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145 lines
5.3 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 dsmc 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 dsmc max_cell_size seed weighting Tref Nrecompute Nsample
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</PRE>
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<UL><LI>max_cell_size = global maximum cell size for DSMC interactions (distance units)
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<LI>seed = random # seed (positive integer)
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<LI>weighting = macroparticle weighting
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<LI>Tref = reference temperature (temperature units)
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<LI>Nrecompute = recompute v*sigma_max every this many timesteps (timesteps)
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<LI>Nsample = sample this many times in recomputing v*sigma_max
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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 dsmc 2.5 34387 10 1.0 100 20
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pair_coeff * * 1.0
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pair_coeff 1 1 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>dsmc</I> computes collisions between pairs of particles for a
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direct simulation Monte Carlo (DSMC) model following the exposition in
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<A HREF = "#Bird">(Bird)</A>. Each collision resets the velocities of the two
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particles involved. The number of pairwise collisions for each pair
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or particle types and the length scale within which they occur are
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determined by the parameters of the pair_style and pair_coeff
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commands.
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</P>
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<P>Stochastic collisions are performed using the variable hard sphere
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(VHS) approach, with the user-defined <I>max_cell_size</I> value used as
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the maximum DSMC cell size, and reference cross-sections for
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collisions given using the pair_coeff command.
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</P>
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<P>There is no pairwise energy or virial contributions associated with
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this pair style.
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</P>
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<P>The following coefficient must be defined for each pair of atoms types
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via the <A HREF = "pair_coeff.html">pair_coeff</A> command as in the examples above,
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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>sigma (area units, i.e. distance-squared)
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</UL>
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<P>The global DSMC <I>max_cell_size</I> determines the maximum cell length
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used in the DSMC calculation. A structured mesh is overlayed on the
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simulation box such that an integer number of cells are created in
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each direction for each processor's sub-domain. Cell lengths are
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adjusted up to the user-specified maximum cell size.
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</P>
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<HR>
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<P>To perform a DSMC simulation with LAMMPS, several additional options
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should be set in your input script, though LAMMPS does not check for
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these settings.
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</P>
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<P>Since this pair style does not compute particle forces, you should use
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the "fix nve/noforce" time integration fix for the DSMC particles,
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e.g.
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</P>
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<PRE>fix 1 all nve/noforce
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</PRE>
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<P>This pair style assumes that all particles will communicated to
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neighboring processors every timestep as they move. This makes it
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possible to perform all collisions between pairs of particles that are
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on the same processor. To ensure this occurs, you should use
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these commands:
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</P>
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<PRE>neighbor 0.0 bin
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neigh_modify every 1 delay 0 check no
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communicate single cutoff 0.0
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</PRE>
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<P>These commands insure that LAMMPS communicates particles to
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neighboring processors every timestep and that no ghost atoms are
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created. The output statistics for a simulation run should indicate
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there are no ghost particles or neighbors.
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</P>
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<HR>
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<P><B>Mixing, shift, table, tail correction, restart, rRESPA info</B>:
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</P>
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<P>This 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 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-specified 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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initially. 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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<P>This pair style can only be used via the <I>pair</I> keyword of the
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<A HREF = "run_style.html">run_style respa</A> command. It does not support the
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<I>inner</I>, <I>middle</I>, <I>outer</I> keywords.
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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 MC package. It is only enabled if LAMMPS
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was built with that package. See the <A HREF = "Section_start.html#start_3">Making
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LAMMPS</A> section for more info.
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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>, <A HREF = "fix_nve_noforce.html">fix nve/noforce</A>,
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<A HREF = "neigh_modify.html">neigh_modify</A>, <A HREF = "neighbor.html">neighbor</A>,
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<A HREF = "communicate.html">communicate</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 = "Bird"></A>
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<P><B>(Bird)</B> G. A. Bird, "Molecular Gas Dynamics and the Direct Simulation
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of Gas Flows" (1994).
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</P>
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</HTML>
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