forked from lijiext/lammps
123 lines
4.2 KiB
Plaintext
123 lines
4.2 KiB
Plaintext
"LAMMPS WWW Site"_lws - "LAMMPS Documentation"_ld - "LAMMPS Commands"_lc :c
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:link(lws,http://lammps.sandia.gov)
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:link(ld,Manual.html)
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:link(lc,Section_commands.html#comm)
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:line
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pair_style tersoff command :h3
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[Syntax:]
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pair_style tersoff :pre
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[Examples:]
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pair_style tersoff
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pair_coeff * * si.tersoff Si
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pair_coeff * * SiC.tersoff Si C Si :pre
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[Description:]
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The {tersoff} style computes a 3-body "Tersoff"_#Tersoff potential
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for the energy E of a system of atoms as
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:c,image(Eqs/pair_tersoff.jpg)
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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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Only a single pair_coeff command is used with the {tersoff} 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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filename
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N element names = mapping of Tersoff elements to atom types :ul
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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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pair_coeff * * SiC.tersoff Si Si Si C :pre
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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 {tersoff} potential is used as part of the {hybrid} 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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Tersoff files in the {potentials} 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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element 1 (the center atom in a 3-body interaction)
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element 2 (the atom bonded to the center atom)
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element 3 (the atom influencing the 1-2 bond in a bond-order sense)
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lambda3 (1/distance units)
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c
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d
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costheta0 (can be a value < -1 or > 1)
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n
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beta
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lambda2 (1/distance units)
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B (energy units)
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R (distance units)
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D (distance units)
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lambda1 (1/distance units)
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A (energy units) :ul
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The n, beta, lambda2, B, R, D, lambda1, and A parameters are for
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two-body interactions. The lambda3, c, d, and costheta0 parameters
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are for three-body interactions. The non-annotated parameters are
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unitless.
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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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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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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. Two-body parameters for an 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 two-body parameters in the SiCC and CSiSi entries should
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thus be the same. Two-body parameters in entries whose 2nd and 3rd
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element are different (e.g. SiCSi) are ignored.
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[Restrictions:]
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This pair potential requires the "newton"_newton.html setting to be
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"on" for pair interactions.
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[Related commands:]
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"pair_coeff"_pair_coeff.html
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[Default:] none
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:line
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:link(Tersoff)
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[(Tersoff)] Tersoff, Phys Rev B, 37, 6991 (1988).
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