2010-08-20 08:37:59 +08:00
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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/comb 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/comb Nevery precision keyword value ...
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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/comb = style name of this fix command
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<LI>Nevery = perform charge equilibration every this many steps
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<LI>precision = convergence criterion for charge equilibration
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<LI>zero or more keyword/value pairs may be appended
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<LI>keyword = <I>file</I>
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<PRE> <I>file</I> value = filename
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filename = name of file to write QEQ equilibration info to
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</PRE>
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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 surface qeq/comb 10 0.0001
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</PRE>
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<P><B>Description:</B>
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</P>
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2011-03-28 22:58:14 +08:00
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<P>Perform charge equilibration (QeQ) in conjunction with the COMB
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(Charge-Optimized Many-Body) potential as described in
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<A HREF = "#COMB_1">(COMB_1)</A> and <A HREF = "#COMB_2">(COMB_2)</A>. It performs the charge
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equilibration portion of the calculation using the so-called QEq
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method, whereby the charge on each atom is adjusted to minimize the
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energy of the system. This fix can only be used with the COMB
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potential; see the <A HREF = "fix_qeq_reqx.html">fix qeq/reax</A> command for a QeQ
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calculation that can be used with any potential.
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2010-08-20 08:37:59 +08:00
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</P>
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<P>Only charges on the atoms in the specified group are equilibrated.
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The fix relies on the pair style (COMB in this case) to calculate the
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per-atom electronegativity (effective force on the charges). An
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electronegativity equalization calculation (or QEq) is performed in an
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interative fashion, which in parallel requires communication at each
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iteration for processors to exchange charge information about nearby
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atoms with each other. See <A HREF = "#Rappe_and_Goddard">Rappe_and_Goddard</A> and
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<A HREF = "#Rick_and_Stuart">Rick_and_Stuart</A> for details.
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</P>
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<P>During a run, charge equilibration is peformed every <I>Nevery</I> time
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steps. Charge equilibration is also always enforced on the first step
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of each run. The <I>precision</I> argument controls the tolerance for the
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difference in electronegativity for all atoms during charge
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equilibration. <I>Precision</I> is a trade-off between the cost of
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performing charge equilibration (more iterations) and accuracy.
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</P>
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<P>If the <I>file</I> keyword is used, then information about each
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equilibration calculation is written to the specifed file.
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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>. None of the <A HREF = "fix_modify.html">fix_modify</A> options
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are relevant to this fix.
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</P>
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<P>This fix produces a per-atom vector which can be accessed by various
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<A HREF = "Section_howto.html#4_15">output commands</A>. The vector stores the
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gradient of the charge on each atom. The per-atom values be accessed
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on any timestep.
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</P>
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<P>No parameter of this fix can be used with the <I>start/stop</I> keywords of
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the <A HREF = "run.html">run</A> command. This fix is not invoked during <A HREF = "minimize.html">energy
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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 command currently only supports <A HREF = "pair_comb.html">pair style <I>comb</I></A>.
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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_comb.html">pair_style comb</A>
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</P>
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<P><B>Default:</B>
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</P>
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<P>No file output is performed.
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</P>
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<HR>
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<A NAME = "COMB_1"></A>
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<P><B>(COMB_1)</B> J. Yu, S. B. Sinnott, S. R. Phillpot, Phys Rev B, 75, 085311 (2007),
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</P>
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<A NAME = "COMB_2"></A>
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<P><B>(COMB_2)</B> T.-R. Shan, B. D. Devine, T. W. Kemper, S. B. Sinnott, S. R.
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Phillpot, Phys Rev B, 81, 125328 (2010).
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</P>
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<A NAME = "Rappe_and_Goddard"></A>
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<P><B>(Rappe_and_Goddard)</B> A. K. Rappe, W. A. Goddard, J Phys Chem 95, 3358
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(1991).
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</P>
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<A NAME = "Rick_and_Stuart"></A>
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<P><B>(Rick_and_Stuart)</B> S. W. Rick, S. J. Stuart, B. J. Berne, J Chem Phys
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101, 16141 (1994).
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</P>
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</HTML>
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