Reviews in Computational Chemistry Volume 18

Applications 123
classical and rigid) water models; it is not necessarily the best estimate of the
actual dipole moment. The dipole moment of liquid water cannot be measured
experimentally, nor can it even be defined unambiguously, since the electronic
density is not zero between molecules. 197,198 Ab initio simulations of liquid
water predict that the average dipole moment varies from 2.4 to 3.0 D depending
on how the density is partitioned, so a value of 2.6 D is consistent with
these studies. 199–201
Dynamic properties, such as the self-diffusion constant, are likewise
strongly correlated with the dipole moment. 5,23 This coupling between the
translational motion and the dipole moment is indicated in the dielectric spectrum.
126 Models that are overpolarized tend to undergo dynamics that are
significantly slower than the real physical system. The inclusion of polarization
can substantially affect the dynamics of a model, although the direction of the
effect can vary. When a nonpolarizable model is reparameterized to include
polarizability, the new model often exhibits faster dynamics, as with polarizable
versions of TIP4P, 202 Reimers–Watts–Klein (RWK), 185,203 and reduced
effective representation (RER) 30 potentials. There are exceptions, however,
such as the polarizable simple point charge (PSPC) 23,57 and fluctuating charge
(FQ) 126 models. The usual explanation for faster dynamics in polarizable
models is that given by Sprik. 202 Events governing dynamical properties,
such as translational diffusion and orientational relaxation, are activated
processes—they depend on relatively infrequent barrier-crossing events.
Adiabatic dynamics of the polarizable degrees of freedom allows for relaxation
of the polarization at the transition, through means that are inaccessible
to nonpolarizable models. This in turn lowers the activation barrier and
increases the number of successful transition attempts. The nonunanimity
of published simulation results concerning dynamic properties is likely due
to such factors as: inconsistent parameterization procedures between the
polarizable and nonpolarizable models; a strong dependence of dynamic
properties on the system pressure (which is often insufficiently controlled
during simulations); and the effects of using point versus diffuse charge
distributions.
Transferability to different temperatures is a particularly difficult task
for polarizable water models. This statement is illustrated by the problems
in predicting the PVT and phase coexistence properties. There are a handful
of polarizable water models—including both dipole- and EE-based models—
that are reasonably successful in predicting some of the structural and energetic
changes in liquid water over a range of several hundred degrees. 53,61,204
Many models fail to capture this behavior, however, so temperature transferability
is far from an automatic feature of polarizable models. 35,52,61,62 Indeed,
it has been demonstrated by several authors 35,52,61 that a point dipole-based
model designed specifically to reproduce properties of the gas-phase monomer
and the bulk liquid at 298 K is doomed to fail at higher temperatures. This
failure could arise from insufficiencies in the Lennard–Jones function typically