Ab initio random structure searching and single-crystal x-ray diffraction have been used to determine the full structures of three phases of lithium, recently discovered at low temperature above 60 GPa. A structure with C2mb symmetry, calculated to be a poor metal, is proposed for the oC88 phase (60-65 GPa). The oC40 phase (65-95 GPa) is found to have a lowest-enthalpy structure with C2cb symmetry, in excellent agreement with the x-ray data. It is calculated to be a semiconductor with a band gap of ∼1  eV at 90 GPa. oC24, stable above 95 GPa, has the space group Cmca, and refined atomic coordinates are in excellent agreement with previous calculations.

Experimentally, we have found that among the "complicated" phases of potassium at intermediate pressures is one which has the same space group as the double hexagonal-close-packed structure, although its atomic coordination is completely different. Calculations on this P6(3)/mmc (hP4) structure as a function of pressure show three isostructural transitions and three distinctive types of chemical bonding: free electron, ionic, and metallic. Interestingly, relationships between localized metallic structures and ionic compounds are found.

We present a detailed study of different exchange-correlation (xc) functionals in describing the dynamical properties of finite systems. For that purpose, we calculated the static polarizabilities, ionization potentials and optical absorption spectrum of four small clusters, Na 2 , Na 4 , SiH 4 and Si 2 H 6 , using a real-space, real-time technique. The computed static polarizabilities and ionization potentials seem to be in rather good agreement with the available experimental data, once the proper asymptotics of the potential are taken into account. The same conclusion holds for the absorption spectra, although the xc kernels in use do not provide a sufficiently strong attractive interaction between electrons and holes, leading to spectra slightly shifted towards higher energies. This deficiency is traced back to the insufficient description of dynamical effects in the correlation functional. Furthermore, it is shown that the xc potential used to obtain the ground state is the key factor to get reasonable spectra, whereas the choice of the xc kernel just amounts to small, although important, quantitative changes.

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The optical response of the lowest energy isomers of the C 20 family is calculated using time-dependent density functional theory within a real-space, real-time scheme. Significant differences are found among the spectra of the different isomers, and thus we propose optical spectroscopy as a tool for experimental investigation of the structure of these important clusters.

The properties of hydrogen at high pressure have wide implications in astrophysics and high-pressure physics. Its phase change in the liquid is variously described as a metallization, H2-dissociation, density discontinuity or plasma phase transition. It has been tacitly assumed that these phenomena coincide at a first-order liquid-liquid transition (LLT). In this work, the relevant pressure-temperature conditions are thoroughly explored with first-principles molecular dynamics. We show there is a large dependency on exchange-correlation functional and significant finite size effects. We use hysteresis in a number of measurable quantities to demonstrate a first-order transition up to a critical point, above which molecular and atomic liquids are indistinguishable. At higher temperature beyond the critical point, H2-dissociation becomes a smooth cross-over in the supercritical region that can be modelled by a pseudo-transition, where the H2→2H transformation is localized and does not cause a density discontinuity at metallization. Thermodynamic anomalies and counter-intuitive transport behavior of protons are also discovered even far beyond the critical point, making this dissociative transition highly relevant to the Supplementary Material. Results of dimer-dimer bond-length distribution (DDLD) calculations, comparison between PBE and vdW-DF results, the thermodynamic modelling of pseudo-transition and phase boundaries, calculated isotherms (equation of state, EOS), transient clusters and their lifetime and charge state, the angular distribution function of H3 clusters, mixing of H and H2 liquid, thermal fluctuation analysis, isotope effect, finite size effects, and the assessment of proton selfdiffusivity and viscosity.

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We have performed in situ synchrotron x-ray diffraction studies of the iridium-hydrogen system up to 125 GPa. At 55 GPa, a phase transition in the metal lattice from the fcc to a distorted simple cubic phase is observed. The new phase is characterized by a drastically increased volume per metal atom, indicating the formation of a metal hydride, and substantially decreased bulk modulus of 190 GPa (383 GPa for pure Ir). Ab initio calculations show that the hydrogen atoms occupy the face-centered positions in the metal matrix, making this the first known noninterstitial noble metal hydride and, with a stoichiometry of IrH 3 , the one with the highest volumetric hydrogen content. Computations also reveal that several energetically competing phases exist, which can all be seen as having distorted simple cubic lattices. Slow kinetics during decomposition at pressures as low as 6 GPa suggest that this material is metastable at ambient pressure and low temperatures.