  Numerous studies have identified large quantum mechanical effects in the
dynamics of liquid water. In this paper, we suggest that these effects may have
been overestimated due to the use of rigid water models and flexible models in
which the intramolecular interactions were described using simple harmonic
functions. To demonstrate this, we introduce a new simple point charge model
for liquid water, q-TIP4P/F, in which the O--H stretches are described by
Morse-type functions. We have parameterized this model to give the correct
liquid structure, diffusion coefficient, and infra-red absorption frequencies
in quantum (path integral-based) simulations. By comparing classical and
quantum simulations of the liquid, we find that quantum mechanical fluctuations
increase the rates of translational diffusion and orientational relaxation in
our model by a factor of around 1.15. This effect is much smaller than that
observed in all previous simulations of simple empirical water models, which
have found a quantum effect of at least 1.4 regardless of the quantum
simulation method or the water model employed. The small quantum effect in our
model is a result of two competing phenomena. Intermolecular zero point energy
and tunneling effects destabilize the hydrogen bonding network, leading to a
less viscous liquid with a larger diffusion coefficient. However this is offset
by intramolecular zero point motion, which changes the average water monomer
geometry resulting in a larger dipole moment, stronger intermolecular
interactions, and slower diffusion. We end by suggesting, on the basis of
simulations of other potential energy models, that the small quantum effect we
find in the diffusion coefficient is associated with the ability of our model
to produce a single broad O-H stretching band in the infra-red absorption
spectrum.
