forked from lijiext/lammps
123 lines
4.5 KiB
HTML
123 lines
4.5 KiB
HTML
<HTML>
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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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<HR>
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<P><B>Mixing, shift, table, tail correction, per-atom energy/stress, and
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restart info</B>:
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</P>
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<P>Thsi pair style does not support mixing. Thus, coefficients for all
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I,J pairs must be specified explicitly.
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</P>
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<P>This pair style does not support the <A HREF = "pair_modify.html">pair_modify</A>
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shift option for the energy of the pair interaction.
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</P>
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<P>The <A HREF = "pair_modify.html">pair_modify</A> table option is not relevant
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for this pair style.
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</P>
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<P>This pair style does not support the <A HREF = "pair_modify.html">pair_modify</A>
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tail option for adding long-range tail corrections to energy and
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pressure.
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</P>
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<P>This pair style can calculate per-atom energy and stress, as used by
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the <A HREF = "compute_epair_atom.html">compute epair/atom</A>, <A HREF = "compute_stress_atom.html">compute
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stress/atom</A>, and <A HREF = "dump.html">dump custom</A>
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commands. However, only the first term with the A coefficienct
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(conservative force) is used for these computations. The drag force
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and random force terms are not.
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</P>
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<P>This pair style writes its information to <A HREF = "restart.html">binary restart
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files</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-specifice 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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intially. 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.
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</P>
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<HR>
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<P><B>Restrictions:</B>
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</P>
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<P>This style is part of the "dpd" package. It is only enabled if
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LAMMPS was built with that package. See the <A HREF = "Section_start.html#2_3">Making
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LAMMPS</A> section for more info.
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</P>
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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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<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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<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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</HTML>
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