The behavior of liquid water under an electric field is a crucial phenomenon in science and engineering. However, its detailed description at a microscopic level is difficult to achieve experimentally. Here we report on the first ab initio molecular-dynamics study on water under an electric field. We observe that the hydrogen-bond length and the molecular orientation are significantly modified at low-to-moderate field intensities. Fields beyond a threshold of about 0.35  V/Å are able to dissociate molecules and sustain an ionic current via a series of correlated proton jumps. Upon applying even more intense fields (∼1.0  V/Å), a 15%-20% fraction of molecules are instantaneously dissociated and the resulting ionic flow yields a conductance of about 7.8  Ω-1 cm-1, in good agreement with experimental values. This result paves the way to quantum-accurate microscopic studies of the effect of electric fields on aqueous solutions and, thus, to massive applications of ab initio molecular dynamics in neurobiology, electrochemistry, and hydrogen economy.

We trace with unprecedented numerical accuracy the phase diagram of the Gaussian-core model, a classical system of point particles interacting via a Gaussian-shaped, purely repulsive potential. This model, which provides a reliable qualitative description of the thermal behavior of interpenetrable globular polymers, is known to exhibit a polymorphic FCC-BCC transition at low densities and reentrant melting at high densities. Extensive Monte Carlo simulations, carried out in conjunction with accurate calculations of the solid free energies, lead to a thermodynamic scenario that is partially modified with respect to previous knowledge. In particular, we find that: i) the fluid-BCC-FCC triple-point temperature is about one third of the maximum freezing temperature; ii) upon isothermal compression, the model exhibits a fluid-BCC-FCC-BCC-fluid sequence of phases in a narrow range of temperatures just above the triple point. We discuss these results in relation to the behavior of star-polymer solutions and of other softly repulsive systems.

The celebrated Miller experiments reported on the spontaneous formation of amino acids from a mixture of simple molecules reacting under an electric discharge, giving birth to the research field of prebiotic chemistry. However, the chemical reactions involved in those experiments have never been studied at the atomic level. Here we report on, to our knowledge, the first ab initio computer simulations of Miller-like experiments in the condensed phase. Our study, based on the recent method of treatment of aqueous systems under electric fields and on metadynamics analysis of chemical reactions, shows that glycine spontaneously forms from mixtures of simple molecules once an electric field is switched on and identifies formic acid and formamide as key intermediate products of the early steps of the Miller reactions, and the crucible of formation of complex biological molecules.I n 1953, Miller reported (1) the surprising results he had achieved by the application of an electric discharge on a mixture of the gases CH 4 , NH 3 , H 2 O, and H 2 that simulated what was considered at the time as a model atmosphere for the primordial Earth. The result of this experiment was a substantial yield of a mixture of amino acids (2-4). Similarly, Orò demonstrated the spontaneous formation of nucleic acid bases from aqueous hydrogen cyanide subject to heating and/or spark discharges (5). Since then, a number of different hypotheses have been formulated to explain the synthesis, from simple molecules (water, ammonia, methane, carbon oxides), of simple organic molecules (formaldehyde, hydrogen cyanide, formic acid), and from the latter ones to biological monomers (amino acids, purines, pyrimidines) up the ladder of the complexity of life (6, 7). However, a "standard model" explaining the emergence of larger and larger molecular assemblies is lacking. Various sources of energy that might have initiated this synthesis have been suggested in the last 60 y, including UV irradiation both in extraterrestrial and terrestrial environments (8-11), hydrothermal energy from deep sea vents (12, 13), shock waves from meteorite impact (14-20), redox energy in the "iron−sulphur world" hypothesis (21), radioactive emission from uranium accumulation (22), and, of course, electric discharges originated by lightning, which motivated the Miller experiment, and are the inspiration of the present work. Despite the historical importance of Miller's experiments, no attempts have been made to explore the role of the electric field, traditionally considered merely in terms of just another thermal activation.A key aspect of that experiment was the formation of hydrogen cyanide, aldehydes, ketones, and amino acids, suggesting that the compounds were produced by the Strecker cyanohydrin reaction (23). Subsequent Miller-like experiments, run with other types and combinations of gases (24, 25) representing plausible geochemical conditions, produced other classes of basic building block molecules, namely sugars and nucleotides. To date, all of the major classes of...

