QUANTUM ESPRESSO is an integrated suite of computer codes for electronic-structure calculations and materials modeling, based on density-functional theory, plane waves, and pseudopotentials (norm-conserving, ultrasoft, and projector-augmented wave). The acronym ESPRESSO stands for opEn Source Package for Research in Electronic Structure, Simulation, and Optimization. It is freely available to researchers around the world under the terms of the GNU General Public License. QUANTUM ESPRESSO builds upon newly-restructured electronic-structure codes that have been developed and tested by some of the original authors of novel electronic-structure algorithms and applied in the last twenty years by some of the leading materials modeling groups worldwide. Innovation and efficiency are still its main focus, with special attention paid to massively parallel architectures, and a great effort being devoted to user friendliness. QUANTUM ESPRESSO is evolving towards a distribution of independent and interoperable codes in the spirit of an open-source project, where researchers active in the field of electronic-structure calculations are encouraged to participate in the project by contributing their own codes or by implementing their own ideas into existing codes.

The structure of self-assembled monolayers (SAMs) of long-chain alkyl sulfides on gold(111) has been resolved by density functional theory-based molecular dynamics simulations and grazing incidence x-ray diffraction for hexanethiol and methylthiol. The analysis of molecular dynamics trajectories and the relative energies of possible SAM structures suggest a competition between SAM ordering, driven by the lateral van der Waals interaction between alkyl chains, and disordering of interfacial Au atoms, driven by the sulfur-gold interaction. We found that the sulfur atoms of the molecules bind at two distinct surface sites, and that the first gold surface layer contains gold atom vacancies (which are partially redistributed over different sites) as well as gold adatoms that are laterally bound to two sulfur atoms.

The phase diagrams of water and ammonia were determined by constant pressure ab initio molecular dynamic simulations at pressures (30 to 300 gigapascal) and temperatures (300 to 7000 kelvin) of relevance for the middle ice layers of the giant planets Neptune and Uranus. Along the planetary isentrope water and ammonia behave as fully dissociated ionic, electronically insulating fluid phases, which turn metallic at temperatures exceeding 7000 kelvin for water and 5500 kelvin for ammonia. At lower temperatures, the phase diagrams of water and ammonia exhibit a superionic solid phase between the solid and the ionic liquid. These simulations improve our understanding of the properties of the middle ice layers of Neptune and Uranus.

Two new transition metal nitrides, IrN2 and OsN2, were synthesized at high pressures and temperatures using laser-heated diamond-anvil cell techniques. Synchrotron x-ray diffraction was used to determine the structures of novel nitrides and the equations of states of both the parent metals as well as the newly synthesized materials. The compounds have bulk moduli comparable with those of the traditional superhard materials. For IrN2, the measured bulk modulus [K0 = 428(12) GPa] is second only to that of diamond (K0 = 440 GPa). Ab initio calculations indicate that both compounds have a metal:nitrogen stoichiometry of 1:2 and that nitrogen intercalates in the lattice of the parent metal in the form of single-bonded N-N units.

The bulk properties of iron at the pressure and temperature conditions of Earth's core were determined by a method that combines first-principles and classical molecular dynamic simulations. The theory indicates that (i) the iron melting temperature at inner-core boundary (ICB) pressure (330 gigapascals) is 5400 (+/-400) kelvin; (ii) liquid iron at ICB conditions is about 6% denser than Earth's outer core; and (iii) the shear modulus of solid iron close to its melting line is 140 gigapascals, consistent with the seismic value for the inner core. These results reconcile melting temperature estimates based on sound velocity shock wave data with those based on diamond anvil cell experiments.

We present a classical interatomic force field for liquid SiO 2 which has been parametrized using the forces, stresses and energies extracted from ab initio calculations. We show how inclusion of more electronic effects in a phenomenological way and parametrization at the relevant conditions of pressure and temperature allow the creation of more accurate force fields. We compare the results of simulations with this force field both to experiment and to the results of ab initio molecular dynamics simulations and show how our procedure leads to comparisons which are greatly improved with respect to the most widely used force fields for silica.

The role of molecular dipole moment, charge transfer, and Pauli repulsion in determining the work-function change (Deltaphi) at organic-metal interfaces has been elucidated by a combined experimental and theoretical study of (CH(3)S)(2)/Au(111) and CH(3)S/Au(111). Comparison between experiment and theory allows us to determine the origin of the interface dipole layer for both phases. For CH(3)S/Au(111), Deltaphi can be ascribed almost entirely to the dipole moment of the CH(3)S layer. For (CH(3)S)(2)/Au(111), a Pauli repulsion mechanism occurs. The implications of these results on the interpretation of Deltaphi in the presence of strongly and weakly adsorbed molecules is discussed.

Among the group IV elements, only carbon forms stable double bonds with oxygen at ambient conditions. At variance with silica and germania, the non-molecular single-bonded crystalline form of carbon dioxide, phase V, only exists at high pressure. The amorphous forms of silica (a-SiO2) and germania (a-GeO2) are well known at ambient conditions; however, the amorphous, non-molecular form of CO2 has so far been described only as a result of first-principles simulations. Here we report the synthesis of an amorphous, silica-like form of carbon dioxide, a-CO2, which we call 'a-carbonia'. The compression of the molecular phase III of CO2 between 40 and 48 GPa at room temperature initiated the transformation to the non-molecular amorphous phase. Infrared spectra measured at temperatures up to 680 K show the progressive formation of C-O single bonds and the simultaneous disappearance of all molecular signatures. Furthermore, state-of-the-art Raman and synchrotron X-ray diffraction measurements on temperature-quenched samples confirm the amorphous character of the material. Comparison with vibrational and diffraction data for a-SiO2 and a-GeO2, as well as with the structure factor calculated for the a-CO2 sample obtained by first-principles molecular dynamics, shows that a-CO2 is structurally homologous to the other group IV dioxide glasses. We therefore conclude that the class of archetypal network-forming disordered systems, including a-SiO2, a-GeO2 and water, must be extended to include a-CO2.