lammps/doc/pair_airebo.txt

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pair_style airebo command :h3

[Syntax:]

pair_style airebo cutoff LJ_flag TORSION_flag :pre

cutoff = LJ cutoff (sigma scale factor)
LJ_flag = 0/1 to turn off/on the LJ term in AIREBO (optional)
TORSION_flag = 0/1 to turn off/on the torsion term in AIREBO (optional) :ul

[Examples:]

pair_style airebo 3.0
pair_style airebo 2.5 1 0
pair_coeff * * ../potentials/CH.airebo H C :pre

[Description:]

The {airebo} pair style computes the Adaptive Intermolecular Reactive
Empirical Bond Order (AIREBO) Potential of "(Stuart)"_#Stuart for a
system of carbon and/or hydrogen atoms.  The potential
consists of three terms:

:c,image(Eqs/pair_airebo.jpg)

By default, all three terms are included.  If the two optional flag
arguments to the pair_style command are included, the LJ and torsional
terms can be turned off.  Note that both or neither of the flags must
be included.

The detailed formulas for this potential are given in
"(Stuart)"_#Stuart; here we provide only a brief description.

The E_REBO term has the same functional form as the hydrocarbon REBO
potential developed in "(Brenner)"_#Brenner.  The coefficients for
E_REBO in AIREBO are essentially the same as Brenner's potential, but
a few fitted spline values are slightly different.  For most cases the
E_REBO term in AIREBO will produce the same energies, forces and
statistical averages as the original REBO potential from which it was
derived.  The E_REBO term in the AIREBO potential gives the model its
reactive capabilities and only describes short-ranged C-C, C-H and H-H
interactions (r < 2 Angstroms). These interactions have strong
coordination-dependence through a bond order parameter, which adjusts
the attraction between the I,J atoms based on the position of other
nearby atoms and thus has 3- and 4-body dependence.

The E_LJ term adds longer-ranged interactions (2 < r < cutoff) using a
form similar to the standard "Lennard Jones potential"_pair_lj.html.
The E_LJ term in AIREBO contains a series of switching functions so
that the short-ranged LJ repulsion (1/r^12) does not interfere with
the energetics captured by the E_REBO term.  The extent of the E_LJ
interactions is determined by the {cutoff} argument to the pair_style
command which is a scale factor.  For each type pair (C-C, C-H, H-H)
the cutoff is obtained by multiplying the scale factor by the sigma
value defined in the potential file for that type pair.  In the
standard AIREBO potential, sigma_CC = 3.4 Angstroms, so with a scale
factor of 3.0 (the argument in pair_style), the resulting E_LJ cutoff
would be 10.2 Angstroms.

The E_TORSION term is an explicit 4-body potential that describes
various dihedral angle preferences in hydrocarbon configurations.

Only a single pair_coeff command is used with the {airebo} style which
specifies an AIREBO potential file with parameters for C and H.  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 AIREBO elements to atom types :ul

As an example, if your LAMMPS simulation has 4 atom types and you want
the 1st 3 to be C, and the 4th to be H, you would use the following
pair_coeff command:

pair_coeff * * CH.airebo C C C H :pre

The 1st 2 arguments must be * * so as to span all LAMMPS atom types.
The first three C arguments map LAMMPS atom types 1,2,3 to the C
element in the AIREBO file.  The final H argument maps LAMMPS atom
type 4 to the H element in the SW file.  If a mapping value is
specified as NULL, the mapping is not performed.  This can be used
when a {airebo} 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.

The parameters/coefficients for the AIREBO potentials are listed in
the CH.airebo file to agree with the original "(Stuart)"_#Stuart
paper.  Thus the parameters are specific to this potential and the way
it was fit, so modifying the file should be done cautiously.  Also
note that the E_LJ and E_TORSION terms in AIREBO are intended to be
used with the E_REBO term and not as stand-alone potentials.  Thus we
don't suggest you use pair_style airebo with the E_REBO term turned
off.

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

This pair style does not support the "pair_modify"_pair_modify.html
mix, 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.

[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#2_3 section for more info.

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

The CH.airebo potential file provided with LAMMPS (see the potentials
directory) is parameterized for metal "units"_units.html.  You can use
the AIREBO potential with any LAMMPS units, but you would need to
create your own AIREBO 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(Stuart)
[(Stuart)] Stuart, Tutein, Harrison, J Chem Phys, 112, 6472-6486
(2000).

:link(Brenner)
[(Brenner)] Brenner, Shenderova, Harrison, Stuart, Ni, Sinnott, J
Physics: Condensed Matter, 14, 783-802 (2002).