Publications

A comparison of DFT methods Density functional theory (DFT) is now routinely used for simulating material properties. Many software packages are available, which makes it challenging to know which are the best to use for a specific calculation. Lejaeghere et al. compared the calculated values for the equation of states for 71 elemental crystals from 15 different widely used DFT codes employing 40 different potentials (see the Perspective by Skylaris). Although there were variations in the calculated values, most recent codes and methods converged toward a single value, with errors comparable to those of experiment. Science , this issue p. 10.1126/science.aad3000 ; see also p. 1394

No abstract is provided for this article.

Well-ordered epitaxial V2O3(0001) films (thickness>80 Å) have been prepared on the clean Rh(111) surface and investigated with scanning tunnelling microscopy, low-energy electron diffraction, high-resolution electron energy loss spectroscopy and high-resolution X-ray photoelectron spectroscopy with synchrotron radiation. Atomically flat V2O3(0001) surfaces, terminated by vanadyl (VO) groups, have been obtained after the reactive oxide deposition and subsequent annealing in vacuum to 600 °C. Annealing the VO termination in oxygen atmosphere (500 °C, 5×10−6 mbar) results in the partial removal of the vanadyl groups and in the formation of an oxygen-richer surface termination, exhibiting a (√3×√3)R30° structure. Structure models for this oxygen-rich termination are proposed, which are characterised by (OV)0.66–O3–V–V⋯ and (OV)0.33–O3–V–V⋯ stacking sequences. Conversely, deposition of submonolayer V coverages onto the VO surface leads to the formation of a metal-rich V2O3(0001) surface, terminated by a close-packed V3 layer on top of the bulk-type O3 plane. Such a VO(111)-layer is only metastable and oxidises back readily upon annealing in vacuum to the vanadyl-terminated V2O3(0001) surface.

Density functional theory (DFT) calculations of charged molecules and surfaces are critical to applications in electrocatalysis, energy materials, and related fields of materials science. DFT implementations such as the Vienna simulation package () compute the electrostatic potential under three-dimensional (3D) periodic boundary conditions, necessitating charge neutrality. In this work, we implement 0D and 2D periodic boundary conditions to facilitate DFT calculations of charged molecules and surfaces, respectively. We implement these boundary conditions using the Coulomb kernel truncation method. Our implementation computes the potential under 0D and 2D boundary conditions by selectively subtracting unwanted long-range interactions in the potential computed under 3D boundary conditions. By combining the Coulomb kernel truncation method with a computationally efficient padding approach, we remove nonphysical potentials from vacuum in 0D and 2D systems. To illustrate the computational efficiency of our method, we perform large supercell calculations of the formation energy of a charged chlorine defect on a sodium chloride (001) surface and perform long-timescale molecular dynamics simulations on a stepped gold (211) water electrode-electrolyte interface.

Reconstructive phase transitions involving breaking and reconstruction of primary chemical bonds are ubiquitous and important for many technological applications. In contrast to displacive phase transitions, the dynamics of reconstructive phase transitions are usually slow due to the large energy barrier. Nevertheless, the reconstructive phase transformation from $\beta$- to $\lambda$-Ti$_3$O$_5$ exhibits an ultrafast and reversible behavior. Despite extensive studies, the underlying microscopic mechanism remains unclear. Here, we discover a kinetically favorable in-plane nucleated layer-by-layer transformation mechanism through metadynamics and large-scale molecular dynamics simulations. This is enabled by developing an efficient machine learning potential with near first-principles accuracy through an on-the-fly active learning method and an advanced sampling technique. Our results reveal that the $\beta$-$\lambda$ phase transformation initiates with the formation of two-dimensional nuclei in the $ab$-plane and then proceeds layer-by-layer through a multistep barrier-lowering kinetic process via intermediate metastable phases. Our work not only provides important insight into the ultrafast and reversible nature of the $\beta$-$\lambda$ transition, but also presents useful strategies and methods for tackling other complex structural phase transitions.

Optical conductivity of an interacting polaron gas is calculated within an extended random phase approximation which takes into account mixing of collective excitations of the electron gas with longitudinal optical (LO) phonons. This mixing is important for the optical response of strongly polar crystals where the static dielectric constant is rather high, as in the case of strontium titanate. The present calculation sheds light on unexplained features of experimentally observed optical conductivity spectra in n-doped SrTiO 3 . These features appear to be due to dynamic screening of the electron–electron interaction by polar optical phonons and hence do not require additional mechanisms for their explanation.

