Single-reference ab initio methods for the calculation of excited states of large molecules.

While platinum has hitherto been the element of choice for catalysing oxygen electroreduction in acidic polymer fuel cells, tremendous progress has been reported for pyrolysed Fe-N-C materials. However, the structure of their active sites has remained elusive, delaying further advance. Here, we synthesized Fe-N-C materials quasi-free of crystallographic iron structures after argon or ammonia pyrolysis. These materials exhibit nearly identical Mossbauer spectra and identical X-ray absorption near-edge spectroscopy (XANES) spectra, revealing the same Fe-centred moieties. However, the much higher activity and basicity of NH3-pyrolysed Fe-N-C materials demonstrates that the turnover frequency of Fe-centred moieties depends on the physico-chemical properties of the support. Following a thorough XANES analysis, the detailed structures of two FeN4 porphyrinic architectures with different O2 adsorption modes were then identified. These porphyrinic moieties are not easily integrated in graphene sheets, in contrast with Fe-centred moieties assumed hitherto for pyrolysed Fe-N-C materials. These new insights open the path to bottom-up synthesis approaches and studies on site-support interactions.

Identification of catalytic sites for oxygen reduction in iron- and nitrogen-doped graphene materials.

We briefly review our implementation of the real-space Green's function (RSGF) approach for calculations of X-ray spectra, focusing on recently developed parameter free models for dominant many-body effects. Although the RSGF approach has been widely used both for near edge (XANES) and extended (EXAFS) ranges, previous implementations relied on semi-phenomenological methods, e.g., the plasmon-pole model for the self-energy, the final-state rule for screened core hole effects, and the correlated Debye model for vibrational damping. Here we describe how these approximations can be replaced by efficient ab initio models including a many-pole model of the self-energy, inelastic losses and multiple-electron excitations; a linear response approach for the core hole; and a Lanczos approach for Debye–Waller effects. We also discuss the implementation of these models and software improvements within the FEFF9 code, together with a number of examples.

Parameter-free calculations of X-ray spectra with FEFF9

The modern version of the KKR (Korringa–Kohn–Rostoker) method represents the electronic structure of a system directly and efficiently in terms of its single-particle Green's function (GF). This is in contrast to its original version and many other traditional wave-function-based all-electron band structure methods dealing with periodically ordered solids. Direct access to the GF results in several appealing features. In addition, a wide applicability of the method is achieved by employing multiple scattering theory. The basic ideas behind the resulting KKR-GF method are outlined and the different techniques to deal with the underlying multiple scattering problem are reviewed. Furthermore, various applications of the KKR-GF method are reviewed in some detail to demonstrate the remarkable flexibility of the approach. Special attention is devoted to the numerous developments of the KKR-GF method, that have been contributed in recent years by a number of work groups, in particular in the following fields: embedding schemes for atoms, clusters and surfaces, magnetic response functions and anisotropy, electronic and spin-dependent transport, dynamical mean field theory, various kinds of spectroscopies, as well as first-principles determination of model parameters.

https://iopscience.iop.org/article/10.1088/0034-4885/74/9/096501/pdf

Calculating condensed matter properties using the KKR-Green's function method—recent developments and applications

Ruthenium compounds, particularly Ru(II) polypyridine complexes, are the class of transition metal complexes which has been most deeply investigated from a photochemical viewpoint. The reason for such great interest stems from a unique combination of chemical stability, redox properties, excited-state reactivity, luminescence emission, and excited-state lifetime. Ruthenium polypyridine complexes are indeed good visible light absorbers, feature relatively intense and long-lived luminescence, and can undergo reversible redox processes in both the ground and excited states. This chapter presents some general concepts on the photochemical properties of Ru(II) polypyridine complexes and gives an overview of various research topics involving ruthenium photochemistry which have emerged in the last 15 years. In particular, aspects connected to supramolecular photochemistry and photophysics are discussed, such as multicomponent systems for light harvesting and photoinduced charge separation, systems for photoinduced multielectron/hole storage, and photocatalytic processes based on supramolecular Ru(II) polypyridine species. Interaction with biological systems and dye-sensitized photoelectrochemical cells are also briefly discussed.

