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WSN: new preprints on-line
Following preprints (full text in Tex + figures) are available from Chemical
Physics Preprint Database (URL: http://www.chem.brown.edu/chem-ph.html):
*** chem-ph/9406002
Dynamical Fluctuating Charge Force Fields: Application to Liquid Water
Steven W. Rick, Steven J. Stuart, and B.J. Berne Department of Chemistry
and Center for Biomolecular Simulation Columbia University, NY 10027
J. Chem. Phys., in press 29 pages + 12 figures
A new molecular dynamics model in which the point charges on atomic sites
are allowed to fluctuate in response to the environment is developed and
applied to water. The idea for treating charges as variables is based on
the concept of electronegativity equalization according to which: (a) The
electronegativity of an atomic site is dependent on the atom's type and
charge and is perturbed by the electrostatic potential it experiences
from its neighbors and (b) Charge is transferred between atomic sites in
such a way that electronegativities are equalized. The charges are
treated as dynamical variables using an extended Lagrangian method in
which the charges are given a fictitious mass, velocities and kinetic
energy and then propagated according to Newtonian mechanics along with
the atomic degrees of freedom. Models for water with fluctuating charges are
developed using the geometries of two common fixed-charge water potentials:
the simple point charge (SPC) and the 4-point transferable intermolecular
potential (TIP4P). Both fluctuating charge models give accurate
predictions for gas-phase and liquid state properties, including radial
distribution functions, the dielectric constant, and the diffusion
constant. The method does not introduce any new intermolecular
interactions beyond those already present in the fixed charge models and
increases the computer time by only a factor of 1.1, making this method
tractable for large systems.
*** chem-ph/9408001
"Theoretical calculation of the water ion product K_W," by Gregory J.
Tawa and Lawrence R. Pratt, Theoretical Division, Los Alamos National
Laboratory, Los Alamos NM 87545. 13 pages and two figures. LA-UR-94-.
A dielectric model is applied to the prediction of the equilibrium
ionization of liquid water over a wide range of density and temperature
with the objective of calibrating that model for the study of ionization
in water of organic acids, e.g., proteins and nucleic acids. The model
includes an approximate description of the polarizability of the
dissociating water molecule. The radii required for the spherical
molecular volumes of the water molecule in order to fit the experimental
ion product are presented and discussed. These radii are larger than
those commonly used. They decrease with increasing density as would be
guessed but the rate of decrease is slight. They increase with increasing
temperature, a variation opposite to what would be guessed if radii were
strictly viewed as a distance of closest approach. The observed
variations with temperature and density have not been explained on the
basis of molecular theory. The theoretical principles upon which such an
explanation might be built are discussed.
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Iosif Vaisman
UNC-Chapel Hill