Electronic structure calculations using the density functional theory (DFT) within the generalized-gradient approximation and ultra-soft pseudopotentials have been used to investigate the adsorption of water on the main cleavage planes of CaO and CaF2. The calculated structural parameters are found to be in good agreement with experiment. The unhydrated surfaces show negligible ionic relaxation from bulk termination due to the minimal distortion of the electron density in the surface layer. Electron density plots show both crystals to be strongly ionic. We found on both mineral surfaces that associative adsorption of water is energetically preferred. Water molecules which were initially dissociatively adsorbed, recombined to form associatively adsorbed species. The water molecules are adsorbed by their oxygen ion to surface calcium ions, but electron density plots show strong interactions between surface anions and hydrogen atoms. The calculated hydration energies of approximately 70kJmol−1 on the CaO {100} surface and 41–53kJmol−1 on the CaF2 {111} surface indicate physisorption on both surfaces.

Density functional theory (DFT) calculations of charged molecules and surfaces are critical to applications in electro-catalysis, energy materials and related fields of materials science. DFT implementations such as the Vienna ab-initio Simulation Package (VASP) compute the electrostatic potential under 3D periodic boundary conditions, necessitating charge neutrality. In this work, we implement 0D and 2D periodic boundary conditions to facilitate DFT calculations of charged molecules and surfaces respectively. We implement these boundary conditions using the Coulomb kernel truncation method. Our implementation computes the potential under 0D and 2D boundary conditions by selectively subtracting unwanted long-range interactions in the potential computed under 3D boundary conditions. By combining the Coulomb kernel truncation method with a computationally efficient padding approach, we remove nonphysical potentials from vacuum in 0D and 2D systems. To illustrate the computational efficiency of our method, we perform large supercell calculations of the formation energy of a charged chlorine defect on a sodium chloride (001) surface and perform long time-scale molecular dynamics simulations on a stepped gold (211) | water electrode-electrolyte interface.

We present a systematic investigation of the role and importance of excitonic effects on the optical properties of transitions metal oxide perovskites. A representative set of 14 compounds has been selected, including $3d$ (${\mathrm{SrTiO}}_{3}$, ${\mathrm{LaScO}}_{3}$, ${\mathrm{LaTiO}}_{3}$, ${\mathrm{LaVO}}_{3}$, ${\mathrm{LaCrO}}_{3}$, ${\mathrm{LaMnO}}_{3}$, ${\mathrm{LaFeO}}_{3}$, and ${\mathrm{SrMnO}}_{3}$), $4d$ (${\mathrm{SrZrO}}_{3}$, ${\mathrm{SrTcO}}_{3}$, and ${\mathrm{Ca}}_{2}{\mathrm{RuO}}_{4}$) and $5d$ (${\mathrm{SrHfO}}_{3}$, ${\mathrm{KTaO}}_{3}$, and ${\mathrm{NaOsO}}_{3}$) perovskites, covering a band gap ranging from 0.1 eV to 6.1 eV and exhibiting different electronic, structural, and magnetic properties. Optical conductivities and optical transitions including electron-hole interactions are calculated through the solution of the Bethe-Salpeter equation (BSE) with quasiparticle energies evaluated by the single-shot ${G}_{0}{W}_{0}$ approximation. The exciton binding energies are computed by means of a model BSE, carefully benchmarked against the full-BSE method, in order to obtain well-converged results in terms of $k$-point sampling. The predicted results are compared with available measured data, with an overall satisfactory agreement between theory and experiment.

Electron-phonon coupling (EPC) is key for understanding many properties of materials such as superconductivity and electric resistivity. Although first principles density-functional-theory (DFT) based EPC calculations are used widely, their efficacy is limited by the accuracy and efficiency of the underlying exchange-correlation functionals. These limitations become exacerbated in complex $d$- and $f$-electron materials, where beyond-DFT approaches and empirical corrections, such as the Hubbard $U$, are commonly invoked. Here, using the examples of CoO and NiO, we show how the efficient r2scan density functional correctly captures strong EPC effects in transition-metal oxides without requiring the introduction of empirical parameters. We also demonstrate the ability of r2scan to accurately model phonon-mediated superconducting properties of the main group compounds (e.g., MgB$_2$), with improved electronic bands and phonon dispersions over those of traditional density functionals. Our study provides a pathway for extending the scope of accurate first principles modeling of electron-phonon interactions to encompass complex $d$-electron materials.