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<TITLE>29. Electrostatic potentials</TITLE>
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<B> Next:</B> <A NAME="tex2html1571"
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HREF="node32.html">30. Prepare</A>
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HREF="node30.html">28. Properties</A>
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  <B> <A NAME="tex2html1569"
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HREF="node2.html">Contents</A></B>
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<A NAME="CHILD_LINKS"><STRONG>Subsections</STRONG></A>
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<UL>
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<LI><A NAME="tex2html1572"
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HREF="node31.html#SECTION003110000000000000000">29.1 Grid specification</A>
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<LI><A NAME="tex2html1573"
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HREF="node31.html#SECTION003120000000000000000">29.2 Constraints</A>
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<LI><A NAME="tex2html1574"
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HREF="node31.html#SECTION003130000000000000000">29.3 Restraints</A>
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<HR>
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<H1><A NAME="SECTION003100000000000000000">
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29. Electrostatic potentials</A>
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</H1>
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<A NAME="sec:esp"></A>
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<P>
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The NWChem Electrostatic Potential (ESP) module derives partial atomic
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charges that fit the quantum mechanical electrostatic potential on selected
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grid points.
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<P>
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The ESP module is specified by the NWChem task directive
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<PRE>
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task esp
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</PRE>
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<P>
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The input for the module is taken from the ESP input block
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<PRE>
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ESP
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...
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END
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</PRE>
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<P>
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<H1><A NAME="SECTION003110000000000000000">
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29.1 Grid specification</A>
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</H1>
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The grid points for which the quantum mechanical electrostatic potential is
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evaluated and used in the fitting procedure of the partial atomic charges
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all lie outside the van der Waals radius of the atoms and within a cutoff
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distance from the atomic centers. The following input parameters determine
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the selection of grid points.
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<UL>
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<LI>If a grid file is found, the grid will be read from that file. If no grid
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file is found, or the keyword
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<PRE>
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recalculate
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</PRE>
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is given, the grid and the electrostatic potential is recalculated.
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</LI>
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<LI>The extent of the grid is determined by
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<PRE>
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range <real rcut>
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</PRE>
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where <code>rcut</code> is the maximum distance in <IMG
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WIDTH="28" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
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SRC="img195.gif"
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ALT="$nm$"> between a grid point and
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any of the atomic centers. When omitted, a default value for <code>rcut</code> of
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0.3 <IMG
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WIDTH="28" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
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SRC="img195.gif"
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ALT="$nm$"> is used.
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</LI>
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<LI>The grid spacing is specified by
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<PRE>
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spacing <real spac>
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</PRE>
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where <code>spac</code> is the grid spacing in <IMG
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WIDTH="28" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
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SRC="img195.gif"
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ALT="$nm$"> for the regularly spaced
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grid points. If not specified, a default spacing of 0.05 <IMG
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WIDTH="28" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
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SRC="img195.gif"
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ALT="$nm$"> is used.
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</LI>
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<LI>The van der Waals radius of an element can be specified by
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<PRE>
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radius <integer iatnum> <real atrad>
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</PRE>
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where <code>iatnum</code> is the atomic number for which a van der Waals radius
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of <code>atrad</code> in <IMG
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WIDTH="28" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
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SRC="img195.gif"
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ALT="$nm$"> will be used in the grid point determination.
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Default values will be used for atoms not specified.
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</LI>
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<LI>The probe radius in nm determining the envelope around the molecule is
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specified by
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<PRE>
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probe <real probe default 0.07>
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</PRE>
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</LI>
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<LI>The distance between atomic center and probe center can be multiplied
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by a constant factor specified by
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<PRE>
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factor <real factor default 1.0>
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</PRE>
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All grid points are discarded that lie within a distance
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<code>factor*(radius(i)+probe)</code> from any atom <IMG
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WIDTH="10" HEIGHT="15" ALIGN="BOTTOM" BORDER="0"
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SRC="img164.gif"
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ALT="$i$">.
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</LI>
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<LI>Schwarz screening is applied using
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<PRE>
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screen [<real scrtol default 1.0D-5>]
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</PRE>
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</LI>
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</UL>
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<P>
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<H1><A NAME="SECTION003120000000000000000">
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29.2 Constraints</A>
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</H1>
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Additional constraints to the partial atomic charges can be imposed during
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the fitting procedure.
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<UL>
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<LI>The net charge of a subset of atoms can be constrained using
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<PRE>
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constrain <real charge> {<integer iatom>}
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</PRE>
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where <code>charge</code> is the net charge of the set of atoms <code>{iatom}</code>.
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A negative atom number <code>iatom</code> can be used to specify that the
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partial charge of that atom is substracted in the sum for the set.
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</LI>
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<LI>The net charge of a sequence of atoms can be constrained using
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<PRE>
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constrain <real charge> <integer iatom> through <integer jatom>
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</PRE>
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where <code>charge</code> is the net charge of the set of atoms <code>{[iatom:jatom]}</code>.
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</LI>
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<LI>A group of atoms can be constrained to have the same charge with
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<PRE>
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constrain equal {<integer iatom>}
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</PRE>
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</LI>
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<LI>The individual charge of a group of atoms can be constrained to be equal to
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those of a second group of atoms with
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<PRE>
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constrain group <integer iatom> <integer jatom> to <integer katom> <integer latom>
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</PRE>
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resulting in the same charge for atoms <code>iatom</code> and <code>katom</code>, for
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atoms <code>iatom+1</code> and <code>katom+1</code>, ... for atoms <code>jatom</code> and <code>latom</code>.
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</LI>
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<LI>A special constraint
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<PRE>
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constrain xhn <integer iatom> {<integer jatom>}
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</PRE>
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can be used to constrain the set <code>{iatom,{jatom}}</code> to zero charge, and
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constrain all atoms in <code>{jatom}</code> to have the same charge. This can be used,
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for example, to restrain a methyl group to zero charge, and have all hydrogen
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carrying identical charges.
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</LI>
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</UL>
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<P>
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<H1><A NAME="SECTION003130000000000000000">
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29.3 Restraints</A>
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</H1>
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Restraints can be applied to each partial charge using the RESP charge
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fitting procedure.
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<UL>
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<LI>The directive for charge restraining is
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<PRE>
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restrain [hfree] (harmonic [<real scale>] | \
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hyperbolic [<real scale> [<real tight>]] \
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[maxiter <integer maxit>] [tolerance <real toler>])
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</PRE>
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where <code>hfree</code> can be specified to exclude hydrogen atoms from the
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restaining procecure. Variable <code>scale</code> is the strength of the
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restraint potential, with a default of <IMG
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WIDTH="58" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
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SRC="img196.gif"
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ALT="$0.005 au$"> for the harmonic
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restraint and a default value of <IMG
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WIDTH="58" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
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SRC="img197.gif"
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ALT="$0.001 au$"> for the hyperbolic restraint.
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For the hyperbolic restraints the tightness <code>tight</code> can be specified
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to change the default value of <IMG
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WIDTH="32" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
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SRC="img198.gif"
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ALT="$0.1 e$">. The iteration count that needs to
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be carried out for the hyperbolic restraint is determined by the
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maximum number of allowed iterations <code>maxiter</code>, with a default value
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of 25, and the tolerance in the convergence of the partial charges
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<code>toler</code>, with a default of <IMG
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WIDTH="48" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
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SRC="img199.gif"
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ALT="$0.001 e$">.
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</LI>
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</UL>
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<P>
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<ADDRESS>
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Edoardo Apra
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2004-05-25
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