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144 lines
5.1 KiB
TeX
144 lines
5.1 KiB
TeX
%
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% $Id$
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%
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\label{sec:esp}
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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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The ESP module is specified by the NWChem task directive
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\begin{verbatim}
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task esp
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\end{verbatim}
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The input for the module is taken from the ESP input block
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\begin{verbatim}
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ESP
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...
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END
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\end{verbatim}
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\section{Grid specification}
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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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\begin{itemize}
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\item
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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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\begin{verbatim}
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recalculate
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\end{verbatim}
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is given, the grid and the electrostatic potential is recalculated.
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\item
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The extent of the grid is determined by
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\begin{verbatim}
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range <real rcut>
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\end{verbatim}
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where \verb+rcut+ is the maximum distance in $nm$ between a grid point and
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any of the atomic centers. When omitted, a default value for \verb+rcut+ of
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0.3 $nm$ is used.
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\item
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The grid spacing is specified by
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\begin{verbatim}
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spacing <real spac>
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\end{verbatim}
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where \verb+spac+ is the grid spacing in $nm$ for the regularly spaced
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grid points. If not specified, a default spacing of 0.05 $nm$ is used.
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\item
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The van der Waals radius of an element can be specified by
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\begin{verbatim}
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radius <integer iatnum> <real atrad>
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\end{verbatim}
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where \verb+iatnum+ is the atomic number for which a van der Waals radius
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of \verb+atrad+ in $nm$ 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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\item
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The probe radius in nm determining the envelope around the molecule is
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specified by
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\begin{verbatim}
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probe <real probe default 0.07>
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\end{verbatim}
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\item
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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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\begin{verbatim}
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factor <real factor default 1.0>
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\end{verbatim}
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All grid points are discarded that lie within a distance
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\verb-factor*(radius(i)+probe)- from any atom $i$.
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\item
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Schwarz screening is applied using
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\begin{verbatim}
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screen [<real scrtol default 1.0D-5>]
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\end{verbatim}
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\end{itemize}
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\section{Constraints}
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Additional constraints to the partial atomic charges can be imposed during
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the fitting procedure. Since point group symmetry is ignored during
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the fitting, contrains must be applied to maintain a symmetric charge.
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\begin{itemize}
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\item
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The net charge of a subset of atoms can be constrained using
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\begin{verbatim}
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constrain <real charge> {<integer iatom>}
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\end{verbatim}
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where \verb+charge+ is the net charge of the set of atoms \verb+{iatom}+.
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A negative atom number \verb+iatom+ 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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\item
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The net charge of a sequence of atoms can be constrained using
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\begin{verbatim}
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constrain <real charge> <integer iatom> through <integer jatom>
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\end{verbatim}
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where \verb+charge+ is the net charge of the set of atoms \verb+{[iatom:jatom]}+.
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\item
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A group of atoms can be constrained to have the same charge with
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\begin{verbatim}
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constrain equal {<integer iatom>}
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\end{verbatim}
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\item
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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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\begin{verbatim}
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constrain group <integer iatom> <integer jatom> to <integer katom> <integer latom>
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\end{verbatim}
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resulting in the same charge for atoms \verb+iatom+ and \verb+katom+, for
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atoms \verb.iatom+1. and \verb.katom+1., ... for atoms \verb+jatom+ and \verb+latom+.
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\item
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A special constraint
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\begin{verbatim}
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constrain xhn <integer iatom> {<integer jatom>}
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\end{verbatim}
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can be used to constrain the set \verb+{iatom,{jatom}}+ to zero charge, and
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constrain all atoms in \verb+{jatom}+ 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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\end{itemize}
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\section{Restraints}
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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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\begin{itemize}
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\item
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The directive for charge restraining is
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\begin{verbatim}
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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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\end{verbatim}
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where \verb+hfree+ can be specified to exclude hydrogen atoms from the
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restaining procecure. Variable \verb+scale+ is the strength of the
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restraint potential, with a default of $0.005 au$ for the harmonic
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restraint and a default value of $0.001 au$ for the hyperbolic restraint.
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For the hyperbolic restraints the tightness \verb+tight+ can be specified
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to change the default value of $0.1 e$. 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 \verb+maxiter+, with a default value
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of 25, and the tolerance in the convergence of the partial charges
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\verb+toler+, with a default of $0.001 e$.
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\end{itemize}
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