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<div class="section" id="pair-style-dpd-command">
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<span id="index-0"></span><h1>pair_style dpd command<a class="headerlink" href="#pair-style-dpd-command" title="Permalink to this headline">¶</a></h1>
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</div>
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<div class="section" id="pair-style-dpd-gpu-command">
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<h1>pair_style dpd/gpu command<a class="headerlink" href="#pair-style-dpd-gpu-command" title="Permalink to this headline">¶</a></h1>
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</div>
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<div class="section" id="pair-style-dpd-omp-command">
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<h1>pair_style dpd/omp command<a class="headerlink" href="#pair-style-dpd-omp-command" title="Permalink to this headline">¶</a></h1>
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</div>
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<div class="section" id="pair-style-dpd-tstat-command">
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<h1>pair_style dpd/tstat command<a class="headerlink" href="#pair-style-dpd-tstat-command" title="Permalink to this headline">¶</a></h1>
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</div>
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<div class="section" id="pair-style-dpd-tstat-gpu-command">
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<h1>pair_style dpd/tstat/gpu command<a class="headerlink" href="#pair-style-dpd-tstat-gpu-command" title="Permalink to this headline">¶</a></h1>
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</div>
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<div class="section" id="pair-style-dpd-tstat-omp-command">
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<h1>pair_style dpd/tstat/omp command<a class="headerlink" href="#pair-style-dpd-tstat-omp-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>pair_style dpd T cutoff seed
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pair_style dpd/tstat Tstart Tstop cutoff seed
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</pre></div>
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</div>
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<ul class="simple">
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<li>T = temperature (temperature units)</li>
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<li>Tstart,Tstop = desired temperature at start/end of run (temperature units)</li>
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<li>cutoff = global cutoff for DPD interactions (distance units)</li>
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<li>seed = random # seed (positive integer)</li>
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</ul>
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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>pair_style dpd 1.0 2.5 34387
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pair_coeff * * 3.0 1.0
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pair_coeff 1 1 3.0 1.0 1.0
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</pre></div>
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</div>
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<div class="highlight-python"><div class="highlight"><pre>pair_style dpd/tstat 1.0 1.0 2.5 34387
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pair_coeff * * 1.0
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pair_coeff 1 1 1.0 1.0
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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>Style <em>dpd</em> computes a force field for dissipative particle dynamics
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(DPD) following the exposition in <a class="reference internal" href="#groot"><span>(Groot)</span></a>.</p>
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<p>Style <em>dpd/tstat</em> invokes a DPD thermostat on pairwise interactions,
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which is equivalent to the non-conservative portion of the DPD force
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field. This pair-wise thermostat can be used in conjunction with any
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<a class="reference internal" href="pair_style.html"><em>pair style</em></a>, and in leiu of per-particle thermostats
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like <a class="reference internal" href="fix_langevin.html"><em>fix langevin</em></a> or ensemble thermostats like
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Nose Hoover as implemented by <a class="reference internal" href="fix_nh.html"><em>fix nvt</em></a>. To use
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<em>dpd/stat</em> as a thermostat for another pair style, use the <a class="reference internal" href="pair_hybrid.html"><em>pair_style hybrid/overlay</em></a> command to compute both the desired
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pair interaction and the thermostat for each pair of particles.</p>
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<p>For style <em>dpd</em>, the force on atom I due to atom J is given as a sum
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of 3 terms</p>
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<img alt="_images/pair_dpd.jpg" class="align-center" src="_images/pair_dpd.jpg" />
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<p>where Fc is a conservative force, Fd is a dissipative force, and Fr is
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a random force. Rij is a unit vector in the direction Ri - Rj, Vij is
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the vector difference in velocities of the two atoms = Vi - Vj, alpha
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is a Gaussian random number with zero mean and unit variance, dt is
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the timestep size, and w(r) is a weighting factor that varies between
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0 and 1. Rc is the cutoff. Sigma is set equal to sqrt(2 Kb T gamma),
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where Kb is the Boltzmann constant and T is the temperature parameter
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in the pair_style command.</p>
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<p>For style <em>dpd/tstat</em>, the force on atom I due to atom J is the same
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as the above equation, except that the conservative Fc term is
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dropped. Also, during the run, T is set each timestep to a ramped
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value from Tstart to Tstop.</p>
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<p>For style <em>dpd</em>, the pairwise energy associated with style <em>dpd</em> is
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only due to the conservative force term Fc, and is shifted to be zero
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at the cutoff distance Rc. The pairwise virial is calculated using
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all 3 terms. For style <em>dpd/tstat</em> there is no pairwise energy, but
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the last two terms of the formula make a contribution to the virial.</p>
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<p>For style <em>dpd</em>, the following coefficients must be defined for each
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pair of atoms types via the <a class="reference internal" href="pair_coeff.html"><em>pair_coeff</em></a> command as in
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the examples above, or in the data file or restart files read by the
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<a class="reference internal" href="read_data.html"><em>read_data</em></a> or <a class="reference internal" href="read_restart.html"><em>read_restart</em></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>gamma (force/velocity 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. Note that sigma is set equal to sqrt(2 T gamma),
