Luca M. Ghiringhelli

Big Data of Materials Science: Critical Role of the Descriptor

Statistical learning of materials properties or functions so far starts with a largely silent, nonchallenged step: the choice of the set of descriptive parameters (termed descriptor). However, when the scientific connection between the descriptor and the actuating mechanisms is unclear, the causality of the learned descriptor-property relation is uncertain. Thus, a trustful prediction of new promising materials, identification of anomalies, and scientific advancement are doubtful. We analyze this issue and define requirements for a suitable descriptor. For a classic example, the energy difference of zinc blende or wurtzite and rocksalt semiconductors, we demonstrate how a meaningful descriptor can be found systematically.

Report on the sixth blind test of organic crystal structure prediction methods

The results of the sixth blind test of organic crystal structure prediction methods are presented and discussed, highlighting progress for salts, hydrates and bulky flexible molecules, as well as on-going challenges.

Surface-induced crystallization in supercooled tetrahedral liquids

Surfaces have long been known to have an intricate role in solid-liquid phase transformations. Whereas melting is often observed to originate at surfaces, freezing usually starts in the bulk, and only a few systems have been reported to exhibit signatures of surface-induced crystallization. These include assembly of chain-like molecules, some liquid metals and alloys and silicate glasses. Here, we report direct computational evidence of surface-induced nucleation in supercooled liquid silicon and germanium, and we illustrate the crucial role of free surfaces in the freezing process of tetrahedral liquids exhibiting a negative slope of their melting lines (dT/dP|coexist<0). Our molecular dynamics simulations show that the presence of free surfaces may enhance the nucleation rates by several orders of magnitude with respect to those found in the bulk. Our findings provide insight, at the atomistic level, into the nucleation mechanism of widely used semiconductors, and support the hypothesis of surface-induced crystallization in other tetrahedrally coordinated systems, in particular water in the atmosphere.

Competing adsorption between hydrated peptides and water onto metal surfaces: From electronic to conformational properties

Inorganic-(bio)organic interfaces are of central importance in many fields of current research. Theoretical and computational tools face the difficult problem of the different time and length scales that are involved and linked in a nontrivial way. In this work, a recently proposed hierarchical quantum-classical scale-bridging approach is further developed to study large flexible molecules. The approach is then applied to study the adsorption of oligopeptides on a hydrophilic Pt(111) surface under complete wetting conditions. We examine histidine sequences, which are well known for their binding affinity to metal surfaces. Based on a comparison with phenylalanine, which binds as strong as histidine under high vacuum conditions but, as we show, has no surface affinity under wet conditions, we illustrate the mediating effects of near-surface water molecules. These contribute significantly to the mechanism and strength of peptide binding. In addition to providing physical-chemical insights in the mechanism of surface binding, our computational approach provides future opportunities for surface-specific sequence design.

State-of-the-art models for the phase diagram of carbon and diamond nucleation

We review recent developments in the modelling of the phase diagram and the kinetics of crystallization of carbon. In particular, we show that a particular class of bond-order potentials (the so-called LCBOP models) account well for many of the known structural and thermodynamic properties of carbon at high pressures and temperatures. We discuss the LCBOP models in some detail. In addition, we briefly review the ‘history’ of experimental and theoretical studies of the phase behaviour of carbon. Using a well-tested version of the LCBOP model (viz. LCBOPI+) we address some of the more controversial hypotheses concerning the phase behaviour of carbon, in particular: the suggestion that liquid carbon can exist in two phases separated by a first-order phase transition and the conjecture that diamonds could have formed by homogeneous nucleation in Uranus and Neptune.

Autocatalytic and Cooperatively Stabilized Dissociation of Water on a Stepped Platinum Surface

Water-metal interfaces are ubiquitous and play a key role in many chemical processes, from catalysis to corrosion. Whereas water adlayers on atomically flat transition metal surfaces have been investigated in depth, little is known about the chemistry of water on stepped surfaces, commonly occurring in realistic situations. Using first-principles simulations, we study the adsorption of water on a stepped platinum surface. We find that water adsorbs preferentially at the step edge, forming linear clusters or chains, stabilized by the cooperative effect of chemical bonds with the substrate and hydrogen bonds. In contrast with their behavior on flat Pt, at steps water molecules dissociate, forming mixed hydroxyl/water structures, through an autocatalytic mechanism promoted by H-bonding. Nuclear quantum effects contribute to stabilize partially dissociated cluster and chains. Together with the recently demonstrated behavior of water chains adsorbed on stepped Pt surfaces to transfer protons via thermally activated hopping, these findings make these systems viable candidates for proton wires.

