QUANTUM ESPRESSO is an integrated suite of computer codes for electronic-structure calculations and materials modeling, based on density-functional theory, plane waves, and pseudopotentials (norm-conserving, ultrasoft, and projector-augmented wave). The acronym ESPRESSO stands for opEn Source Package for Research in Electronic Structure, Simulation, and Optimization. It is freely available to researchers around the world under the terms of the GNU General Public License. QUANTUM ESPRESSO builds upon newly-restructured electronic-structure codes that have been developed and tested by some of the original authors of novel electronic-structure algorithms and applied in the last twenty years by some of the leading materials modeling groups worldwide. Innovation and efficiency are still its main focus, with special attention paid to massively parallel architectures, and a great effort being devoted to user friendliness. QUANTUM ESPRESSO is evolving towards a distribution of independent and interoperable codes in the spirit of an open-source project, where researchers active in the field of electronic-structure calculations are encouraged to participate in the project by contributing their own codes or by implementing their own ideas into existing codes.

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QUANTUM ESPRESSO: a modular and open-source software project for quantum simulations of materials

Quantum ESPRESSO is an integrated suite of open-source computer codes for quantum simulations of materials using state-of-the-art electronic-structure techniques, based on density-functional theory, density-functional perturbation theory, and many-body perturbation theory, within the plane-wave pseudopotential and projector-augmented-wave approaches Quantum ESPRESSO owes its popularity to the wide variety of properties and processes it allows to simulate, to its performance on an increasingly broad array of hardware architectures, and to a community of researchers that rely on its capabilities as a core open-source development platform to implement their ideas In this paper we describe recent extensions and improvements, covering new methodologies and property calculators, improved parallelization, code modularization, and extended interoperability both within the distribution and with external software

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Advanced capabilities for materials modelling with Quantum ESPRESSO.

Quantum ESPRESSO is an integrated suite of open-source computer codes for quantum simulations of materials using state-of-the art electronic-structure techniques, based on density-functional theory, density-functional perturbation theory, and many-body perturbation theory, within the plane-wave pseudo-potential and projector-augmented-wave approaches. Quantum ESPRESSO owes its popularity to the wide variety of properties and processes it allows to simulate, to its performance on an increasingly broad array of hardware architectures, and to a community of researchers that rely on its capabilities as a core open-source development platform to implement theirs ideas. In this paper we describe recent extensions and improvements, covering new methodologies and property calculators, improved parallelization, code modularization, and extended interoperability both within the distribution and with external software.

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Advanced capabilities for materials modelling with Quantum ESPRESSO

In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.

Ferromagnetic materials

X-ray absorption Fe−K edge data on ferrous and ferric model complexes have been studied to establish a detailed understanding of the 1s → 3d pre-edge feature and its sensitivity to the electronic structure of the iron site. The energy position and splitting, and intensity distribution, of the pre-edge feature were found to vary systematically with spin state, oxidation state, geometry, and bridging ligation (for binuclear complexes). A methodology for interpreting the energy splitting and intensity distribution of the 1s → 3d pre-edge features was developed for high-spin ferrous and ferric complexes in octahedral, tetrahedral, and square pyramidal environments and low-spin ferrous and ferric complexes in octahedral environments. In each case, the allowable many-electron excited states were determined using ligand field theory. The energies of the excited states were calculated and compared to the energy splitting in the 1s → 3d pre-edge features. The relative intensities of electric quadrupole transitions...

A Multiplet Analysis of Fe K-Edge 1s → 3d Pre-Edge Features of Iron Complexes

Full Multiple Scattering and Crystal Field Multiplet Calculations Performed on the Spin Transition FeII(phen)2(NCS)2 Complex at the Iron K and L2,3 X-ray Absorption Edges

Pre-edge features in x-ray absorption spectroscopy contain key information about the lowest excited states and thus on the most interesting physical properties of the system In transition-metal oxides they are particularly structured but extracting physical parameters by comparison with a calculation is not easy due to several computational challenges By combining core-hole attraction and correlation effects in a first-principles approach, we calculate $\text{Ni}\text{ }K$-edge x-ray absorption spectra in NiO We obtain a striking parameter-free agreement with experimental data and show that dipolar pre-edge features above the correlation gap are due to nonlocal excitations largely unaffected by the core hole We show that in charge transfer insulators, this property can be used to measure the correlation gap and probe the intrinsic position of the upper Hubbard band

Intrinsic charge transfer gap in NiO from Ni K-edge x-ray absorption spectroscopy

An implementation of the multiple-scattering approach to x-ray magnetic circular dichroism (XMCD) in K edge x-ray absorption spectroscopy is presented. The convergence problems due to the cluster size and the relativistic corrections are solved using an expansion of the Dirac Green function for complex energies up to second order in 1/c. The Fermi energy is dealt with via a complex plane integration. Numerical methods used to obtain the semirelativistic Green function in the whole complex plane are explained. We present a calculation of the magnetic circular dichroism at the K edge of bcc iron including the core hole effect. A good agreement is found at high energy. The physical origins of the XMCD spectrum near the edge and far from the edge are analyzed. The influence of the core hole, the possibility of a multiple-scattering expansion, and the relation of XMCD with the spin polarized density of states are discussed. A simple interpretation of XMCD at the K edge is presented in terms of a rigid-band model. \textcopyright{} 1996 The American Physical Society.

Multiple-scattering theory of x-ray magnetic circular dichroism: Implementation and results for the iron K edge.

For the first time, commonly unaccessible local electronic structure parameters of Fe2+ and Fe3+ in minerals are derived from the calculation of the pre-edge features of X-ray absorption spectra at the Fe K edge. The Ligand Field Multiplet approach is used to calculate the eigenstates of the ions and the absolute intensities of the electric-quadrupole and dipole transitions involved in the pre-edge. For ions in tetrahedral symmetry, a new model for p-d hybridization is developed. The degree of admixture between 3d and 4p levels is derived and local structure parameters (crystal field, bond covalency) are obtained.

Calculation of multipole transitions at the Fe K pre-edge through p-d hybridization in the Ligand Field Multiplet model

The Butcher group and its underlying Hopf algebra of rooted trees were originally formulated to describe Runge–Kutta methods in numerical analysis. In the past few years, these concepts turned out to have far-reaching applications in several areas of mathematics and physics: they were rediscovered in noncommutative geometry, they describe the combinatorics of renormalization in quantum field theory. The concept of Hopf algebra is introduced using a familiar example and the Hopf algebra of rooted trees is defined. Its role in Runge–Kutta methods, renormalization theory and noncommutative geometry is described.

Ch. Brouder

Papers

Full Multiple Scattering and Crystal Field Multiplet Calculations Performed on the Spin Transition FeII(phen)2(NCS)2 Complex at the Iron K and L2,3 X-ray Absorption Edges

Intrinsic charge transfer gap in NiO from Ni K-edge x-ray absorption spectroscopy

Multiple-scattering theory of x-ray magnetic circular dichroism: Implementation and results for the iron K edge.

Calculation of multipole transitions at the Fe K pre-edge through p-d hybridization in the Ligand Field Multiplet model

Trees, renormalization and differential equations