forked from lijiext/lammps
164 lines
5.9 KiB
HTML
164 lines
5.9 KiB
HTML
<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 colloid command
|
|
</H3>
|
|
<P><B>Syntax:</B>
|
|
</P>
|
|
<PRE>pair_style colloid cutoff
|
|
</PRE>
|
|
<UL><LI>cutoff = global cutoff for colloidal interactions (distance units)
|
|
</UL>
|
|
<P><B>Examples:</B>
|
|
</P>
|
|
<PRE>pair_style colloid 10.0
|
|
pair_coeff * * 25 1.0 10.0 10.0
|
|
pair_coeff 1 1 144 1.0 0.0 0.0 3.0
|
|
pair_coeff 1 2 75.398 1.0 0.0 10.0 9.0
|
|
pair_coeff 2 2 39.478 1.0 10.0 10.0 25.0
|
|
</PRE>
|
|
<P><B>Description:</B>
|
|
</P>
|
|
<P>Style <I>colloid</I> computes pairwise interactions between large colloidal
|
|
particles and small solvent particles using 3 formulas. A colloidal
|
|
particle has a size > sigma; a solvent particle is the usual
|
|
Lennard-Jones particle of size sigma.
|
|
</P>
|
|
<P>The colloid-colloid interaction energy is given by
|
|
</P>
|
|
<CENTER><IMG SRC = "Eqs/pair_colloid_cc.jpg">
|
|
</CENTER>
|
|
<P>A_cc is the Hamaker constant, a1 and a2 are the radii of the two
|
|
colloidal particles, and Rc is the cutoff. This equation results from
|
|
describing each colloidal particle as an integrated collection of
|
|
Lennard-Jones particles of size sigma and is derived in
|
|
<A HREF = "#Everaers">(Everaers)</A>.
|
|
</P>
|
|
<P>The colloid-solvent interaction energy is given by
|
|
</P>
|
|
<CENTER><IMG SRC = "Eqs/pair_colloid_cs.jpg">
|
|
</CENTER>
|
|
<P>A_cs is the Hamaker constant, a is the radius of the colloidal
|
|
particle, and Rc is the cutoff. This formula is derived from the
|
|
colloid-colloid interaction, letting one of the particle sizes go to
|
|
zero.
|
|
</P>
|
|
<P>The solvent-solvent interaction energy is given by the usual
|
|
Lennard-Jones formula
|
|
</P>
|
|
<CENTER><IMG SRC = "Eqs/pair_colloid_ss.jpg">
|
|
</CENTER>
|
|
<P>with A_ss set appropriately, which results from letting both particle
|
|
sizes go to zero.
|
|
</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 units)
|
|
<LI>sigma (distance units)
|
|
<LI>d1 (distance units)
|
|
<LI>d2 (distance units)
|
|
<LI>cutoff (distance units)
|
|
</UL>
|
|
<P>A is the Hamaker energy prefactor and should typically be set as
|
|
follows:
|
|
</P>
|
|
<UL><LI>A_cc = colloid/colloid = 4 pi^2 = 39.5
|
|
<LI>A_cs = colloid/solvent = sqrt(A_cc*A_ss)
|
|
<LI>A_ss = solvent/solvent = 144 (assuming epsilon = 1, so that 144/36 = 4)
|
|
</UL>
|
|
<P>Sigma is the size of the solvent particle or the constituent particles
|
|
integrated over in the colloidal particle and should typically be set
|
|
as follows:
|
|
</P>
|
|
<UL><LI>Sigma_cc = colloid/colloid = 1.0
|
|
<LI>Sigma_cs = colloid/solvent = arithmetic mixing between colloid sigma and solvent sigma
|
|
<LI>Sigma_ss = solvent/solvent = 1.0 or whatever size the solvent particle is
|
|
</UL>
|
|
<P>Thus typically Sigma_cs = 1.0, unless the solvent particle's size !=
|
|
1.0.
|
|
</P>
|
|
<P>D1 and d2 are particle diameters, so that d1 = 2*a1 and d2 = 2*a2 in
|
|
the formulas above. Both d1 and d2 must be values >= 0. If d1 > 0
|
|
and d2 > 0, then the pair interacts via the colloid-colloid formula
|
|
above. If d1 = 0 and d2 = 0, then the pair interacts via the
|
|
solvent-solvent formula. I.e. a d value of 0 is a Lennard-Jones
|
|
particle of size sigma. If either d1 = 0 or d2 = 0 and the other is
|
|
larger, then the pair interacts via the colloid-solvent formula.
|
|
</P>
|
|
<P>Note that the diameter of a particular particle type may appear in
|
|
multiple pair_coeff commands, as it interacts with other particle
|
|
types. You should insure the particle diameter is specified
|
|
consistently each time it appears.
|
|
</P>
|
|
<P>The last coefficient is optional. If not specified, the global cutoff
|
|
specified in the pair_style command is used. However, you typically
|
|
want different cutoffs for interactions between different particle
|
|
sizes. E.g. if colloidal particles of diameter 10 are used with
|
|
solvent particles of diameter 1, then a solvent-solvent cutoff of 2.5
|
|
would correspond to a colloid-colloid cutoff of 25. A good
|
|
rule-of-thumb is to use a colloid-solvent cutoff that is half the big
|
|
diameter + 4 times the small diameter. I.e. 9 = 5 + 4 for the
|
|
colloid-solvent cutoff in this case.
|
|
</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, sigma, d1, and d2
|
|
coefficients and cutoff distance for this pair style can be mixed. A
|
|
is an energy value mixed like a LJ epsilon. D1 and d2 are distance
|
|
values and are mixed like sigma. 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><B>Related commands:</B>
|
|
</P>
|
|
<P><A HREF = "pair_coeff.html">pair_coeff</A>
|
|
</P>
|
|
<P><B>Default:</B> none
|
|
</P>
|
|
<HR>
|
|
|
|
<A NAME = "Everaers"></A>
|
|
|
|
<P><B>(Everaers)</B> Everaers, Ejtehadi, Phys Rev E, 67, 041710 (2003).
|
|
</P>
|
|
</HTML>
|