150 lines
4.7 KiB
ReStructuredText
150 lines
4.7 KiB
ReStructuredText
.. index:: pair_style yukawa/colloid
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.. index:: pair_style yukawa/colloid/gpu
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.. index:: pair_style yukawa/colloid/omp
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pair_style yukawa/colloid command
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=================================
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Accelerator Variants: *yukawa/colloid/gpu*, *yukawa/colloid/omp*
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Syntax
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""""""
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.. code-block:: LAMMPS
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pair_style yukawa/colloid kappa cutoff
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* kappa = screening length (inverse distance units)
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* cutoff = global cutoff for colloidal Yukawa interactions (distance units)
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Examples
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""""""""
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.. code-block:: LAMMPS
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pair_style yukawa/colloid 2.0 2.5
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pair_coeff 1 1 100.0 2.3
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pair_coeff * * 100.0
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Description
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"""""""""""
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Style *yukawa/colloid* computes pairwise interactions with the formula
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.. math::
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E = \frac{A}{\kappa} e^{- \kappa (r - (r_i + r_j))} \qquad r < r_c
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where :math:`r_i` and :math:`r_j` are the radii of the two particles
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and :math:`r_c` is the cutoff.
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In contrast to :doc:`pair_style yukawa <pair_yukawa>`, this functional
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form arises from the Coulombic interaction between two colloid
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particles, screened due to the presence of an electrolyte, see the
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book by :ref:`Safran <Safran>` for a derivation in the context of DLVO
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theory. :doc:`Pair_style yukawa <pair_yukawa>` is a screened Coulombic
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potential between two point-charges and uses no such approximation.
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This potential applies to nearby particle pairs for which the Derjagin
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approximation holds, meaning :math:`h << r_i + r_j`, where *h* is the
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surface-to-surface separation of the two particles.
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When used in combination with :doc:`pair_style colloid <pair_colloid>`,
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the two terms become the so-called DLVO potential, which combines
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electrostatic repulsion and van der 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/distance units)
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* cutoff (distance units)
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The prefactor A is determined from the relationship between surface
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charge and surface potential due to the presence of electrolyte. Note
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that the A for this potential style has different units than the A
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used in :doc:`pair_style yukawa <pair_yukawa>`. For low surface
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potentials, i.e. less than about 25 mV, A can be written as:
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.. math::
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A = 2 \pi R\varepsilon\varepsilon_0 \kappa \psi^2
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where
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* *R* = colloid radius (distance units)
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* :math:`\varepsilon_0` = permittivity of free space (charge\^2/energy/distance units)
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* :math:`\varepsilon` = relative permittivity of fluid medium (dimensionless)
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* :math:`\kappa` = inverse screening length (1/distance units)
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* :math:`\psi` = surface potential (energy/charge units)
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The last coefficient is optional. If not specified, the global
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yukawa/colloid cutoff is used.
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----------
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.. include:: accel_styles.rst
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----------
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Mixing, shift, table, tail correction, restart, rRESPA info
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"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
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For atom type pairs I,J and I != J, the A coefficient and cutoff
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distance for this pair style can be mixed. A is an energy value mixed
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like a LJ epsilon. The default mix value is *geometric*\ . See the
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"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 :doc:`Build package <Build_package>` page for more info.
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This pair style requires that atoms be finite-size spheres with a
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diameter, as defined by the :doc:`atom_style sphere <atom_style>`
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command.
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Per-particle polydispersity is not yet supported by this pair style;
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per-type polydispersity is allowed. This means all particles of the
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same type must have the same diameter. Each type can have a different
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diameter.
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Related commands
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""""""""""""""""
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:doc:`pair_coeff <pair_coeff>`
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Default
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"""""""
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none
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----------
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.. _Safran:
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**(Safran)** Safran, Statistical Thermodynamics of Surfaces, Interfaces,
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And Membranes, Westview Press, ISBN: 978-0813340791 (2003).
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