2007-06-20 21:15:18 +08:00
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"LAMMPS WWW Site"_lws - "LAMMPS Documentation"_ld - "LAMMPS Commands"_lc :c
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:link(lws,http://lammps.sandia.gov)
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:link(ld,Manual.html)
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:link(lc,Section_commands.html#comm)
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:line
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pair_style colloid command :h3
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[Syntax:]
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pair_style colloid cutoff :pre
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cutoff = global cutoff for colloidal interactions (distance units) :ul
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[Examples:]
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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 :pre
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[Description:]
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Style {colloid} 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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The colloid-colloid interaction energy is given by
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:c,image(Eqs/pair_colloid_cc.jpg)
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2009-11-07 06:32:29 +08:00
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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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"(Everaers)"_#Everaers.
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The colloid-solvent interaction energy is given by
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:c,image(Eqs/pair_colloid_cs.jpg)
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2009-11-07 06:32:29 +08:00
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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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The solvent-solvent interaction energy is given by the usual
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Lennard-Jones formula
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:c,image(Eqs/pair_colloid_ss.jpg)
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2007-10-23 06:21:53 +08:00
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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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2009-11-07 05:01:28 +08:00
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When used in combination with "pair_style
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yukawa/colloid"_pair_colloid.html, 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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2007-06-20 21:15:18 +08:00
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The following coefficients must be defined for each pair of atoms
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types via the "pair_coeff"_pair_coeff.html 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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"read_data"_read_data.html or "read_restart"_read_restart.html
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2007-06-26 08:03:39 +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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A (energy units)
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sigma (distance units)
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d1 (distance units)
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d2 (distance units)
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cutoff (distance units) :ul
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2007-10-23 06:21:53 +08:00
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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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A_cc = colloid/colloid = 4 pi^2 = 39.5
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A_cs = colloid/solvent = sqrt(A_cc*A_ss)
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A_ss = solvent/solvent = 144 (assuming epsilon = 1, so that 144/36 = 4) :ul
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2007-06-20 21:15:18 +08:00
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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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Sigma_cc = colloid/colloid = 1.0
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Sigma_cs = colloid/solvent = arithmetic mixing between colloid sigma and solvent sigma
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Sigma_ss = solvent/solvent = 1.0 or whatever size the solvent particle is :ul
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Thus typically Sigma_cs = 1.0, unless the solvent particle's size !=
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1.0.
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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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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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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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2008-06-23 23:02:54 +08:00
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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: "neighbor multi"_neighbor.html and
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"communicate multi"_communicate.html.
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2007-06-26 08:03:39 +08:00
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:line
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2008-01-22 02:12:34 +08:00
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[Mixing, shift, table, tail correction, restart, rRESPA info]:
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2007-06-26 08:03:39 +08:00
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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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{geometric}. See the "pair_modify" command for details.
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This pair style supports the "pair_modify"_pair_modify.html shift
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option for the energy of the pair interaction.
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The "pair_modify"_pair_modify.html table option is not relevant
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for this pair style.
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This pair style does not support the "pair_modify"_pair_modify.html
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tail option for adding long-range tail corrections to energy and
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pressure.
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This pair style writes its information to "binary restart
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files"_restart.html, 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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2007-10-04 08:21:14 +08:00
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This pair style can only be used via the {pair} keyword of the
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"run_style respa"_run_style.html command. It does not support the
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2007-10-12 07:09:49 +08:00
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{inner}, {middle}, {outer} keywords.
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2007-10-04 08:21:14 +08:00
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2007-06-26 08:03:39 +08:00
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:line
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2007-06-20 21:15:18 +08:00
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[Restrictions:]
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2007-06-26 08:03:39 +08:00
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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 "Making
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2007-06-20 21:15:18 +08:00
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LAMMPS"_Section_start.html#2_3 section for more info.
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[Related commands:]
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"pair_coeff"_pair_coeff.html
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[Default:] none
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:line
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:link(Everaers)
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[(Everaers)] Everaers, Ejtehadi, Phys Rev E, 67, 041710 (2003).
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