2007-08-13 22:49:11 +08:00
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<CENTER><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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<HR>
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<H3>pair_style airebo 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 airebo cutoff LJ_flag TORSION_flag
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</PRE>
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<UL><LI>cutoff = LJ cutoff (sigma scale factor)
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<LI>LJ_flag = 0/1 to turn off/on the LJ term in AIREBO (optional)
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<LI>TORSION_flag = 0/1 to turn off/on the torsion term in AIREBO (optional)
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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 airebo 3.0
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pair_style airebo 2.5 1 0
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pair_coeff * * ../potentials/CH.airebo H C
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</PRE>
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<P><B>Description:</B>
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</P>
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<P>The <I>airebo</I> pair style computes the Adaptive Intermolecular Reactive
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Empirical Bond Order (AIREBO) Potential of <A HREF = "#Stuart">(Stuart)</A> for a
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system of carbon and/or hydrogen atoms. The potential
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consists of three terms:
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</P>
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<CENTER><IMG SRC = "Eqs/pair_airebo.jpg">
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</CENTER>
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<P>By default, all three terms are included. If the two optional flag
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arguments to the pair_style command are included, the LJ and torsional
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terms can be turned off. Note that both or neither of the flags must
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be included.
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</P>
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<P>The detailed formulas for this potential are given in
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<A HREF = "#Stuart">(Stuart)</A>; here we provide only a brief description.
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</P>
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<P>The E_REBO term has the same functional form as the hydrocarbon REBO
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potential developed in <A HREF = "#Brenner">(Brenner)</A>. The coefficients for
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E_REBO in AIREBO are essentially the same as Brenner's potential, but
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a few fitted spline values are slightly different. For most cases the
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E_REBO term in AIREBO will produce the same energies, forces and
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statistical averages as the original REBO potential from which it was
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derived. The E_REBO term in the AIREBO potential gives the model its
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reactive capabilities and only describes short-ranged C-C, C-H and H-H
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interactions (r < 2 Angstroms). These interactions have strong
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coordination-dependence through a bond order parameter, which adjusts
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the attraction between the I,J atoms based on the position of other
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nearby atoms and thus has 3- and 4-body dependence.
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</P>
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<P>The E_LJ term adds longer-ranged interactions (2 < r < cutoff) using a
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form similar to the standard <A HREF = "pair_lj.html">Lennard Jones potential</A>.
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The E_LJ term in AIREBO contains a series of switching functions so
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that the short-ranged LJ repulsion (1/r^12) does not interfere with
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the energetics captured by the E_REBO term. The extent of the E_LJ
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interactions is determined by the <I>cutoff</I> argument to the pair_style
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command which is a scale factor. For each type pair (C-C, C-H, H-H)
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the cutoff is obtained by multiplying the scale factor by the sigma
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value defined in the potential file for that type pair. In the
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standard AIREBO potential, sigma_CC = 3.4 Angstroms, so with a scale
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factor of 3.0 (the argument in pair_style), the resulting E_LJ cutoff
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would be 10.2 Angstroms.
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</P>
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<P>The E_TORSION term is an explicit 4-body potential that describes
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various diheadral angle preferences in hydrocarbon configurations.
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</P>
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<P>Only a single pair_coeff command is used with the <I>airebo</I> style which
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specifies an AIREBO potential file with parameters for C and H. These
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are mapped to LAMMPS atom types by specifying N additional arguments
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after the filename in the pair_coeff command, where N is the number of
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LAMMPS atom types:
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</P>
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<UL><LI>filename
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<LI>N element names = mapping of AIREBO elements to atom types
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</UL>
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<P>As an example, if your LAMMPS simulation has 4 atoms types and you
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want the 1st 3 to be C, and the 4th to be H, you would use the
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following pair_coeff command:
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</P>
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<PRE>pair_coeff * * CH.airebo C C C H
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</PRE>
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<P>The 1st 2 arguments must be * * so as to span all LAMMPS atom types.
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The first three C arguments map LAMMPS atom types 1,2,3 to the C
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element in the AIREBO file. The final H argument maps LAMMPS atom
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type 4 to the H element in the SW file. If a mapping value is
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specified as NULL, the mapping is not performed. This can be used
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when a <I>airebo</I> potential is used as part of the <I>hybrid</I> pair style.
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The NULL values are placeholders for atom types that will be used with
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other potentials.
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</P>
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<P>The parameters/coefficients for the AIREBO potentials are listed in
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the CH.airebo file to agree with the original <A HREF = "#Stuart">(Stuart)</A>
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paper. Thus the parameters are specific to this potential and the way
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it was fit, so modifying the file should be done cautiously. Also
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note that the E_LJ and E_TORSION terms in AIREBO are intended to be
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used with the E_REBO term and not as stand-alone potentials. Thus we
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don't suggest you use pair_style airebo with the E_REBO term turned
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off.
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</P>
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<HR>
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2007-10-04 08:21:14 +08:00
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<P><B>Mixing, shift, table, tail correction, per-atom energy/stress,
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restart, and rRESPA info</B>:
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2007-08-13 22:49:11 +08:00
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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 calculate per-atom energy and stress, as used
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by the <A HREF = "compute_epair_atom.html">compute epair/atom</A>, <A HREF = "compute_stress_atom.html">compute
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stress/atom</A>, and <A HREF = "dump.html">dump custom</A>
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commands.
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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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2007-10-04 08:21:14 +08:00
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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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2007-10-12 07:09:49 +08:00
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<I>inner</I>, <I>middle</I>, <I>outer</I> keywords.
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2007-10-04 08:21:14 +08:00
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</P>
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2007-08-13 22:49:11 +08:00
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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 "manybody" package. It is only enabled
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if LAMMPS was built with that package (which it is by default). See
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the <A HREF = "Section_start.html#2_3">Making LAMMPS</A> section for more info.
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</P>
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<P>This pair potential requires the <A HREF = "newton.html">newton</A> setting to be
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"on" for pair interactions.
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</P>
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<P>The CH.airebo potential file provided with LAMMPS (see the potentials
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directory) is parameterized for metal <A HREF = "units.html">units</A>. You can use
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the AIREBO potential with any LAMMPS units, but you would need to
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create your own AIREBO potential file with coefficients listed in the
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appropriate units if your simulation doesn't use "metal" units.
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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 = "Stuart"></A>
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<P><B>(Stuart)</B> Stuart, Tutein, Harrison, J Chem Phys, 112, 6472-6486
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(2000).
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</P>
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<A NAME = "Brenner"></A>
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<P><B>(Brenner)</B> Brenner, Shenderova, Harrison, Stuart, Ni, Sinnott, J
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Physics: Condensed Matter, 14, 783-802 (2002).
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</P>
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</HTML>
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