<p>Style <em>reax/c</em> computes the ReaxFF potential of van Duin, Goddard and
co-workers. ReaxFF uses distance-dependent bond-order functions to
represent the contributions of chemical bonding to the potential
energy. There is more than one version of ReaxFF. The version
implemented in LAMMPS uses the functional forms documented in the
supplemental information of the following paper: <aclass="reference internal"href="#chenoweth-2008"><spanclass="std std-ref">(Chenoweth et al., 2008)</span></a>. The version integrated into LAMMPS matches
the most up-to-date version of ReaxFF as of summer 2010. For more
technical details about the pair reax/c implementation of ReaxFF, see
the <aclass="reference internal"href="#aktulga"><spanclass="std std-ref">(Aktulga)</span></a> paper.</p>
<p>The <em>reax/c</em> style differs from the <aclass="reference internal"href="pair_reax.html"><spanclass="doc">pair_style reax</span></a>
command in the lo-level implementation details. The <em>reax</em> style is a
Fortran library, linked to LAMMPS. The <em>reax/c</em> style was initially
implemented as stand-alone C code and is now integrated into LAMMPS as
a package.</p>
<p>LAMMPS provides several different versions of ffield.reax in its
potentials dir, each called potentials/ffield.reax.label. These are
documented in potentials/README.reax. The default ffield.reax
contains parameterizations for the following elements: C, H, O, N.</p>
<p>The format of these files is identical to that used originally by van
Duin. We have tested the accuracy of <em>pair_style reax/c</em> potential
against the original ReaxFF code for the systems mentioned above. You
can use other ffield files for specific chemical systems that may be
available elsewhere (but note that their accuracy may not have been
tested).</p>
<divclass="admonition note">
<pclass="first admonition-title">Note</p>
<pclass="last">We do not distribute a wide variety of ReaxFF force field files
with LAMMPS. Adri van Duin’s group at PSU is the central repository
for this kind of data as they are continuously deriving and updating
parameterizations for different classes of materials. You can submit
a contact request at the Materials Computation Center (MCC) website
<p>Use of this pair style requires that a charge be defined for every
atom. See the <aclass="reference internal"href="atom_style.html"><spanclass="doc">atom_style</span></a> and
<aclass="reference internal"href="read_data.html"><spanclass="doc">read_data</span></a> commands for details on how to specify
charges.</p>
<p>The ReaxFF parameter files provided were created using a charge
equilibration (QEq) model for handling the electrostatic interactions.
Therefore, by default, LAMMPS requires that the <aclass="reference internal"href="fix_qeq_reax.html"><spanclass="doc">fix qeq/reax</span></a> command be used with <em>pair_style reax/c</em>
when simulating a ReaxFF model, to equilibrate charge each timestep.
Using the keyword <em>checkqeq</em> with the value <em>no</em>
turns off the check for <em>fix qeq/reax</em>,
allowing a simulation to be run without charge equilibration.
In this case, the static charges you
assign to each atom will be used for computing the electrostatic
interactions in the system.
See the <aclass="reference internal"href="fix_qeq_reax.html"><spanclass="doc">fix qeq/reax</span></a> command for details.</p>
<p>Using the optional keyword <em>lgvdw</em> with the value <em>yes</em> turns on
the low-gradient correction of the ReaxFF/C for long-range
London Dispersion, as described in the <aclass="reference internal"href="#liu-2011"><spanclass="std std-ref">(Liu)</span></a> paper. Force field
file <em>ffield.reax.lg</em> is designed for this correction, and is trained
for several energetic materials (see “Liu”). When using lg-correction,
recommended value for parameter <em>thb</em> is 0.01, which can be set in the
control file. Note: Force field files are different for the original
or lg corrected pair styles, using wrong ffield file generates an error message.</p>
<p>Optional keywords <em>safezone</em> and <em>mincap</em> are used for allocating
reax/c arrays. Increase these values can avoid memory problems, such
as segmentation faults and bondchk failed errors, that could occur under
certain conditions.</p>
<p>The thermo variable <em>evdwl</em> stores the sum of all the ReaxFF potential
energy contributions, with the exception of the Coulombic and charge
equilibration contributions which are stored in the thermo variable
<em>ecoul</em>. The output of these quantities is controlled by the
<p>This pair style tallies a breakdown of the total ReaxFF potential
energy into sub-categories, which can be accessed via the <aclass="reference internal"href="compute_pair.html"><spanclass="doc">compute pair</span></a> command as a vector of values of length 14.
The 14 values correspond to the following sub-categories (the variable
names in italics match those used in the original FORTRAN ReaxFF code):</p>
<olclass="arabic simple">
<li><em>eb</em> = bond energy</li>
<li><em>ea</em> = atom energy</li>
<li><em>elp</em> = lone-pair energy</li>
<li><em>emol</em> = molecule energy (always 0.0)</li>
<p>This pair style does not support the <aclass="reference internal"href="pair_modify.html"><spanclass="doc">pair_modify</span></a>
mix, shift, table, and tail options.</p>
<p>This pair style does not write its information to <aclass="reference internal"href="restart.html"><spanclass="doc">binary restart files</span></a>, 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.</p>
<p>This pair style can only be used via the <em>pair</em> keyword of the
<aclass="reference internal"href="run_style.html"><spanclass="doc">run_style respa</span></a> command. It does not support the
<p>This pair style is part of the USER-REAXC package. It is only enabled
if LAMMPS was built with that package. See the <aclass="reference internal"href="Section_start.html#start-3"><spanclass="std std-ref">Making LAMMPS</span></a> section for more info.</p>
<p>The ReaxFF potential files provided with LAMMPS in the potentials
directory are parameterized for real <aclass="reference internal"href="units.html"><spanclass="doc">units</span></a>. You can use
the ReaxFF potential with any LAMMPS units, but you would need to
create your own potential file with coefficients listed in the
appropriate units if your simulation doesn’t use “real” units.</p>
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