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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<H3>pair_style colloid command
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</H3>
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2011-10-07 01:32:51 +08:00
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<H3>pair_style colloid/omp command
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</H3>
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2007-06-20 21:15:18 +08:00
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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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2009-11-07 06:32:29 +08:00
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<P>where A_cc is the Hamaker constant, a1 and a2 are the radii of the two
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2007-06-20 21:15:18 +08:00
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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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2009-11-07 06:32:29 +08:00
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<P>where A_cs is the Hamaker constant, a is the radius of the colloidal
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2007-10-23 06:21:53 +08:00
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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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2009-11-07 05:01:28 +08:00
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<P>When used in combination with <A HREF = "pair_colloid.html">pair_style
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yukawa/colloid</A>, the two terms become the so-called
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DLVO potential, which combines electrostatic repulsion and van der
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Waals attraction.
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</P>
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2007-06-20 21:15:18 +08:00
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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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2007-06-20 21:15:18 +08:00
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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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2007-10-23 06:21:53 +08:00
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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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2007-06-20 21:15:18 +08:00
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</P>
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<UL><LI>A_cc = colloid/colloid = 4 pi^2 = 39.5
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2007-10-23 06:21:53 +08:00
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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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2007-06-20 21:15:18 +08:00
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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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2008-03-01 09:13:20 +08:00
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diameter + 4 times the small diameter. I.e. 9 = 5 + 4 for the
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2007-06-20 21:15:18 +08:00
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colloid-solvent cutoff in this case.
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</P>
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2008-06-23 23:02:54 +08:00
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<P>IMPORTANT NOTE: When using pair_style colloid for a mixture with 2 (or
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more) widely different particles sizes (e.g. sigma=10 colloids in a
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background sigam=1 LJ fluid), you will likely want to use these
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commands for efficiency: <A HREF = "neighbor.html">neighbor multi</A> and
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<A HREF = "communicate.html">communicate multi</A>.
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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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2011-10-07 01:32:51 +08:00
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<P>Styles with a <I>cuda</I>, <I>gpu</I>, <I>omp</I>, or <I>opt</I> suffix are functionally
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the same as the corresponding style without the suffix. They have
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been optimized to run faster, depending on your available hardware,
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as discussed in <A HREF = "Section_accelerate.html">this section</A> of the manual.
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The accelerated styles take the same arguments and should produce the
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same results, except for round-off and precision issues.
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</P>
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<P>These accelerated styles are part of the USER-CUDA, GPU, USER-OMP and OPT
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packages, respectively. They are only enabled if LAMMPS was built with
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those packages. See the <A HREF = "Section_start.html#start_3">Making LAMMPS</A>
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section for more info.
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</P>
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<P>You can specify the accelerated styles explicitly in your input script
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by including their suffix, or you can use the <A HREF = "Section_start.html#start_6">-suffix command-line
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switch</A> when you invoke LAMMPS, or you can
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use the <A HREF = "suffix.html">suffix</A> command in your input script.
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</P>
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<P>See <A HREF = "Section_accelerate.html">this section</A> of the manual for more
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instructions on how to use the accelerated styles effectively.
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</P>
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<HR>
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2008-01-22 02:12:34 +08:00
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<P><B>Mixing, shift, table, tail correction, 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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2007-06-20 21:15:18 +08:00
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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 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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2007-10-12 07:09:49 +08:00
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<I>inner</I>, <I>middle</I>, <I>outer</I> keywords.
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2007-10-04 08:21:14 +08:00
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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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2011-08-27 02:53:00 +08:00
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<P>This style is part of the COLLOID package. It is only enabled if
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2011-08-26 00:46:23 +08:00
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LAMMPS 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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2011-04-14 05:39:34 +08:00
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<P>Normally, this pair style should be used with finite-size particles
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which have a diameter, e.g. see the <A HREF = "atom_style.html">atom_style
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sphere</A> command. However, this is not a requirement,
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since the only definition of particle size is via the pair_coeff
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parameters for each type. In other words, the physical radius of the
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particle is ignored. Thus you should insure that the d1,d2 parameters
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you specify are consistent with the physical size of the particles of
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that type.
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</P>
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<P>Per-particle polydispersity is not yet supported by this pair style;
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only per-type polydispersity is enabled via the pair_coeff parameters.
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</P>
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2007-06-20 21:15:18 +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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<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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