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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 (integer > 0 and < 900000000)
</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>
<P><B>Restrictions:</B> none
</P>
<P>This style is part of the "dpd" package. It is only enabled if LAMMPS
was built with those package. See the <A HREF = "Section_start.html#2_2">Making
LAMMPS</A> section for more info.
</P>
<P>The <I>dpd</I> potential does not support the
<A HREF = "pair_modify.hmtl">pair_modify</A> <I>mix</I> option. Coefficients for all i,j
pairs must be specified explicitly.
</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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