2009-11-07 05:01:28 +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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<HR>
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<H3>pair_style yukawa/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 yukawa/colloid kappa cutoff
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</PRE>
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<UL><LI>kappa = screening length (inverse distance units)
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<LI>cutoff = global cutoff for colloidal Yukawa 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 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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</PRE>
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<P><B>Description:</B>
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</P>
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<P>Style <I>yukawa/colloid</I> computes pairwise interactions with the formula
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</P>
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<CENTER><IMG SRC = "Eqs/pair_yukawa_colloid.jpg">
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</CENTER>
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<P>where Ri and Rj are the radii of the two particles and Rc is the
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cutoff.
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</P>
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<P>In contrast to <A HREF = "pair_yukawa.html">pair_style yukawa</A>, 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.
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<A HREF = "pair_yukawa.html">Pair_style yukawa</A> is a screened Coulombic potential
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between two point-charges and uses no such approximation.
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</P>
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<P>This potential applies to nearby particle pairs for which the Derjagin
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approximation holds, meaning h << Ri + Rj, where h is the
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surface-to-surface separation of the two particles.
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</P>
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<P>When used in combination with <A HREF = "pair_colloid.html">pair_style colloid</A>,
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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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</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, or by mixing as described below:
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</P>
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<UL><LI>A (energy/distance units)
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<LI>cutoff (distance units)
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</UL>
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<P>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 <A HREF = "pair_yukawa.html">pair_style yukawa</A>. For low surface
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potentials, i.e. less than about 25 mV, A can be written as:
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</P>
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<PRE>A = 2 * PI * R*eps*eps0 * kappa * psi^2
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</PRE>
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<P>where
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</P>
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<UL><LI>R = colloid radius (distance units)
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<LI>eps0 = permittivity of free space (charge^2/energy/distance units)
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<LI>eps = relative permittivity of fluid medium (dimensionless)
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<LI>kappa = inverse screening length (1/distance units)
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<LI>psi = surface potential (energy/charge units)
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</UL>
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<P>The last coefficient is optional. If not specified, the global
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yukawa/colloid cutoff is used.
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</P>
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<HR>
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<P><B>Mixing, shift, table, tail correction, restart, rRESPA info</B>:
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</P>
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<P>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 <I>geometric</I>. See the
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"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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</P>
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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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<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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<I>inner</I>, <I>middle</I>, <I>outer</I> keywords.
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</P>
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<HR>
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<P><B>Restrictions:</B>
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</P>
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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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2009-11-07 05:01:28 +08:00
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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>This pair style requires that atoms be finite-size spheres with a
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diameter, as defined by the <A HREF = "atom_style.html">atom_style sphere</A>
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command.
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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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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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2009-11-07 05:01:28 +08:00
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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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</HTML>
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