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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>fix qeq/reax command
</H3>
<P><B>Syntax:</B>
</P>
<PRE>fix ID group-ID qeq/reax Nevery cutlo cuthi tolerance params
</PRE>
<UL><LI>ID, group-ID are documented in <A HREF = "fix.html">fix</A> command
<LI>qeq/reax = style name of this fix command
<LI>Nevery = perform QEq every this many steps
<LI>cutlo,cuthi = lo and hi cutoff for Taper radius
<LI>tolerance = precision to which charges will be equilibrated
<LI>params = reax/c or a filename
</UL>
<P><B>Examples:</B>
</P>
<PRE>fix 1 all qeq/reax 1 0.0 10.0 1.0e-6 reax/c
fix 1 all qeq/reax 1 0.0 10.0 1.0e-6 param.qeq
</PRE>
<P><B>Description:</B>
</P>
<P>Perform the charge equilibration (QEq) method as described in <A HREF = "#Rappe_1991">(Rappe
and Goddard, 1991)</A> and formulated in <A HREF = "#Nakano_1997">(Nakano,
1997)</A>. It is typically used in conjunction with the
ReaxFF force field model as implemented in the <A HREF = "pair_reax_c.html">pair_style
reax/c</A> command, but it can be used with any
potential in LAMMPS, so long as it defines and uses charges on each
atom. The <A HREF = "fix_qeq_comb.html">fix qeq/comb</A> command should be used to
perform charge equliibration with the <A HREF = "pair_comb.html">COMB potential</A>.
For more technical details about the charge equilibration performed by
fix qeq/reax, see the "(Aktulga)" paper.
</P>
<P>The QEq method minimizes the electrostatic energy of the system by
adjusting the partial charge on individual atoms based on interactions
with their neighbors. It reqires some parameters for each atom type.
If the <I>params</I> setting above is the word "reax/c", then these are
extracted from the <A HREF = "pair_reax_c.html">pair_style reax/c</A> command and
the ReaxFF force field file it reads in. If a file name is specified
for <I>params</I>, then the parameters are taken from the specified file
and the file must contain one line for each atom type. The latter
form must be used when performing QeQ with a non-ReaxFF potential.
Each line should be formatted as follows:
</P>
<PRE>itype chi eta gamma
</PRE>
<P>where <I>itype</I> is the atom type from 1 to Ntypes, <I>chi</I> denotes the
electronegativity in eV, <I>eta</I> denotes the self-Coulomb
potential in eV, and <I>gamma</I> denotes the valence orbital
exponent. Note that these 3 quantities are also in the ReaxFF
potential file, except that eta is defined here as twice the eta value
in the ReaxFF file. Note that unlike the rest of LAMMPS, the units
of this fix are hard-coded to be A, eV, and electronic charge.
</P>
<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>. No global scalar or vector 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.
</P>
<P>This fix is invoked during <A HREF = "minimize.html">energy minimization</A>.
</P>
<P><B>Restrictions:</B>
</P>
<P>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><B>Related commands:</B>
</P>
<P><A HREF = "pair_reax_c.html">pair_style reax/c</A>
</P>
<P><B>Default:</B> none
</P>
<HR>
<A NAME = "Rappe_1991"></A>
<P><B>(Rappe)</B> Rappe and Goddard III, Journal of Physical Chemistry, 105,
3358-3363 (1991).
</P>
<A NAME = "Nakano_1997"></A>
<P><B>(Nakano)</B> Nakano, Computer Physics Communications, 104, 59-69 (1997).
</P>
<P>:link(Aktulga) <B>(Aktulga)</B> Aktulga, Fogarty, Pandit, Grama, Parallel
Computing, to appear (2011).
</P>
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