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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>
</CENTER>
<HR>
<H3>pair_style yukawa/colloid command
</H3>
<H3>pair_style yukawa/colloid/omp command
</H3>
<P><B>Syntax:</B>
</P>
<PRE>pair_style yukawa/colloid kappa cutoff
</PRE>
<UL><LI>kappa = screening length (inverse distance units)
<LI>cutoff = global cutoff for colloidal Yukawa interactions (distance units)
</UL>
<P><B>Examples:</B>
</P>
<PRE>pair_style yukawa/colloid 2.0 2.5
pair_coeff 1 1 100.0 2.3
pair_coeff * * 100.0
</PRE>
<P><B>Description:</B>
</P>
<P>Style <I>yukawa/colloid</I> computes pairwise interactions with the formula
</P>
<CENTER><IMG SRC = "Eqs/pair_yukawa_colloid.jpg">
</CENTER>
<P>where Ri and Rj are the radii of the two particles and Rc is the
cutoff.
</P>
<P>In contrast to <A HREF = "pair_yukawa.html">pair_style yukawa</A>, this functional
form arises from the Coulombic interaction between two colloid
particles, screened due to the presence of an electrolyte.
<A HREF = "pair_yukawa.html">Pair_style yukawa</A> is a screened Coulombic potential
between two point-charges and uses no such approximation.
</P>
<P>This potential applies to nearby particle pairs for which the Derjagin
approximation holds, meaning h << Ri + Rj, where h is the
surface-to-surface separation of the two particles.
</P>
<P>When used in combination with <A HREF = "pair_colloid.html">pair_style colloid</A>,
the two terms become the so-called DLVO potential, which combines
electrostatic repulsion and van der Waals attraction.
</P>
<P>The following coefficients must be defined for each pair of atoms
types via the <A HREF = "pair_coeff.html">pair_coeff</A> command as in the examples
above, or in the data file or restart files read by the
<A HREF = "read_data.html">read_data</A> or <A HREF = "read_restart.html">read_restart</A>
commands, or by mixing as described below:
</P>
<UL><LI>A (energy/distance units)
<LI>cutoff (distance units)
</UL>
<P>The prefactor A is determined from the relationship between surface
charge and surface potential due to the presence of electrolyte. Note
that the A for this potential style has different units than the A
used in <A HREF = "pair_yukawa.html">pair_style yukawa</A>. For low surface
potentials, i.e. less than about 25 mV, A can be written as:
</P>
<PRE>A = 2 * PI * R*eps*eps0 * kappa * psi^2
</PRE>
<P>where
</P>
<UL><LI>R = colloid radius (distance units)
<LI>eps0 = permittivity of free space (charge^2/energy/distance units)
<LI>eps = relative permittivity of fluid medium (dimensionless)
<LI>kappa = inverse screening length (1/distance units)
<LI>psi = surface potential (energy/charge units)
</UL>
<P>The last coefficient is optional. If not specified, the global
yukawa/colloid cutoff is used.
</P>
<HR>
<P>Styles with a <I>cuda</I>, <I>gpu</I>, <I>omp</I>, or <I>opt</I> suffix are functionally
the same as the corresponding style without the suffix. They have
been optimized to run faster, depending on your available hardware, as
discussed in <A HREF = "Section_accelerate.html">Section_accelerate</A> of the
manual. The accelerated styles take the same arguments and should
produce the same results, except for round-off and precision issues.
</P>
<P>These accelerated styles are part of the USER-CUDA, GPU, USER-OMP and OPT
packages, respectively. They are only enabled if LAMMPS was built with
those packages. See the <A HREF = "Section_start.html#start_3">Making LAMMPS</A>
section for more info.
</P>
<P>You can specify the accelerated styles explicitly in your input script
by including their suffix, or you can use the <A HREF = "Section_start.html#start_6">-suffix command-line
switch</A> when you invoke LAMMPS, or you can
use the <A HREF = "suffix.html">suffix</A> command in your input script.
</P>
<P>See <A HREF = "Section_accelerate.html">Section_accelerate</A> of the manual for
more instructions on how to use the accelerated styles effectively.
</P>
<HR>
<P><B>Mixing, shift, table, tail correction, restart, rRESPA info</B>:
</P>
<P>For atom type pairs I,J and I != J, the A coefficient and cutoff
distance for this pair style can be mixed. A is an energy value mixed
like a LJ epsilon. The default mix value is <I>geometric</I>. See the
"pair_modify" command for details.
</P>
<P>This pair style supports the <A HREF = "pair_modify.html">pair_modify</A> shift
option for the energy of the pair interaction.
</P>
<P>The <A HREF = "pair_modify.html">pair_modify</A> table option is not relevant
for this pair style.
</P>
<P>This pair style does not support the <A HREF = "pair_modify.html">pair_modify</A>
tail option for adding long-range tail corrections to energy and
pressure.
</P>
<P>This pair style writes its information to <A HREF = "restart.html">binary restart
files</A>, so pair_style and pair_coeff commands do not need
to be specified in an input script that reads a restart file.
</P>
<P>This pair style can only be used via the <I>pair</I> keyword of the
<A HREF = "run_style.html">run_style respa</A> command. It does not support the
<I>inner</I>, <I>middle</I>, <I>outer</I> keywords.
</P>
<HR>
<P><B>Restrictions:</B>
</P>
<P>This style is part of the COLLOID package. It is only enabled if
LAMMPS was built with that package. See the <A HREF = "Section_start.html#start_3">Making
LAMMPS</A> section for more info.
</P>
<P>This pair style requires that atoms be finite-size spheres with a
diameter, as defined by the <A HREF = "atom_style.html">atom_style sphere</A>
command.
</P>
<P>Per-particle polydispersity is not yet supported by this pair style;
per-type polydispersity is allowed. This means all particles of the
same type must have the same diameter. Each type can have a different
diameter.
</P>
<P><B>Related commands:</B>
</P>
<P><A HREF = "pair_coeff.html">pair_coeff</A>
</P>
<P><B>Default:</B> none
</P>
</HTML>
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