2014-09-08 23:46:08 +08:00
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<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/point command
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</H3>
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<H3>fix qeq/shielded command
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</H3>
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<H3>fix qeq/slater command
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</H3>
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<H3>fix qeq/dynamic 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 style Nevery cutoff tolerance maxiter qfile
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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>style = <I>qeq/point</I> or <I>qeq/shielded</I> or <I>qeq/slater</I> or <I>qeq/dynamic</I>
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<LI>Nevery = perform charge equilibration every this many steps
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<LI>cutoff = global cutoff for charge-charge interactions (distance unit)
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<LI>tolerance = precision to which charges will be equilibrated
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<LI>maxiter = maximum iterations to perform charge equilibration
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<LI>qfile = a filename with QEq parameters
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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/point 1 10 1.0e-6 200 param.qeq1
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fix 1 qeq qeq/shielded 1 8 1.0e-6 100 param.qeq2
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fix 1 all qeq/slater 5 10 1.0e-6 100 params
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fix 1 qeq qeq/dynamic 1 12 1.0e-3 100 my_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> (also known
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as the matrix inversion method) and in <A HREF = "#Rick">(Rick and Stuart)</A> (also
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known as the extended Lagrangian method) based on the
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2014-09-09 23:17:43 +08:00
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electronegativity equilization principle.
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</P>
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<P>These fixes can be used with any <A HREF = "pair_style.html">pair style</A> in
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LAMMPS, so long as per-atom charges are defined. The most typical
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use-case is in conjunction with a <A HREF = "pair_style.html">pair style</A> that
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performs charge equilibration periodically (e.g. every timestep), such
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as the ReaxFF or Streitz-Mintmire potential (the latter is not yet
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implemented in LAMMPS). But these fixes can also be used with
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potentials that normally assume per-atom charges are fixed, e.g. a
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<A HREF = "pair_buck.html">Buckingham</A> or <A HREF = "pair_lj.html">LJ/Coulombic</A> potential.
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</P>
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<P>Because the charge equilibration calculation is effectively
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independent of the pair style, these fixes can also be used to perform
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a one-time assignment of charges to atoms. For example, you could
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define the QEq fix, perform a zero-timestep run via the <A HREF = "run.html">run</A>
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command without any pair style defined which would set per-atom
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charges (based on the current atom configuration), then remove the fix
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via the <A HREF = "unfix.html">unfix</A> command before performing further dynamics.
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</P>
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<P>IMPORTANT NOTE: Computing and using charge values different from
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published values defined for a fixed-charge potential like Buckingham
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or CHARMM or AMBER, can have a strong effect on energies and forces,
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and produces a different model than the published versions.
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</P>
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<P>IMPORTANT NOTE: The <A HREF = "fix_qeq_comb.html">fix qeq/comb</A> command must
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still be used to perform charge equliibration with the <A HREF = "pair_comb.html">COMB
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potential</A>. The <A HREF = "fix_qeq_reax.html">fix qeq/reax</A>
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command can be used to perform charge equilibration with the <A HREF = "pair_reax_c.html">ReaxFF
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force field</A>, although fix qeq/shielded yields the
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same results as fix qeq/reax if <I>Nevery</I>, <I>cutoff</I>, and <I>tolerance</I> are
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the same. Eventually the fix qeq/reax command will be deprecated.
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</P>
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<P>The QEq method minimizes the electrostatic energy of the system (or
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equalizes the derivative of energy with respect to charge of all the
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atoms) by adjusting the partial charge on individual atoms based on
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interactions with their neighbors within <I>cutoff</I>. It reqires a few
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parameters, in <I>metal</I> units, for each atom type which provided in a
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file specified by <I>qfile</I>. The file has the following format
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</P>
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<PRE>1 chi eta gamma zeta qcore
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2 chi eta gamma zeta qcore
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...
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Ntype chi eta gamma zeta qcore
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</PRE>
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<P>There is one line per atom type with the following parameters.
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Only a subset of the parameters is used by each QEq style as descibed
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below, thus the others can be set to 0.0 if desired.
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</P>
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<UL><LI><I>chi</I> = electronegativity in energy units
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<LI><I>eta</I> = self-Coulomb potential in energy units
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<LI><I>gamma</I> = shielded Coulomb constant defined by <A HREF = "#vanDuin">ReaxFF force field</A> in distance units
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<LI><I>zeta</I> = Slater type orbital exponent defined by the <A HREF = "#Streitz">Streitz-Mintmire</A> potential in reverse distance units
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<LI><I>qcore</I> = charge of the nucleus defined by the <A HREF = "#Streitz">Streitz-Mintmire potential</A> potential in charge units
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</UL>
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<P>The <I>qeq/point</I> style describes partial charges on atoms as point
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charges. Interaction between a pair of charged particles is 1/r,
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which is the simplest description of the interaction between charges.
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Only the <I>chi</I> and <I>eta</I> parameters from the <I>qfile</I> file are used.
