mirror of https://github.com/lammps/lammps.git
138 lines
4.9 KiB
HTML
138 lines
4.9 KiB
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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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</CENTER>
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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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<P>A 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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<P>A is the Hamaker constant, a is the radius of the colloidal particle,
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and Rc is the cutoff. This formula is derived from the
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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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<P>which results from letting both particle sizes go to zero.
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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 (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 energy prefactor and should typically be set as 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_ss = solvent/solvent = 144 (assuming epsilon = 1, so that 144/36 = 4)
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<LI>A_cs = colloid/solvent = sqrt(A_cc*A_ss)
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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_ss = solvent/solvent = 1.0 or whatever size the solvent particle is
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<LI>Sigma_cs = colloid/solvent = arithmetic mixing between colloid sigma and solvent sigma
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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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<P>If a pair_coeff command is not specified for I != J, then the
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coefficients are mixed according the mixing rules defined by the
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<A HREF = "pair_modify.html">pair_modify</A> command. The prefactor A is mixed like
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the Lennard-Jones epsilon; sigma,d1,d2 are all mixed like the
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Lennard-Jones sigma.
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</P>
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<P><B>Restrictions:</B>
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</P>
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<P>The <I>colloid</I> style is part of the "colloid" package. It is only
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enabled if 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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