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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>
</CENTER>
<HR>
<H3>fix species command
</H3>
<P><B>Syntax:</B>
</P>
<PRE>fix ID group-ID species Nevery Nrepeat Nfreq filename keyword value ...
</PRE>
<UL><LI>ID, group-ID are documented in <A HREF = "fix.html">fix</A> command
<LI>species = style name of this command
<LI>Nevery = sample bond-order every this many timesteps
<LI>Nrepeat = # of bond-order samples used for calculating averages
<LI>Nfreq = calculate average bond-order every this many timesteps
<LI>filename = name of output file
<LI>zero or more keyword/value pairs may be appended
<LI>keyword = <I>cutoff</I> or <I>element</I>
<PRE> <I>cutoff</I> value = I J Cutoff
I, J = atom types
Cutoff = Bond-order cutoff value for this pair of atom types:pre
<I>element</I> value = Element1, Element2, ...
</PRE>
</UL>
<P><B>Examples:</B>
</P>
<PRE>fix 1 all species 10 10 100 species.out
fix 1 all species 1 2 20 species.out cutoff 1 1 0.40 1 2 0.55
fix 1 all species 10 10 100 species.out element Au O H
</PRE>
<P><B>Description:</B>
</P>
<P>Write out the chemical species information computed by the ReaxFF potential
specified by <A HREF = "pair_reax_c.html">pair_style reax/c</A>. Bond-order values
(either averaged or instantaneous, depending on value of <I>Nrepeat</I>)
are used to determine chemical bonds. Every <I>Nfreq</I> timesteps,
chemical species information is written to <I>filename</I> as a two lines.
The first line is a header containing labels. The second line consists
of the following: timestep, total number of molecules,
total number of distinct species, number of molecules of each species.
The chemical formula of each species is given in the first line.
</P>
<P>Optional keyword <I>cutoff</I> can be assigned to change the minimum bond-order values
used in identifying chemical bonds between pairs of atoms. Bond-order cutoffs
should be carefully chosen, as bond-order cutoffs that are too small may include
too many bonds (which will result in an error), while too-large cutoffs will
result in fragmented molecules. The default cutoff of 0.3 usually gives good
estimate.
</P>
<P>Optional keyword <I>element</I> can be used to specify the chemical symbol printed for
each LAMMPS atom type. The number of symbols must match the number of LAMMPS atom types
and each symbol must consist of 1 or 2 alphanumeric characters. Normally, these
symbols should be chosen to match the chemical identity of each LAMMPS atom type,
as specified using the <A HREF = "pair_reax_c.html">reax/c pair_coeff</A> command and
the ReaxFF force field file.
</P>
<HR>
<P>The <I>Nevery</I>, <I>Nrepeat</I>, and <I>Nfreq</I> arguments specify on what
timesteps the bond-order values are sampled to get the average bond
order. The species analysis is performed using the average bond-order
on timesteps
that are a multiple of <I>Nfreq</I>. The average is over <I>Nrepeat</I>
bond-order samples, computed in the preceding portion of the simulation every
<I>Nevery</I> timesteps. <I>Nfreq</I> must be a multiple of <I>Nevery</I> and
<I>Nevery</I> must be non-zero even if <I>Nrepeat</I> is 1. Also, the timesteps
contributing to the average bond-order cannot overlap, i.e. Nfreq >
(Nrepeat-1)*Nevery is required.
</P>
<P>For example, if Nevery=2, Nrepeat=6, and Nfreq=100, then bond-order values on
timesteps 90,92,94,96,98,100 will be used to compute the average bond-order
for the species analysis output on timestep 100.
</P>
<HR>
<P><B>Restart, fix_modify, output, run start/stop, minimize info:</B>
</P>
<P>No information about this fix is written to <A HREF = "restart.html">binary restart
files</A>. None of the <A HREF = "fix_modify.html">fix_modify</A> options
are relevant to this fix. No global or per-atom quantities are stored
by this fix for access by various <A HREF = "Section_howto.html#howto_15">output
commands</A>. No parameter of this fix can
be used with the <I>start/stop</I> keywords of the <A HREF = "run.html">run</A> command.
This fix is not invoked during <A HREF = "minimize.html">energy minimization</A>.
</P>
<P><B>Restrictions:</B>
</P>
<P>The fix species currently only works with
<A HREF = "pair_reax_c.html">pair_style reax/c</A> and it requires that the <A HREF = "pair_reax_c.html">pair_style
reax/c</A> be invoked. This fix is part of the
USER-REAXC package. It is only enabled if LAMMPS was built with that
package. See the <A HREF = "Section_start.html#start_3">Making LAMMPS</A> section
for more info.
</P>
<P>It should be possible to extend it to other reactive pair_styles (such as
<A HREF = "pair_airebo.html">rebo</A>, <A HREF = "pair_airebo.html">airebo</A>,
<A HREF = "pair_comb.html">comb</A>, and <A HREF = "pair_bop.html">bop</A>), but this has not yet been done.
</P>
<P><B>Related commands:</B>
</P>
<P><A HREF = "pair_reax_c.html">pair_style reax/c</A>, <A HREF = "fix_reax_bonds.html">fix
reax/bonds</A>
</P>
<P><B>Default:</B>
</P>
<P>The default values for bond-order cutoffs are 0.3 for all I-J pairs. The
default element symbols are C, H, O, N.
</P>
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