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where T is the temperature set by the <a class="reference internal" href="pair_style.html"><em>pair_style</em></a>
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command so it does not need to be specified.</p>
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<p>For style <em>dpd/tstat</em>, the coefficiencts defined for each pair of
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atoms types via the <a class="reference internal" href="pair_coeff.html"><em>pair_coeff</em></a> command is the same,
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except that A is not included.</p>
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<p>The GPU-accelerated versions of these styles are implemented based on
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the work of <a class="reference internal" href="#afshar"><span>(Afshar)</span></a> and <a class="reference internal" href="#phillips"><span>(Phillips)</span></a>.</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 you are modeling DPD polymer chains, you may want to use the
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<a class="reference internal" href="pair_srp.html"><em>pair_style srp</em></a> command in conjuction with these pair
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styles. It is a soft segmental repulsive potential (SRP) that can
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prevent DPD polymer chains from crossing each other.</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">The virial calculation for pressure when using this pair style
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includes all the components of force listed above, including the
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random force.</p>
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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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<hr class="docutils" />
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<p><strong>Mixing, shift, table, tail correction, restart, rRESPA info</strong>:</p>
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<p>These pair styles do not support mixing. Thus, coefficients for all
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I,J pairs must be specified explicitly.</p>
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<p>These pair styles do not support the <a class="reference internal" href="pair_modify.html"><em>pair_modify</em></a>
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shift option for the energy of the pair interaction. Note that as
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discussed above, the energy due to the conservative Fc term is already
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shifted to be 0.0 at the cutoff distance Rc.</p>
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<p>The <a class="reference internal" href="pair_modify.html"><em>pair_modify</em></a> table option is not relevant
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for these pair styles.</p>
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<p>These pair style do not support the <a class="reference internal" href="pair_modify.html"><em>pair_modify</em></a>
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tail option for adding long-range tail corrections to energy and
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pressure.</p>
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<p>These pair styles writes their information to <a class="reference internal" href="restart.html"><em>binary restart files</em></a>, so pair_style and pair_coeff commands do not need
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to be specified in an input script that reads a restart file. Note
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that the user-specified random number seed is stored in the restart
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file, so when a simulation is restarted, each processor will
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re-initialize its random number generator the same way it did
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initially. This means the random forces will be random, but will not
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be the same as they would have been if the original simulation had
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continued past the restart time.</p>
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<p>These pair styles can only be used via the <em>pair</em> keyword of the
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<a class="reference internal" href="run_style.html"><em>run_style respa</em></a> command. They do not support the
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<em>inner</em>, <em>middle</em>, <em>outer</em> keywords.</p>
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<p>The <em>dpd/tstat</em> style can ramp its target temperature over multiple
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runs, using the <em>start</em> and <em>stop</em> keywords of the <a class="reference internal" href="run.html"><em>run</em></a>
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command. See the <a class="reference internal" href="run.html"><em>run</em></a> command for details of how to do
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this.</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<a class="headerlink" href="#restrictions" title="Permalink to this headline">¶</a></h2>
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<p>The default frequency for rebuilding neighbor lists is every 10 steps
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(see the <a class="reference internal" href="neigh_modify.html"><em>neigh_modify</em></a> command). This may be too
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infrequent for style <em>dpd</em> simulations since particles move rapidly
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and can overlap by large amounts. If this setting yields a non-zero
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number of “dangerous” reneighborings (printed at the end of a
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simulation), you should experiment with forcing reneighboring more
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often and see if system energies/trajectories change.</p>
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<p>These pair styles requires you to use the <a class="reference internal" href="comm_modify.html"><em>comm_modify vel yes</em></a> command so that velocites are stored by ghost
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atoms.</p>
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<p>These pair styles will not restart exactly when using the
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<a class="reference internal" href="read_restart.html"><em>read_restart</em></a> command, though they should provide
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statistically similar results. This is because the forces they
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compute depend on atom velocities. See the
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<a class="reference internal" href="read_restart.html"><em>read_restart</em></a> command for more details.</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="pair_coeff.html"><em>pair_coeff</em></a>, <a class="reference internal" href="fix_nh.html"><em>fix nvt</em></a>, <a class="reference internal" href="fix_langevin.html"><em>fix langevin</em></a>, <a class="reference internal" href="pair_srp.html"><em>pair_style srp</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="groot"><strong>(Groot)</strong> Groot and Warren, J Chem Phys, 107, 4423-35 (1997).</p>
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<p id="afshar"><strong>(Afshar)</strong> Afshar, F. Schmid, A. Pishevar, S. Worley, Comput Phys
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Comm, 184, 1119-1128 (2013).</p>
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<p id="phillips"><strong>(Phillips)</strong> C. L. Phillips, J. A. Anderson, S. C. Glotzer, Comput
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Phys Comm, 230, 7191-7201 (2011).</p>
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