forked from lijiext/lammps
164 lines
6.6 KiB
Plaintext
164 lines
6.6 KiB
Plaintext
"LAMMPS WWW Site"_lws - "LAMMPS Documentation"_ld - "LAMMPS Commands"_lc :c
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:link(lws,http://lammps.sandia.gov)
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:link(ld,Manual.html)
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:link(lc,Section_commands.html#comm)
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:line
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pair_style airebo command :h3
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pair_style rebo command :h3
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[Syntax:]
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pair_style style cutoff LJ_flag TORSION_flag :pre
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style = {airebo} or {rebo}
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cutoff = LJ cutoff (sigma scale factor) (AIREBO only)
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LJ_flag = 0/1 to turn off/on the LJ term (AIREBO only, optional)
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TORSION_flag = 0/1 to turn off/on the torsion term (AIREBO only, optional) :ul
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[Examples:]
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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 :pre
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pair_style rebo
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pair_coeff * * ../potentials/CH.airebo H C :pre
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[Description:]
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The {airebo} pair style computes the Adaptive Intermolecular Reactive
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Empirical Bond Order (AIREBO) Potential of "(Stuart)"_#Stuart 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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{rebo} pair style computes the Reactive Empirical Bond Order (REBO)
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Potential of "(Brenner)"_#Brenner. 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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The AIREBO potential consists of three terms:
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:c,image(Eqs/pair_airebo.jpg)
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By default, all three terms are included. For the {airebo} 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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The detailed formulas for this potential are given in
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"(Stuart)"_#Stuart; here we provide only a brief description.
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The E_REBO term has the same functional form as the hydrocarbon REBO
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potential developed in "(Brenner)"_#Brenner. 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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The E_LJ term adds longer-ranged interactions (2 < r < cutoff) using a
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form similar to the standard "Lennard Jones potential"_pair_lj.html.
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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 {cutoff} 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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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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Only a single pair_coeff command is used with the {airebo} or {rebo}
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style which specifies an AIREBO potential file with parameters for C
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and H. Note that the {rebo} 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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filename
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N element names = mapping of AIREBO elements to atom types :ul
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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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pair_coeff * * CH.airebo C C C H :pre
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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 {airebo} potential is used as part of the {hybrid} 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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The parameters/coefficients for the AIREBO potentials are listed in
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the CH.airebo file to agree with the original "(Stuart)"_#Stuart
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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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:line
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[Mixing, shift, table, tail correction, restart, rRESPA info]:
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These pair styles do not support the "pair_modify"_pair_modify.html
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mix, shift, table, and tail options.
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These pair styles do not write their information to "binary restart
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files"_restart.html, 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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These pair styles can only be used via the {pair} keyword of the
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"run_style respa"_run_style.html command. They do not support the
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{inner}, {middle}, {outer} keywords.
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[Restrictions:]
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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 "Making LAMMPS"_Section_start.html#start_3 section
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for more info.
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These pair potentials require the "newton"_newton.html setting to be
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"on" for pair interactions.
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The CH.airebo potential file provided with LAMMPS (see the potentials
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directory) is parameterized for metal "units"_units.html. 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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[Related commands:]
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"pair_coeff"_pair_coeff.html
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[Default:] none
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:line
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:link(Stuart)
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[(Stuart)] Stuart, Tutein, Harrison, J Chem Phys, 112, 6472-6486
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(2000).
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:link(Brenner)
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[(Brenner)] Brenner, Shenderova, Harrison, Stuart, Ni, Sinnott, J
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Physics: Condensed Matter, 14, 783-802 (2002).
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