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Abstract:
Hydrogen bonds (HBs) involving water molecules are ubiquitous in nature. However an
accurate description of HBs with simulation techniques, including even quantum mechanical
approaches such as density-functional theory (DFT), is a major challenge. Mainly
because of a good balance between computational cost and accuracy, DFT has been
routinely applied to study water in various environments, for example, liquid water, ice,
adsorbed, and confined water, yet how well DFT exchange-correlation (xc) functionals
describe HBs between water molecules is unknown and indeed controversial. To address
this issue a series of systematic studies on water from different environments (representative
of gas phase clusters, liquid water, and various phases of ice) have been performed
with a range of DFT xc functionals and, in principle, more accurate explicitly correlated
quantum chemistry methods.
For small gas phase water clusters (dimer to pentamer in their global minimum configurations)
several hybrid xc functionals (where a fraction of exact exchange is included)
are found to be far superior to the more common and widely used pure DFT xc functionals.
Similarly on water clusters extracted from a simulation of liquid water the hybrid
functionals offer much improved performance. It is shown that the poor performance of
generalized gradient approximation (GGA) xc functionals for liquid water is because of
a poor description of the covalent O-H bond stretching of water molecules with GGA xc
functionals. This provides a possible explanation for the predicted low diffusion coefficients
obtained in many previous GGA simulations of liquid water and raises a general
concern over the ability of pure GGA xc functionals to describe the intra-molecular deformation
in other molecular liquids too, highlighting the importance of exact exchange
in simulations of molecular liquids.
Aiming to finally understand the significance of van der Waals (vdW) dispersion forces
in holding water molecules together, a systematic study of the four low-lying isomers of
the gas phase water hexamer was performed. This revealed that due to the lack of vdW
interactions no xc functional tested found the correct lowest energy structure of the
water hexamers. More open structures (“cyclic” or “book”) were favored over the more
compact “prism” isomer which is known (from explicitly correlated calculations) to be
the lowest energy isomer. This clearly indicates the importance of vdW forces in holding
water molecules together and indicates a need for an improved account of vdW forces
in conjunction with DFT xc functionals. A similar conclusion has been reached through
simulations on a range of ambient and high pressure phases of ice, where it is found that
vdW forces play a crucial role in determining the relative stabilities of the high density
phases.
Overall, significant contributions have been made to both better understand the nature
of the interactions between water molecules and to pinpoint the shortcomings in DFT xc
functionals to describe HBs among water molecules. This will aid in the development of
improved xc functionals and deepens our understanding of the gas and condensed phases
of water and other hydrogen bonded systems too.
