First principles study of electronic and structural properties of CuO

We investigate the electronic and structural properties of CuO, which shows significant deviations from the trends obeyed by other transition-metal monoxides. Using an extended Hubbard corrective functional, we uncover an orbitally ordered insulating ground state for the cubic phase of this material, which was expected but never found before. This insulating state results from a fine balance between the tendency of Cu to complete its d-shell and Hund's rule magnetism. Starting from the ground state for the cubic phase, we also study tetragonal distortions of the unit cell (recently reported in experiments), the consequent electronic reorganizations and identify the equilibrium structure. Our calculations reveal an unexpected richness of possible magnetic and orbital orders, relatively close in energy to the ground state, whose stability depends on the sign and entity of distortion.Comment: 9 pages, 9 figure

Structure and energetics of solvated ferrous and ferric ions: Car-Parrinello molecular dynamics in the DFT+U formalism

We implemented a rotationally-invariant Hubbard U extension to density-functional theory in the Car-Parrinello molecular dynamics framework, with the goal of bringing the accuracy of the DFT+U approach to finite-temperature simulations, especially for liquids or solids containing transition-metal ions. First, we studied the effects on the Hubbard U on the static equilibrium structure of the hexa-aqua ferrous and ferric ions, and the inner-sphere reorganization energy for the electron-transfer reaction between aqueous ferrous and ferric ions. It is found that the reorganization energy is increased, mostly as a result of the Fe-O distance elongation in the hexa-aqua ferrous ion. Second, we performed a first-principles molecular dynamics study of the solvation structure of the two aqueous ferrous and ferric ions. The Hubbard term is found to change the Fe-O radial distribution function for the ferrous ion, while having a negligible effect on the aqueous ferric ion. Moreover, the frequencies of vibrations between Fe and oxygen atoms in the first-solvation shell are shown to be unaffected by the Hubbard corrections for both ferrous and ferric ions.Comment: 13 pages, 2 figures, 1 table. Submitted to Journal of Electroanalytical Chemistr

Simulation of Heme using DFT+U: a step toward accurate spin-state energetics

We investigate the DFT+U approach as a viable solution to describe the low-lying states of ligated and unligated iron heme complexes. Besides their central role in organometallic chemistry, these compounds represent a paradigmatic case where LDA, GGA, and common hybrid functionals fail to reproduce the experimental magnetic splittings. In particular, the imidazole pentacoordinated heme is incorrectly described as a triplet by all usual DFT flavors. In this study we show that a U parameter close to 4 eV leads to spin transitions and molecular geometries in quantitative agreement with experiments, and that DFT+U represents an appealing tool in the description of iron porphyrin complexes, at a much reduced cost compared to correlated quantum-chemistry methods. The possibility of obtaining the U parameter from first-principles is explored through a self-consistent linear-response formulation. We find that this approach, which proved to be successful in other iron systems, produces in this case some overestimation with respect to the optimal values of U.Comment: To be published in The Journal of Physical Chemistry B 30 pages, 15 figure

Energetics and cathode voltages of LiMPO4_4 olivines (M = Fe, Mn) from extended Hubbard functionals

Transition-metal compounds pose serious challenges to first-principles calculations based on density-functional theory (DFT), due to the inability of most approximate exchange-correlation functionals to capture the localization of valence electrons on their $d$ states, essential for a predictive modeling of their properties. In this work we focus on two representatives of a well known family of cathode materials for Li-ion batteries, namely the orthorhombic LiMPO$_4$ olivines (M = Fe, Mn). We show that extended Hubbard functionals with on-site ($U$) and inter-site ($V$) interactions (so called DFT+U+V) can predict the electronic structure of the mixed-valence phases, the formation energy of the materials with intermediate Li contents, and the overall average voltage of the battery with remarkable accuracy. We find, in particular, that the inclusion of inter-site interactions in the corrective Hamiltonian improves considerably the prediction of thermodynamic quantities when electronic localization occurs in the presence of significant interatomic hybridization (as is the case for the Mn compound), and that the self-consistent evaluation of the effective interaction parameters as material- and ground-state-dependent quantities allows the prediction of energy differences between different phases and concentrations

Electronic-enthalpy functional for finite systems under pressure

We introduce the notion of electronic enthalpy for first-principles structural and dynamical calculations of finite systems under pressure. An external pressure field is allowed to act directly on the electronic structure of the system studied via the ground-state minimization of the functional $E+PV_{q}$, where $V_{q}$ is the quantum volume enclosed by a charge isosurface. The Hellmann-Feynman theorem applies, and assures that the ionic equations of motion follow an isoenthalpic dynamics. No pressurizing medium is explicitly required, while coatings of environmental ions or ligands can be introduced if chemically relevant. We apply this novel approach to the study of group-IV nanoparticles during a shock wave, highlighting the significant differences inthe plastic or elastic response of the diamond cage under load, and their potential use as novel nanostructured impact-absorbing materials.Comment: 4 pages, 4 figure

Spin-state crossover and hyperfine interactions of ferric iron in MgSiO3_3 perovskite

Using density functional theory plus Hubbard $U$ calculations, we show that the ground state of (Mg,Fe)(Si,Fe)O$_3$ perovskite, a major mineral phase in the Earth's lower mantle, has high-spin ferric iron ($S=5/2$) at both the dodecahedral (A) and octahedral (B) site. As the pressure increases, the B-site iron undergoes a spin-state crossover to the low-spin state ($S=1/2$), while the A-site iron remains in the high-spin state. Our calculation shows that the B-site spin-state crossover in the pressure range of 40-70 GPa is accompanied by a noticeable volume reduction and an increase in quadrupole splitting, consistent with recent X-ray diffraction and M\"ossbauer spectroscopy measurements. The volume reduction leads to a significant softening in the bulk modulus, which suggests a possible source of seismic velocity anomalies in the lower mantle.Comment: 11 pages, 4 figures, 1 tabl

Hubbard parameters from density-functional perturbation theory

We present a transparent and computationally efficient approach for the first-principles calculation of Hubbard parameters from linear-response theory. This approach is based on density-functional perturbation theory and the use of monochromatic perturbations. In addition to delivering much improved efficiency, the present approach makes it straightforward to calculate automatically these Hubbard parameters for any given system, with tight numerical control on convergence and precision. The effectiveness of the method is showcased in three case studies - Cu$_2$O, NiO, and LiCoO$_2$ - and by the direct comparison with finite differences in supercell calculations.Comment: 17 pages, 6 figure

Self-consistent Hubbard parameters from density-functional perturbation theory in the ultrasoft and projector-augmented wave formulations

The self-consistent evaluation of Hubbard parameters using linear-response theory is crucial for quantitatively predictive calculations based on Hubbard-corrected density-functional theory. Here, we extend a recently-introduced approach based on density-functional perturbation theory (DFPT) for the calculation of the on-site Hubbard $U$ to also compute the inter-site Hubbard $V$. DFPT allows to reduce significantly computational costs, improve numerical accuracy, and fully automate the calculation of the Hubbard parameters by recasting the linear response of a localized perturbation into an array of monochromatic perturbations that can be calculated in the primitive cell. In addition, here we generalize the entire formalism from norm-conserving to ultrasoft and projector-augmented wave formulations, and to metallic ground states. After benchmarking DFPT against the conventional real-space Hubbard linear response in a supercell, we demonstrate the effectiveness of the present extended Hubbard formulation in determining the equilibrium crystal structure of Li$_x$MnPO$_4$ (x=0,1) and the subtle energetics of Li intercalation.Comment: 15 pages, 3 figure