Structural and Vibrational Properties of Liquid Water and Aqueous Solutions from First Principles Molecular Dynamics

We carried out a series of ab initio molecular dynamics simulations of liquid water and simple ions in water using density functional theory (DFT). Our study had a two-fold objective: (i) establish which level of ab-initio DFT is appropriate to describe not only the structure of water, but also its vibrational, electronic and some of its thermodynamic properties; (ii) interpret experimental data available for infrared spectra (IR) of the pure liquid and liquid solutions. We first studied IR spectra of water using semilocal functionals. We showed that intermolecular dipolar correlations play a key role in determining the complex shape and width of the IR stretching band and those correlations are long-ranged, extending to the second coordination shell. Using a molecular orbital picture, we identified specific features of the band arising from correlations of electronic contributions to the IR activities. These features are robust and do not depend on the functional used in our simulations. The IR spectra of liquid water obtained with the hybrid functional PBE0 yielded a much better agreement with experimental results than a semilocal functional description (PBE). Such an improved description stems from a two-fold effect: a more accurate account, at the PBE0 level of theory, of the vibrational properties of the water monomer and dimer, and an underlying structural model for the liquid with a smaller number of hydrogen bonds and oxygen coordination than those obtained with semilocal functionals. Also with van der Waals density functionals, liquid water displays a hydrogen-bonded network less tightly bound than when using semilocal and hybrid functionals, and yielded IR spectra in good agreement with experiment. However, PBE0 greatly improves the description of water electronic properties (e.g. the quasi-particle gap), while van der Waals functionals yielded results similar to PBE. In spite of improvements in the description of the structural and vibrational properties of water found when using hybrid or van der Waals functionals, our study of the entropy of the liquid indicated that in the vicinity of room temperature and at experimental equilibrium density, all functionals severely underestimate the total entropy and the liquid exhibits a degree of tetrahedral order higher than in experiment. In the case of simple solutions, we focused on anions that are technically more demanding to simulate than cations and in particular on the chloride ion. Compared with the pure liquid, we found an overall softening of the hydrogen bonding structure in the chloride solution, with a smaller average dipole moment of water molecules and an increased number of broken or distorted hydrogen bonds. The ion effects on the hydrogen bonding network are significant in the first chloride solvation shell and become weak beyond that, although non-negligible. Overall, our findings represent a significant step forward in the theoretical modeling of water from first principles, as one may extend the framework adopted here with hybrid or van der Waals functionals to other aqueous environments, such as water under confinement or in contact with surfaces.