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<div class="section" id="fix-nvt-eff-command">
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<span id="index-0"></span><h1>fix nvt/eff command</h1>
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</div>
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<div class="section" id="fix-npt-eff-command">
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<h1>fix npt/eff command</h1>
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</div>
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<div class="section" id="fix-nph-eff-command">
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<h1>fix nph/eff 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">style_name</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>style_name = <em>nvt/eff</em> or <em>npt/eff</em> or <em>nph/eff</em></li>
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</ul>
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<pre class="literal-block">
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one or more keyword value pairs may be appended
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keyword = <em>temp</em> or <em>iso</em> or <em>aniso</em> or <em>tri</em> or <em>x</em> or <em>y</em> or <em>z</em> or <em>xy</em> or <em>yz</em> or <em>xz</em> or <em>couple</em> or <em>tchain</em> or <em>pchain</em> or <em>mtk</em> or <em>tloop</em> or <em>ploop</em> or <em>nreset</em> or <em>drag</em> or <em>dilate</em>
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<em>temp</em> values = Tstart Tstop Tdamp
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Tstart,Tstop = external temperature at start/end of run
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Tdamp = temperature damping parameter (time units)
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<em>iso</em> or <em>aniso</em> or <em>tri</em> values = Pstart Pstop Pdamp
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Pstart,Pstop = scalar external pressure at start/end of run (pressure units)
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Pdamp = pressure damping parameter (time units)
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<em>x</em> or <em>y</em> or <em>z</em> or <em>xy</em> or <em>yz</em> or <em>xz</em> values = Pstart Pstop Pdamp
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Pstart,Pstop = external stress tensor component at start/end of run (pressure units)
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Pdamp = stress damping parameter (time units)
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<em>couple</em> = <em>none</em> or <em>xyz</em> or <em>xy</em> or <em>yz</em> or <em>xz</em>
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<em>tchain</em> value = length of thermostat chain (1 = single thermostat)
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<em>pchain</em> values = length of thermostat chain on barostat (0 = no thermostat)
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<em>mtk</em> value = <em>yes</em> or <em>no</em> = add in MTK adjustment term or not
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<em>tloop</em> value = number of sub-cycles to perform on thermostat
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<em>ploop</em> value = number of sub-cycles to perform on barostat thermostat
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<em>nreset</em> value = reset reference cell every this many timesteps
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<em>drag</em> value = drag factor added to barostat/thermostat (0.0 = no drag)
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<em>dilate</em> value = <em>all</em> or <em>partial</em>
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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">nvt</span><span class="o">/</span><span class="n">eff</span> <span class="n">temp</span> <span class="mf">300.0</span> <span class="mf">300.0</span> <span class="mf">0.1</span>
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<span class="n">fix</span> <span class="mi">1</span> <span class="n">part</span> <span class="n">npt</span><span class="o">/</span><span class="n">eff</span> <span class="n">temp</span> <span class="mf">300.0</span> <span class="mf">300.0</span> <span class="mf">0.1</span> <span class="n">iso</span> <span class="mf">0.0</span> <span class="mf">0.0</span> <span class="mf">1.0</span>
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<span class="n">fix</span> <span class="mi">2</span> <span class="n">part</span> <span class="n">npt</span><span class="o">/</span><span class="n">eff</span> <span class="n">temp</span> <span class="mf">300.0</span> <span class="mf">300.0</span> <span class="mf">0.1</span> <span class="n">tri</span> <span class="mf">5.0</span> <span class="mf">5.0</span> <span class="mf">1.0</span>
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<span class="n">fix</span> <span class="mi">2</span> <span class="n">ice</span> <span class="n">nph</span><span class="o">/</span><span class="n">eff</span> <span class="n">x</span> <span class="mf">1.0</span> <span class="mf">1.0</span> <span class="mf">0.5</span> <span class="n">y</span> <span class="mf">2.0</span> <span class="mf">2.0</span> <span class="mf">0.5</span> <span class="n">z</span> <span class="mf">3.0</span> <span class="mf">3.0</span> <span class="mf">0.5</span> <span class="n">yz</span> <span class="mf">0.1</span> <span class="mf">0.1</span> <span class="mf">0.5</span> <span class="n">xz</span> <span class="mf">0.2</span> <span class="mf">0.2</span> <span class="mf">0.5</span> <span class="n">xy</span> <span class="mf">0.3</span> <span class="mf">0.3</span> <span class="mf">0.5</span> <span class="n">nreset</span> <span class="mi">1000</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>These commands perform time integration on Nose-Hoover style
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non-Hamiltonian equations of motion for nuclei and electrons in the
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group for the <a class="reference internal" href="pair_eff.html"><span class="doc">electron force field</span></a> model. The fixes
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are designed to generate positions and velocities sampled from the
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canonical (nvt), isothermal-isobaric (npt), and isenthalpic (nph)
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ensembles. This is achieved by adding some dynamic variables which
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are coupled to the particle velocities (thermostatting) and simulation
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domain dimensions (barostatting). In addition to basic thermostatting
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and barostatting, these fixes can also create a chain of thermostats
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coupled to the particle thermostat, and another chain of thermostats
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coupled to the barostat variables. The barostat can be coupled to the
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overall box volume, or to individual dimensions, including the <em>xy</em>,
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<em>xz</em> and <em>yz</em> tilt dimensions. The external pressure of the barostat
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can be specified as either a scalar pressure (isobaric ensemble) or as
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components of a symmetric stress tensor (constant stress ensemble).
