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pair_yukawa_colloid.html
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rLAMMPS lammps
pair_yukawa_colloid.html
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<HTML>
<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>
<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><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#2_3"
>
Making
LAMMPS
</A>
section for more info.
</P>
<P>
Because this potential uses the radii of the particles, the atom style
must support particles whose size is set via the
<A
HREF =
"shape.html"
>
shape
</A>
command. For example
<A
HREF =
"atom_style.html"
>
atom_style
</A>
colloid or
ellipsoid. Only spherical mono-disperse particles are currently
allowed for pair_style yukawa/colloid, which means the 3 shape
diameters for all particle types must be the same.
</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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