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<div role="main" class="document" itemscope="itemscope" itemtype="http://schema.org/Article">
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<div class="section" id="fix-heat-command">
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<span id="index-0"></span><h1>fix heat 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">heat</span> <span class="n">N</span> <span class="n">eflux</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>heat = style name of this fix command</li>
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<li>N = add/subtract heat every this many timesteps</li>
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<li>eflux = rate of heat addition or subtraction (energy/time units)</li>
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<li>eflux can be a variable (see below)</li>
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<li>zero or more keyword/value pairs may be appended to args</li>
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<li>keyword = <em>region</em></li>
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</ul>
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<pre class="literal-block">
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<em>region</em> value = region-ID
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region-ID = ID of region atoms must be in to have added force
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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">3</span> <span class="n">qin</span> <span class="n">heat</span> <span class="mi">1</span> <span class="mf">1.0</span>
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<span class="n">fix</span> <span class="mi">3</span> <span class="n">qin</span> <span class="n">heat</span> <span class="mi">10</span> <span class="n">v_flux</span>
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<span class="n">fix</span> <span class="mi">4</span> <span class="n">qout</span> <span class="n">heat</span> <span class="mi">1</span> <span class="o">-</span><span class="mf">1.0</span> <span class="n">region</span> <span class="n">top</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>Add non-translational kinetic energy (heat) to a group of atoms in a
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manner that conserves their aggregate momentum. Two of these fixes
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can be used to establish a temperature gradient across a simulation
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domain by adding heat (energy) to one group of atoms (hot reservoir)
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and subtracting heat from another (cold reservoir). E.g. a simulation
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sampling from the McDLT ensemble.</p>
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<p>If the <em>region</em> keyword is used, the atom must be in both the group
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and the specified geometric <a class="reference internal" href="region.html"><span class="doc">region</span></a> in order to have
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energy added or subtracted to it. If not specified, then the atoms in
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the group are affected wherever they may move to.</p>
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<p>Heat addition/subtraction is performed every N timesteps. The <em>eflux</em>
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parameter can be specified as a numeric constant or as a variable (see
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below). If it is a numeric constant or equal-style variable which
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evaluates to a scalar value, then the <em>eflux</em> determines the change in
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aggregate energy of the entire group of atoms per unit time, e.g. in
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eV/psec for <a class="reference internal" href="units.html"><span class="doc">metal units</span></a>. In this case it is an
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“extensive” quantity, meaning its magnitude should be scaled with the
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number of atoms in the group. Note that since <em>eflux</em> has per-time
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units (i.e. it is a flux), this means that a larger value of N will
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add/subtract a larger amount of energy each time the fix is invoked.</p>
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<div class="admonition note">
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<p class="first admonition-title">Note</p>
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<p class="last">The heat-exchange (HEX) algorithm implemented by this fix is
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known to exhibit a pronounced energy drift. An improved algorithm
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(eHEX) is available as a <a class="reference internal" href="fix_ehex.html"><span class="doc">fix ehex</span></a> command and might be
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preferable if energy conservation is important.</p>
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</div>
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<p>If <em>eflux</em> is specified as an atom-style variable (see below), then
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the variable computes one value per atom. In this case, each value is
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the energy flux for a single atom, again in units of energy per unit
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time. In this case, each value is an “intensive” quantity, which need
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not be scaled with the number of atoms in the group.</p>
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<p>As mentioned above, the <em>eflux</em> parameter can be specified as an
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equal-style or atom_style <a class="reference internal" href="variable.html"><span class="doc">variable</span></a>. If the value is a
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variable, it should be specified as v_name, where name is the variable
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name. In this case, the variable will be evaluated each timestep, and
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its value(s) used to determine the flux.</p>
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<p>Equal-style variables can specify formulas with various mathematical
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functions, and include <a class="reference internal" href="thermo_style.html"><span class="doc">thermo_style</span></a> command
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keywords for the simulation box parameters and timestep and elapsed
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time. Thus it is easy to specify a time-dependent flux.</p>
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<p>Atom-style variables can specify the same formulas as equal-style
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variables but can also include per-atom values, such as atom
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coordinates. Thus it is easy to specify a spatially-dependent flux
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with optional time-dependence as well.</p>
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<div class="admonition note">
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<p class="first admonition-title">Note</p>
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<p class="last">If heat is subtracted from the system too aggressively so that
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the group’s kinetic energy would go to zero, or any individual atom’s
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kinetic energy would go to zero for the case where <em>eflux</em> is an
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atom-style variable, then LAMMPS will halt with an error message.</p>
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</div>
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<p>Fix heat is different from a thermostat such as <a class="reference internal" href="fix_nh.html"><span class="doc">fix nvt</span></a>
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or <a class="reference internal" href="fix_temp_rescale.html"><span class="doc">fix temp/rescale</span></a> in that energy is
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added/subtracted continually. Thus if there isn’t another mechanism
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in place to counterbalance this effect, the entire system will heat or
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cool continuously. You can use multiple heat fixes so that the net
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energy change is 0.0 or use <a class="reference internal" href="fix_viscous.html"><span class="doc">fix viscous</span></a> to drain
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energy from the system.</p>
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<p>This fix does not change the coordinates of its atoms; it only scales
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their velocities. Thus you must still use an integration fix
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(e.g. <a class="reference internal" href="fix_nve.html"><span class="doc">fix nve</span></a>) on the affected atoms. This fix should
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not normally be used on atoms that have their temperature controlled
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by another fix - e.g. <a class="reference internal" href="fix_nh.html"><span class="doc">fix nvt</span></a> or <a class="reference internal" href="fix_langevin.html"><span class="doc">fix langevin</span></a> fix.</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>No information about this fix is written to <a class="reference internal" href="restart.html"><span class="doc">binary restart files</span></a>. None of the <a class="reference internal" href="fix_modify.html"><span class="doc">fix_modify</span></a> options
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are relevant to this fix.</p>
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<p>This fix computes a global scalar which can be accessed by various
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<a class="reference internal" href="Section_howto.html#howto-15"><span class="std std-ref">output commands</span></a>. This scalar is the
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most recent value by which velocites were scaled. The scalar value
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calculated by this fix is “intensive”. If <em>eflux</em> is specified as
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an atom-style variable, this fix computes the average value by which
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the velocities were scaled for all of the atoms that had their
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velocities scaled.</p>
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<p>No parameter of this fix can be used with the <em>start/stop</em> keywords of
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the <a class="reference internal" href="run.html"><span class="doc">run</span></a> command. This fix is not invoked during <a class="reference internal" href="minimize.html"><span class="doc">energy minimization</span></a>.</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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<blockquote>
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<div>none</div></blockquote>
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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_ehex.html"><span class="doc">fix ehex</span></a>, <a class="reference internal" href="compute_temp.html"><span class="doc">compute temp</span></a>, <a class="reference internal" href="compute_temp_region.html"><span class="doc">compute temp/region</span></a></p>
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<p><strong>Default:</strong> none</p>
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