Juan A. Santana

Structural stability and defect energetics of ZnO from diffusion quantum Monte Carlo

We have applied the many-body ab initio diffusion quantum Monte Carlo (DMC) method to study Zn and ZnO crystals under pressure and the energetics of the oxygen vacancy, zinc interstitial, and hydrogen impurities in ZnO. We show that DMC is an accurate and practical method that can be used to characterize multiple properties of materials that are challenging for density functional theory (DFT) approximations. DMC agrees with experimental measurements to within 0.3 eV, including the band-gap of ZnO, the ionization potential of O and Zn, and the atomization energy of O2, ZnO dimer, and wurtzite ZnO. DMC predicts the oxygen vacancy as a deep donor with a formation energy of 5.0(2) eV under O-rich conditions and thermodynamic transition levels located between 1.8 and 2.5 eV from the valence band maximum. Our DMC results indicate that the concentration of zinc interstitial and hydrogen impurities in ZnO should be low under n-type and Zn- and H-rich conditions because these defects have formation energies above 1.4 eV under these conditions. Comparison of DMC and hybrid functionals shows that these DFT approximations can be parameterized to yield a general correct qualitative description of ZnO. However, the formation energy of defects in ZnO evaluated with DMC and hybrid functionals can differ by more than 0.5 eV.

Cohesive energy and structural parameters of binary oxides of groups IIA and IIIB from diffusion quantum Monte Carlo

We have applied the diffusion quantum Monte Carlo (DMC) method to calculate the cohesive energy and the structural parameters of the binary oxides CaO, SrO, BaO, Sc2O3, Y2O3, and La2O3. The aim of our calculations is to systematically quantify the accuracy of the DMC method to study this type of metal oxides. The DMC results were compared with local, semi-local, and hybrid Density Functional Theory (DFT) approximations as well as with experimental measurements. The DMC method yields cohesive energies for these oxides with a mean absolute deviation from experimental measurements of 0.18(2) eV, while with local, semi-local, and hybrid DFT approximations, the deviation is 3.06, 0.94, and 1.23 eV, respectively. For lattice constants, the mean absolute deviations in DMC, local, semi-local, and hybrid DFT approximations are 0.017(1), 0.07, 0.05, and 0.04 Å, respectively. DMC is a highly accurate method, outperforming the DFT approximations in describing the cohesive energies and structural parameters of these binary oxides.

Many-body ab initio diffusion quantum Monte Carlo applied to the strongly correlated oxide NiO

We present a many-body diffusion quantum Monte Carlo (DMC) study of the bulk and defect properties of NiO. We find excellent agreement with experimental values, within 0.3%, 0.6%, and 3.5% for the lattice constant, cohesive energy, and bulk modulus, respectively. The quasiparticle bandgap was also computed, and the DMC result of 4.72 (0.17) eV compares well with the experimental value of 4.3 eV. Furthermore, DMC calculations of excited states at the L, Z, and the gamma point of the Brillouin zone reveal a flat upper valence band for NiO, in good agreement with Angle Resolved Photoemission Spectroscopy results. To study defect properties, we evaluated the formation energies of the neutral and charged vacancies of oxygen and nickel in NiO. A formation energy of 7.2 (0.15) eV was found for the oxygen vacancy under oxygen rich conditions. For the Ni vacancy, we obtained a formation energy of 3.2 (0.15) eV under Ni rich conditions. These results confirm that NiO occurs as a p-type material with the dominant intrinsic vacancy defect being Ni vacancy.

Multireference Møller–Plesset perturbation theory results on levels and transition rates in Al-like ions of iron group elements

Ground configuration and low-lying levels of Al-like ions contribute to a variety of laboratory and solar spectra, but the available information in databases are neither complete nor necessarily correct. We have performed multireference M?ller?Plesset perturbation theory calculations that approach spectroscopic accuracy in order to check the information that databases hold on the 40 lowest levels of Al-like ions of iron group elements (K through Ge), and to provide input for the interpretation of concurrent experiments. Our results indicate problems of the database holdings on the levels of the lowest quartet levels in the lighter elements of the range studied. The results of our calculations of the decay rates of five long-lived levels (3s23p?2Po3/2, 3s3p2?4PoJ and 3s3p3d 4Fo9/2) are compared with lifetime data from beam-foil, electron beam ion trap and heavy-ion storage ring experiments.

Hydrogen adsorption on and spillover from Au- and Cu-supported Pt3 and Pd3 clusters: a density functional study

Motivated by the use of electrodes modified at the nanoscale by supported metal species, we studied computationally how the reactivity changes in such a composite system compared to the reactivity of the individual systems, metal clusters and metal surfaces. Specifically, we examined hydrogen adsorption on and hydrogen spillover from Au- and Cu-supported Pt(3) and Pd(3) clusters, using a method based on Density Functional Theory. Two distinctive types of sites were found for the adsorption of atomic hydrogen: (i) on the supported clusters and (ii) at the cluster-substrate interfaces. The adsorption energy of hydrogen on the supported clusters is ∼20 kJ mol(-1) smaller when the cluster is supported by Cu instead of Au. In contrast, the substrate has no effect on hydrogen adsorbed at the cluster-substrate interfaces. Adsorbed Pt(3) and Pd(3) clusters locally modify the reactivity of the substrates as quantified by the reduced adsorption energy of hydrogen compared to the corresponding clean substrate. Hydrogen dissociative adsorption followed by spillover is thermodynamically and kinetically favored for clusters supported on a Cu surface, but not on Au. Moreover, spillover of hydrogen is more likely from metal-supported Pd than Pt clusters as revealed by barriers that are calculated 40-50 kJ mol(-1) lower in energy.

