25. Electrostatic potentials

The NWChem Electrostatic Potential (ESP) module derives partial atomic charges that fit the quantum mechanical electrostatic potential on selected grid points.

The ESP module is specified by the NWChem task directive

task esp

The input for the module is taken from the ESP input block

esp
end

25.1 Grid specification

The grid points for which the quantum mechanical electrostatic potential is evaluated and used in the fitting procedure of the partial atomic charges all lie outside the van der Waals radius of the atoms and within a cutoff distance from the atomic centers. The following input parameters determine the selection of grid points.

• If a grid file is found, the grid will be read from that file. If no grid file is found, or the keyword

    recalculate
  

  is given, the grid and the electrostatic potential is recalculated.
• The extent of the grid is determined by

    range <real rcut>
  

  where rcut is the maximum distance in $nm$ between a grid point and any of the atomic centers. When omitted, a default value for rcut of 0.3 $nm$ is used.
• The grid spacing is specified by

    spacing <real spac>
  

  where spac is the grid spacing in $nm$ for the regularly spaced grid points. If not specified, a default spacing of 0.05 $nm$ is used.
• The van der Waals radius of an element can be specified by

    radius <integer iatnum> <real atrad>
  

  where iatnum is the atomic number for which a van der Waals radius of atrad in $nm$ will be used in the grid point determination. Default values will be used for atoms not specified.
• The probe radius in nm determining the envelope around the molecule is specified by

    probe <real probe default 0.07>
  

• The distance between atomic center and probe center can be multiplied by a constant factor specified by

    factor <real factor default 1.0>
  

  All grid points are discarded that lie within a distance factor*(radius(i)+probe) from any atom $i$.
• Schwarz screening is applied using

    screen [<real scrtol default 1.0D-5>]
  

25.2 Constraints

Additional constraints to the partial atomic charges can be imposed during the fitting procedure.

• The net charge of a subset of atoms can be constrained using

    constrain <real charge> {<integer iatom>}
  

  where charge is the net charge of the set of atoms {iatom}. A negative atom number iatom can be used to specify that the partial charge of that atom is substracted in the sum for the set.
• The net charge of a sequence of atoms can be constrained using

    constrain <real charge> <integer iatom> through <integer jatom>
  

  where charge is the net charge of the set of atoms {[iatom:jatom]}.
• A group of atoms can be constrained to have the same charge with

    constrain equal {<integer iatom>}
  

• The individual charge of a group of atoms can be constrained to be equal to those of a second group of atoms with

    constrain group <integer iatom> <integer jatom> to <integer katom> <integer latom>
  

  resulting in the same charge for atoms iatom and katom, for atoms iatom+1 and katom+1, ... for atoms jatom and latom.
• A special constraint

    constrain xhn <integer iatom> {<integer jatom>}
  

  can be used to constrain the set {iatom,{jatom}} to zero charge, and constrain all atoms in {jatom} to have the same charge. This can be used, for example, to restrain a methyl group to zero charge, and have all hydrogen carrying identical charges.

25.3 Restraints

Restraints can be applied to each partial charge using the RESP charge fitting procedure.

• The directive for charge restraining is

    restrain [hfree] (harmonic [<real scale>] | \
     hyperbolic [<real scale> [<real tight>]]  \
      [maxiter <integer maxit>]  [tolerance <real toler>])
  

  where hfree can be specified to exclude hydrogen atoms from the restaining procecure. Variable scale is the strength of the restraint potential, with a default of $0.005 au$ for the harmonic restraint and a default value of $0.001 au$ for the hyperbolic restraint. For the hyperbolic restraints the tightness tight can be specified to change the default value of $0.1 e$. The iteration count that needs to be carried out for the hyperbolic restraint is determined by the maximum number of allowed iterations maxiter, with a default value of 25, and the tolerance in the convergence of the partial charges toler, with a default of $0.001 e$.