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Note that Coulomb catastrophe can occur if repulsion between the pair
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of charged particles is too weak. This style solves partial charges
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on atoms via the matrix inversion method. A tolerance of 1.0e-6 is
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usually a good number.
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</P>
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<P>The <I>qeq/shielded</I> style describes partial charges on atoms also as
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point charges, but uses a shielded Coulomb potential to describe the
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interaction between a pair of charged particles. Interaction through
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the shielded Coulomb is given by equation (13) of the <A HREF = "#vanDuin">ReaxFF force
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field</A> paper. The shielding accounts for charge overlap
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between charged particles at small separation. This style is the same
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as <A HREF = "fix_qeq_reax.html">fix qeq/reax</A>, and can be used with <A HREF = "pair_reax_c.html">pair_style
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reax/c</A>. Only the <I>chi</I>, <I>eta</I>, and <I>gamma</I>
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parameters from the <I>qfile</I> file are used. This style solves partial
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charges on atoms via the matrix inversion method. A tolerance of
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1.0e-6 is usually a good number.
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</P>
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<P>The <I>qeq/slater</I> style describes partial charges on atoms as spherical
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charge densities centered around atoms via the Slater 1<I>s</I> orbital, so
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that the interaction between a pair of charged particles is the
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product of two Slater 1<I>s</I> orbitals. The expression for the Slater
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1<I>s</I> orbital is given under equation (6) of the
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<A HREF = "#Streitz">Streitz-Mintmire</A> paper. Only the <I>chi</I>, <I>eta</I>, <I>zeta</I>, and
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<I>qcore</I> parameters from the <I>qfile</I> file are used. This style solves
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partial charges on atoms via the matrix inversion method. A tolerance
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of 1.0e-6 is usually a good number.
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</P>
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<P>The <I>qeq/dynamic</I> style describes partial charges on atoms as point
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charges that interact through 1/r, but the extended Lagrangian method
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is used to solve partial charges on atoms. Only the <I>chi</I> and <I>eta</I>
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parameters from the <I>qfile</I> file are used. Note that Coulomb
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catastrophe can occur if repulsion between the pair of charged
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particles is too weak. A tolerance of 1.0e-3 is usually a good
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number.
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</P>
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<P>Note that <I>qeq/point</I>, <I>qeq/shielded</I>, and <I>qeq/slater</I> describe
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different charge models, whereas the matrix inversion method and the
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extended Lagrangian method (<I>qeq/dynamic</I>) are different solvers.
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</P>
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<P>Note that the <I>qeq/point</I> and the <I>qeq/dynamic</I> styles both describe
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charges as point charges that interact through 1/r relationship, but
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solve partial charges on atoms using different solvers. Styles
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<I>qeq/point</I> and the <I>qeq/dynamic</I> should yield comparable results if
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the QEq parameters and <I>Nevery</I>, <I>cutoff</I>, and <I>tolerance</I> are the
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same. Style <I>qeq/point</I> is typically faster, but <I>qeq/dynamic</I> scales
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better on larger sizes.
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</P>
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<P>IMPORTANT NOTE: To avoid the evaluation of the derivative of charge
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with respect to position, which is typically ill-defined, the system
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should have a zero net charge.
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</P>
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<P>IMPORTANT NOTE: Developing QEq parameters (chi, eta, gamma, zeta, and
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qcore) is an "art". Charges on atoms are not guaranteed to
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equilibrate with arbitrary choices of these parameters. We do not
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develop these QEq paramters. See the examples/qeq directory for some
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examples.
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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 these fixes 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 these fixes for access by various <A HREF = "Section_howto.html#howto_15">output
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commands</A>. No parameter of these fixes
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can be used with the <I>start/stop</I> keywords of the <A HREF = "run.html">run</A>
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command.
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</P>
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<P>Thexe fixes are 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>These fixes are part of the QEQ package. They are 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><B>Related commands:</B>
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</P>
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<P><A HREF = "fix_qeq_reax.html">fix qeq/reax</A>, <A HREF = "fix_qeq_comb.html">fix qeq/comb</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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2014-09-08 23:56:56 +08:00
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<P><B>(Rappe and Goddard)</B> A. K. Rappe and W. A. Goddard III, J Physical
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Chemistry, 95, 3358-3363 (1991).
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</P>
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<A NAME = "Nakano"></A>
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<P><B>(Nakano)</B> A. Nakano, Computer Physics Communications, 104, 59-69 (1997).
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</P>
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<A NAME = "Rick"></A>
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<P><B>(Rick and Stuart)</B> S. W. Rick, S. J. Stuart, B. J. Berne, J Chemical Physics
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101, 16141 (1994).
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</P>
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<A NAME = "Streitz"></A>
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<P><B>(Streitz-Mintmire)</B> F. H. Streitz, J. W. Mintmire, Physical Review B, 50,
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16, 11996 (1994)
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</P>
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<A NAME = "vanDuin"></A>
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<P><B>(ReaxFF)</B> A. C. T. van Duin, S. Dasgupta, F. Lorant, W. A. Goddard III, J
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Physical Chemistry, 105, 9396-9049 (2001)
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</P>
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</HTML>
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