<p>Style <em>dpd</em> computes a force field for dissipative particle dynamics
(DPD) following the exposition in <aclass="reference internal"href="#groot"><spanclass="std std-ref">(Groot)</span></a>.</p>
<p>Style <em>dpd/tstat</em> invokes a DPD thermostat on pairwise interactions,
which is equivalent to the non-conservative portion of the DPD force
field. This pair-wise thermostat can be used in conjunction with any
<aclass="reference internal"href="pair_style.html"><spanclass="doc">pair style</span></a>, and in leiu of per-particle thermostats
like <aclass="reference internal"href="fix_langevin.html"><spanclass="doc">fix langevin</span></a> or ensemble thermostats like
Nose Hoover as implemented by <aclass="reference internal"href="fix_nh.html"><spanclass="doc">fix nvt</span></a>. To use
<em>dpd/tstat</em> as a thermostat for another pair style, use the <aclass="reference internal"href="pair_hybrid.html"><spanclass="doc">pair_style hybrid/overlay</span></a> command to compute both the desired
pair interaction and the thermostat for each pair of particles.</p>
<p>For style <em>dpd</em>, the force on atom I due to atom J is given as a sum
<p>where Fc is a conservative force, Fd is a dissipative force, and Fr is
a random force. Rij is a unit vector in the direction Ri - Rj, Vij is
the vector difference in velocities of the two atoms = Vi - Vj, alpha
is a Gaussian random number with zero mean and unit variance, dt is
the timestep size, and w(r) is a weighting factor that varies between
0 and 1. Rc is the cutoff. Sigma is set equal to sqrt(2 Kb T gamma),
where Kb is the Boltzmann constant and T is the temperature parameter
in the pair_style command.</p>
<p>For style <em>dpd/tstat</em>, the force on atom I due to atom J is the same
as the above equation, except that the conservative Fc term is
dropped. Also, during the run, T is set each timestep to a ramped
value from Tstart to Tstop.</p>
<p>For style <em>dpd</em>, the pairwise energy associated with style <em>dpd</em> is
only due to the conservative force term Fc, and is shifted to be zero
at the cutoff distance Rc. The pairwise virial is calculated using
all 3 terms. For style <em>dpd/tstat</em> there is no pairwise energy, but
the last two terms of the formula make a contribution to the virial.</p>
<p>For style <em>dpd</em>, the following coefficients must be defined for each
pair of atoms types via the <aclass="reference internal"href="pair_coeff.html"><spanclass="doc">pair_coeff</span></a> command as in
the examples above, or in the data file or restart files read by the
<aclass="reference internal"href="read_data.html"><spanclass="doc">read_data</span></a> or <aclass="reference internal"href="read_restart.html"><spanclass="doc">read_restart</span></a>
commands:</p>
<ulclass="simple">
<li>A (force units)</li>
<li>gamma (force/velocity units)</li>
<li>cutoff (distance units)</li>
</ul>
<p>The last coefficient is optional. If not specified, the global DPD
cutoff is used. Note that sigma is set equal to sqrt(2 T gamma),
where T is the temperature set by the <aclass="reference internal"href="pair_style.html"><spanclass="doc">pair_style</span></a>
command so it does not need to be specified.</p>
<p>For style <em>dpd/tstat</em>, the coefficiencts defined for each pair of
atoms types via the <aclass="reference internal"href="pair_coeff.html"><spanclass="doc">pair_coeff</span></a> command is the same,
except that A is not included.</p>
<p>The GPU-accelerated versions of these styles are implemented based on
the work of <aclass="reference internal"href="#afshar"><spanclass="std std-ref">(Afshar)</span></a> and <aclass="reference internal"href="#phillips"><spanclass="std std-ref">(Phillips)</span></a>.</p>
<divclass="admonition note">
<pclass="first admonition-title">Note</p>
<pclass="last">If you are modeling DPD polymer chains, you may want to use the
<aclass="reference internal"href="pair_srp.html"><spanclass="doc">pair_style srp</span></a> command in conjuction with these pair
styles. It is a soft segmental repulsive potential (SRP) that can
prevent DPD polymer chains from crossing each other.</p>
</div>
<divclass="admonition note">
<pclass="first admonition-title">Note</p>
<pclass="last">The virial calculation for pressure when using this pair style
includes all the components of force listed above, including the
random force.</p>
</div>
<hrclass="docutils"/>
<p>Styles with a <em>gpu</em>, <em>intel</em>, <em>kk</em>, <em>omp</em>, or <em>opt</em> suffix are
functionally the same as the corresponding style without the suffix.
