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.. index:: pair_style airebo
pair_style airebo command
=========================
pair_style airebo/omp command
=============================
pair_style airebo/morse command
===============================
pair_style airebo/morse/omp command
===================================
pair_style rebo command
=======================
pair_style rebo/omp command
===========================
Syntax
""""""
.. parsed-literal::
pair_style style cutoff LJ_flag TORSION_flag
* style = *airebo* or *airebo/morse* or *rebo*
* cutoff = LJ or Morse cutoff (sigma scale factor) (AIREBO and AIREBO-M only)
* LJ_flag = 0/1 to turn off/on the LJ or Morse term (AIREBO and AIREBO-M only, optional)
* TORSION_flag = 0/1 to turn off/on the torsion term (AIREBO and AIREBO-M only, optional)
Examples
""""""""
.. parsed-literal::
pair_style airebo 3.0
pair_style airebo 2.5 1 0
pair_coeff * * ../potentials/CH.airebo H C
.. parsed-literal::
pair_style airebo/morse 3.0
pair_coeff * * ../potentials/CH.airebo-m H C
.. parsed-literal::
pair_style rebo
pair_coeff * * ../potentials/CH.airebo H C
Description
"""""""""""
The *airebo* pair style computes the Adaptive Intermolecular Reactive
Empirical Bond Order (AIREBO) Potential of :ref:`(Stuart) <Stuart>` for a
system of carbon and/or hydrogen atoms. Note that this is the initial
formulation of AIREBO from 2000, not the later formulation.
The *airebo/morse* pair style computes the AIREBO-M potential, which
is equivalent to AIREBO, but replaces the LJ term with a Morse potential.
The Morse potentials are parameterized by high-quality quantum chemistry
(MP2) calculations and do not diverge as quickly as particle density
increases. This allows AIREBO-M to retain accuracy to much higher pressures
than AIREBO (up to 40 GPa for Polyethylene). Details for this potential
and its parameterization are given in :ref:`(O'Conner) <OConnor>`.
The *rebo* pair style computes the Reactive Empirical Bond Order (REBO)
Potential of :ref:`(Brenner) <Brenner>`. Note that this is the so-called
2nd generation REBO from 2002, not the original REBO from 1990.
As discussed below, 2nd generation REBO is closely related to the
intial AIREBO; it is just a subset of the potential energy terms.
The AIREBO potential consists of three terms:
.. image:: Eqs/pair_airebo.jpg
:align: center
By default, all three terms are included. For the *airebo* style, 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. If both of the LJ an
torsional terms are turned off, it becomes the 2nd-generation REBO
potential, with a small caveat on the spline fitting procedure
mentioned below. This can be specified directly as pair_style *rebo*
with no additional arguments.
The detailed formulas for this potential are given in
:ref:`(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 :ref:`(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 :doc:`Lennard Jones potential <pair_lj>`.
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*\ , *airebo*
or *rebo* style which specifies an AIREBO or AIREBO-M potential file
with parameters for C and H. Note that the *rebo* style in LAMMPS
uses the same AIREBO-formatted potential file. 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
See the :doc:`pair_coeff <pair_coeff>` doc page for alternate ways
to specify the path for the potential file.
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:
.. parsed-literal::
pair_coeff * * CH.airebo C C C H
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 :ref:`(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.
Similarly the parameters/coefficients for the AIREBO-M potentials are
listed in the CH.airebo-m file to agree with the :ref:`(O'Connor) <OConnor>`
paper. Thus the parameters are specific to this potential and the way
it was fit, so modifying the file should be done cautiously. The
AIREBO-M Morse potentials were parameterized using a cutoff of
3.0 (sigma). Modifying this cutoff may impact simulation accuracy.
This pair style tallies a breakdown of the total AIREBO potential
energy into sub-categories, which can be accessed via the :doc:`compute pair <compute_pair>` command as a vector of values of length 3.
The 3 values correspond to the following sub-categories:
1. *E_REBO* = REBO energy
2. *E_LJ* = Lennard-Jones energy
3. *E_TORSION* = Torsion energy
To print these quantities to the log file (with descriptive column
headings) the following commands could be included in an input script:
.. parsed-literal::
compute 0 all pair airebo
variable REBO equal c_0[1]
variable LJ equal c_0[2]
variable TORSION equal c_0[3]
thermo_style custom step temp epair v_REBO v_LJ v_TORSION
----------
Styles with a *gpu*\ , *intel*\ , *kk*\ , *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 :doc:`Section_accelerate <Section_accelerate>`
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 GPU, USER-INTEL, KOKKOS,
USER-OMP and OPT packages, respectively. They are only enabled if
LAMMPS was built with those packages. See the :ref:`Making LAMMPS <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 :ref:`-suffix command-line switch <start_7>` when you invoke LAMMPS, or you can
use the :doc:`suffix <suffix>` command in your input script.
See :doc:`Section_accelerate <Section_accelerate>` of the manual for
more instructions on how to use the accelerated styles effectively.
----------
**Mixing, shift, table, tail correction, restart, rRESPA info**\ :
These pair styles do not support the :doc:`pair_modify <pair_modify>`
mix, shift, table, and tail options.
These pair styles do not write their information to :doc:`binary restart files <restart>`, 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.
These pair styles can only be used via the *pair* keyword of the
:doc:`run_style respa <run_style>` command. They do not support the
*inner*\ , *middle*\ , *outer* keywords.
Restrictions
""""""""""""
These pair styles are part of the MANYBODY package. They are only
enabled if LAMMPS was built with that package (which it is by
default). See the :ref:`Making LAMMPS <start_3>` section
for more info.
These pair potentials require the :doc:`newton <newton>` setting to be
"on" for pair interactions.
The CH.airebo and CH.airebo-m potential files provided with LAMMPS
(see the potentials directory) are parameterized for metal :doc:`units <units>`.
You can use the AIREBO, AIREBO-M or REBO potential with any LAMMPS units,
but you would need to create your own AIREBO or AIREBO-M potential file
with coefficients listed in the appropriate units, if your simulation
doesn't use "metal" units.
Related commands
""""""""""""""""
:doc:`pair_coeff <pair_coeff>`
**Default:** none
----------
.. _Stuart:
**(Stuart)** Stuart, Tutein, Harrison, J Chem Phys, 112, 6472-6486
(2000).
.. _Brenner:
**(Brenner)** Brenner, Shenderova, Harrison, Stuart, Ni, Sinnott, J
Physics: Condensed Matter, 14, 783-802 (2002).
.. _OConnor:
**(O'Connor)** O'Connor et al., J. Chem. Phys. 142, 024903 (2015).
.. _lws: http://lammps.sandia.gov
.. _ld: Manual.html
.. _lc: Section_commands.html#comm
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