1%
2% $Id$
3%
4\label{sec:esp}
5
6The NWChem Electrostatic Potential (ESP) module derives partial atomic
7charges that fit the quantum mechanical electrostatic potential on selected
8grid points.
9
10The ESP module is specified by the NWChem task directive
11\begin{verbatim}
12task esp
13\end{verbatim}
14
15The input for the module is taken from the ESP input block
16\begin{verbatim}
17ESP
18  ...
19END
20\end{verbatim}
21
22\section{Grid specification}
23The grid points for which the quantum mechanical electrostatic potential is
24evaluated and used in the fitting procedure of the partial atomic charges
25all lie outside the van der Waals radius of the atoms and within a cutoff
26distance from the atomic centers. The following input parameters determine
27the selection of grid points.
28\begin{itemize}
29\item
30If a grid file is found, the grid will be read from that file. If no grid
31file is found, or the keyword
32\begin{verbatim}
33  recalculate
34\end{verbatim}
35is given, the grid and the electrostatic potential is recalculated.
36\item
37The extent of the grid is determined by
38\begin{verbatim}
39  range <real rcut>
40\end{verbatim}
41where \verb+rcut+ is the maximum distance in $nm$ between a grid point and
42any of the atomic centers. When omitted, a default value for \verb+rcut+ of
430.3 $nm$ is used.
44\item
45The grid spacing is specified by
46\begin{verbatim}
47  spacing <real spac>
48\end{verbatim}
49where \verb+spac+ is the grid spacing in $nm$ for the regularly spaced
50grid points. If not specified, a default spacing of 0.05 $nm$ is used.
51\item
52The van der Waals radius of an element can be specified by
53\begin{verbatim}
54  radius <integer iatnum> <real atrad>
55\end{verbatim}
56where \verb+iatnum+ is the atomic number for which a van der Waals radius
57of \verb+atrad+ in $nm$ will be used in the grid point determination.
58Default values will be used for atoms not specified.
59\item
60The probe radius in nm determining the envelope around the molecule is
61specified by
62\begin{verbatim}
63  probe <real probe default 0.07>
64\end{verbatim}
65\item
66The distance between atomic center and probe center can be multiplied
67by a constant factor specified by
68\begin{verbatim}
69  factor <real factor default 1.0>
70\end{verbatim}
71All grid points are discarded that lie within a distance
72\verb-factor*(radius(i)+probe)- from any atom $i$.
73\item
74Schwarz screening is applied using
75\begin{verbatim}
76  screen [<real scrtol default 1.0D-5>]
77\end{verbatim}
78\end{itemize}
79
80\section{Constraints}
81Additional constraints to the partial atomic charges can be imposed during
82the fitting procedure.  Since point group symmetry is ignored during
83the fitting, contrains must be applied to maintain a symmetric charge.
84\begin{itemize}
85\item
86The net charge of a subset of atoms can be constrained using
87\begin{verbatim}
88  constrain <real charge> {<integer iatom>}
89\end{verbatim}
90where \verb+charge+ is the net charge of the set of atoms \verb+{iatom}+.
91A negative atom number \verb+iatom+ can be used to specify that the
92partial charge of that atom is substracted in the sum for the set.
93\item
94The net charge of a sequence of atoms can be constrained using
95\begin{verbatim}
96  constrain <real charge> <integer iatom> through <integer jatom>
97\end{verbatim}
98where \verb+charge+ is the net charge of the set of atoms \verb+{[iatom:jatom]}+.
99\item
100A group of atoms can be constrained to have the same charge with
101\begin{verbatim}
102  constrain equal {<integer iatom>}
103\end{verbatim}
104\item
105The individual charge of a group of atoms can be constrained to be equal to
106those of a second group of atoms with
107\begin{verbatim}
108  constrain group <integer iatom> <integer jatom> to <integer katom> <integer latom>
109\end{verbatim}
110resulting in the same charge for atoms \verb+iatom+ and \verb+katom+, for
111atoms \verb.iatom+1. and \verb.katom+1., ... for atoms \verb+jatom+ and \verb+latom+.
112\item
113A special constraint
114\begin{verbatim}
115  constrain xhn <integer iatom> {<integer jatom>}
116\end{verbatim}
117can be used to constrain the set \verb+{iatom,{jatom}}+ to zero charge, and
118constrain all atoms in \verb+{jatom}+ to have the same charge. This can be used,
119for example, to restrain a methyl group to zero charge, and have all hydrogen
120carrying identical charges.
121\end{itemize}
122
123\section{Restraints}
124Restraints can be applied to each partial charge using the RESP charge
125fitting procedure.
126\begin{itemize}
127\item
128The directive for charge restraining is
129\begin{verbatim}
130  restrain [hfree] (harmonic [<real scale>] | \
131   hyperbolic [<real scale> [<real tight>]]  \
132    [maxiter <integer maxit>]  [tolerance <real toler>])
133\end{verbatim}
134where \verb+hfree+ can be specified to exclude hydrogen atoms from the
135restaining procecure. Variable \verb+scale+ is the strength of the
136restraint potential, with a default of $0.005 au$ for the harmonic
137restraint and a default value of $0.001 au$ for the hyperbolic restraint.
138For the hyperbolic restraints the tightness \verb+tight+ can be specified
139to change the default value of $0.1 e$. The iteration count that needs to
140be carried out for the hyperbolic restraint is determined by the
141maximum number of allowed iterations \verb+maxiter+, with a default value
142of 25, and the tolerance in the convergence of the partial charges
143\verb+toler+, with a default of $0.001 e$.
144\end{itemize}
145