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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 lj/class2 command
</H3>
<H3>pair_style lj/class2/coul/cut command
</H3>
<H3>pair_style lj/class2/coul/long command
</H3>
<P><B>Syntax:</B>
</P>
<PRE>pair_style style args
</PRE>
<UL><LI>style = <I>lj/class2</I> or <I>lj/class2/coul/cut</I> or <I>lj/class2/coul/long</I>
<LI>args = list of arguments for a particular style
</UL>
<PRE> <I>lj/class2</I> args = cutoff
cutoff = global cutoff for class 2 interactions (distance units)
<I>lj/class2/coul/cut</I> args = cutoff (cutoff2)
cutoff = global cutoff for class 2 (and Coulombic if only 1 arg) (distance units)
cutoff2 = global cutoff for Coulombic (optional) (distance units)
<I>lj/class2/coul/long</I> args = cutoff (cutoff2)
cutoff = global cutoff for class 2 (and Coulombic if only 1 arg) (distance units)
cutoff2 = global cutoff for Coulombic (optional) (distance units)
</PRE>
<P><B>Examples:</B>
</P>
<PRE>pair_style lj/class2 10.0
pair_coeff * * 100.0 2.5
pair_coeff 1 2* 100.0 2.5 9.0
</PRE>
<PRE>pair_style lj/class2/coul/cut 10.0
pair_style lj/class2/coul/cut 10.0 8.0
pair_coeff * * 100.0 3.0
pair_coeff 1 1 100.0 3.5 9.0
pair_coeff 1 1 100.0 3.5 9.0 9.0
</PRE>
<PRE>pair_style lj/class2/coul/long 10.0
pair_style lj/class2/coul/long 10.0 8.0
pair_coeff * * 100.0 3.0
pair_coeff 1 1 100.0 3.5 9.0
</PRE>
<P><B>Description:</B>
</P>
<P>The <I>lj/class2</I> styles compute a 6/9 Lennard-Jones potential given by
</P>
<CENTER><IMG SRC = "Eqs/pair_class2.jpg">
</CENTER>
<P>Rc is the cutoff.
</P>
<P>The <I>lj/class2/coul/cut</I> and <I>lj/class2/coul/long</I> styles add a
Coulombic term as described for the <A HREF = "pair_lj.html">lj/cut</A> pair
styles.
</P>
<P>See <A HREF = "#Sun">(Sun)</A> for a description of the COMPASS class2 force field.
</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>epsilon (energy units)
<LI>sigma (distance units)
<LI>cutoff1 (distance units)
<LI>cutoff2 (distance units)
</UL>
<P>The latter 2 coefficients are optional. If not specified, the global
class 2 and Coulombic cutoffs are used. If only one cutoff is
specified, it is used as the cutoff for both class 2 and Coulombic
interactions for this type pair. If both coefficients are specified,
they are used as the class 2 and Coulombic cutoffs for this type pair.
You cannot specify 2 cutoffs for style <I>lj/class2</I>, since it has no
Coulombic terms.
</P>
<P>For <I>lj/class2/coul/long</I> only the class 2 cutoff can be specified
since a Coulombic cutoff cannot be specified for an individual I,J
type pair. All type pairs use the same global Coulombic cutoff
specified in the pair_style command.
</P>
<P>: line
</P>
<P>If the pair_coeff command is not used to define coefficients for a
particular I != J type pair, the mixing rule for epsilon and sigma for
all class2 potentials is to use the <I>sixthpower</I> formulas documented
by the <A HREF = "pair_modify.html">pair_modify</A> command. The <A HREF = "pair_modify.html">pair_modify
mix</A> setting is thus ignored for class2 potentials
for epsilon and sigma. However it is still followed for mixing the
cutoff distance.
</P>
<P><B>Mixing, shift, table, tail correction, restart, rRESPA info</B>:
</P>
<P>For atom type pairs I,J and I != J, the epsilon and sigma coefficients
and cutoff distance for all of the lj/class2 pair styles can be mixed.
Epsilon and sigma are always mixed with the value <I>sixthpower</I>. The
cutoff distance is mixed by whatever option is set by the pair_modify
command (default = geometric). See the "pair_modify" command for
details.
</P>
<P>All of the lj/class2 pair styles support the
<A HREF = "pair_modify.html">pair_modify</A> shift option for the energy of the
Lennard-Jones portion of the pair interaction.
</P>
<P>The <I>lj/class2/coul/long</I> pair style does not support the
<A HREF = "pair_modify.html">pair_modify</A> table option since a tabulation
capability has not yet been added to this potential.
</P>
<P>All of the lj/class2 pair styles support the
<A HREF = "pair_modify.html">pair_modify</A> tail option for adding a long-range
tail correction to the energy and pressure of the Lennard-Jones
portion of the pair interaction.
</P>
<P>All of the lj/class2 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>All of the lj/class2 pair styles can only be used via the <I>pair</I>
keyword of the <A HREF = "run_style.html">run_style respa</A> command. They do not
support the <I>inner</I>, <I>middle</I>, <I>outer</I> keywords.
</P>
<P><B>Restrictions:</B>
</P>
<P>All of these pair styles are part of the "class2" package. They are
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 = "Sun"></A>
<P><B>(Sun)</B> Sun, J Phys Chem B 102, 7338-7364 (1998).
</P>
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