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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 sw command
</H3>
<P><B>Syntax:</B>
</P>
<PRE>pair_style sw
</PRE>
<P><B>Examples:</B>
</P>
<PRE>pair_style sw
pair_coeff * * si.sw Si
pair_coeff * * SiC.sw Si C Si
</PRE>
<P><B>Description:</B>
</P>
<P>The <I>sw</I> style computes a 3-body <A HREF = "#Stillinger">Stillinger-Weber</A>
potential for the energy E of a system of atoms as
</P>
<CENTER><IMG SRC = "Eqs/pair_sw.jpg">
</CENTER>
<P>where phi2 is a two-body term and phi3 is a three-body term. The
summations in the formula are over all neighbors J and K of atom I
within a cutoff distance = a*sigma.
</P>
<P>Only a single pair_coeff command is used with the <I>sw</I> style which
specifies a Stillinger-Weber potential file with parameters for all
needed elements. These are mapped to LAMMPS atom types by specifying
N additional arguments after the filename in the pair_coeff command,
where N is the number of LAMMPS atom types:
</P>
<UL><LI>filename
<LI>N element names = mapping of SW elements to atom types
</UL>
<P>As an example, imagine the SiC.sw file has Stillinger-Weber values for
Si and C. If your LAMMPS simulation has 4 atoms types and you want
the 1st 3 to be Si, and the 4th to be C, you would use the following
pair_coeff command:
</P>
<PRE>pair_coeff * * SiC.sw Si Si Si C
</PRE>
<P>The 1st 2 arguments must be * * so as to span all LAMMPS atom types.
The first three Si arguments map LAMMPS atom types 1,2,3 to the Si
element in the SW file. The final C argument maps LAMMPS atom type 4
to the C element in the SW file. If a mapping value is specified as
NULL, the mapping is not performed. This can be used when a <I>sw</I>
potential is used as part of the <I>hybrid</I> pair style. The NULL values
are placeholders for atom types that will be used with other
potentials.
</P>
<P>Stillinger-Weber files in the <I>potentials</I> directory of the LAMMPS
distribution have a ".sw" suffix. Lines that are not blank or
comments (starting with #) define parameters for a triplet of
elements. The parameters in a single entry correspond to the two-body
and three-body coefficients in the formula above:
</P>
<UL><LI>element 1 (the center atom in a 3-body interaction)
<LI>element 2
<LI>element 3
<LI>epsilon (energy units)
<LI>sigma (distnace units)
<LI>a
<LI>lambda
<LI>gamma
<LI>costheta0
<LI>A
<LI>B
<LI>p
<LI>q
</UL>
<P>The non-annotated parameters are unitless.
</P>
<P>The Stillinger-Weber potential file must contain entries for all the
elements listed in the pair_coeff command. It can also contain
entries for additional elements not being used in a particular
simulation; LAMMPS ignores those entries.
</P>
<P>For a single-element simulation, only a single entry is required
(e.g. SiSiSi). For a two-element simulation, the file must contain 8
entries (for SiSiSi, SiSiC, SiCSi, SiCC, CSiSi, CSiC, CCSi, CCC), that
specify SW parameters for all permutations of the two elements
interacting in three-body configurations. Thus for 3 elements, 27
entries would be required, etc.
</P>
<P>As annotated above, the first element in the entry is the center atom
in a three-body interaction. Thus an entry for SiCC means a Si atom
with 2 C atoms as neighbors. By symmetry, three-body parameters for
SiCSi and SiSiC entries should be the same. Two-body parameters for
an interaction come from the entry where the 2nd element is repeated.
Thus the two-body parameters for Si interacting with C, comes from the
SiCC entry. Again by symmetry, the two-body parameters in the SiCC
and CSiSi entries should thus be the same. Two-body parameters in
entries whose 2nd and 3rd element are different (e.g. SiCSi) are
ignored.
</P>
<P><B>Restrictions:</B>
</P>
<P>This pair potential requires the <A HREF = "newton.html">newton</A> setting to be
"on" for pair interactions.
</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 = "Stillinger"></A>
<P><B>(Stillinger)</B> Stillinger and Weber, Phys Rev B, 31, 5262 (1985).
</P>
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