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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 soft command
</H3>
<P><B>Syntax:</B>
</P>
<PRE>pair_style soft cutoff
</PRE>
<UL><LI>cutoff = global cutoff for soft interactions (distance units)
</UL>
<P><B>Examples:</B>
</P>
<PRE>pair_style soft 2.5
pair_coeff * * 0.0 60.0
pair_coeff 1 1 0.0 60.0 3.0
</PRE>
<P><B>Description:</B>
</P>
<P>Style <I>soft</I> computes pairwise interactions with the formula
</P>
<CENTER><IMG SRC = "Eqs/pair_soft.jpg">
</CENTER>
<P>It is useful for pushing apart overlapping atoms, since it does not
blow up as r goes to 0. A is a pre-factor that varies in time from
the start to the end of the run. The <A HREF = "run.html">run</A> command documents
how to make the ramping take place across multiple runs. Rc is the
cutoff. See the <A HREF = "fix_nve_limit.html">fix nve/limit</A> command for
another way to push apart overlapping atoms.
</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>Astart (energy units)
<LI>Astop (energy units)
<LI>cutoff (distance units)
</UL>
<P>Astart and Astop are the values of the prefactor at the start and end
of the next run. At intermediate times the value of A will be ramped
between these 2 values. Note that before performing a 2nd run, you
will want to adjust the values of Astart and Astop for all type pairs,
or switch to a new pair style.
</P>
<P>The last coefficient is optional. If not specified, the global soft
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 Astart, Astop coefficients and
cutoff distance for this pair style can be mixed. Astart and Atop are
always mixed via a <I>geometric</I> rule. The cutoff is mixed according to
the pair_modify mix value. The default mix value is <I>geometric</I>. See
the "pair_modify" command for details.
</P>
<P>This pair styles does not support the <A HREF = "pair_modify.html">pair_modify</A>
shift option, since the pair interaction is goes to 0.0 at the cutoff.
</P>
<P>The <A HREF = "pair_modify.html">pair_modify</A> table and tail options are not
relevant for this pair style.
</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> none
</P>
<P><B>Related commands:</B>
</P>
<P><A HREF = "pair_coeff.html">pair_coeff</A>, <A HREF = "fix_nve_limit.html">fix nve/limit</A>
</P>
<P><B>Default:</B> none
</P>
</HTML>
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