lammps/doc/pair_tersoff_mod.txt

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pair_style tersoff/mod command :h3
pair_style tersoff/mod/omp command :h3

[Syntax:]

pair_style tersoff/mod :pre

[Examples:]

pair_style tersoff/mod
pair_coeff * * Si.tersoff.mod Si Si :pre

[Description:]

The {tersoff/mod} style computes a bond-order type interatomic
potential "(Kumagai)"_#Kumagai based on a 3-body Tersoff potential
"(Tersoff_1)"_#Tersoff_1, "(Tersoff_2)"_#Tersoff_2 with modified
cutoff function and angular-dependent term, giving the energy E of a
system of atoms as

:c,image(Eqs/pair_tersoff_mod.jpg)

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.

The modified cutoff function f_C proposed by "(Murty)"_#Murty and
having a continuous second-order differential is employed. The
angular-dependent term g(theta) was modified to increase the
flexibility of the potential.

The {tersoff/mod} potential is fitted to both the elastic constants
and melting point by employing the modified Tersoff potential function
form in which the angular-dependent term is improved. The model
performs extremely well in describing the crystalline, liquid, and
amorphous phases "(Schelling)"_#Schelling.

Only a single pair_coeff command is used with the {tersoff/mod} style
which specifies a Tersoff/MOD 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:

filename
N element names = mapping of Tersoff/MOD elements to atom types :ul

As an example, imagine the Si.tersoff_mod file has Tersoff values for Si.
If your LAMMPS simulation has 3 Si atoms types, you would use the following
pair_coeff command:

pair_coeff * * Si.tersoff_mod Si Si Si :pre

The 1st 2 arguments must be * * so as to span all LAMMPS atom types.
The three Si arguments map LAMMPS atom types 1,2,3 to the Si element
in the Tersoff/MOD file. If a mapping value is specified as NULL, the
mapping is not performed.  This can be used when a {tersoff/mod}
potential is used as part of the {hybrid} pair style. The NULL values
are placeholders for atom types that will be used with other
potentials.

Tersoff/MOD file in the {potentials} directory of the LAMMPS
distribution have a ".tersoff.mod" 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:

element 1 (the center atom in a 3-body interaction)
element 2 (the atom bonded to the center atom)
element 3 (the atom influencing the 1-2 bond in a bond-order sense)
beta
alpha
h
eta
beta_ters = 1 (dummy parameter)
lambda2 (1/distance units)
B (energy units)
R (distance units)
D (distance units)
lambda1 (1/distance units)
A (energy units)
n
c1
c2
c3
c4
c5 :ul

The n, eta, lambda2, B, lambda1, and A parameters are only used for
two-body interactions.  The beta, alpha, c1, c2, c3, c4, c5, h
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.

The Tersoff/MOD 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.

For a single-element simulation, only a single entry is required
(e.g. SiSiSi). 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 SiSiSi means Si bonded to a Si with another Si atom influencing the bond.

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Styles with a {cuda}, {gpu}, {omp}, or {opt} suffix are functionally
the same as the corresponding style without the suffix.  They have
been optimized to run faster, depending on your available hardware, as
discussed in "Section_accelerate"_Section_accelerate.html of the
manual.  The accelerated styles take the same arguments and should
produce the same results, except for round-off and precision issues.

These accelerated styles are part of the USER-CUDA, GPU, USER-OMP and OPT
packages, respectively.  They are only enabled if LAMMPS was built with
those packages.  See the "Making LAMMPS"_Section_start.html#start_3
section for more info.

You can specify the accelerated styles explicitly in your input script
by including their suffix, or you can use the "-suffix command-line
switch"_Section_start.html#start_7 when you invoke LAMMPS, or you can
use the "suffix"_suffix.html command in your input script.

See "Section_accelerate"_Section_accelerate.html of the manual for
more instructions on how to use the accelerated styles effectively.

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[Mixing, shift, table, tail correction, restart, rRESPA info]:

This pair style does not support the "pair_modify"_pair_modify.html
shift, table, and tail options.

This pair style does not write its information to "binary restart
files"_restart.html, since it is stored in potential files.  Thus, you
need to re-specify the pair_style and pair_coeff commands in an input
script that reads a restart file.

This pair style can only be used via the {pair} keyword of the
"run_style respa"_run_style.html command.  It does not support the
{inner}, {middle}, {outer} keywords.

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[Restrictions:]

This pair style is part of the MANYBODY package.  It is only enabled
if LAMMPS was built with that package (which it is by default).  See
the "Making LAMMPS"_Section_start.html#start_3 section for more info.

This pair style requires the "newton"_newton.html setting to be "on"
for pair interactions.

The Tersoff/MOD potential files provided with LAMMPS (see the potentials
directory) are parameterized for metal "units"_units.html.  You can
use the Tersoff/MOD potential with any LAMMPS units, but you would need to
create your own Tersoff/MOD potential file with coefficients listed in the
appropriate units if your simulation doesn't use "metal" units.

[Related commands:]

"pair_coeff"_pair_coeff.html

[Default:] none

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:link(Kumagai)
[(Kumagai)] T. Kumagai, S. Izumi, S. Hara, S. Sakai,
Comp. Mat. Science, 39, 457 (2007).

:link(Tersoff_1)
[(Tersoff_1)] J. Tersoff, Phys Rev B, 37, 6991 (1988). 

:link(Tersoff_2)
[(Tersoff_2)] J. Tersoff, Phys Rev B, 38, 9902 (1988). 

:link(Murty)
[(Murty)] M.V.R. Murty, H.A. Atwater, Phys Rev B, 51, 4889 (1995). 

:link(Schelling)
[(Schelling)] Patrick K. Schelling, Comp. Mat. Science, 44, 274 (2008).