Reviews in Computational Chemistry Volume 18

124 Polarizability in Computer Simulations
used for the short-range repulsion, as well as from the use of point charges or
dipoles rather than diffuse charge distributions. Evidence exists showing that
diffuse charge distributions are necessary to ensure transferability, in both
polarizable and nonpolarizable models. 35,53,205
Predicting phase coexistence behavior near the critical point seems to be
a particularly difficult task, even for the best polarizable models. Almost no
existing model that works well at ambient conditions has been demonstrated
to predict the critical temperature and density to better than 10% accuracy.
61,206 And those that are specifically designed to work well near the critical
point seem to do a poor job of reproducing the liquid structure at lower
temperatures. 61 Part of the problem is that the simulations required to measure
phase coexistence properties are computationally expensive due to the extensive
sampling required. Because of this expense, phase coexistence properties
have not typically been included in the list of target properties when parameterizing
new water models. Thus, it is only now becoming clear how to construct
a model that is transferable across hundreds of degrees, from
supercooled liquid to supercritical fluid. It is not yet clear whether one particular
type of polarizable model is better able to capture the variation of water
properties under varying temperatures and densities than another. However,
the current situation clearly underscores the considerable flexibility and ambiguities
involved in parameterizing polarizable potentials.
Transferability from the solid state to the liquid state is equally problematic.
A truly transferable potential in this region of the phase diagram must
reproduce not only the freezing point, but also the temperature of maximum
density and the relative stability of the various phases of ice. This goal remains
out of reach at present, and few existing models demonstrate acceptable transferability
from solid to liquid phases. 33,52,207 One feature of water that has
been demonstrated by both an EE model study 207 and an ab initio study 200
is that the dipole moments of the liquid and the solid are different, so polarization
is likely to be important for an accurate reproduction of both phases.
In addition, while many nonpolarizable water models exhibit a computed
temperature of maximum density for the liquid, the temperature is not near
the experimental value of 277 K. 53,62,208–215 For example, TIP4P 192 and
SPC/E 4 models have a temperature of maximum density, TMD, near 248
K. 211,213,215 Several EE models 53,147,207 and one EE–PPD hybrid model 166
yield a TMD right at 277 K, suggesting that polarizability may be an important
factor for this property as well. However, PPD models do not reproduce the
T MD maximum density very well; one model does not even have a T MD 212 and
another has a temperature dependence on the density that is much too
strong. 62 One nonpolarizable model, the TIP5P model, which includes lonepair
interaction sites, has been successfully constructed to have the correct
TMD. 216
The successful transferability of water models from the bulk phases to
more heterogeneous conditions is another important goal for scientists