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When used correctly, the time-averaged temperature and stress tensor
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of the particles will match the target values specified by
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Tstart/Tstop and Pstart/Pstop.</p>
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<p>The operation of these fixes is exactly like that described by the
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<a class="reference internal" href="fix_nh.html"><span class="doc">fix nvt, npt, and nph</span></a> commands, except that the radius
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and radial velocity of electrons are also updated. Likewise the
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temperature and pressure calculated by the fix, using the computes it
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creates (as discussed in the <a class="reference internal" href="fix_nh.html"><span class="doc">fix nvt, npt, and nph</span></a>
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doc page), are performed with computes that include the eFF contribution
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to the temperature or kinetic energy from the electron radial velocity.</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">there are two different pressures that can be reported for eFF
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when defining the pair_style (see <span class="xref doc">pair eff/cut</span> to
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understand these settings), one (default) that considers electrons do
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not contribute radial virial components (i.e. electrons treated as
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incompressible ‘rigid’ spheres) and one that does. The radial
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electronic contributions to the virials are only tallied if the
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flexible pressure option is set, and this will affect both global and
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per-atom quantities. In principle, the true pressure of a system is
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somewhere in between the rigid and the flexible eFF pressures, but,
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for most cases, the difference between these two pressures will not be
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significant over long-term averaged runs (i.e. even though the energy
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partitioning changes, the total energy remains similar).</p>
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</div>
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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">currently, there is no available option for the user to set or
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create temperature distributions that include the radial electronic
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degrees of freedom with the <a class="reference internal" href="velocity.html"><span class="doc">velocity</span></a> command, so the
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the user must allow for these degrees of freedom to equilibrate
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(i.e. equi-partitioning of energy) through time integration.</p>
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</div>
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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>See the doc page for the <a class="reference internal" href="fix_nh.html"><span class="doc">fix nvt, npt, and nph</span></a> commands
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for details.</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 is part of the USER-EFF 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>Other restriction discussed on the doc page for the <a class="reference internal" href="fix_nh.html"><span class="doc">fix nvt, npt, and nph</span></a> commands also apply.</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 temperature for systems (regions or groups) with only
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electrons and no nuclei is 0.0 (i.e. not defined) in the current
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temperature calculations, a practical example would be a uniform
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electron gas or a very hot plasma, where electrons remain delocalized
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from the nuclei. This is because, even though electron virials are
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included in the temperature calculation, these are averaged over the
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nuclear degrees of freedom only. In such cases a corrective term must
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be added to the pressure to get the correct kinetic contribution.</p>
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</div>
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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_nh.html"><span class="doc">fix nvt</span></a>, <a class="reference internal" href="fix_nh.html"><span class="doc">fix nph</span></a>, <a class="reference internal" href="fix_nh.html"><span class="doc">fix npt</span></a>,
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<a class="reference internal" href="fix_modify.html"><span class="doc">fix_modify</span></a>, <a class="reference internal" href="run_style.html"><span class="doc">run_style</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 tchain = 3, pchain = 3, mtk = yes, tloop =
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ploop = 1, nreset = 0, drag = 0.0, dilate = all, and couple = none.</p>
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<hr class="docutils" />
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<p id="martyna"><strong>(Martyna)</strong> Martyna, Tobias and Klein, J Chem Phys, 101, 4177 (1994).</p>
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<p id="parrinello"><strong>(Parrinello)</strong> Parrinello and Rahman, J Appl Phys, 52, 7182 (1981).</p>
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<p id="tuckerman"><strong>(Tuckerman)</strong> Tuckerman, Alejandre, Lopez-Rendon, Jochim, and
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Martyna, J Phys A: Math Gen, 39, 5629 (2006).</p>
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<p id="shinoda"><strong>(Shinoda)</strong> Shinoda, Shiga, and Mikami, Phys Rev B, 69, 134103 (2004).</p>
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