We investigate the importance of the far red wing of the Lyman $ \alpha$ line of hydrogen in the atmospheres of cool white dwarfs of pure hydrogen composition. We find that this absorption process dominates all important sources of opacity in the blue part of the optical spectrum of these stars. Our successful fits to the spectra of cool DA/DC white dwarfs indicate that the far red wing of the $\rm Ly \alpha$ line is the source of opacity that had been missing in the models. The observed sequence of cool white dwarfs in color-color diagrams is very well reproduced by our new pure hydrogen atmosphere models, suggesting that the atmospheric composition of the coolest DC white dwarfs must be revisited.Comment: 11 pages, 4 figures, accepted for publication in The Astrophysical Journal Letter

A comprehensive phase diagram of lowest-energy structures and compositions of the rutile ${\text{TiO}}_2\left(110\right)$ surface in equilibrium with a surrounding gas phase at finite temperatures and pressures has been determined using density-functional theory in combination with a thermodynamic formalism. The exchange of oxygen, hydrogen, and water molecules with the gas phase is considered. Particular attention is given to the convergence of all calculations with respect to lateral system size and slab thickness. In addition, the reliability of semilocal density functionals in describing the energetics of the reduced surfaces is critically evaluated. For ambient conditions the surface is found to be fully covered by molecularly adsorbed water. At low coverages, in the limit of single isolated water molecules, molecular and dissociative adsorption modes become energetically degenerate. Oxygen vacancies form in strongly reducing, oxygen-poor environments. However, already at slightly more moderate conditions it is shown that removing full ${\text{TiO}}_2$ units from the surface is thermodynamically preferred. In agreement with recent experimental observations it is furthermore confirmed that even under extremely hydrogen-rich environments the surface cannot be fully hydroxylated, but only a maximum coverage with hydrogen of about 0.6–0.7 monolayer can be reached. Finally, calculations of migration paths strongly suggest that hydrogen prefers diffusing into the bulk over desorbing from the surface into the gas phase.

The dynamics of an F center created by an oxygen vacancy on the TiO2(110) rutile surface has been investigated using ab~initio molecular dynamics. These simulations uncover a truly complex, time-dependent behavior of fluctuating electron localization topologies in the vicinity of the oxygen vacancy. Although the two excess electrons are found to populate preferentially the second subsurface layer, they occasionally visit surface sites and also the third subsurface layer. This dynamical behavior of the excess charge explains hitherto conflicting interpretations of both theoretical findings and experimental data.

Using the linear response-based constrained local density approximation (cLDA) approach we systematically computed the Hubbard $U$ parameters for series of $3d, 4d,$ and $5d$ transition metals. We compare the results with estimations by the constrained random phase approximation (cRPA) method and discuss the performance of the self-consistent density functional theory + $U$ ($\mathrm{DFT}+U$) method for prediction of lattice parameters, work functions, $d$-bandwidths and $d$-band centers. Interestingly, we found that blindly applied the standard, fully localized limit (FLL) version of the $\mathrm{DFT}+U$ approach heavily overestimates the positions of $d$-band centers with respect to the Fermi level, but much better agreement with experiment is obtained when applying a more realistic, Wannier-type representation of $d$ orbitals for projection of $d$ states occupancies. We present another, independent estimate of the Hubbard $U$ parameter based on the comparison of Hartree-Fock and DFT eigenvalues, and positions of $d$-band centers. The so-derived estimates are surprisingly well consistent with the ones derived from the above-mentioned first principles approaches, and allow for validation of cRPA or cLDA results for the disputed cases, including Cu, Ag, and Au for which large $U$ parameters are obtained from the cLDA method.

Doping with Fe boosts the electrocatalytic performance of NiOOH for the oxygen evolution reaction (OER). To understand this effect, we have employed state-of-the-art electronic structure calculations and thermodynamic modeling. Our study reveals that at low concentrations Fe exists in a low-spin state. Only this spin state explains the large solubility limit of Fe and similarity of Fe-O and Ni-O bond lengths measured in the Fe-doped NiOOH phase. The low-spin state renders the surface Fe sites highly active for the OER. The low-to-high spin transition at the Fe concentration of ~ 25% is consistent with the experimentally determined solubility limit of Fe in NiOOH. The thermodynamic overpotentials computed for doped and pure materials, η = 0.42 V and 0.77 V, agree well with the measured values. Our results indicate a key role of the low-spin state of Fe for the OER activity of Fe-doped NiOOH electrocatalysts.