Water and simple aqueous solutions

First principles simulations of liquid water are an ongoing challenge.

Accounting for the properties of water requires the ability to describe different types of bonds: intra-molecular covalent bonds, inter-molecular hydrogen bonds and van der Waals interactions. None of the known density functionals or hybrid functionals is capable of giving an equally accurate description of all these bonds. Additional difficult tasks include accounting for proton quantum effects, and efficiently sampling the complex potential energy surface of water as a function of time.

In spite of these challenges, we are making progress in understanding solvation properties of simple ions, unraveling interactions between water and surfaces, and in predicting spectroscopic properties of aqueous systems and of water and ice under pressure. We mostly use ab initio molecular dynamics (MD) and the Qbox code, but also classical simulations to investigate ice nucleation processes and some of the properties of water yet inaccessible to ab initio MD.

The early life of an electron in water

Understanding redox and photochemical reactions in aqueous environments requires a precise knowledge of the ionization potential and electron affinity of liquid water. The former has been measured, but not the latter. We predict the electron affinity of liquid water and of its surface from first principles, coupling path-integral molecular dynamics with ab initio potentials, and many-body perturbation theory. Our results for the surface (0.8 eV) agree well with recent pump-probe spectroscopy measurements on amorphous ice. Those for the bulk (0.1-0.3 eV) differ from several estimates adopted in the literature, which we critically revisit. We show that the ionization potential of the bulk and surface are almost identical; instead their electron affinities differ substantially, with the conduction band edge of the surface much deeper in energy than that of the bulk. We also discuss the significant impact of nuclear quantum effects on the fundamental gap and band edges of the liquid.

Electronic properties of simple solvated anions

We have proposed efficient and accurate approaches to predict the electronic properties of aqueous solutions, on the basis of the combination of first-principles methods and experimental validation using state-of-the-art spectroscopic measurements. We are studying a broad range of solvated ions: our results show showing first-principles molecular dynamics simulations and electronic structure calculations using hybrid functionals provide a quantitative description of the electronic properties of the solvent and solutes, including excitation energies. Our computational framework is general and applicable to a variety of liquids, thereby offering promise in understanding and engineering solutions and liquid electrolytes for a variety of important energy technologies.

Structure and bonding of salt solutions

Determining how the structure of water is modified by the presence of salts is instrumental to understanding the solvation of biomolecules and in general, the role played by salts in biochemical processes. Yet the extent of hydrogen bonding disruption induced by salts remains controversial. We performed extensive first-principles simulations of solutions of a simple salt (NaCl) and found that while the cation does not significantly change the structure of water beyond the first solvation shell, the anion has a further reaching effect, modifying the hydrogen-bond network even outside its second solvation shell. We found that a distinctive fingerprint of hydrogen bonding modification is the change in polarizability of water molecules. Molecular dipole moments are instead insensitive probes of long-range modifications induced by Na+ and Cl- ions. Though noticeable, the long-range effect of Cl- is expected to be too weak to affect solubility of large biomolecules. We obtained results using both PBE and hybrid functionals.

Photoelectron Spectra of Aqueous Solutions

We investigated the photoelectron spectrum of a simple aqueous solution of NaCl, by combining ab initio calculations and experiments. Measurements were conducted on microjets, and first-principles calculations were performed using hybrid functionals and many-body perturbation theory at the G0W0 level, starting with wave functions computed in ab initio molecular dynamics simulations. We found excellent agreement between theory and experiments for the positions of both the solute and solvent excitation energies on an absolute energy scale and for peak intensities. The best comparison was obtained using wave functions obtained with dielectric-dependent self-consistent and range-separated hybrid functionals. Our computational protocol opens the way to accurate, predictive calculations of the electronic properties of electrolytes, of interest to a variety of energy problems.

Density and compressibility of pure water

We determined the equilibrium density and compressibility of water and ice from first-principles molecular dynamics simulations using gradient-corrected (PBE) and hybrid (PBE0) functionals and the Qbox code. Both functionals predicted the density of ice to be larger than that of water, by 15 (PBE) and 35% (PBE0). The PBE0 functional yielded a lower density of both ice and water with respect to PBE, leading to better agreement with experiment for ice but not for liquid water. Approximate inclusion of dispersion interactions on computed molecular-dynamics trajectories led to a substantial improvement of the PBE0 results for the density of liquid water, which, however, resulted to be slightly lower than that of ice.

Water vibrational spectra with hybrid and vdW functionals

The use of hybrid functionals - in particular PBE0 - improves the description of the structural, diffusion and vibrational properties of liquid water, with respect to that provided by semi-local density functionals (e.g. PBE). Such an improved description stems from two effects: a more accurate account, at the PBE0 level of theory, of the vibrational properties of the monomer and dimer, and a structural model for the liquid with a smaller number of hydrogen bonds and oxygen coordination than those obtained with semilocal functionals. Van der Waals (vdW) functionals may also improve the description of water and ice properties, with respect to PBE, depending on the choice of the local exchange functional.

Hydrophobic substrates

Ab initio infrared spectra of water confined between non polar surfaces show that electronic charge fluctuations at the interface occur even in the case of highly hydrophobic substrates. These results indicate that hydrophobic surfaces may not be regarded as hard, inert walls. Electronic fluctuations are responsible for specific features present in IR signals and for important differences between IR spectra and vibrational densities of states.

Confined pure and salty water

The model of water confined between non polar substrates obtained from ab initio simulations points at a highly inhomogeneous, mobile interfacial layer. This layer is composed of a region of about 2.5 Å of zero particle density (and depleted electronic density), followed by a region with particle density higher than in the bulk, with molecules exhibiting a smaller dipole moment than in bulk water. In confined salty water simulated from first principles, simple ions such as Na+ and Cl- preferentially reside at the interface.

X-Ray absorption spectra

First principles molecular dynamics simulations and ab initio calculations of X-ray absorption spectra were used to investigate the aqueous solvation of cations in MgCl2, CaCl2, and NaCl solutions and of salty water confined in nanotubes. The results helped rationalize and interpret several experimental findings. In particular spectral signatures of single donor, double donor and acceptor HBs were identified, and assigned to specific configurations of water molecules in the solvation shells of the mono- and di-valent cations.

Dielectric screening

The calculation of the static dielectric screening in water and ice allowed us to identify intra- and inter-molecular screening modes (as linear combinations of the eigenvectors of the dielectric matrix). The results indicate that in ice and water screening has strong intermolecular components and cannot be simply derived from perturbed values of gas phase molecular polarizabilities. The work on dielectric screening led to find efficient ways to represent large dielectric matrices, that are now used in many body perturbation theory calculations (BSE and GW) of absorption and emission spectra of molecules and nanostructures.