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<CENTER>M<A HREF = "http://lammps.sandia.gov">LAMMPS WWW Site</A> - <A HREF = "Manual.html">LAMMPS Documentation</A> - <A HREF = "Section_commands.html#comm">LAMMPS Commands</A>
</CENTER>
<HR>
<H3>pair_style reax command
</H3>
<P><B>Syntax:</B>
</P>
<PRE>pair_style reax hbcut precision
</PRE>
<UL><LI>hbcut = hydrogen-bond cutoff (distance units)
<LI>precision = precision for charge equilibration
</UL>
<P><B>Examples:</B>
</P>
<PRE>pair_style reax
pair_style reax 10.0 1.0e-5
pair_coeff * * ffield.reax 3 1 2 2
pair_coeff * * ffield.reax 3 NULL NULL 3
</PRE>
<P><B>Description:</B>
</P>
<P>The pair style 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: <A HREF = "#Chenoweth_2008">(Chenoweth et al.,
2008)</A>.
</P>
<P>LAMMPS requires that a file called ffield.reax be provided, containing the
ReaxFF parameters for each atom type, bond type, etc. The format is identical to
the ffield file used by van Duin and co-workers. The filename is also
reuqired as an argument in the pair_coeff command. Any value other than
ffield.reax will be rejected (see below).
</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, S. You can use one of the other files in place of it,
or obtain a file from elsewhere.
</P>
<P>The <I>hbcut</I> and <I>precision</I> settings are optional arguments. If
neither is provided, default settings are used: <I>hbcut</I> = 10 (which is
Angstroms in real units) and <I>precision</I> = 1.0e-6 (one part in 10^6).
If you wish to override either of these defaults, then both settings
must be specified.
</P>
<P>Use of this pair style requires that a charge be defined for every
atom since the potential performs charge equilibration. See the
<A HREF = "atom_style.html">atom_style</A> and <A HREF = "read_data.html">read_data</A> commands
for details on how to specify charges.
</P>
<P>The thermo variable <I>evdwl</I> 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
<I>ecoul</I>. The output of these quantities is controlled by the
<A HREF = "thermo.html">thermo</A> command.
</P>
<P>Only a single pair_coeff command is used with the <I>reax</I> style which
specifies a ReaxFF 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:
</P>
<UL><LI>filename
<LI>N indices = mapping of ReaxFF elements to atom types
</UL>
<P>The specification of the filename and the mapping of LAMMPS atom types
recognized by the ReaxFF is done differently than for other LAMMPS
potentials, due to the non-portable difficulty of passing character
strings (e.g. filename, element names) between C++ and Fortran.
</P>
<P>The filename has to be "ffield.reax" and it has to exist in the
directory you are running LAMMPS in. This means you cannot prepend a
path to the file in the potentials dir. Rather, you should copy that
file into the directory you are running from. If you wish to use
another ReaxFF potential file, then name it "ffield.reax" and put it
in the directory you run from.
</P>
<P>In the ReaxFF potential file, near the top, is a section that contains
element names, each with a couple dozen numeric parameters. The
default ffield.reax provided with LAMMPS lists 5 elements: C, H, O, N,
S. Think of these as numbered 1 to 5. When using this file each of
the N indices you specify for the N atom types of LAMMPS atoms must be
an integer from 1 to 5. Atoms with LAMMPS type 1 will be mapped to
whatever element you specify as the first index value, etc. If a
mapping value is specified as NULL, the mapping is not performed.
This can be used when a ReaxFF potential is used as part of the
<I>hybrid</I> pair style. The NULL values are placeholders for atom types
that will be used with other potentials.
</P>
<P>In the pair_coeff example above with 3 1 2 2, the LAMMPS simulation
has 4 atoms types and they are set as follows:
</P>
<PRE>type 1 = O
type 2 = C
type 3 = H
type 4 = H
</PRE>
<HR>
<P><B>Mixing, shift, table, tail correction, restart, rRESPA info</B>:
</P>
<P>This pair style does not support the <A HREF = "pair_modify.html">pair_modify</A>
mix, shift, table, and tail options.
</P>
<P>This pair style does not write its information to <A HREF = "restart.html">binary restart
files</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 <I>pair</I> keyword of the
<A HREF = "run_style.html">run_style respa</A> command. It does not support the
<I>inner</I>, <I>middle</I>, <I>outer</I> keywords.
</P>
<P><B>Restrictions:</B>
</P>
<P>This pair style is part of the "reax" package. It is only enabled if
LAMMPS was built with that package, which also requires the REAX
library be built and linked with LAMMPS. See the <A HREF = "Section_start.html#2_3">Making
LAMMPS</A> section for more info.
</P>
<P>The ffield.reax potential file provided with LAMMPS in the potentials
directory is parameterized for real <A HREF = "units.html">units</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. This
would be somewhat tricky, so contact the LAMMPS authors if you wish to
do this.
</P>
<P><B>Related commands:</B>
</P>
<P><A HREF = "pair_coeff.html">pair_coeff</A>
</P>
<P><B>Default:</B> none
</P>
<HR>
<A NAME = "Chenoweth_2008"></A>
<P><B>(Chenoweth)</B> Chenoweth, van Duin and Goddard III,
Journal of Physical Chemistry A, 112, 1040-1053 (2008).
</P>
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