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<li class="toctree-l1"><a class="reference internal" href="Section_intro.html">1. Introduction</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Section_errors.html">12. Errors</a></li>
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<li>pair_style reax command</li>
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<div role="main" class="document" itemscope="itemscope" itemtype="http://schema.org/Article">
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<div class="section" id="pair-style-reax-command">
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<span id="index-0"></span><h1>pair_style reax command</h1>
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<div class="section" id="syntax">
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<h2>Syntax</h2>
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<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">pair_style</span> <span class="n">reax</span> <span class="n">hbcut</span> <span class="n">hbnewflag</span> <span class="n">tripflag</span> <span class="n">precision</span>
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</pre></div>
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</div>
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<ul class="simple">
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<li>hbcut = hydrogen-bond cutoff (optional) (distance units)</li>
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<li>hbnewflag = use old or new hbond function style (0 or 1) (optional)</li>
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<li>tripflag = apply stabilization to all triple bonds (0 or 1) (optional)</li>
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<li>precision = precision for charge equilibration (optional)</li>
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</ul>
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</div>
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<div class="section" id="examples">
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<h2>Examples</h2>
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<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">pair_style</span> <span class="n">reax</span>
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<span class="n">pair_style</span> <span class="n">reax</span> <span class="mf">10.0</span> <span class="mi">0</span> <span class="mi">1</span> <span class="mf">1.0e-5</span>
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<span class="n">pair_coeff</span> <span class="o">*</span> <span class="o">*</span> <span class="n">ffield</span><span class="o">.</span><span class="n">reax</span> <span class="mi">3</span> <span class="mi">1</span> <span class="mi">2</span> <span class="mi">2</span>
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<span class="n">pair_coeff</span> <span class="o">*</span> <span class="o">*</span> <span class="n">ffield</span><span class="o">.</span><span class="n">reax</span> <span class="mi">3</span> <span class="n">NULL</span> <span class="n">NULL</span> <span class="mi">3</span>
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</pre></div>
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</div>
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</div>
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<div class="section" id="description">
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<h2>Description</h2>
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<p>Style <em>reax</em> computes the ReaxFF potential of van Duin, Goddard and
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co-workers. ReaxFF uses distance-dependent bond-order functions to
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represent the contributions of chemical bonding to the potential
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energy. There is more than one version of ReaxFF. The version
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implemented in LAMMPS uses the functional forms documented in the
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supplemental information of the following paper:
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<a class="reference internal" href="pair_reax_c.html#chenoweth-2008"><span class="std std-ref">(Chenoweth)</span></a>. The version integrated into LAMMPS matches
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the most up-to-date version of ReaxFF as of summer 2010.</p>
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<p>WARNING: pair style reax is now deprecated and will soon be retired. Users
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should switch to <a class="reference internal" href="pair_reax_c.html"><span class="doc">pair_style reax/c</span></a>. The <em>reax</em> style
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differs from the <em>reax/c</em> style in the lo-level implementation details.
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The <em>reax</em> style is a
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Fortran library, linked to LAMMPS. The <em>reax/c</em> style was initially
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implemented as stand-alone C code and is now integrated into LAMMPS as
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a package.</p>
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<p>LAMMPS requires that a file called ffield.reax be provided, containing
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the ReaxFF parameters for each atom type, bond type, etc. The format
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is identical to the ffield file used by van Duin and co-workers. The
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filename is required as an argument in the pair_coeff command. Any
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value other than “ffield.reax” will be rejected (see below).</p>
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<p>LAMMPS provides several different versions of ffield.reax in its
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potentials dir, each called potentials/ffield.reax.label. These are
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documented in potentials/README.reax. The default ffield.reax
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contains parameterizations for the following elements: C, H, O, N.</p>
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<div class="admonition note">
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<p class="first admonition-title">Note</p>
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<p class="last">We do not distribute a wide variety of ReaxFF force field files
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with LAMMPS. Adri van Duin’s group at PSU is the central repository
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for this kind of data as they are continuously deriving and updating
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parameterizations for different classes of materials. You can submit
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a contact request at the Materials Computation Center (MCC) website
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<a class="reference external" href="https://www.mri.psu.edu/materials-computation-center/connect-mcc">https://www.mri.psu.edu/materials-computation-center/connect-mcc</a>,
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describing the material(s) you are interested in modeling with ReaxFF.