Photochemistry and Photophysics of Coordination Compounds: Ruthenium

We present a general multiple-scattering (MS) scheme utilizing complex potentials for the calculation and interpretation of inner-shell x-ray-absorption fine structure for clusters of atoms in condensed matter with application to the case of the molecules SF 6 , GeCl 4 , and Br 2 . The method is based on the solution of the Dyson equation with a complex self-energy of the Hedin-Lundqvist type for the single-particle Green's function describing the propagation of the excited photoelectron through the system

General multiple-scattering scheme for the computation and interpretation of x-ray-absorption fine structure in atomic clusters with applications to SF6, GeCl4, and Br2 molecules.

A combined x-ray-absorption near-edge structure and extended x-ray-absorption fine-structure quantitative analysis, based on fitting procedures of both low- and high-energy ranges, has been carried out at the Cu K edge. This allowed an accurate determination of the structural parameters associated with the first hydration shell of the Cu 2 + aqua complex. The existence of an average fivefold coordination has been evidenced, ruling out the recently proposed regular pyramidal configuration with five equal Cu-O distances. Our structural results are compared with recent neutron-diffraction data performed by the second-difference isotopic substitution method.

Evidence of distorted fivefold coordination of the Cu 2+ aqua ion from an x-ray-absorption spectroscopy quantitative analysis

A combined x-ray-absorption near edge structure (XANES) and extended x-ray-absorption fine-structure (EXAFS) quantitative analysis of ${\mathrm{Co}}^{2+},$ ${\mathrm{Ni}}^{2+},$ and ${\mathrm{Zn}}^{2+}$ in water solutions based on fitting procedures of both low- and high-energy ranges has been carried out. The hydrogen contribution has been accounted for in both analyses and the effect of its inclusion on the structural parameters has been highlighted. The structural results obtained from the XANES and EXAFS analyses are in good agreement, confirming the validity of the application of the multiple-scattering theory in the low-energy range of the x-ray-absorption spectra. A systematic shortening of the ion-water first shell distance of about $0.03 \mathrm{\AA{}}$ is obtained from the XANES analyses, as compared to the EXAFS one. The origin of this systematic effect has been deeply investigated and it has been found to be due to the low-energy behavior of the real part of the Hedin-Lundqvist potential used in the calculations.

Combined XANES and EXAFS analysis of Co 2 + , Ni 2 + , and Zn 2 + aqueous solutions

Quantitative Analysis of X-Ray Absorption Near Edge Structure Data by a Full Multiple Scattering Procedure

A quite unexpected sevenfold coordination of the hydrated Hg(II) complex in aqueous solution is revealed by an extensive study combining X-ray absorption spectroscopy (XAS) and quantum mechanics/molecular dynamics (QM/MD) calculations. As a matter of fact, the generally accepted octahedral solvation of Hg(II) ion cannot be reconciled with XAS results. Next, refined QM computations point out the remarkable stability of a heptacoordinated structure with C 2 symmetry, and long-time MD simulations by new interaction potentials including many-body effects reveal that the hydrated complex has a quite flexible structure, corresponding for most of the time to heptacoordinated species. This picture is fully consistent with X-ray absorption near-edge structure experimental data which unambiguously show the preference for a sevenfold instead of a sixfold coordination.

M. Benfatto

Papers

General multiple-scattering scheme for the computation and interpretation of x-ray-absorption fine structure in atomic clusters with applications to SF6, GeCl4, and Br2 molecules.

Evidence of distorted fivefold coordination of the Cu 2+ aqua ion from an x-ray-absorption spectroscopy quantitative analysis

Combined XANES and EXAFS analysis of Co 2 + , Ni 2 + , and Zn 2 + aqueous solutions

Quantitative Analysis of X-Ray Absorption Near Edge Structure Data by a Full Multiple Scattering Procedure

Evidence for sevenfold coordination in the first solvation shell of Hg(II) aqua ion.