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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>
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<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">
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<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 n, beta, lambda2, B, lambda1, and A parameters are only used for
two-body interactions. The lambda3, c, d, and costheta0 parameters
are only used for three-body interactions. The R and D parameters
are used for both two-body and three-body interactions. 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. The parameters used for the two-body 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 twobody parameters in the SiCC and CSiSi entries should
thus be the same. The parameters used for a particular three-body
interaction come from the entry with the corresponding three elements.
The parameters used only for two-body interactions
(n, beta, lambda2, B, lambda1, and A)
in entries whose 2nd and 3rd element are different (e.g. SiCSi)
are not used for anything and can be set to 0.0 if desired.
</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>The Tersoff potential files provided with LAMMPS (see the potentials
directory) are parameterized for metal <A HREF = "units.html">units</A>. You can
use the Tersoff potential with any LAMMPS units, but you need to
create your own Tersoff potential file if your simulation doesn't use
"metal" units.
</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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