Spherical momentum distribution of the protons in hexagonal ice from modeling of inelastic neutron scattering data.

The spherical momentum distribution of the protons in ice is extracted from a high resolution deep inelastic neutron scattering experiment. Following a recent path integral Car-Parrinello molecular dynamics study, data were successfully interpreted in terms of an anisotropic Gaussian model, with a statistical accuracy comparable to that of the model independent scheme used previously, but providing more detailed information on the three dimensional potential energy surface experienced by the proton. A recently proposed theoretical concept is also employed to directly calculate the mean force from the experimental neutron Compton profile, and to evaluate the accuracy required to unambiguously resolve and extract the effective proton potential from the experimental data.

Melting of diamond at high pressure.

Melting of diamond at high pressure and the properties of liquid carbon at pressures greater than 1 megabar were investigated with a first-principles molecular dynamics technique. The results indicate an increase of the diamond melting temperature with pressure, which is opposite to the behavior of silicon and germanium. This is contrary to long-held assumptions, but agrees with recent experiments, and has important implications for geology and astrophysics. As is the case for the solid phase of carbon at low temperature, which changes greatly with pressure from graphite to diamond, the structural and bonding properties of liquid carbon vary strongly with pressure.

First solvation shell of the Cu(II) aqua ion: evidence for fivefold coordination.

We determined the structure of the hydrated Cu(II) complex by both neutron diffraction and first-principles molecular dynamics. In contrast with the generally accepted picture, which assumes an octahedrally solvated Cu(II) ion, our experimental and theoretical results favor fivefold coordination. The simulation reveals that the solvated complex undergoes frequent transformations between square pyramidal and trigonal bipyramidal configurations. We argue that this picture is also consistent with experimental data obtained previously by visible near-infrared absorption, x-ray absorption near-edge structure, and nuclear magnetic resonance methods. The preference of the Cu(II) ion for fivefold instead of sixfold coordination, which occurs for other cations of comparable charge and size, results from a Jahn-Teller destabilization of the octahedral complex.

Advanced capabilities for materials modelling with Quantum ESPRESSO.

Quantum EXPRESSO is an integrated suite of open-source computer codes for quantum simulations of materials using state-of-the-art electronic-structure techniques, based on density-functional theory, density-functional perturbation theory, and many-body perturbation theory, within the plane-wave pseudopotential and projector-augmented-wave approaches. Quantum EXPRESSO owes its popularity to the wide variety of properties and processes it allows to simulate, to its performance on an increasingly broad array of hardware architectures, and to a community of researchers that rely on its capabilities as a core open-source development platform to implement their ideas. In this paper we describe recent extensions and improvements, covering new methodologies and property calculators, improved parallelization, code modularization, and extended interoperability both within the distribution and with external software.

Dangling bond defects at Si-SiO2 interfaces: atomic structure of the P(b1) center

Using a first-principles approach, we characterize dangling bond defects at Si-SiO2 interfaces by calculating hyperfine parameters for several relaxed structures. Interface models, in which defect Si atoms remain close to crystalline sites of the substrate upon relaxation, successfully describe P(b) and P(b0) defects at (111) and (100) interfaces, respectively. On the basis of calculated hyperfine parameters, we discard models of the P(b1) defect containing a first neighbor shell with an O atom or a strained bond. A novel model consisting of an asymmetrically oxidized dimer yields hyperfine parameters in excellent agreement with experiment and is proposed as the structure of the P(b1) center.