value of <em>q</em> that provides volume
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compression on the timescale of 100 fs to 1 ps. If the volume is not
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compressing, either the shock speed is chosen to be below the material
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sound speed or <em>p0</em> has been chosen inaccurately. Volume compression
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at the start can be sped up by using a non-zero value of <em>tscale</em>. Use
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the smallest value of <em>tscale</em> that results in compression.</p>
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<p>Under some special high-symmetry conditions, the pressure (volume)
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and/or temperature of the system may oscillate for many cycles even
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with an appropriate choice of mass-like parameter <em>q</em>. Such
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oscillations have physical significance in some cases. The optional
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<em>mu</em> keyword adds an artificial viscosity that helps break the system
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symmetry to equilibrate to the shock Hugoniot and Rayleigh line more
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rapidly in such cases.</p>
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<p><em>tscale</em> is a factor between 0 and 1 that determines what fraction of
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thermal kinetic energy is converted to compressive strain kinetic
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energy at the start of the simulation. Setting this parameter to a
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non-zero value may assist in compression at the start of simulations
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where it is slow to occur.</p>
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<p>If keywords <em>e0</em>, <em>p0</em>,or <em>v0</em> are not supplied, these quantities will
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be calculated on the first step, after the energy specified by
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<em>tscale</em> is removed. The value of <em>e0</em> is not used in the dynamical
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equations, but is used in calculating the deviation from the Hugoniot.</p>
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<p>Values of shockvel less than a critical value determined by the
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material response will not have compressive solutions. This will be
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reflected in lack of significant change of the volume in the MSST.</p>
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<p>For all pressure styles, the simulation box stays orthogonal in shape.
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Parrinello-Rahman boundary conditions (tilted box) are supported by
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LAMMPS, but are not implemented for MSST.</p>
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<p>This fix computes a temperature and pressure each timestep. To do
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this, the fix creates its own computes of style “temp” and “pressure”,
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as if these commands had been issued:</p>
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<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">compute</span> <span class="n">fix</span><span class="o">-</span><span class="n">ID_temp</span> <span class="n">group</span><span class="o">-</span><span class="n">ID</span> <span class="n">temp</span>
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<span class="n">compute</span> <span class="n">fix</span><span class="o">-</span><span class="n">ID_press</span> <span class="n">group</span><span class="o">-</span><span class="n">ID</span> <span class="n">pressure</span> <span class="n">fix</span><span class="o">-</span><span class="n">ID_temp</span>
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</pre></div>
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</div>
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<p>See the <a class="reference internal" href="compute_temp.html"><span class="doc">compute temp</span></a> and <a class="reference internal" href="compute_pressure.html"><span class="doc">compute pressure</span></a> commands for details. Note that the
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IDs of the new computes are the fix-ID + underscore + “temp” or fix_ID
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+ underscore + “press”. The group for the new computes is “all”.</p>
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</div>
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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</h2>
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<p>This fix writes the state of all internal variables to <a class="reference internal" href="restart.html"><span class="doc">binary restart files</span></a>. See the <a class="reference internal" href="read_restart.html"><span class="doc">read_restart</span></a> command
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for info on how to re-specify a fix in an input script that reads a
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restart file, so that the operation of the fix continues in an
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uninterrupted fashion.</p>
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<p>The progress of the MSST can be monitored by printing the global
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scalar and global vector quantities computed by the fix.</p>
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<p>The scalar is the cumulative energy change due to the fix. This is
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also the energy added to the potential energy by the
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<a class="reference internal" href="fix_modify.html"><span class="doc">fix_modify</span></a> <em>energy</em> command. With this command, the
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thermo keyword <em>etotal</em> prints the conserved quantity of the MSST
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dynamic equations. This can be used to test if the MD timestep is
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sufficiently small for accurate integration of the dynamic
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equations. See also <a class="reference internal" href="thermo_style.html"><span class="doc">thermo_style</span></a> command.</p>
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<p>The global vector contains four values in this order:</p>
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<p>[<em>dhugoniot</em>, <em>drayleigh</em>, <em>lagrangian_speed</em>, <em>lagrangian_position</em>]</p>
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<ol class="arabic simple">
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<li><em>dhugoniot</em> is the departure from the Hugoniot (temperature units).</li>
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<li><em>drayleigh</em> is the departure from the Rayleigh line (pressure units).</li>
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<li><em>lagrangian_speed</em> is the laboratory-frame Lagrangian speed (particle velocity) of the computational cell (velocity units).</li>
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<li><em>lagrangian_position</em> is the computational cell position in the reference frame moving at the shock speed. This is usually a good estimate of distance of the computational cell behind the shock front.</li>
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</ol>
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<p>To print these quantities to the log file with descriptive column
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headers, the following LAMMPS commands are suggested:</p>
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<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">fix</span> <span class="n">msst</span> <span class="nb">all</span> <span class="n">msst</span> <span class="n">z</span>
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<span class="n">fix_modify</span> <span class="n">msst</span> <span class="n">energy</span> <span class="n">yes</span>
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<span class="n">variable</span> <span class="n">dhug</span> <span class="n">equal</span> <span class="n">f_msst</span><span class="p">[</span><span class="mi">1</span><span class="p">]</span>
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<span class="n">variable</span> <span class="n">dray</span> <span class="n">equal</span> <span class="n">f_msst</span><span class="p">[</span><span class="mi">2</span><span class="p">]</span>
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<span class="n">variable</span> <span class="n">lgr_vel</span> <span class="n">equal</span> <span class="n">f_msst</span><span class="p">[</span><span class="mi">3</span><span class="p">]</span>
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<span class="n">variable</span> <span class="n">lgr_pos</span> <span class="n">equal</span> <span class="n">f_msst</span><span class="p">[</span><span class="mi">4</span><span class="p">]</span>
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<span class="n">thermo_style</span> <span class="n">custom</span> <span class="n">step</span> <span class="n">temp</span> <span class="n">ke</span> <span class="n">pe</span> <span class="n">lz</span> <span class="n">pzz</span> <span class="n">etotal</span> <span class="n">v_dhug</span> <span class="n">v_dray</span> <span class="n">v_lgr_vel</span> <span class="n">v_lgr_pos</span> <span class="n">f_msst</span>
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</pre></div>
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</div>
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<p>These fixes compute a global scalar and a global vector of 4
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quantities, 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 values calculated
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by this fix are “extensive”; the vector values are “intensive”.</p>
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</div>
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<div class="section" id="restrictions">
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<h2>Restrictions</h2>
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<p>This fix style is part of the SHOCK package. It is only enabled if
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LAMMPS was built with that package. See the <a class="reference internal" href="Section_start.html#start-3"><span class="std std-ref">Making LAMMPS</span></a> section for more info.</p>
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<p>All cell dimensions must be periodic. This fix can not be used with a
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triclinic cell. The MSST fix has been tested only for the group-ID
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all.</p>
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</div>
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<div class="section" id="related-commands">
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<h2>Related commands</h2>
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<p><a class="reference internal" href="fix_nphug.html"><span class="doc">fix nphug</span></a>, <a class="reference internal" href="fix_deform.html"><span class="doc">fix deform</span></a></p>
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</div>
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<div class="section" id="default">
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<h2>Default</h2>
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<p>The keyword defaults are q = 10, mu = 0, tscale = 0.01. p0, v0, and e0
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are calculated on the first step.</p>
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<hr class="docutils" />
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<p id="reed"><strong>(Reed)</strong> Reed, Fried, and Joannopoulos, Phys. Rev. Lett., 90, 235503 (2003).</p>
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