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They can tell
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you what is currently available or what it would take to create a
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suitable ReaxFF parameterization.</p>
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</div>
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<p>The format of these files is identical to that used originally by van
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Duin. We have tested the accuracy of <em>pair_style reax</em> potential
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against the original ReaxFF code for the systems mentioned above. You
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can use other ffield files for specific chemical systems that may be
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available elsewhere (but note that their accuracy may not have been
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tested).</p>
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<p>The <em>hbcut</em>, <em>hbnewflag</em>, <em>tripflag</em>, and <em>precision</em> settings are
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optional arguments. If none are provided, default settings are used:
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<em>hbcut</em> = 6 (which is Angstroms in real units), <em>hbnewflag</em> = 1 (use
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new hbond function style), <em>tripflag</em> = 1 (apply stabilization to all
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triple bonds), and <em>precision</em> = 1.0e-6 (one part in 10^6). If you
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wish to override any of these defaults, then all of the settings must
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be specified.</p>
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<p>Two examples using <em>pair_style reax</em> are provided in the examples/reax
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sub-directory, along with corresponding examples for
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<a class="reference internal" href="pair_reax_c.html"><span class="doc">pair_style reax/c</span></a>. Note that while the energy and force
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calculated by both of these pair styles match very closely, the
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contributions due to the valence angles differ slightly due to
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the fact that with <em>pair_style reax/c</em> the default value of <em>thb_cutoff_sq</em>
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is 0.00001, while for <em>pair_style reax</em> it is hard-coded to be 0.001.</p>
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<p>Use of this pair style requires that a charge be defined for every
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atom since the <em>reax</em> pair style performs a charge equilibration (QEq)
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calculation. See the <a class="reference internal" href="atom_style.html"><span class="doc">atom_style</span></a> and
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<a class="reference internal" href="read_data.html"><span class="doc">read_data</span></a> commands for details on how to specify
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charges.</p>
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<p>The thermo variable <em>evdwl</em> stores the sum of all the ReaxFF potential
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energy contributions, with the exception of the Coulombic and charge
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equilibration contributions which are stored in the thermo variable
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<em>ecoul</em>. The output of these quantities is controlled by the
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<a class="reference internal" href="thermo.html"><span class="doc">thermo</span></a> command.</p>
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<p>This pair style tallies a breakdown of the total ReaxFF potential
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energy into sub-categories, which can be accessed via the <a class="reference internal" href="compute_pair.html"><span class="doc">compute pair</span></a> command as a vector of values of length 14.
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The 14 values correspond to the following sub-categories (the variable
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names in italics match those used in the ReaxFF FORTRAN library):</p>
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<ol class="arabic simple">
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<li><em>eb</em> = bond energy</li>
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<li><em>ea</em> = atom energy</li>
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<li><em>elp</em> = lone-pair energy</li>
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<li><em>emol</em> = molecule energy (always 0.0)</li>
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<li><em>ev</em> = valence angle energy</li>
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<li><em>epen</em> = double-bond valence angle penalty</li>
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<li><em>ecoa</em> = valence angle conjugation energy</li>
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<li><em>ehb</em> = hydrogen bond energy</li>
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<li><em>et</em> = torsion energy</li>
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<li><em>eco</em> = conjugation energy</li>
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<li><em>ew</em> = van der Waals energy</li>
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<li><em>ep</em> = Coulomb energy</li>
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<li><em>efi</em> = electric field energy (always 0.0)</li>
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<li><em>eqeq</em> = charge equilibration energy</li>
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</ol>
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<p>To print these quantities to the log file (with descriptive column
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headings) the following commands could be included in an input script:</p>
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<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">compute</span> <span class="n">reax</span> <span class="nb">all</span> <span class="n">pair</span> <span class="n">reax</span>
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<span class="n">variable</span> <span class="n">eb</span> <span class="n">equal</span> <span class="n">c_reax</span><span class="p">[</span><span class="mi">1</span><span class="p">]</span>
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<span class="n">variable</span> <span class="n">ea</span> <span class="n">equal</span> <span class="n">c_reax</span><span class="p">[</span><span class="mi">2</span><span class="p">]</span>
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<span class="o">...</span>
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<span class="n">variable</span> <span class="n">eqeq</span> <span class="n">equal</span> <span class="n">c_reax</span><span class="p">[</span><span class="mi">14</span><span class="p">]</span>
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<span class="n">thermo_style</span> <span class="n">custom</span> <span class="n">step</span> <span class="n">temp</span> <span class="n">epair</span> <span class="n">v_eb</span> <span class="n">v_ea</span> <span class="o">...</span> <span class="n">v_eqeq</span>
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</pre></div>
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</div>
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<p>Only a single pair_coeff command is used with the <em>reax</em> style which
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specifies a ReaxFF potential file with parameters for all needed
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elements. These are mapped to LAMMPS atom types by specifying N
