forked from lijiext/lammps
154 lines
6.3 KiB
Plaintext
154 lines
6.3 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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fix reax/c/species command :h3
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[Syntax:]
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fix ID group-ID reax/c/species Nevery Nrepeat Nfreq filename keyword value ... :pre
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ID, group-ID are documented in "fix"_fix.html command :ulb,l
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reax/c/species = style name of this command :l
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Nevery = sample bond-order every this many timesteps :l
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Nrepeat = # of bond-order samples used for calculating averages :l
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Nfreq = calculate average bond-order every this many timesteps :l
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filename = name of output file :l
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zero or more keyword/value pairs may be appended :l
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keyword = {cutoff} or {element} or {position} :l
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{cutoff} value = I J Cutoff
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I, J = atom types
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Cutoff = Bond-order cutoff value for this pair of atom types
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{element} value = Element1, Element2, ...
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{position} value = posfreq filepos
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posfreq = write position files every this many timestep
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filepos = name of position output file :pre
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:ule
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[Examples:]
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fix 1 all reax/c/species 10 10 100 species.out
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fix 1 all reax/c/species 1 2 20 species.out cutoff 1 1 0.40 cutoff 1 2 0.55
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fix 1 all reax/c/species 1 100 100 species.out element Au O H position 1000 AuOH.pos :pre
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[Description:]
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Write out the chemical species information computed by the ReaxFF
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potential specified by "pair_style reax/c"_pair_reax_c.html.
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Bond-order values (either averaged or instantaneous, depending on
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value of {Nrepeat}) are used to determine chemical bonds. Every
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{Nfreq} timesteps, chemical species information is written to
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{filename} as a two line output. The first line is a header
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containing labels. The second line consists of the following:
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timestep, total number of molecules, total number of distinct species,
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number of molecules of each species. In this context, "species" means
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a unique molecule. The chemical formula of each species is given in
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the first line.
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Optional keyword {cutoff} can be assigned to change the minimum
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bond-order values used in identifying chemical bonds between pairs of
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atoms. Bond-order cutoffs should be carefully chosen, as bond-order
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cutoffs that are too small may include too many bonds (which will
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result in an error), while cutoffs that are too large will result in
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fragmented molecules. The default cutoff of 0.3 usually gives good
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results.
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The optional keyword {element} can be used to specify the chemical
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symbol printed for each LAMMPS atom type. The number of symbols must
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match the number of LAMMPS atom types and each symbol must consist of
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1 or 2 alphanumeric characters. Normally, these symbols should be
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chosen to match the chemical identity of each LAMMPS atom type, as
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specified using the "reax/c pair_coeff"_pair_reax_c.html command and
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the ReaxFF force field file.
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The optional keyword {position} writes center-of-mass positions of
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each identified molecules to file {filepos} every {posfreq} timesteps.
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The first line contains information on timestep, total number of
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molecules, total number of distinct species, and box dimensions. The
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second line is a header containing labels. From the third line
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downward, each molecule writes a line of output containing the
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following information: molecule ID, number of atoms in this molecule,
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chemical formula, total charge, and center-of-mass xyz positions of
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this molecule. The xyz positions are in fractional coordinates
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relative to the box dimensions.
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For the keyword {position}, the {filepos} is the name of the output
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file. It can contain the wildcard character "*". If the "*"
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character appears in {filepos}, then one file per snapshot is written
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at {posfreq} and the "*" character is replaced with the timestep
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value. For example, AuO.pos.* becomes AuO.pos.0, AuO.pos.1000, etc.
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:line
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The {Nevery}, {Nrepeat}, and {Nfreq} arguments specify on what
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timesteps the bond-order values are sampled to get the average bond
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order. The species analysis is performed using the average bond-order
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on timesteps that are a multiple of {Nfreq}. The average is over
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{Nrepeat} bond-order samples, computed in the preceding portion of the
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simulation every {Nevery} timesteps. {Nfreq} must be a multiple of
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{Nevery} and {Nevery} must be non-zero even if {Nrepeat} is 1. Also,
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the timesteps contributing to the average bond-order cannot overlap,
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i.e. Nfreq > (Nrepeat-1)*Nevery is required.
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For example, if Nevery=2, Nrepeat=6, and Nfreq=100, then bond-order
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values on timesteps 90,92,94,96,98,100 will be used to compute the
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average bond-order for the species analysis output on timestep 100.
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:line
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[Restart, fix_modify, output, run start/stop, minimize info:]
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No information about this fix is written to "binary restart
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files"_restart.html. None of the "fix_modify"_fix_modify.html options
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are relevant to this fix.
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This fix computes both a global vector of length 2 and a per-atom
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vector, either of which can be accessed by various "output
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commands"_Section_howto.html#howto_15. The values in the global
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vector are "intensive".
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The 2 values in the global vector are as follows:
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1 = total number of molecules
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2 = total number of distinct species :ul
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The per-atom vector stores the molecule ID for each atom as identified
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by the fix. If an atom is not in a molecule, its ID will be 0.
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For atoms in the same molecule, the molecule ID for all of them
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will be the same and will be equal to the smallest atom ID of
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any atom in the molecule.
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No parameter of this fix can be used with the {start/stop} keywords of
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the "run"_run.html command. This fix is not invoked during "energy
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minimization"_minimize.html.
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[Restrictions:]
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The fix species currently only works with
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"pair_style reax/c"_pair_reax_c.html and it requires that the "pair_style
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reax/c"_pair_reax_c.html be invoked. This fix is part of the
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USER-REAXC package. It is only enabled if LAMMPS was built with that
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package. See the "Making LAMMPS"_Section_start.html#start_3 section
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for more info.
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It should be possible to extend it to other reactive pair_styles (such as
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"rebo"_pair_airebo.html, "airebo"_pair_airebo.html,
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"comb"_pair_comb.html, and "bop"_pair_bop.html), but this has not yet been done.
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[Related commands:]
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"pair_style reax/c"_pair_reax_c.html, "fix
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reax/bonds"_fix_reax_bonds.html
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[Default:]
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The default values for bond-order cutoffs are 0.3 for all I-J pairs. The
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default element symbols are C, H, O, N. Position files are not written
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by default.
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