2007-06-20 21:15:18 +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 colloid 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 colloid cutoff
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</PRE>
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<UL><LI>cutoff = global cutoff for colloidal interactions (distance units)
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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 colloid 10.0
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pair_coeff * * 25 1.0 10.0 10.0
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pair_coeff 1 1 144 1.0 0.0 0.0 3.0
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pair_coeff 1 2 75.398 1.0 0.0 10.0 9.0
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pair_coeff 2 2 39.478 1.0 10.0 10.0 25.0
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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>colloid</I> computes pairwise interactions between large colloidal
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particles and small solvent particles using 3 formulas. A colloidal
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particle has a size > sigma; a solvent particle is the usual
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Lennard-Jones particle of size sigma.
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</P>
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<P>The colloid-colloid interaction energy is given by
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</P>
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<CENTER><IMG SRC = "Eqs/pair_colloid_cc.jpg">
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</CENTER>
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2007-10-23 06:21:53 +08:00
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<P>A_cc is the Hamaker constant, a1 and a2 are the radii of the two
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colloidal particles, and Rc is the cutoff. This equation results from
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describing each colloidal particle as an integrated collection of
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Lennard-Jones particles of size sigma and is derived in
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<A HREF = "#Everaers">(Everaers)</A>.
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</P>
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<P>The colloid-solvent interaction energy is given by
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</P>
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<CENTER><IMG SRC = "Eqs/pair_colloid_cs.jpg">
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</CENTER>
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2007-10-23 06:21:53 +08:00
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<P>A_cs is the Hamaker constant, a is the radius of the colloidal
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particle, and Rc is the cutoff. This formula is derived from the
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2007-06-20 21:15:18 +08:00
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colloid-colloid interaction, letting one of the particle sizes go to
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zero.
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</P>
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<P>The solvent-solvent interaction energy is given by the usual
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Lennard-Jones formula
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</P>
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<CENTER><IMG SRC = "Eqs/pair_colloid_ss.jpg">
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</CENTER>
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2007-10-23 06:21:53 +08:00
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<P>with A_ss set appropriately, which results from letting both particle
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sizes go to zero.
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2007-06-20 21:15:18 +08:00
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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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2007-06-25 22:36:36 +08:00
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commands, or by mixing as described below:
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</P>
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<UL><LI>A (energy units)
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<LI>sigma (distance units)
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<LI>d1 (distance units)
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<LI>d2 (distance units)
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<LI>cutoff (distance units)
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</UL>
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<P>A is the Hamaker energy prefactor and should typically be set as
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follows:
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</P>
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<UL><LI>A_cc = colloid/colloid = 4 pi^2 = 39.5
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<LI>A_cs = colloid/solvent = sqrt(A_cc*A_ss)
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<LI>A_ss = solvent/solvent = 144 (assuming epsilon = 1, so that 144/36 = 4)
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2007-06-20 21:15:18 +08:00
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</UL>
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<P>Sigma is the size of the solvent particle or the constituent particles
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integrated over in the colloidal particle and should typically be set
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as follows:
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</P>
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<UL><LI>Sigma_cc = colloid/colloid = 1.0
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<LI>Sigma_cs = colloid/solvent = arithmetic mixing between colloid sigma and solvent sigma
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<LI>Sigma_ss = solvent/solvent = 1.0 or whatever size the solvent particle is
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</UL>
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<P>Thus typically Sigma_cs = 1.0, unless the solvent particle's size !=
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1.0.
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</P>
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<P>D1 and d2 are particle diameters, so that d1 = 2*a1 and d2 = 2*a2 in
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the formulas above. Both d1 and d2 must be values >= 0. If d1 > 0
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and d2 > 0, then the pair interacts via the colloid-colloid formula
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above. If d1 = 0 and d2 = 0, then the pair interacts via the
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solvent-solvent formula. I.e. a d value of 0 is a Lennard-Jones
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particle of size sigma. If either d1 = 0 or d2 = 0 and the other is
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larger, then the pair interacts via the colloid-solvent formula.
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</P>
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<P>Note that the diameter of a particular particle type may appear in
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multiple pair_coeff commands, as it interacts with other particle
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types. You should insure the particle diameter is specified
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consistently each time it appears.
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</P>
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<P>The last coefficient is optional. If not specified, the global cutoff
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specified in the pair_style command is used. However, you typically
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want different cutoffs for interactions between different particle
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sizes. E.g. if colloidal particles of diameter 10 are used with
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solvent particles of diameter 1, then a solvent-solvent cutoff of 2.5
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would correspond to a colloid-colloid cutoff of 25. A good
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rule-of-thumb is to use a colloid-solvent cutoff that is half the big
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diamter + 4 times the small diamter. I.e. 9 = 5 + 4 for the
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colloid-solvent cutoff in this case.
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</P>
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2007-06-25 22:36:36 +08:00
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<HR>
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2007-10-04 08:21:14 +08:00
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<P><B>Mixing, shift, table, tail correction, per-atom energy/stress,
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restart, rRESPA info</B>:
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2007-06-25 22:36:36 +08:00
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</P>
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<P>For atom type pairs I,J and I != J, the A, sigma, d1, and d2
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coefficients and cutoff distance for this pair style can be mixed. A
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is an energy value mixed like a LJ epsilon. D1 and d2 are distance
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values and are mixed like sigma. The default mix value is
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<I>geometric</I>. See the "pair_modify" command for details.
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</P>
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<P>This pair style supports the <A HREF = "pair_modify.html">pair_modify</A> shift
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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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2007-06-25 22:36:36 +08:00
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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.
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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.
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</P>
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2007-10-04 08:21:14 +08:00
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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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2007-06-25 22:36:36 +08:00
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<HR>
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2007-06-20 21:15:18 +08:00
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<P><B>Restrictions:</B>
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</P>
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2007-06-25 22:36:36 +08:00
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<P>This style is part of the "colloid" 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><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 = "Everaers"></A>
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<P><B>(Everaers)</B> Everaers, Ejtehadi, Phys Rev E, 67, 041710 (2003).
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</P>
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</HTML>
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