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<li>pair_style dpd/fdt command</li>
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<div class="section" id="pair-style-dpd-fdt-command">
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<span id="index-0"></span><h1>pair_style dpd/fdt command</h1>
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</div>
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<div class="section" id="pair-style-dpd-fdt-energy-command">
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<h1>pair_style dpd/fdt/energy 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">pair_style</span> <span class="n">style</span> <span class="n">args</span>
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</pre></div>
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</div>
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<ul class="simple">
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<li>style = <em>dpd/fdt</em> or <em>dpd/fdt/energy</em></li>
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<li>args = list of arguments for a particular style</li>
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</ul>
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<pre class="literal-block">
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<em>dpd/fdt</em> args = T cutoff seed
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T = temperature (temperature units)
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cutoff = global cutoff for DPD interactions (distance units)
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seed = random # seed (positive integer)
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<em>dpd/fdt/energy</em> args = cutoff seed
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cutoff = global cutoff for DPD interactions (distance units)
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seed = random # seed (positive integer)
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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">pair_style</span> <span class="n">dpd</span><span class="o">/</span><span class="n">fdt</span> <span class="mf">300.0</span> <span class="mf">2.5</span> <span class="mi">34387</span>
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<span class="n">pair_coeff</span> <span class="o">*</span> <span class="o">*</span> <span class="mf">3.0</span> <span class="mf">1.0</span> <span class="mf">2.5</span>
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</pre></div>
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</div>
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<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">pair_style</span> <span class="n">dpd</span><span class="o">/</span><span class="n">fdt</span><span class="o">/</span><span class="n">energy</span> <span class="mf">2.5</span> <span class="mi">34387</span>
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<span class="n">pair_coeff</span> <span class="o">*</span> <span class="o">*</span> <span class="mf">3.0</span> <span class="mf">1.0</span> <span class="mf">0.1</span> <span class="mf">2.5</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>Styles <em>dpd/fdt</em> and <em>dpd/fdt/energy</em> set the fluctuation-dissipation
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theorem parameters and compute the conservative force for dissipative
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particle dynamics (DPD). The conservative force on atom I due to atom
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J is given by</p>
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<img alt="_images/pair_dpd_conservative.jpg" class="align-center" src="_images/pair_dpd_conservative.jpg" />
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<p>where the weighting factor, omega_ij, varies between 0 and 1, and is
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chosen to have the following functional form:</p>
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<img alt="_images/pair_dpd_omega.jpg" class="align-center" src="_images/pair_dpd_omega.jpg" />
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<p>where Rij is a unit vector in the direction Ri - Rj, and Rc is the
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cutoff. Note that alternative definitions of the weighting function
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exist, but would have to be implemented as a separate pair style
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command.</p>
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<p>These pair style differ from the other dpd styles in that the
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dissipative and random forces are not computed within the pair style.
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This style can be combined with the <a class="reference internal" href="fix_shardlow.html"><span class="doc">fix shardlow</span></a>
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to perform the stochastic integration of the dissipative and random
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forces through the Shardlow splitting algorithm approach.</p>
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<p>The pairwise energy associated with styles <em>dpd/fdt</em> and
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<em>dpd/fdt/energy</em> is only due to the conservative force term Fc, and is
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shifted to be zero at the cutoff distance Rc. The pairwise virial is
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calculated using only the conservative term.</p>
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<p>For style <em>dpd/fdt</em>, the fluctuation-dissipation theorem defines gamma
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to be set equal to sigma*sigma/(2 T), where T is the set point
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temperature specified as a pair style parameter in the above examples.
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This style can be combined with <a class="reference internal" href="fix_shardlow.html"><span class="doc">fix shardlow</span></a> to
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perform DPD simulations under isothermal and isobaric conditions (see
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<a class="reference internal" href="#lisal"><span class="std std-ref">(Lisal)</span></a>). The following coefficients must be defined for
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each pair of atoms types via the <a class="reference internal" href="pair_coeff.html"><span class="doc">pair_coeff</span></a> command
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as in the examples above, or in the data file or restart files read by
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the <a class="reference internal" href="read_data.html"><span class="doc">read_data</span></a> or <a class="reference internal" href="read_restart.html"><span class="doc">read_restart</span></a>
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commands:</p>
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<ul class="simple">
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<li>A (force units)</li>
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<li>sigma (force*time^(1/2) units)</li>
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<li>cutoff (distance units)</li>
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</ul>
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<p>The last coefficient is optional. If not specified, the global DPD
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cutoff is used.</p>
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<p>For style <em>dpd/fdt/energy</em>, the fluctuation-dissipation theorem
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defines gamma to be set equal to sigma*sigma/(2 dpdTheta), where
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dpdTheta is the average internal temperature for the pair.
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Furthermore, the fluctuation-dissipation defines alpha*alpha to be set
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equal to 2*kB*kappa, where kappa is the mesoparticle thermal
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conductivity parameter. This style can be combined with <a class="reference internal" href="fix_shardlow.html"><span class="doc">fix shardlow</span></a> to perform DPD simulations under
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isoenergetic and isoenthalpic conditions (see <a class="reference internal" href="#lisal"><span class="std std-ref">(Lisal)</span></a>). The
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following coefficients must be defined for each pair of atoms types
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via the <a class="reference internal" href="pair_coeff.html"><span class="doc">pair_coeff</span></a> command as in the examples above,
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or in the data file or restart files read by the
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<a class="reference internal" href="read_data.html"><span class="doc">read_data</span></a> or <a class="reference internal" href="read_restart.html"><span class="doc">read_restart</span></a>
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commands:</p>
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<ul class="simple">
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<li>A (force units)</li>
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<li>sigma (force*time^(1/2) units)</li>
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<li>kappa (1/time units)</li>
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<li>cutoff (distance units)</li>
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</ul>
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<p>The last coefficient is optional. If not specified, the global DPD
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cutoff is used.</p>
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<p>For style <em>dpd/fdt/energy</em>, the particle internal temperature is
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related to the particle internal energy through a mesoparticle
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equation of state. Thus, an an additional <a class="reference internal" href="fix.html"><span class="doc">fix eos</span></a> must be
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specified.</p>
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</div>
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<hr class="docutils" />
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<div class="section" id="restrictions">
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<h2>Restrictions</h2>
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<p>Pair styles <em>dpd/fdt</em> and <em>dpd/fdt/energy</em> are only available if
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LAMMPS is built with the USER-DPD package.</p>
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<p>Pair styles <em>dpd/fdt</em> and <em>dpd/fdt/energy</em> require use of the
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<span class="xref doc">communicate vel yes</span> option so that velocites are
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stored by ghost atoms.</p>
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<p>Pair style <em>dpd/fdt/energy</em> requires <a class="reference internal" href="atom_style.html"><span class="doc">atom_style dpd</span></a>
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to be used in order to properly account for the particle internal
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energies and temperatures.</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="pair_coeff.html"><span class="doc">pair_coeff</span></a>, <a class="reference internal" href="fix_shardlow.html"><span class="doc">fix shardlow</span></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="lisal"><strong>(Lisal)</strong> M. Lisal, J.K. Brennan, J. Bonet Avalos, “Dissipative
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particle dynamics as isothermal, isobaric, isoenergetic, and
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isoenthalpic conditions using Shardlow-like splitting algorithms.”,
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J. Chem. Phys., 135, 204105 (2011).</p>
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