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doc/html/_sources/pair_colloid.txt
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doc/html/_sources/pair_colloid.txt
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.. index:: pair_style colloid
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pair_style colloid command
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==========================
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pair_style colloid/gpu command
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==============================
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pair_style colloid/omp command
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==============================
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Syntax
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""""""
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.. parsed-literal::
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pair_style colloid cutoff
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* cutoff = global cutoff for colloidal interactions (distance units)
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Examples
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""""""""
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.. parsed-literal::
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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
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Description
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"""""""""""
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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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.. image:: Eqs/pair_colloid_cc.jpg
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:align: center
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where 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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:ref:`(Everaers) <Everaers>`.
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The colloid-solvent interaction energy is given by
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.. image:: Eqs/pair_colloid_cs.jpg
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:align: center
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where 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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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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.. image:: Eqs/pair_colloid_ss.jpg
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:align: center
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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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When used in combination with :doc:`pair_style yukawa/colloid <pair_colloid>`, 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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The following coefficients must be defined for each pair of atoms
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types via the :doc:`pair_coeff <pair_coeff>` 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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:doc:`read_data <read_data>` or :doc:`read_restart <read_restart>`
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commands, or by mixing as described below:
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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)
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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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* 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)
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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
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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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diameter + 4 times the small diameter. I.e. 9 = 5 + 4 for the
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colloid-solvent cutoff in this case.
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.. note::
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When using pair_style colloid for a mixture with 2 (or more)
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widely different particles sizes (e.g. sigma=10 colloids in a
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background sigma=1 LJ fluid), you will likely want to use these
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commands for efficiency: :doc:`neighbor multi <neighbor>` and
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:doc:`comm_modify multi <comm_modify>`.
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----------
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Styles with a *cuda*\ , *gpu*\ , *intel*\ , *kk*\ , *omp*\ , or *opt* suffix are
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functionally the same as the corresponding style without the suffix.
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They have been optimized to run faster, depending on your available
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hardware, as discussed in :doc:`Section_accelerate <Section_accelerate>`
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of the manual. The accelerated styles take the same arguments and
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should produce the same results, except for round-off and precision
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issues.
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These accelerated styles are part of the USER-CUDA, GPU, USER-INTEL,
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KOKKOS, USER-OMP and OPT packages, respectively. They are only
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enabled if LAMMPS was built with those packages. See the :ref:`Making LAMMPS <start_3>` section for more info.
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You can specify the accelerated styles explicitly in your input script
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by including their suffix, or you can use the :ref:`-suffix command-line switch <start_7>` when you invoke LAMMPS, or you can
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use the :doc:`suffix <suffix>` command in your input script.
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See :doc:`Section_accelerate <Section_accelerate>` of the manual for
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more instructions on how to use the accelerated styles effectively.
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----------
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**Mixing, shift, table, tail correction, restart, rRESPA info**\ :
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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 :doc:`pair_modify <pair_modify>` shift
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option for the energy of the pair interaction.
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The :doc:`pair_modify <pair_modify>` table option is not relevant
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for this pair style.
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This pair style does not support the :doc:`pair_modify <pair_modify>`
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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 :doc:`binary restart files <restart>`, 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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This pair style can only be used via the *pair* keyword of the
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:doc:`run_style respa <run_style>` command. It does not support the
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*inner*\ , *middle*\ , *outer* keywords.
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----------
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Restrictions
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""""""""""""
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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 :ref:`Making LAMMPS <start_3>` section for more info.
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Normally, this pair style should be used with finite-size particles
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which have a diameter, e.g. see the :doc:`atom_style sphere <atom_style>` 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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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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Related commands
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""""""""""""""""
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:doc:`pair_coeff <pair_coeff>`
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**Default:** none
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----------
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.. _Everaers:
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**(Everaers)** Everaers, Ejtehadi, Phys Rev E, 67, 041710 (2003).
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.. _lws: http://lammps.sandia.gov
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.. _ld: Manual.html
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.. _lc: Section_commands.html#comm
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