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<CENTER><A HREF = "http://lammps.sandia.gov">LAMMPS WWW Site</A> - <A HREF = "Manual.html">LAMMPS Documentation</A> - <A HREF = "Section_commands.html#comm">LAMMPS Commands</A>
</CENTER>
<HR>
<H3>pair_style dpd command
</H3>
<P><B>Syntax:</B>
</P>
<PRE>pair_style dpd T cutoff seed
</PRE>
<UL><LI>T = temperature (temperature units)
<LI>cutoff = global cutoff for DPD interactions (distance units)
<LI>seed = random # seed (positive integer)
</UL>
<P><B>Examples:</B>
</P>
<PRE>pair_style dpd 1.0 2.5 34387
pair_coeff * * 3.0 1.0
pair_coeff 1 1 3.0 1.0 1.0
</PRE>
<P><B>Description:</B>
</P>
<P>Style <I>dpd</I> computes a force field for dissipative particle dynamics
(DPD) following the exposition in <A HREF = "#Groot">(Groot)</A>. The force
on atom I due to atom J is given as a sum of 3 terms
</P>
<CENTER><IMG SRC = "Eqs/pair_dpd.jpg">
</CENTER>
<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 T gamma),
where T is a parameter in the pair_style command.
</P>
<P>The following coefficients must be defined for each pair of atoms
types via the <A HREF = "pair_coeff.html">pair_coeff</A> command as in the examples
above, or in the data file or restart files read by the
<A HREF = "read_data.html">read_data</A> or <A HREF = "read_restart.html">read_restart</A>
commands:
</P>
<UL><LI>A (force units)
<LI>gamma (force/velocity units)
<LI>cutoff (distance units)
</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 <A HREF = "pair_style.html">pair_style</A>
command so it does not need to be specified.
</P>
<HR>
<P><B>Mixing, shift, table, tail correction, restart, rRESPA info</B>:
</P>
<P>This pair style does not support mixing. Thus, coefficients for all
I,J pairs must be specified explicitly.
</P>
<P>This pair style does not support the <A HREF = "pair_modify.html">pair_modify</A>
shift option for the energy of the pair interaction.
</P>
<P>The <A HREF = "pair_modify.html">pair_modify</A> table option is not relevant
for this pair style.
</P>
<P>This pair style does not support the <A HREF = "pair_modify.html">pair_modify</A>
tail option for adding long-range tail corrections to energy and
pressure.
</P>
<P>This pair style writes its information to <A HREF = "restart.html">binary restart
files</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>This pair style can only be used via the <I>pair</I> keyword of the
<A HREF = "run_style.html">run_style respa</A> command. It does not support the
<I>inner</I>, <I>middle</I>, <I>outer</I> keywords.
</P>
<HR>
<P><B>Restrictions:</B>
</P>
<P>This style is part of the "dpd" package. It is only enabled if
LAMMPS was built with that package. See the <A HREF = "Section_start.html#2_3">Making
LAMMPS</A> section for more info.
</P>
<P>The default frequency for rebuilding neighbor lists is every 10 steps
(see the <A HREF = "neigh_modify.html">neigh_modify</A> command). This may be too
infrequent for DPD 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><B>Related commands:</B>
</P>
<P><A HREF = "pair_coeff.html">pair_coeff</A>
</P>
<P><B>Default:</B> none
</P>
<HR>
<A NAME = "Groot"></A>
<P><B>(Groot)</B> Groot and Warren, J Chem Phys, 107, 4423-35 (1997).
</P>
</HTML>
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