Phenylalanine near inorganic surfaces: Conformational statistics vs specific chemistry

We present a first principle density functional study of phenylalanine interacting with three different classes of surfaces, namely a purely repulsive hard wall, mildly interacting close packed surfaces of group 11 metals (Cu(111), Ag(111), and Au(111)), and strongly interacting close packed surfaces of group 10 metals (Ni(111), Pd(111), and Pt(111)). In particular, we characterize, by changing the substrate, the passage from the statistical behavior of a flexible molecule in the presence of the topological confinement of a hard wall to a purely chemical behavior where the molecule, highly deformed compared to the free state, strongly binds to the surface and statistical conformations play no longer a role. Such a comparative study allows for characterization of some of the key aspects of the adsorption process for a prototype of flexible amino acids on experimentally and technologically relevant metal surfaces.

Local Structure of Liquid Carbon Controls Diamond Nucleation

Diamonds melt at temperatures above 4000 K. There are no measurements of the steady-state rate of the reverse process, i.e., diamond nucleation from the melt, because experiments are difficult at these extreme temperatures and pressures. Using numerical simulations, we estimate the diamond nucleation rate and find that it increases by many orders of magnitude when the pressure is increased at constant supersaturation. The reason is that by increasing the pressure the local coordination of the liquid changes from threefold to fourfold, and we show that the free-energy cost to create a diamond-liquid interface is lower in the fourfold than in the threefold liquid. We speculate that this mechanism for nucleation control is relevant for crystallization in many network-forming liquids. We conclude that homogeneous diamond nucleation is likely in carbon-rich stars and unlikely in gaseous planets.

Stability and metastability of clusters in a reactive atmosphere: Theoretical evidence for unexpected stoichiometries of MgMOx

By applying a genetic algorithm and ab initio atomistic thermodynamics, we identify the stable and metastable compositions and structures of MgMOx clusters at realistic temperatures and oxygen pressures. We find that small clusters (M≲5) are in thermodynamic equilibrium when x>M. The nonstoichiometric clusters exhibit peculiar magnetic behavior, suggesting the possibility of tuning magnetic properties by changing environmental pressure and temperature conditions. Furthermore, we show that density-functional theory with a hybrid exchange-correlation functional is needed for predicting accurate phase diagrams of metal-oxide clusters. Neither a (sophisticated) force field nor density-functional theory with (semi)local exchange-correlation functionals is sufficient for even a qualitative prediction.

Not so loosely bound rare gas atoms: finite-temperature vibrational fingerprints of neutral gold-cluster complexes

We present an experimental and theoretical study of the structure of small, neutral gold clusters?Au3, Au4 and Au7??tagged? by krypton atoms. Infrared (IR) spectra of AuN?KrM complexes formed at 100?K are obtained via far-IR multiple photon dissociation in a molecular beam. The theoretical study is based on a statistical (canonical) sampling of the AuN?KrM complexes through ab initio molecular dynamics using density-functional theory in the generalized gradient approximation, explicitly corrected for long-range van-der-Waals (vdW) interactions. The choice of the functional is validated against higher-level first-principle methods. Thereby finite-temperature theoretical vibrational spectra are obtained that are compared with the experimental spectra. This enables us to identify which structures are present in the experimental molecular beam for a given cluster size. For Au2, Au3 and Au4, the predicted vibrational spectra of the Kr-complexed and pristine species differ. For Au7, the presence of Kr influences the vibrational spectra only marginally. This behavior is explained in terms of the formation of a weak chemical bond between Kr and small gold clusters that localizes the Kr atom at a defined adsorption site, whereas for bigger clusters the vdW interactions prevail and the Kr adatom is delocalized and orbits the gold cluster. In all cases, at temperatures as low as T?=?100?K, vibrational spectra already display a notable anharmonicity and show, in comparison with harmonic spectra, different position of the peaks, different intensities and broadenings, and even the appearance of new peaks.