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203 lines
8.6 KiB
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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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<H3>pair_style airebo command
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</H3>
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<H3>pair_style airebo/omp command
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</H3>
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<H3>pair_style rebo command
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</H3>
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<H3>pair_style rebo/omp 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 style cutoff LJ_flag TORSION_flag
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</PRE>
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<UL><LI>style = <I>airebo</I> or <I>rebo</I>
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<LI>cutoff = LJ cutoff (sigma scale factor) (AIREBO only)
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<LI>LJ_flag = 0/1 to turn off/on the LJ term (AIREBO only, optional)
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<LI>TORSION_flag = 0/1 to turn off/on the torsion term (AIREBO only, 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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<PRE>pair_style rebo
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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. Note that this is the initial
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formulation of AIREBO from 2000, not the later formulation. The
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<I>rebo</I> pair style computes the Reactive Empirical Bond Order (REBO)
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Potential of <A HREF = "#Brenner">(Brenner)</A>. Note that this is the so-called
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2nd generation REBO from 2002, not the original REBO from 1990. As
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discussed below, 2nd generation REBO is closely related to the intial
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AIREBO; it is just a subset of the potential energy terms.
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</P>
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<P>The AIREBO potential 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. For the <I>airebo</I> style, if
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the two optional flag arguments to the pair_style command are
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included, the LJ and torsional terms can be turned off. Note that
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both or neither of the flags must be included. If both of the LJ an
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torsional terms are turned off, it becomes the 2nd-generation REBO
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potential, with a small caveat on the spline fitting procedure
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mentioned below. This can be specified directly as pair_style rebo
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with no additional arguments.
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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 dihedral angle preferences in hydrocarbon configurations.
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</P>
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<HR>
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<P>Only a single pair_coeff command is used with the <I>airebo</I> or <I>rebo</I>
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style which specifies an AIREBO potential file with parameters for C
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and H. Note that the <I>rebo</I> style in LAMMPS uses the same
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AIREBO-formatted potential file. These are mapped to LAMMPS atom
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types by specifying N additional arguments after the filename in the
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pair_coeff command, 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 element names = mapping of AIREBO elements to atom types
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</UL>
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<P>See the <A HREF = "pair_coeff.html">pair_coeff</A> doc page for alternate ways
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to specify the path for the potential file.
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</P>
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<P>As an example, if your LAMMPS simulation has 4 atom types and you want
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the 1st 3 to be C, and the 4th to be H, you would use the following
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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.
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</P>
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<HR>
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<P>Styles with a <I>cuda</I>, <I>gpu</I>, <I>omp</I>, or <I>opt</I> suffix are functionally
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the same as the corresponding style without the suffix. They have
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been optimized to run faster, depending on your available hardware, as
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discussed in <A HREF = "Section_accelerate.html">Section_accelerate</A> of the
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manual. The accelerated styles take the same arguments and should
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produce the same results, except for round-off and precision issues.
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</P>
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<P>These accelerated styles are part of the USER-CUDA, GPU, USER-OMP and OPT
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packages, respectively. They are only enabled if LAMMPS was built with
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those packages. See the <A HREF = "Section_start.html#start_3">Making LAMMPS</A>
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section for more info.
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</P>
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<P>You can specify the accelerated styles explicitly in your input script
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by including their suffix, or you can use the <A HREF = "Section_start.html#start_7">-suffix command-line
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switch</A> when you invoke LAMMPS, or you can
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use the <A HREF = "suffix.html">suffix</A> command in your input script.
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</P>
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<P>See <A HREF = "Section_accelerate.html">Section_accelerate</A> of the manual for
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more instructions on how to use the accelerated styles effectively.
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</P>
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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>These pair styles do 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>These pair styles do not write their 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>These pair styles 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. They do 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>These pair styles are part of the MANYBODY package. They are only
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enabled if LAMMPS was built with that package (which it is by
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default). See the <A HREF = "Section_start.html#start_3">Making LAMMPS</A> section
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for more info.
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</P>
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<P>These pair potentials require 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 or REBO potential with any LAMMPS units, but you would need
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to create your own AIREBO potential file with coefficients listed in
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the 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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