Kevin Jorissen

Parameter-free calculations of X-ray spectra with FEFF9

We briefly review our implementation of the real-space Greens 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.

A high performance scientific cloud computing environment for materials simulations

a b s t r a c t We describe the development of a scientific cloud computing (SCC) platform that offers high performance computation capability. The platform consists of a scientific virtual machine prototype containing a UNIX operating system and several materials science codes, together with essential interface tools (an SCC toolset) that offers functionality comparable to local compute clusters. In particular, our SCC toolset provides automatic creation of virtual clusters for parallel computing, including tools for execution and monitoring performance, as well as efficient I/O utilities that enable seamless connections to and from the cloud. Our SCC platform is optimized for the Amazon Elastic Compute Cloud (EC2). We present benchmarks for prototypical scientific applications and demonstrate performance comparable to local compute clusters. To facilitate code execution and provide user-friendly access, we have also integrated cloud computing capability in a JAVA-based GUI. Our SCC platform may be an alternative to traditional HPC resources for materials science or quantum chemistry applications.

Polarization Dependent High Energy Resolution X-ray Absorption Study of Dicesium Uranyl Tetrachloride

Dicesium uranyl tetrachloride (Cs2UO2Cl4) has been a model compound for experimental and theoretical studies of electronic structure of U(VI) in the form of UO2(2+) (uranyl ion) for decades. We have obtained angle-resolved electronic structure information for oriented Cs2UO2Cl4 crystal, specifically relative energies of 5f and 6d valence orbitals probed with extraordinary energy resolution by polarization dependent high energy resolution X-ray absorption near edge structure (PD-HR-XANES) and compare these with predictions from quantum chemical Amsterdam density functional theory (ADF) and ab initio real space multiple-scattering Greens function based FEFF codes. The obtained results have fundamental value but also demonstrate an experimental approach, which offers great potential to benchmark and drive improvement in theoretical calculations of electronic structures of actinide elements.

Actinide and lanthanide speciation with high-energy resolution X-ray techniques

High-energy resolution X-ray absorption spectroscopy (HR-XAS) and Resonant inelastic X-ray scattering (RIXS) combined with quantum theoretical tools are gaining importance for understanding electronic and coordination structures of actinide (An) and lanthanide (Ln) materials. HR-XAS is successfully used to remove lifetime broadening by registering the partial fluorescence yield emitted by the sample, thereby yielding highly resolved X-ray absorption near edge structure spectra (HR-XANES), which often display resonant features not observed in conventional XANES. We demonstrate the structural characterization capabilities of these novel techniques by comparative discussion of U M4/L3-HR-XANES and L3-valence band RIXS (L3-VB-RIXS) spectra of two model U(VI) minerals. We show that the ab initio multiple scattering theory based code FEFF9.5 is an effective tool for calculations of An and Ln L3-HR-XANES and L3-RIXS spectra as it successfully reproduces dipole and quadrupole transitions in the same spectrum.

High-performance computing without commitment: SC2IT: A cloud computing interface that makes computational science available to non-specialists

Computational work is a vital part of many scientific studies. In materials science research in particular, theoretical models are often needed to understand measurements. There is currently a double barrier that keeps a broad class of researchers from using state-of-the-art materials science codes: the software typically lacks user-friendliness, and the hardware requirements can demand a significant investment, e.g. the purchase of a Beowulf cluster. Scientific Cloud Computing has the potential to remove this barrier and make computational science accessible to a wider class of scientists who are not computational specialists. We present a set of interface tools, SC2IT, that enables seamless control of virtual compute clusters in the Amazon EC2 cloud and is designed to be embedded in user-friendly Java GUIs. We present applications of our Scientific Cloud Computing method to the materials science codes FEFF9, WIEN2k, and MEEP-mpi. SC2IT and the paradigm described here are applicable to other fields of research outside materials science within current Cloud Computing capability.

New Developments in FEFF: FEFF9 and JFEFF

The ab initio core-level spectroscopy code FEFF9 has seen many new developments in recent years. We describe the addition of new physics and new features designed to calculate more accurate spectra. We also present the user-friendly Java-based GUI JFEFF that simplifies running FEFF on platforms ranging from personal computers to high-performance parallel systems and virtual cloud platforms.

Comparative investigation of N donor ligand-lanthanide complexes from the metal and ligand point of view

N-donor ligands such as n-Pr-BTP (2,6-bis(5,6-dipropyl-1,2,4-triazin-3-yl)pyridine) studied here preferentially bind An(III) over Ln(III) in liquid-liquid separation of trivalent ac-tinides from spent nuclear fuel. The chemical and physical processes responsible for this selectivity are not yet well understood. We present systematic comparative near-edge X-ray absorption structure (XANES) spectroscopy investigations at the Gd L3 edge of [GdBTP3](NO3)3, [Gd(BTP)3](OTf)3, Gd(NO3)3, Gd(OTf)3 and N K edge of [Gd(BTP)3](NO3)3, Gd(NO3)3 complexes. The pre-edge absorption resonance in Gd L3 edge high-energy resolution X-ray absorption near edge structure spectra (HR-XANES) is explained as arising from 2p3/2 → 4f/5d electronic transitions by calculations with the FEFF9.5 code. Experimental evidence is found for higher electronic density on Gd in [Gd(BTP)3](NO3)3 and [Gd(BTP)3](OTf)3 compared to Gd in Gd(NO3)3 and Gd(OTf)3, and on N in [Gd(BTP)3](NO3)3 compared to n-Pr-BTP. The origin of the pre-edge structure in the N K edge XANES is explained by density functional theory (DFT) with the ORCA code. Results at the N K edge suggest a change in ligand orbital occupancies and mixing upon complexation but further work is necessary to interpret observed spectral variations.

Real-space multiple-scattering Hubbard model calculations for d- and f-state materials.

Calculations are presented of the electronic structure and X-ray spectra of materials with correlated d- and f-electron states based on the Hubbard model, a real-space multiple-scattering formalism and a rotationally invariant local density approximation. Values of the Hubbard parameter are calculated ab initio using the constrained random-phase approximation. The combination of the real-space Greens function with Hubbard model corrections provides an efficient approach to describe localized correlated electron states in these systems, and their effect on core-level X-ray spectra. Results are presented for the projected density of states and X-ray absorption spectra for transition metal- and lanthanide-oxides. Results are found to be in good agreement with experiment.

Ab initio Real Space Calculations of Electron Energy Loss Spectra

We briefly discuss recent progress in the theory of electron energy loss spectra (EELS) and related spectroscopies such as x‐ray absorption spectra (XAS). In particular we review how the real‐space Green/s function (RSGF) approach and its extensions described below can provide quantitative ab initio treatments of both XAS and EELS. We also discuss differences in these spectra due to relativistic effects and finite momentum transfer.