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pair_coul.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 coul/cut command
</H3>
<H3>pair_style coul/debye command
</H3>
<H3>pair_style coul/long command
</H3>
<H3>pair_style coul/long/gpu command
</H3>
<P><B>Syntax:</B>
</P>
<PRE>pair_style coul/cut cutoff
pair_style coul/debye kappa cutoff
pair_style coul/long cutoff
pair_style coul/long/gpu cutoff
</PRE>
<UL><LI>cutoff = global cutoff for Coulombic interactions
<LI>kappa = Debye length (inverse distance units)
</UL>
<P><B>Examples:</B>
</P>
<PRE>pair_style coul/cut 2.5
pair_coeff * *
pair_coeff 2 2 3.5
</PRE>
<PRE>pair_style coul/debye 1.4 3.0
pair_coeff * *
pair_coeff 2 2 3.5
</PRE>
<PRE>pair_style coul/long 10.0
pair_coeff * *
</PRE>
<P><B>Description:</B>
</P>
<P>The <I>coul/cut</I> style computes the standard Coulombic interaction
potential given by
</P>
<CENTER><IMG SRC = "Eqs/pair_coulomb.jpg">
</CENTER>
<P>where C is an energy-conversion constant, Qi and Qj are the charges on
the 2 atoms, and epsilon is the dielectric constant which can be set
by the <A HREF = "dielectric.html">dielectric</A> command. The cutoff Rc truncates
the interaction distance.
</P>
<P>Style <I>coul/debye</I> adds an additional exp() damping factor to the
Coulombic term, given by
</P>
<CENTER><IMG SRC = "Eqs/pair_debye.jpg">
</CENTER>
<P>where kappa is the Debye length. This potential is another way to
mimic the screening effect of a polar solvent.
</P>
<P>Style <I>coul/long</I> computes the same Coulombic interactions as style
<I>coul/cut</I> except that an additional damping factor is applied so it
can be used in conjunction with the <A HREF = "kspace_style.html">kspace_style</A>
command and its <I>ewald</I> or <I>pppm</I> option. The Coulombic cutoff
specified for this style means that pairwise interactions within this
distance are computed directly; interactions outside that distance are
computed in reciprocal space.
</P>
<P>These potentials are designed to be combined with other pair
potentials via the <A HREF = "pair_hybrid.html">pair_style hybrid/overlay</A>
command. This is because they have no repulsive core. Hence if they
are used by themselves, there will be no repulsion to keep two
oppositely charged particles from overlapping each other.
</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>cutoff (distance units)
</UL>
<P>For <I>coul/cut</I> and <I>coul/debye</I>, the cutoff coefficient is optional.
If it is not used (as in some of the examples above), the default
global value specified in the pair_style command is used.
</P>
<P>For <I>coul/long</I> no cutoff can be specified for an individual I,J type
pair via the pair_coeff command. All type pairs use the same global
Coulombic cutoff specified in the pair_style command.
</P>
<HR>
<P>Styles with a <I>cuda</I>, <I>gpu</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">this section</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", 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">this section</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 cutoff distance for the
<I>coul/cut</I> style can be mixed. The default mix value is <I>geometric</I>.
See the "pair_modify" command for details.
</P>
<P>The <A HREF = "pair_modify.html">pair_modify</A> shift option is not relevant
for these pair styles.
</P>
<P>The <I>coul/long</I> style supports the <A HREF = "pair_modify.html">pair_modify</A>
table option for tabulation of the short-range portion of the
long-range Coulombic interaction.
</P>
<P>These pair styles do 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>These pair styles write their 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>The <I>coul/long</I> style is part of the "kspace" package. It is only
enabled if LAMMPS was built with that package (which it is by
default). See the <A HREF = "Section_start.html#start_3">Making LAMMPS</A> section
for more info.
</P>
<P><B>Related commands:</B>
</P>
<P><A HREF = "pair_coeff.html">pair_coeff</A>, <A HREF = "pair_hybrid.html">pair_style
hybrid/overlay</A>
</P>
<P><B>Default:</B> none
</P>
</HTML>
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