They have been optimized to run faster, depending on your available
hardware, as discussed in <aclass="reference internal"href="Section_accelerate.html"><spanclass="doc">Section_accelerate</span></a>
of the manual. The accelerated styles take the same arguments and
should produce the same results, except for round-off and precision
issues.</p>
<p>These accelerated styles are part of the GPU, USER-INTEL, KOKKOS,
USER-OMP and OPT packages, respectively. They are only enabled if
LAMMPS was built with those packages. See the <spanclass="xref std std-ref">Making LAMMPS</span> section for more info.</p>
<p>You can specify the accelerated styles explicitly in your input script
by including their suffix, or you can use the <spanclass="xref std std-ref">-suffix command-line switch</span> when you invoke LAMMPS, or you can
use the <aclass="reference internal"href="suffix.html"><spanclass="doc">suffix</span></a> command in your input script.</p>
<p>See <aclass="reference internal"href="Section_accelerate.html"><spanclass="doc">Section_accelerate</span></a> of the manual for
more instructions on how to use the accelerated styles effectively.</p>
<p>These pair styles do not support mixing. Thus, coefficients for all
I,J pairs must be specified explicitly.</p>
<p>These pair styles do not support the <aclass="reference internal"href="pair_modify.html"><spanclass="doc">pair_modify</span></a>
shift option for the energy of the pair interaction. Note that as
discussed above, the energy due to the conservative Fc term is already
shifted to be 0.0 at the cutoff distance Rc.</p>
<p>The <aclass="reference internal"href="pair_modify.html"><spanclass="doc">pair_modify</span></a> table option is not relevant
for these pair styles.</p>
<p>These pair style do not support the <aclass="reference internal"href="pair_modify.html"><spanclass="doc">pair_modify</span></a>
tail option for adding long-range tail corrections to energy and
pressure.</p>
<p>These pair styles writes their information to <aclass="reference internal"href="restart.html"><spanclass="doc">binary restart files</span></a>, so pair_style and pair_coeff commands do not need
to be specified in an input script that reads a restart file. Note
that the user-specified random number seed is stored in the restart
file, so when a simulation is restarted, each processor will
re-initialize its random number generator the same way it did
initially. This means the random forces will be random, but will not
be the same as they would have been if the original simulation had
continued past the restart time.</p>
<p>These pair styles can only be used via the <em>pair</em> keyword of the
<aclass="reference internal"href="run_style.html"><spanclass="doc">run_style respa</span></a> command. They do not support the
<p>The <em>dpd/tstat</em> style can ramp its target temperature over multiple
runs, using the <em>start</em> and <em>stop</em> keywords of the <aclass="reference internal"href="run.html"><spanclass="doc">run</span></a>
command. See the <aclass="reference internal"href="run.html"><spanclass="doc">run</span></a> command for details of how to do
this.</p>
</div>
<hrclass="docutils"/>
<divclass="section"id="restrictions">
<h2>Restrictions</h2>
<p>The default frequency for rebuilding neighbor lists is every 10 steps
(see the <aclass="reference internal"href="neigh_modify.html"><spanclass="doc">neigh_modify</span></a> command). This may be too
infrequent for style <em>dpd</em> simulations since particles move rapidly
and can overlap by large amounts. If this setting yields a non-zero
number of “dangerous” reneighborings (printed at the end of a
simulation), you should experiment with forcing reneighboring more
often and see if system energies/trajectories change.</p>
<p>These pair styles requires you to use the <aclass="reference internal"href="comm_modify.html"><spanclass="doc">comm_modify vel yes</span></a> command so that velocites are stored by ghost
atoms.</p>
<p>These pair styles will not restart exactly when using the
<aclass="reference internal"href="read_restart.html"><spanclass="doc">read_restart</span></a> command, though they should provide
statistically similar results. This is because the forces they
compute depend on atom velocities. See the
<aclass="reference internal"href="read_restart.html"><spanclass="doc">read_restart</span></a> command for more details.</p>
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