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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 tersoff command
</H3>
<P><B>Syntax:</B>
</P>
<PRE>pair_style tersoff
</PRE>
<P><B>Examples:</B>
</P>
<PRE>pair_style tersoff
pair_coeff * * si.tersoff Si
pair_coeff * * SiC.tersoff Si C Si
</PRE>
<P><B>Description:</B>
</P>
<P>The <I>tersoff</I> style computes a 3-body <A HREF = "#Tersoff">Tersoff</A> potential
for the energy E of a system of atoms as
</P>
<CENTER><IMG SRC = "Eqs/pair_tersoff.jpg">
</CENTER>
<P>where f_R is a two-body term and f_A includes three-body interactions.
The summations in the formula are over all neighbors J and K of atom I
within a cutoff distance = R + D.
</P>
<P>Only a single pair_coeff command is used with the <I>tersoff</I> style
which specifies a Tersoff 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 Tersoff elements to atom types
</UL>
<P>As an example, imagine the SiC.tersoff file has Tersoff 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.tersoff 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 Tersoff file. The final C argument maps LAMMPS atom
type 4 to the C element in the Tersoff file. If a mapping value is
specified as NULL, the mapping is not performed. This can be used
when a <I>tersoff</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>Tersoff files in the <I>potentials</I> directory of the LAMMPS distribution
have a ".tersoff" 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 coefficients in the formula
above:
</P>
<UL><LI>element 1 (the center atom in a 3-body interaction)
<LI>element 2 (the atom bonded to the center atom)
<LI>element 3 (the atom influencing the 1-2 bond in a bond-order sense)
<LI>lambda3 (1/distance units)
<LI>c
<LI>d
<LI>costheta0 (can be a value < -1 or > 1)
<LI>n
<LI>beta
<LI>lambda2 (1/distance units)
<LI>B (energy units)
<LI>R (distance units)
<LI>D (distance units)
<LI>lambda1 (1/distance units)
<LI>A (energy units)
</UL>
<P>The non-annotated parameters are unitless.
</P>
<P>The Tersoff 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 Tersoff 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 and it is bonded to the 2nd atom and the
bond is influenced by the 3rd atom. Thus an entry for SiCC means Si
bonded to a C with another C atom influencing the bond. Thus
three-body parameters for SiCSi and SiSiC entries will not, in
general, 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. 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 = "Tersoff"></A>
<P><B>(Tersoff)</B> Tersoff, Phys Rev B, 37, 6991 (1988).
</P>
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