Anhydrous sodium hydroxide, a common and structurally simple compound, shows spectacular isotope effects: NaOD undergoes a first-order transition, which is absent in NaOH. By combining ab initio electronic structure calculations with Feynman path integrals, we show that NaOH is an unusual example of a quantum paraelectric: zero-point quantum fluctuations stretch the weak hydrogen bonds (HBs) into a region where they are unstable and break. By strengthening the HBs via isotope substitution or applied pressure, the system can be driven to a broken-symmetry antiferroelectric phase. In passing, we provide a simple quantitative criterion for HB breaking in layered crystals and show that nuclear quantum effects are crucial in paraelectric to ferroelectric transitions in hydrogen-bonded hydroxides.

Transport of hydrogen in hydrous minerals under high pressure is a key step for the water cycle within the Earth interior. Brucite Mg(OH) 2 is one of the simplest minerals containing hydroxyl groups and is believed to decompose under the geological condition of the deep Earth’s mantle. In the present study, we investigate the proton diffusion in brucite under high pressure, which results from a complex interplay between two processes: the O–H reorientations motion around the c axis and O–H covalent bond dissociations. First-principle path-integral molecular dynamics simulations reveal that the increasing pressure tends to lock the former motion, while, in contrast, it activates the latter which is mainly triggered by nuclear quantum effects. These two competing effects therefore give rise to a pressure sweet spot for proton diffusion within the mineral. In brucite Mg(OH) 2 , proton diffusion reaches a maximum for pressures close to 70GPa, while the structurally similar portlandite Ca(OH) 2 never shows proton diffusion within the pressure range and time scale that we explored. We analyze the different behavior of brucite and portlandite, which might constitute two prototypes for other minerals with same structure.

Methane hydrate was recently shown, both experimentally and through simulations, to be stable up to the remarkably high pressure of 150 GPa. A new methane hydrate high-pressure (MH-IV) phase, reminiscent of ice at ambient pressure, was described for pressures above approximately 40 GPa. We disentangle here the main contributions to the relative stability of the lower pressure, denoted MH-III, and the high-pressure MH-IV structures. Through several simulation techniques, including metadynamics and path integral molecular dynamics for nuclear quantum effects, we analyze the phase transition mechanism, which implies hydrogen bond breaking and reforming, as well as methane reordering. The transition pathway is far from trivial, and the quantum delocalization of the hydrogen nuclei plays a significant role.

We propose an efficient method to explore the configuration space of nanoclusters by combining together ab initio molecular dynamics, metadynamics, and data clustering algorithms. On the one side, we employ collective variables sensitive to topological changes in the network of interatomic connections to map the configuration space; on the other side, we introduce an automatic approach to select, in such a space, representative structures to be optimized. In this way, we show that it is possible to sample thoroughly the set of relevant nanocluster geometries at a limited computational cost. We apply our method to explore MoS2 clusters that recently raised a sizable interest due to their remarkable electronic and catalytic properties. We demonstrate that the unsupervised algorithm is able to find a large number of low-energy structures at different cluster sizes, including both bulk-like geometries and very different topologies. We are thus able to recapitulate, in a single computational study on technologically relevant MoS2 clusters, the results of all previous works that employed distinct techniques like genetic algorithms or heuristic hypotheses. Furthermore, we found several new structures not previously reported. The ensemble of MoS2 cluster structures is deposited in a publicly accessible database.

Gas hydrates consist of hydrogen-bonded water frameworks enclosing guest gas molecules and have been the focus of intense research for almost 40 y, both for their fundamental role in the understanding of hydrophobic interactions and for gas storage and energy-related applications. The stable structure of methane hydrate above 2 GPa, where CH4 molecules are located within H2O or D2O channels, is referred to as methane hydrate III (MH-III). The stability limit of MH-III and the existence of a new high-pressure phase above 40 to 50 GPa, although recently conjectured, remain unsolved to date. We report evidence for a further high-pressure, room-temperature phase of the CH4-D2O hydrate, based on Raman spectroscopy in diamond anvil cell and ab initio molecular dynamics simulations including nuclear quantum effects. Our results reveal that a methane hydrate IV (MH-IV) structure, where the D2O network is isomorphic with ice Ih, forms at ∼40 GPa and remains stable up to 150 GPa at least. Our proposed MH-IV structure is fully consistent with previous unresolved X-ray diffraction patterns at 55 GPa [T. Tanaka et al., J. Chem. Phys. 139, 104701 (2013)]. The MH-III → MH-IV transition mechanism, as suggested by the simulations, is complex. The MH-IV structure, where methane molecules intercalate the tetrahedral network of hexagonal ice, represents the highest-pressure gas hydrate known up to now. Repulsive interactions between methane and water dominate at the very high pressure probed here and the tetrahedral topology outperforms other possible arrangements in terms of space filling.

We investigate the effects of high pressure on the reorientational and vibrational dynamics of methane molecules embedded in methane hydrate III—the stable form of methane for pressures above 2 GPa at room temperature—by combining high-pressure Raman spectroscopy with ab initio simulations including nuclear quantum effects. We observe a clear evolution of the system from a gas-filled ice structure, where methane molecules occupy the channels of the ice skeleton and rotate almost freely, to a CH4:D2O compound where methane rotations are hindered, and methane and water dynamics are tightly coupled. The gradual orientational ordering of the guest molecules results in a complete locking-in at approximately 20 GPa. This happens along with a progressive distortion of the guest molecules. Finally, as pressure increases beyond 20 GPa, the system enters a strong mode coupling regime where methane guests and water hosts dynamics are intimately paired.

First-principle molecular dynamics simulations of methane hydrate MH-III, including the quantum properties of the hydrogen nuclei, were carried out at pressures in the 5-65 GPa range, in order to observe the H-bond symmetrization at high pressure and at room temperature. According to our simulations, the symmetrization transition takes place around 40 GPa and is little dependent on isotope substitution. We find that, consistently with the rather complex crystal structure of MH-III, the transition is much more convoluted in the hydrate than in ices VII and X. In methane hydrate, due to the presence of non equivalent O ions with distinct O-O distances, the dissimilar H bonds symmetrize in a pressure domain rather than at a single critical pressure.