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149 lines
5.9 KiB
HTML
<HTML>
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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>
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<H3>pair_style reax command
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</H3>
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<P><B>Syntax:</B>
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</P>
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<PRE>pair_style reax hbcut precision
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</PRE>
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<UL><LI>hbcut = hydrogen-bond cutoff (distance units)
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<LI>precision = precision for charge equilibration
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</UL>
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<P><B>Examples:</B>
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</P>
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<PRE>pair_style reax
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pair_style reax 10.0 1.0e-5
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pair_coeff * * ffield.reax 3 1 2 2
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</PRE>
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<P><B>Description:</B>
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</P>
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<P>The pair style 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 implemented
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in LAMMPS uses the functional forms documented in the
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supplemental information of the following paper: <A HREF = "#Chenoweth_2008">(Chenoweth et al.,
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2008)</A>.
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</P>
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<P>LAMMPS requires that a file called ffield.reax be provided, containing the
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ReaxFF parameters for each atom type, bond type, etc. The format is identical to
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the ffield file used by van Duin and co-workers. The filename is also
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reuqired as an argument in the pair_coeff command. Any value other than
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ffield.reax will be rejected (see below).
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</P>
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<P>LAMMPS provides several different versions of ffield.reax
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in its potentials dir, each called potentials/ffield.reax.label.
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These are documented in potentials/README.reax.
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The default ffield.reax contains parameterizations for the following
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elements: C, H, O, N, S. You can use one of the other files in place of it,
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or obtain a file from elsewhere.
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</P>
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<P>The <I>hbcut</I> and <I>precision</I> settings are optional arguments. If
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neither is provided, default settings are used: <I>hbcut</I> = 10 (which is
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Angstroms in real units) and <I>precision</I> = 1.0e-6 (one part in 10^6).
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If you wish to override either of these defaults, then both settings
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must be specified.
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</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 potential performs charge equilibration. See the
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<A HREF = "atom_style.html">atom_style</A> and <A HREF = "read_data.html">read_data</A> commands
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for details on how to specify charges.
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</P>
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<P>The thermo variable <I>evdwl</I> 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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<I>ecoul</I>. The output of these quantities is controlled by the
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<A HREF = "thermo.html">thermo</A> command.
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</P>
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<P>Only a single pair_coeff command is used with the <I>reax</I> 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:
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</P>
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<UL><LI>filename
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<LI>N indices = mapping of ReaxFF elements to atom types
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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.
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</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.
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</P>
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<P>In the ReaxFF potential file, near the top, is a section that contains
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element names, each with a couple dozen numeric parameters. The default
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ffield.reax provided with LAMMPS lists 5 elements: C, H, O, N, S.
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Think of these as numbered 1 to 6. Each of the N indices you specify for the N atom types of
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LAMMPS atoms must be an integer from 1 to 6. Atoms with LAMMPS type 1
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will be mapped to whatever element you specify as the first index
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value, etc.
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</P>
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<P>In the pair_coeff example above, the LAMMPS simulation has 4 atoms
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types and they are set as follows:
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</P>
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<PRE>type 1 = O
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type 2 = C
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type 3 = H
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type 4 = H
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</PRE>
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<HR>
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<P><B>Mixing, shift, table, tail correction, restart, rRESPA info</B>:
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</P>
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<P>This pair style does not support the <A HREF = "pair_modify.html">pair_modify</A>
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mix, shift, table, and tail options.
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</P>
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<P>This pair style does not write its information to <A HREF = "restart.html">binary restart
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files</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.
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</P>
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<P>This pair style can only be used via the <I>pair</I> keyword of the
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<A HREF = "run_style.html">run_style respa</A> command. It does not support the
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<I>inner</I>, <I>middle</I>, <I>outer</I> keywords.
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</P>
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<P><B>Restrictions:</B>
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</P>
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<P>This pair style is part of the "reax" package. It is only enabled if
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LAMMPS was built with that package, which also requires the REAX
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library be built and linked with LAMMPS. See the <A HREF = "Section_start.html#2_3">Making
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LAMMPS</A> section for more info.
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</P>
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<P>The ffield.reax potential file provided with LAMMPS in the potentials
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directory is parameterized for real <A HREF = "units.html">units</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. This
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would be somewhat tricky, so contact the LAMMPS authors if you wish to
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do this.
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</P>
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<P><B>Related commands:</B>
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</P>
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<P><A HREF = "pair_coeff.html">pair_coeff</A>
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</P>
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<P><B>Default:</B> none
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</P>
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<HR>
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<A NAME = "Chenoweth_2008"></A>
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<P><B>(Chenoweth)</B> Chenoweth, van Duin and Goddard III,
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Journal of Physical Chemistry A, 112, 1040-1053 (2008).
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</P>
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</HTML>
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