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additional arguments after the filename in the pair_coeff command,
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where N is the number of LAMMPS atom types:</p>
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<ul class="simple">
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<li>filename</li>
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<li>N indices = mapping of ReaxFF elements to atom types</li>
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</ul>
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<p>The specification of the filename and the mapping of LAMMPS atom types
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recognized by the ReaxFF is done differently than for other LAMMPS
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potentials, due to the non-portable difficulty of passing character
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strings (e.g. filename, element names) between C++ and Fortran.</p>
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<p>The filename has to be “ffield.reax” and it has to exist in the
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directory you are running LAMMPS in. This means you cannot prepend a
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path to the file in the potentials dir. Rather, you should copy that
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file into the directory you are running from. If you wish to use
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another ReaxFF potential file, then name it “ffield.reax” and put it
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in the directory you run from.</p>
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<p>In the ReaxFF potential file, near the top, after the general
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parameters, is the atomic parameters section that contains element
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names, each with a couple dozen numeric parameters. If there are M
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elements specified in the <em>ffield</em> file, think of these as numbered 1
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to M. Each of the N indices you specify for the N atom types of LAMMPS
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atoms must be an integer from 1 to M. Atoms with LAMMPS type 1 will
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be mapped to whatever element you specify as the first index value,
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etc. If a mapping value is specified as NULL, the mapping is not
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performed. This can be used when a ReaxFF potential is used as part
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of the <em>hybrid</em> pair style. The NULL values are placeholders for atom
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types that will be used with other potentials.</p>
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<div class="admonition note">
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<p class="first admonition-title">Note</p>
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<p class="last">Currently the reax pair style cannot be used as part of the
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<em>hybrid</em> pair style. Some additional changes still need to be made to
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enable this.</p>
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</div>
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<p>As an example, say your LAMMPS simulation has 4 atom types and the
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elements are ordered as C, H, O, N in the <em>ffield</em> file. If you want
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the LAMMPS atom type 1 and 2 to be C, type 3 to be N, and type 4 to be
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H, you would use the following pair_coeff command:</p>
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<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">pair_coeff</span> <span class="o">*</span> <span class="o">*</span> <span class="n">ffield</span><span class="o">.</span><span class="n">reax</span> <span class="mi">1</span> <span class="mi">1</span> <span class="mi">4</span> <span class="mi">2</span>
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</pre></div>
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</div>
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<hr class="docutils" />
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<p><strong>Mixing, shift, table, tail correction, restart, rRESPA info</strong>:</p>
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<p>This pair style does not support the <a class="reference internal" href="pair_modify.html"><span class="doc">pair_modify</span></a>
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mix, shift, table, and tail options.</p>
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<p>This pair style does not write its information to <a class="reference internal" href="restart.html"><span class="doc">binary restart files</span></a>, since it is stored in potential files. Thus, you
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need to re-specify the pair_style and pair_coeff commands in an input
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script that reads a restart file.</p>
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<p>This pair style can only be used via the <em>pair</em> keyword of the
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<a class="reference internal" href="run_style.html"><span class="doc">run_style respa</span></a> command. It does not support the
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<em>inner</em>, <em>middle</em>, <em>outer</em> keywords.</p>
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</div>
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<div class="section" id="restrictions">
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<h2>Restrictions</h2>
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<p>The ReaxFF potential files provided with LAMMPS in the potentials
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directory are parameterized for real <a class="reference internal" href="units.html"><span class="doc">units</span></a>. You can use
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the ReaxFF potential with any LAMMPS units, but you would need to
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create your own potential file with coefficients listed in the
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appropriate units if your simulation doesn’t use “real” units.</p>
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</div>
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<div class="section" id="related-commands">
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<h2>Related commands</h2>
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<p><a class="reference internal" href="pair_coeff.html"><span class="doc">pair_coeff</span></a>, <a class="reference internal" href="pair_reax_c.html"><span class="doc">pair_style reax/c</span></a>,
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<a class="reference internal" href="fix_reax_bonds.html"><span class="doc">fix_reax_bonds</span></a></p>
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</div>
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<div class="section" id="default">
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<h2>Default</h2>
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<p>The keyword defaults are <em>hbcut</em> = 6, <em>hbnewflag</em> = 1, <em>tripflag</em> = 1,
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<em>precision</em> = 1.0e-6.</p>
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<hr class="docutils" />
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<p id="chenoweth-2008"><strong>(Chenoweth_2008)</strong> Chenoweth, van Duin and Goddard,
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Journal of Physical Chemistry A, 112, 1040-1053 (2008).</p>
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