2006-09-22 00:22:34 +08:00
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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>
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<HR>
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<H3>pair_style dpd command
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</H3>
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<P><B>Syntax:</B>
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</P>
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<PRE>pair_style dpd T cutoff seed
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</PRE>
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<UL><LI>T = temperature (temperature units)
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<LI>cutoff = global cutoff for DPD interactions (distance units)
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<LI>seed = random # seed (integer > 0 and < 900000000)
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</UL>
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<P><B>Examples:</B>
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</P>
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<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>
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<P><B>Description:</B>
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</P>
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<P>Style <I>dpd</I> computes a force field for dissipative particle dynamics
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(DPD) following the exposition in <A HREF = "#Groot">(Groot)</A>. The force
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on atom I due to atom J is given as a sum of 3 terms
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</P>
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<CENTER><IMG SRC = "Eqs/pair_dpd.jpg">
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</CENTER>
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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 T gamma),
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where T is a parameter in the pair_style command.
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</P>
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<P>The following coefficients must be defined for each pair of atoms
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types via the <A HREF = "pair_coeff.html">pair_coeff</A> command as in the examples
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above, or in the data file or restart files read by the
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<A HREF = "read_data.html">read_data</A> or <A HREF = "read_restart.html">read_restart</A>
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commands:
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</P>
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<UL><LI>A (force units)
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<LI>gamma (force/velocity units)
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<LI>cutoff (distance units)
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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 HREF = "pair_style.html">pair_style</A>
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command so it does not need to be specified.
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</P>
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<P><B>Restrictions:</B> none
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</P>
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<P>This style is part of the "dpd" package. It is only enabled if LAMMPS
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was built with those package. See the <A HREF = "Section_start.html#2_2">Making
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LAMMPS</A> section for more info.
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</P>
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2006-09-28 03:12:31 +08:00
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<P>The <I>dpd</I> potential does not support the
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<A HREF = "pair_modify.hmtl">pair_modify</A> <I>mix</I> option. Coefficients for all i,j
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pairs must be specified explicitly.
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</P>
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2007-02-23 00:52:24 +08:00
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<P>The default frequency for rebuilding neighbor lists is every 10 steps
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(see the <A HREF = "neigh_modify.html">neigh_modify</A> command). This may be too
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infrequent for DPD simulations since particles move rapidly and can
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overlap by large amounts. If this setting yields a non-zero number of
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"dangerous" reneighborings (printed at the end of a simulation), you
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should experiment with forcing reneighboring more often and see if
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system energies/trajectories change.
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</P>
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2006-09-22 00:22:34 +08:00
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<P><B>Related commands:</B>
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</P>
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<P><A HREF = "pair_coeff.html">pair_coeff</A>
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</P>
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<P><B>Default:</B> none
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</P>
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2007-01-30 07:28:45 +08:00
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<HR>
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<A NAME = "Groot"></A>
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<P><B>(Groot)</B> Groot and Warren, J Chem Phys, 107, 4423-35 (1997).
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</P>
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2006-09-22 00:22:34 +08:00
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</HTML>
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