arXiv subjects

Nicolas Richard

Publications and source records attributed to Nicolas Richard.

4 recordsLinked to original sources

arXiv2025

An accurate alternative to hybrid functionals for germanium: DFT+$\alpha$

The accuracy of bulk property predictions in density functional theory (DFT) calculations depends on the choice of exchange-correlation functional. While the Perdew-Burke-Ernzerhof (PBE) functional systematically overestimates lattice parameters and strongly underestimates electronic band gaps, hybrid functionals such as Heyd-Scuseria-Ernzerhof (HSE) offer better overall agreement across a broad range of materials. Using germanium as a critical test case, we challenge the ability of both functionals to capture semiconductor properties. Although HSE improves PBE's gap error, it fails to reproduce germanium's correct $\Gamma$-L indirect and $\Gamma$-$\Gamma$ band gaps simultaneously. Noting that the PBE underestimated energy separation between the 4p valence-band maximum and 4s conduction-band minimum causes unphysical $sp$ mixing, we propose DFT+$\alpha$, a semi-empirical correction scheme applied selectively to 4s-like orbitals. For germanium, DFT+$\alpha$ restores the proper ordering and orbital character of the band edges and yields accurate lattice constant, bulk modulus, elastic constants and phonon frequencies at a fraction of hybrid-functional computational cost.

cond-mat.mtrl-sci↗

arXiv2025

QE-CONVERSE: An open-source package for the Quantum ESPRESSO distribution to compute non-perturbatively orbital magnetization from first principles, including NMR chemical shifts and EPR parameters

Orbital magnetization, a key property arising from the orbital motion of electrons, plays a crucial role in determining the magnetic behavior of molecules and solids. Despite its straightforward calculation in finite systems, the computation in periodic systems poses challenges due to the ill-defined position operator and surface current contributions. The modern theory of orbital magnetization, implemented in the Density Functional Theory (DFT) framework, offers an accurate solution via the "converse approach." Here, we introduce QE-CONVERSE, a refactored, modular implementation of this method, replacing outdated routines from Quantum ESPRESSO (version 3.2). QE-CONVERSE integrates modern computational libraries like scaLAPACK and ELPA, enhancing scalability, especially for large supercell calculations. This work focuses on providing the community with a reliable, accurate orbital magnetization package for properties such as Electron Paramagnetic Resonance (EPR) g-tensors and Nuclear Magnetic Resonance (NMR) chemical shifts, particularly where perturbative methods fail. We demonstrate QE-CONVERSE's effectiveness with benchmark cases, including the NMR shifts of ${}^{27}$Al in alumina and ${}^{17}$O and ${}^{29}$Si in $\alpha$-quartz, as well as the EPR g-tensor for $_{ }^{n}\Sigma(n\geq 2)$ radicals and nitrogen defects in silicon. Results show excellent agreement with theoretical and experimental data, with improved accuracy in EPR calculations over linear response methods. QE-CONVERSE is fully compatible with recent Quantum ESPRESSO versions, enabling new possibilities for studying complex materials

cond-mat.mtrl-sci↗

arXiv2023

On the effect of electronic stopping in molecular dynamics simulations of collision cascades in Gallium Arsenide

Understanding the generation and evolution of defects induced in matter by ion irradiation is of fundamental importance to estimate the degradation of functional properties of materials. Computational approaches used in dierent communities, from space radiation eects to nuclear energy experiments, are based on a number of approximations that, among others, traditionally neglect the coupling between electronic and ionic degrees of freedom in the description of displacements. In this work, we study collision cascades in GaAs, including the electronic stopping power for selfprojectiles in dierent directions obtained via real time Time Dependent Density Functional Theory in Molecular Dynamics simulations of collision cascades, using the recent electron-phonon model and the previously developed two-temperature model. We show that the former can be well applied to describe the eects of electronic stopping in molecular dynamics simulations of collision cascades in a multielement semiconductor and that the number of defects is considerably aected by electronic stopping eects. The results are also discussed in the wider context of the commonly used non-ionizing energy loss model to estimate degradation of materials by cumulative displacements.

physics.comp-ph↗

arXiv2008

Extrinsic symplectic symmetric spaces

We define the notion of extrinsic symplectic symmetric spaces and exhibit some of their properties. We construct large families of examples and show how they fit in the perspective of a complete classification of these manifolds. We also build a natural star-quantization on a class of examples.

math.SG↗