2006-11-04 01:09:44 +08:00
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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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<HR>
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<H3>pair_style tersoff command
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</H3>
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<P><B>Syntax:</B>
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</P>
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<PRE>pair_style tersoff
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</PRE>
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<P><B>Examples:</B>
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</P>
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<PRE>pair_style tersoff
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pair_coeff * * si.tersoff Si
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pair_coeff * * SiC.tersoff Si C Si
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</PRE>
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<P><B>Description:</B>
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</P>
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<P>The <I>tersoff</I> style computes a 3-body <A HREF = "#Tersoff">Tersoff</A> potential
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for the energy E of a system of atoms as
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</P>
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<CENTER><IMG SRC = "Eqs/pair_tersoff.jpg">
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</CENTER>
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<P>where f_R is a two-body term and f_A includes three-body interactions.
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The summations in the formula are over all neighbors J and K of atom I
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within a cutoff distance = R + D.
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</P>
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<P>Only a single pair_coeff command is used with the <I>tersoff</I> style
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which specifies a Tersoff potential file with parameters for all
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needed elements. These are mapped to LAMMPS atom types by specifying
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N additional arguments after the filename in the pair_coeff command,
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where N is the number of LAMMPS atom types:
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</P>
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<UL><LI>filename
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<LI>N element names = mapping of Tersoff elements to atom types
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</UL>
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<P>As an example, imagine the SiC.tersoff file has Tersoff values for Si
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and C. If your LAMMPS simulation has 4 atoms types and you want the
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1st 3 to be Si, and the 4th to be C, you would use the following
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pair_coeff command:
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</P>
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<PRE>pair_coeff * * SiC.tersoff Si Si Si C
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</PRE>
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<P>The 1st 2 arguments must be * * so as to span all LAMMPS atom types.
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The first three Si arguments map LAMMPS atom types 1,2,3 to the Si
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element in the Tersoff file. The final C argument maps LAMMPS atom
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type 4 to the C element in the Tersoff file. If a mapping value is
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specified as NULL, the mapping is not performed. This can be used
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when a <I>tersoff</I> potential is used as part of the <I>hybrid</I> pair style.
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The NULL values are placeholders for atom types that will be used with
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other potentials.
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</P>
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<P>Tersoff files in the <I>potentials</I> directory of the LAMMPS distribution
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have a ".tersoff" suffix. Lines that are not blank or comments
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(starting with #) define parameters for a triplet of elements. The
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parameters in a single entry correspond to coefficients in the formula
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above:
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</P>
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<UL><LI>element 1 (the center atom in a 3-body interaction)
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2007-01-18 04:23:28 +08:00
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<LI>element 2 (the atom bonded to the center atom)
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<LI>element 3 (the atom influencing the 1-2 bond in a bond-order sense)
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<LI>lambda3 (1/distance units)
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<LI>c
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<LI>d
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<LI>costheta0 (can be a value < -1 or > 1)
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<LI>n
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<LI>beta
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<LI>lambda2 (1/distance units)
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<LI>B (energy units)
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<LI>R (distance units)
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<LI>D (distance units)
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<LI>lambda1 (1/distance units)
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<LI>A (energy units)
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</UL>
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<P>The n, beta, lambda2, B, lambda1, and A parameters are only used for
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two-body interactions. The lambda3, c, d, and costheta0 parameters
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are only used for three-body interactions. The R and D parameters
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are used for both two-body and three-body interactions. The
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non-annotated parameters are unitless.
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2006-11-04 01:09:44 +08:00
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</P>
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<P>The Tersoff potential file must contain entries for all the elements
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listed in the pair_coeff command. It can also contain entries for
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additional elements not being used in a particular simulation; LAMMPS
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ignores those entries.
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</P>
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<P>For a single-element simulation, only a single entry is required
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(e.g. SiSiSi). For a two-element simulation, the file must contain 8
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entries (for SiSiSi, SiSiC, SiCSi, SiCC, CSiSi, CSiC, CCSi, CCC), that
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specify Tersoff parameters for all permutations of the two elements
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interacting in three-body configurations. Thus for 3 elements, 27
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entries would be required, etc.
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</P>
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<P>As annotated above, the first element in the entry is the center atom
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in a three-body interaction and it is bonded to the 2nd atom and the
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bond is influenced by the 3rd atom. Thus an entry for SiCC means Si
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bonded to a C with another C atom influencing the bond. Thus
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three-body parameters for SiCSi and SiSiC entries will not, in
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general, be the same. The parameters used for the two-body interaction come
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from the entry where the 2nd element is repeated. Thus the two-body
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parameters for Si interacting with C, comes from the SiCC entry. By
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symmetry, the twobody parameters in the SiCC and CSiSi entries should
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thus be the same. The parameters used for a particular three-body
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interaction come from the entry with the corresponding three elements.
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The parameters used only for two-body interactions
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(n, beta, lambda2, B, lambda1, and A)
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in entries whose 2nd and 3rd element are different (e.g. SiCSi)
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are not used for anything and can be set to 0.0 if desired.
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2006-11-04 01:09:44 +08:00
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</P>
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<P><B>Restrictions:</B>
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</P>
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<P>This pair potential requires the <A HREF = "newton.html">newton</A> setting to be
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"on" for pair interactions.
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</P>
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2007-04-24 00:55:05 +08:00
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<P>The Tersoff potential files provided with LAMMPS (see the potentials
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directory) are parameterized for metal <A HREF = "units.html">units</A>. You can
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use the Tersoff potential with any LAMMPS units, but you need to
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create your own Tersoff potential file if your simulation doesn't use
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"metal" units.
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</P>
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2006-11-04 01:09:44 +08:00
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<P><B>Related commands:</B>
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</P>
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<P><A HREF = "pair_coeff.html">pair_coeff</A>
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</P>
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<P><B>Default:</B> none
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</P>
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<HR>
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<A NAME = "Tersoff"></A>
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<P><B>(Tersoff)</B> Tersoff, Phys Rev B, 37, 6991 (1988).
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</P>
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</HTML>
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