We study the phase behavior of systems of particles interacting through pair potentials with a hard core plus a soft repulsive component. We consider several different forms of soft repulsion, including a square shoulder, a linear ramp and a quasi-exponential tail. The common feature of these potentials is the presence of two repulsive length scales, which may be the origin of unusual phase behaviors such as polyamorphism both in the equilibrium liquid phase and in the glassy state, water-like anomalies in the liquid state and anomalous melting at very high pressures.

We redraw, using state-of-the-art methods for free-energy calculations, the phase diagrams of two reference models for the liquid state: the Gaussian and inverse-power-law repulsive potentials.Notwithstanding the different behavior of the two potentials for vanishing interparticle distances, their thermodynamic properties are similar in a range of densities and temperatures, being ruled by the competition between the body-centered-cubic (BCC) and face-centered-cubic (FCC) crystalline structures and the fluid phase. We confirm the existence of a reentrant BCC phase in the phase diagram of the Gaussian-core model, just above the triple point . We also trace the BCC-FCC coexistence line of the inverse-power-law model as a function of the power exponent n and relate the common features in the phase diagrams of such systems to the softness degree of the interaction.

Effective pair interactions with a soft-repulsive component are a well-known feature of polymer solutions and colloidal suspensions, but they also provide a key to interpret the high-pressure behaviour of simple elements. We have computed the zero-temperature phase diagram of four different model potentials with various degrees of core softness. Among the reviewed crystal structures, there are also a number of non-Bravais lattices, chosen among those observed in real systems.Some of these crystals are indeed found to be stable for the selected potentials. We recognize an apparently universal trend for unbounded potentials, going from high-to low-coordinated crystal phases and back upon increasing the pressure. Conversely, a bounded repulsion may lead to intermittent appearance of compact structures with compression and no eventual settling down in a specific phase. In both cases, the fluid phase repeatedly reenters at intermediate pressures, as suggested by a cell-theory treatment of the solids. These findings are of relevance for soft matter in general, but they also offer fresh insight into the mechanisms subtended to solid polymorphism in elemental substances.

We present a Monte Carlo simulation study of the phase behavior of two-dimensional classical particles repelling each other through an isotropic Gaussian potential. As in the analogous three-dimensional case, a reentrant-melting transition occurs upon compression for not too high temperatures, along with a spectrum of waterlike anomalies in the fluid phase. However, in two dimensions melting is a continuous two-stage transition, with an intermediate hexatic phase which becomes increasingly more definite as pressure grows. All available evidence supports the Kosterlitz-Thouless-Halperin-Nelson-Young scenario for this melting transition. We expect that such a phenomenology can be checked in confined monolayers of charge-stabilized colloids with a softened core.

Two-dimensional crystals of classical particles are very peculiar in that melting may occur in two steps, in a continuous fashion, via an intermediate hexatic fluid phase exhibiting quasi-long-range orientational order. On the other hand, three-dimensional spheres repelling each other through a fast-decaying bounded potential of generalized-exponential shape (GEM4 potential) can undergo freezing into cluster crystals, allowing for more that one particle per lattice site. We hereby study the combined effect of low spatial dimensionality and extreme potential softness, by investigating the phase behavior of the two-dimensional (2D) GEM4 system. Using a combination of densityfunctional theory and numerical free-energy calculations, we show that the 2D GEM4 system displays one ordinary and several cluster triangular-crystal phases, and that only the ordinary crystal first melts into a hexatic phase. Upon heating, the difference between the various cluster crystals fades away, eventually leaving a single undifferentiated cluster phase with a pressuremodulated site occupancy.