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.

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Iron in the major lower mantle (LM) minerals undergoes a high spin (HS) to low spin (LS) transition at relevant pressures (23-135 GPa). Previous failures of standard first principles approaches to describe this phenomenon have hindered its investigation and the clarification of important consequences. Using a rotationally invariant formulation of LDA + U we report a successful study of this transition in low solute concentration magnesiowüstite, (Mg(1-x)Fe(x)(O), (x < 0.2) the second most abundant LM phase. We show that the HS-LS transition goes through an insulating (semiconducting) intermediate mixed spins (MS) state without discontinuous changes in properties, as seen experimentally. We show that the HS state crosses over smoothly to the LS state passing through an insulating MS state where properties change continuously, as seen experimentally.

Using density functional theory plus Hubbard U calculations, we show that the ground state of (Mg,Fe)(Si,Fe)O(3) perovskite, the major mineral phase in Earth's lower mantle, has high-spin ferric iron (S=5/2) at both dodecahedral (A) and octahedral (B) sites. With increasing pressure, the B-site iron undergoes a spin-state crossover to the low-spin state (S=1/2) between 40 and 70 GPa, while the A-site iron remains in the high-spin state. This B-site spin-state crossover is accompanied by a noticeable volume reduction and an increase in quadrupole splitting, consistent with recent x-ray diffraction and Mössbauer spectroscopy measurements. The anomalous volume reduction leads to a significant softening in bulk modulus during the crossover, suggesting a possible source of seismic-velocity anomalies in the lower mantle.

The thermoelastic properties of ferropericlase Mg1؊xFexO (x ‫؍‬ 0.1875) throughout the iron high-to-low spin cross-over have been investigated by first principles at Earth's lower mantle conditions. This cross-over has important consequences for elasticity such as an anomalous bulk modulus (K S) reduction. At room temperature the anomaly is somewhat sharp in pressure but broadens with increasing temperature. Along a typical geotherm it occurs across most of the lower mantle with a more significant K S reduction at Ϸ1,400 -1,600 km depth. This anomaly might also cause a reduction in the effective activation energy for diffusion creep and lead to a viscosity minimum in the mid-lower mantle, in apparent agreement with results from inversion of data related with mantle convection and postglacial rebound.Earth's lower mantle ͉ viscosity ͉ thermodynamics ͉ thermal expansivity U nderstanding of the Earth's lower mantle relies on indirect lines of evidence. Comparison of elastic properties extracted from seismic models with computed or measured elastic properties of candidate minerals at mantle conditions is a fruitful line of enquiry. For instance, it has shed light on the lower mantle composition (1-3) and on the nature of the DЉ layer (4, 5). Such comparisons support the notion that the lower mantle consists primarily of ferrosilicate perovskite, Mg 1Ϫy Fe y SiO 3 , and ferropericlase, Mg 1Ϫx Fe x O (hereafter, Pv and Fp, respectively). In contrast, evidence based on solar and chondritic abundances suggests a deep lower mantle chemical transition into a pure Pv composition at Ϸ1,000 km depth (6). A chemical transition with wide topography, gentle, and diffuse changes in elasticity and density is also supported by geodynamic modeling (7). The discovery of the spin cross-over in Fp and Pv at lower mantle pressures (8, 9) introduces a new dramatic ingredient that demands a careful reexamination of these phases' elastic properties at appropriate conditions, the consequences for mantle elasticity, and reanalysis of lower mantle properties. This may, after all, support lower mantle models containing a chemical transition. Here, we show the effect of the spin cross-over on the bulk modulus and bulk velocity of Fp at high temperatures. We also show the effect it should have on the bulk modulus of a homogeneous lower mantle with pyrolite composition and confirm and justify the origin of anomalies in the elasticity of Fp recently demonstrated at room temperature (10). We point out that such an elastic anomaly might alter the activation energy for diffusion creep (11,12) in Fp, which might affect mantle viscosity. Results and DiscussionsThe high-spin (HS) to low-spin (LS) cross-over (13) in ferrous iron in Fp has been detected by several techniques at room temperature (8,10,(14)(15)(16)(17)(18) and recently up to 2,000 K (19). For typical mantle compositions the cross-over may start as low as Ϸ35 GPa (18) and end as high as 75 GPa (8) at room temperature. The observed variations in the pressure range of the transition seem to b...

We have investigated by first principles the electronic, vibrational, and structural properties of bct C4, a new form of crystalline sp{3} carbon recently found in molecular dynamics simulations of carbon nanotubes under pressure. This phase is transparent, dynamically stable at zero pressure, and more stable than graphite beyond 18.6 GPa. Coexistence of bct C4 with M carbon can explain better the x-ray diffraction pattern of a transparent and hard phase of carbon produced by the cold compression of graphite. Its structure appears to be intermediate between that of graphite and hexagonal diamond. These facts suggest that bct C4 is an accessible form of sp{3} carbon along the graphite-to-hexagonal diamond transformation path.

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