11 research outputs found
. Rearrangements of Water Dimer and Hexamer
Abstract. Rearrangement mechanisms of the water dimer and the cage form of the water hexamer are examined theoretically with particular reference to tunneling splittings and spectroscopy. The three lowest barrier rearrangements of the water dimer are characterized by ab initio methods and compared with the results of previous constrained calculations. The acceptor-tunneling pathway does not proceed via a direct rotation around the C2 axis of the acceptor, but rather via relatively asynchronous rotation of the donor about the hydrogen bond and an associated âwagâ of the acceptor. Rearrangements between different cage isomers of the water hexamer are studied for two empirical potentials. The experimentally observed triplet splittings may be the result of flip and bifurcation rearrangements of the two single-donor, single-acceptor monomers. Twodimensional quantum calculations of the nuclear dynamics suggest that delocalization over more than one cage isomer may occur, especially in excited states
Revised Parameters for the AMOEBA Polarizable Atomic Multipole Water Model
A set of improved parameters for
the AMOEBA polarizable atomic
multipole water model is developed. An automated procedure, ForceBalance,
is used to adjust model parameters to enforce agreement with <i>ab initio</i>-derived results for water clusters and experimental
data for a variety of liquid phase properties across a broad temperature
range. The values reported here for the new AMOEBA14 water model represent
a substantial improvement over the previous AMOEBA03 model. The AMOEBA14
model accurately predicts the temperature of maximum density and qualitatively
matches the experimental density curve across temperatures from 249
to 373 K. Excellent agreement is observed for the AMOEBA14 model in
comparison to experimental properties as a function of temperature,
including the second virial coefficient, enthalpy of vaporization,
isothermal compressibility, thermal expansion coefficient, and dielectric
constant. The viscosity, self-diffusion constant, and surface tension
are also well reproduced. In comparison to high-level <i>ab initio</i> results for clusters of 2â20 water molecules, the AMOEBA14
model yields results similar to AMOEBA03 and the direct polarization
iAMOEBA models. With advances in computing power, calibration data,
and optimization techniques, we recommend the use of the AMOEBA14
water model for future studies employing a polarizable water model