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