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Download file# Partial Atomic Charges and Screened Charge Models of the Electrostatic Potential

journal contribution

posted on 2012-06-12, 00:00 authored by Bo Wang, Donald G. TruhlarWe propose a new screened charge method for calculating
partial
atomic charges in molecules by electrostatic potential (ESP) fitting.
The model, called full density screening (FDS), is used to approximate
the screening effect of full charge densities of atoms in molecules.
The results are compared to the conventional ESP fitting method based
on point charges and to our previously proposed outer density screening
(ODS) method, in which the parameters are reoptimized for the present
purpose. In ODS, the charge density of an atom is represented by the
sum of a point charge and a smeared negative charge distributed in
a Slater-type orbital (STO). In FDS, the charge density of an atom
is taken to be the sum of the charge density of the neutral atom and
a partial atomic charge (of either sign) distributed in an STO. The
ζ values of the STOs used in these two models are optimized
in the present study to best reproduce the electrostatic potentials.
The quality of the fit to the electrostatics is improved in the screened
charge methods, especially for the regions that are within one van
der Waals radius of the centers of atoms. It is also found that the
charges derived by fitting electrostatic potentials with screened
charges are less sensitive to the positions of the fitting points
than are those derived with conventional electrostatic fitting. Moreover,
we found that the electrostatic-potential-fitted (ESP) charges from
the screened charge methods are similar to those from the point-charge
method except for molecules containing the methyl group, where we
have explored the use of restraints on nonpolar H atoms. We recommend
the FDS model if the only goal is ESP fitting to obtain partial atomic
charges or a fit to the ESP field. However, the ODS model is more
accurate for electronic embedding in combined quantum mechanical and
molecular mechanical (QM/MM) modeling and is more accurate than point-charge
models for ESP fitting, and it is recommended for applications involving
QM/MM methods. Since the screened charges describe the electrostatic
potentials more accurately than point charges, since they asymptotically
act as point charges at long distances, and since the electrostatic
potential in terms of the screened charges is still a sum of functions
centered at the atoms, the screened-charge representation of the electrostatic
potential can be used in the same way as the conventional point-charge
representation to model the electrostatic interactions, but it is
more realistic. For the H atom and p block elements, the error in
the fit to the electrostatic potential is reduced by about a factor
of 3, and the sensitivity of the derived partial atomic charges to
the choice of fitting points is reduced by about a factor of 2. For
s and d block elements, there are also improvements in the inner regions
but not necessarily in the outer regions.