Successes and failures of Hubbard-corrected density functional theory. The case of Mg doped LiCoO2

We have evaluated the successes and failures of the Hubbard-corrected density functional theory approach to study Mg doping of LiCoO2. We computed the effect of the U parameter on the energetic, geometric, and electronic properties of two possible doping mechanisms: (1) substitution of Mg onto a Co (or Li) site with an associated impurity state and (2) formation of impurity-state-free complexes of substitutional Mg and point defects in LiCoO2. We find that formation of impurity states results in changes on the valency of Co in LiCoO2. Variation of the Co U shifts the energy of the impurity state, resulting in energetic, geometric, and electronic properties that depend significantly on the specific value of U. In contrast, the properties of the impurity-state-free complexes are insensitive to U. These results identify reasons for the strong dependence on the doping properties on the chosen value of U and for the overall difficulty of achieving agreement with the experimentally known energetic and electronic properties of doped transition metal oxides such as LiCoO2.

Relativistic multireference many-body perturbation theory for open-shell ions with multiple valence shell electrons: the transition rates and lifetimes of the excited levels in chlorinelike Fe X

A recently developed relativistic multireference many-body perturbation theory based on multireference configuration-interaction wavefunctions as zeroth-order wavefunctions is outlined. The perturbation theory employs a general class of configuration-interaction wavefunctions as reference functions, and thus is applicable to multiple open valence shell systems with near degeneracy of a manifold of strongly interacting configurations. Multireference many-body perturbation calculations are reported for the ground and excited states of chlorinelike Fe X in which the near degeneracy of a manifold of strongly interacting configurations mandates a multireference treatment. Term energies of a total of 83 excited levels arising from the 3s23p5, 3s3p6, 3s23p43d, 3s3p53d, and 3s23p33d2 configurations of the ion are evaluated to high accuracy. Transition rates associated with E1/M1/E2/M2/E3 radiative decays and lifetimes of a number of excited levels are calculated and compared with laboratory measurements to critically evaluate recent experiments.

Atomic lifetime measurements on forbidden transitions of Al-, Si-, P- and S-like ions at a heavy-ion storage ring

Lifetimes of 3s23pk ground configuration levels of Al-, Si-, P- and S-like ions of Fe, Co and Ni have been measured at a heavy-ion storage ring. Some of the observed decay curves show strong evidence of cascade repopulation from specific 3d levels that feature lifetimes in the same multi-millisecond range as the levels of the ground configuration. We identify the foil-stripping process in the ion production as the cause of the cascade level population and assess the importance of specific cascades for the measurement technique.

Diffusion quantum Monte Carlo calculations of SrFeO3 and LaFeO3

The equations of state, formation energy, and migration energy barrier of the oxygen vacancy in SrFeO3 and LaFeO3 were calculated with the diffusion quantum Monte Carlo (DMC) method. Calculations were also performed with various Density Functional Theory (DFT) approximations for comparison. DMC reproduces the measured cohesive energies of these materials with errors below 0.23(5) eV and the structural properties within 1% of the experimental values. The DMC formation energies of the oxygen vacancy in SrFeO3 and LaFeO3 under oxygen-rich conditions are 1.3(1) and 6.24(7) eV, respectively. Similar calculations with semi-local DFT approximations for LaFeO3 yielded vacancy formation energies 1.5 eV lower. Comparison of charge density evaluated with DMC and DFT approximations shows that DFT tends to overdelocalize the electrons in defected SrFeO3 and LaFeO3. Calculations with DMC and local density approximation yield similar vacancy migration energy barriers, indicating that steric/electrostatic effects mainly determine migration barriers in these materials.

Isoelectronic trends of the E1-forbidden decay rates of Al-, Si-, P-, and S-like ions of Cl, Ti, Mn, Cu, and Ge

Working towards a reliable grid of atomic lifetime data on 3s23pk ground configuration levels of Al-, Si-, P-, and S-like ions of iron group elements, measurements on electric-dipole forbidden decays in ions of Cl, Ti, Mn, Cu, and Ge have been done at a heavy-ion storage ring that complement earlier measurements on Fe, Co, Ni, and Cu ions. The data are compared isoelectronically with various predictions, especially those made by the multi-reference Moller–Plesset technique, and with the few results available from other experimental approaches. Apparent perturbations of otherwise smooth isoelectronic trends of some of the data are identified with the occurrence of specific cascades from long-lived 3d levels.