forked from lijiext/lammps
107 lines
4.1 KiB
HTML
107 lines
4.1 KiB
HTML
<HTML>
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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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<HR>
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<H3>fix qeq/reax command
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</H3>
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<P><B>Syntax:</B>
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</P>
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<PRE>fix ID group-ID qeq/reax Nevery cutlo cuthi tolerance params
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</PRE>
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<UL><LI>ID, group-ID are documented in <A HREF = "fix.html">fix</A> command
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<LI>qeq/reax = style name of this fix command
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<LI>Nevery = perform QEq every this many steps
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<LI>cutlo,cuthi = lo and hi cutoff for Taper radius
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<LI>tolerance = precision to which charges will be equilibrated
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<LI>params = reax/c or a filename
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</UL>
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<P><B>Examples:</B>
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</P>
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<PRE>fix 1 all qeq/reax 1 0.0 10.0 1.0e-6 reax/c
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fix 1 all qeq/reax 1 0.0 10.0 1.0e-6 param.qeq
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</PRE>
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<P><B>Description:</B>
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</P>
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<P>Perform the charge equilibration (QEq) method as described in <A HREF = "#Rappe">(Rappe
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and Goddard)</A> and formulated in <A HREF = "#Nakano">(Nakano)</A>. It is
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typically used in conjunction with the ReaxFF force field model as
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implemented in the <A HREF = "pair_reax_c.html">pair_style reax/c</A> command, but
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it can be used with any potential in LAMMPS, so long as it defines and
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uses charges on each atom. The <A HREF = "fix_qeq_comb.html">fix qeq/comb</A>
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command should be used to perform charge equliibration with the <A HREF = "pair_comb.html">COMB
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potential</A>. For more technical details about the
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charge equilibration performed by fix qeq/reax, see the
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<A HREF = "#Aktulga">(Aktulga)</A> paper.
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</P>
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<P>The QEq method minimizes the electrostatic energy of the system by
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adjusting the partial charge on individual atoms based on interactions
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with their neighbors. It reqires some parameters for each atom type.
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If the <I>params</I> setting above is the word "reax/c", then these are
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extracted from the <A HREF = "pair_reax_c.html">pair_style reax/c</A> command and
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the ReaxFF force field file it reads in. If a file name is specified
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for <I>params</I>, then the parameters are taken from the specified file
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and the file must contain one line for each atom type. The latter
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form must be used when performing QeQ with a non-ReaxFF potential.
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Each line should be formatted as follows:
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</P>
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<PRE>itype chi eta gamma
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</PRE>
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<P>where <I>itype</I> is the atom type from 1 to Ntypes, <I>chi</I> denotes the
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electronegativity in eV, <I>eta</I> denotes the self-Coulomb
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potential in eV, and <I>gamma</I> denotes the valence orbital
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exponent. Note that these 3 quantities are also in the ReaxFF
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potential file, except that eta is defined here as twice the eta value
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in the ReaxFF file. Note that unlike the rest of LAMMPS, the units
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of this fix are hard-coded to be A, eV, and electronic charge.
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</P>
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<P><B>Restart, fix_modify, output, run start/stop, minimize info:</B>
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</P>
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<P>No information about this fix is written to <A HREF = "restart.html">binary restart
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files</A>. No global scalar or vector or per-atom
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quantities are stored by this fix for access by various <A HREF = "Section_howto.html#howto_15">output
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commands</A>. No parameter of this fix can
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be used with the <I>start/stop</I> keywords of the <A HREF = "run.html">run</A> command.
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</P>
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<P>This fix is invoked during <A HREF = "minimize.html">energy minimization</A>.
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</P>
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<P><B>Restrictions:</B>
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</P>
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<P>This fix is part of the USER-REAXC package. It is only enabled if
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LAMMPS was built with that package. See the <A HREF = "Section_start.html#start_3">Making
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LAMMPS</A> section for more info.
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</P>
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<P>This fix does not correctly handle interactions
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involving multiple periodic images of the same atom. Hence, it should not
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be used for periodic cell dimensions less than 10 angstroms.
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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_reax_c.html">pair_style reax/c</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 = "Rappe"></A>
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<P><B>(Rappe)</B> Rappe and Goddard III, Journal of Physical Chemistry, 105,
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3358-3363 (1991).
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</P>
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<A NAME = "Nakano"></A>
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<P><B>(Nakano)</B> Nakano, Computer Physics Communications, 104, 59-69 (1997).
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</P>
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<A NAME = "Aktulga"></A>
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<P><B>(Aktulga)</B> Aktulga, Fogarty, Pandit, Grama, Parallel Computing, to
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appear (2011).
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</P>
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</HTML>
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