arXiv subjects

Matteo Cococcioni

Publications and source records attributed to Matteo Cococcioni.

At least 19 recordsLinked to original sources

arXiv2026

Magnetic interactions and spin orders in Cr$_8$ and V$_8$ ring-shaped molecular magnets from non-collinear ab initio calculations

We employ density functional theory within a non-collinear framework to investigate the magnetic properties of the octanuclear molecular rings Cr$_8$ and V$_8$. Our aim is to generalize the evaluation of the effective magnetic interactions by explicitly including non-collinear spin configurations, thereby refining our understanding of their dependence upon the underlying electronic structure and molecular geometry. By analyzing the energetics of a variety of magnetic configurations, particularly non-collinear arrangements with neighboring spins oriented along different directions, we move beyond the exchange-only Heisenberg Hamiltonian describing the low-energy sector of the excitation spectrum. This approach enables us to distinguish between in-plane and out-of-plane exchange interactions, and to incorporate biquadratic coupling terms into the effective spin Hamiltonian. We reveal significant antisymmetric exchange interactions of the Dzyaloshinskii-Moriya (DM) type whose dependence on the curvature of the annular structure is clarified by a comparison with the results obtained from linear chains of equal composition. Our work demonstrates that interactions beyond conventional exchange, particularly biquadratic anisotropic terms, in the spin Hamiltonian are essential for accurately capturing the low-energy excitations of these systems. The closest quantitative agreement with experimental results (particularly for the case of Cr$_8$) is achieved when extended Hubbard functionals are used for the evaluation of the effective magnetic couplings.

cond-mat.mtrl-sci↗

arXiv2026

Spin-orbit coupling and beyond in Chiral-Induced Spin Selectivity

Chiral-Induced Spin Selectivity (CISS) describes the emergence of spin-polarized electron transport in chiral systems without magnetic fields, a remarkable effect in light-element materials with weak intrinsic spin-orbit coupling (SOC). This mini-review analyzes the microscopic origins of CISS, highlighting how molecular chirality, local electric fields, and dynamic distortions enhance effective SOC and drive spin-dependent transport. We critically assess existing models in terms of their symmetry constraints, phenomenological assumptions, and compliance with Onsager reciprocity. Recent developments combining relativistic quantum mechanics and complete multipole representations reveal a direct link between chirality density and spin current pseudoscalars, suggesting a field-theoretic foundation for CISS. These insights could help position light-element chiral nanomaterials as tunable platforms for probing and engineering spin-selective phenomena at the nanoscale.

cond-mat.other↗

arXiv2025

Magnetic properties of Cr$_8$ and V$_8$ molecular rings from ab initio calculations

Molecular nanomagnets are systems with a vast phenomenology and are very promising for a variety of technological applications, most notably spintronics and quantum information. Their low-energy spectrum and magnetic properties can be modeled using effective spin Hamiltonians, once the exchange coupling parameters between the localized magnetic moments are determined. In this work we employ density functional theory (DFT) to compute the exchange parameters between the atomic spins for two representative ring-shaped molecules containing eight transition-metal magnetic ions: Cr$_8$ and V$_8$. Considering a set of properly chosen spin configurations and mapping their DFT energies on the corresponding expressions from a Heisenberg Hamiltonian, we compute the exchange couplings between magnetic ions which are first, second and further neighbors on the rings. In spite of their chemical and structural similarities the two systems exhibit very different ground states: antiferromagnetic for Cr$_8$, ferromagnetic for V$_8$, which also features non-negligible couplings between second nearest neighbors. A rationalization of these results is proposed that is based on a multi-band Hubbard model with less-than-half filled shells on magnetic ions.

cond-mat.mtrl-sci↗

arXiv2024

Relativistic Dynamics and Electron Transport in Isolated Chiral Molecules

The Chirality-Induced Spin Selectivity (CISS) effect describes the ability of chiral molecules and crystals to transmit spin-polarized currents, a phenomenon first identified in 1999. Although this effect holds great promise for a broad spectrum of different applications in device physics and synthetic chemistry (including, e.g., spintronics, quantum computing, spin- and enantio-selective chemistry), its underlying mechanisms remain incompletely understood. The prevailing hypothesis attributes the CISS effect to enhanced spin-orbit coupling (SOC) within chiral molecules. However, the SOC magnitude required to align with experimental observations significantly exceeds the values derived from conventional atomic-scale calculations, particularly for systems composed of light atoms. In this work, we leverage the implementation of \texttt{fully relativistic density functional theory (DFT)} equations, as available in the \texttt{Dirac code}, to investigate how molecular chirality manifests itself in the chirality density of electronic states. We further explore how this responds to an applied external electric field. To assess spin-dependent transport, we employ the \texttt{Landauer-Imry-B\"uttiker} formalism, examining the dependence of spin transmission on the twist angle of the molecular structure that defines its geometrical chirality. While our findings qualitatively align with experimental trends, they point to the necessity of a more general treatment of SOC, \textit{e.g.}, including geometrical terms or through the dependence of advanced exchange-correlation functionals on the electronic spin-current density.

cond-mat.mes-hall↗

arXiv2023

Ternary oxides of $\textit{s}$- and $\textit{p}$-block metals for photocatalytic solar-to-hydrogen conversion

Oxides containing metals or metalloids from the {\it p}-block of the periodic table ({\it e.g.}, In, Sn, Sb, Pb, Bi) are of technological interest as transparent conductors and light absorbers for solar energy conversion due to the tunability of their electronic conductivity and optical absorption. Comparatively, these oxides have found limited applications in hydrogen photoelectrolysis primarily due to their high electronegativity, which impedes electron transfer for reducing protons into hydrogen. We have shown recently that inserting {\it s}-block cations into {\it p}-block metal oxides is effective at lowering electronegativities while affording further control of band gaps. Here, we explain the origins of this dual tunability by demonstrating the mediator role of {\it s}-block cations in modulating orbital hybridization while not contributing to frontier electronic states. From this result, we carry out a comprehensive computational study of 109 ternary oxides of {\it s}- and {\it p}-block metal elements as candidate photocatalysts for solar hydrogen generation. We downselect the most desirable materials using band gaps and band edges obtained from Hubbard-corrected density-functional theory with Hubbard parameters computed entirely from first principles, evaluate the stability of these oxides in aqueous conditions, and characterize experimentally four of the remaining materials, synthesized with high phase uniformity, to validate and further develop the computational models. We thus propose nine oxide semiconductors, including CsIn$_3$O$_5$, Sr$_2$In$_2$O$_5$, and KSbO$_2$ which, to the extent of our literature review, have not been previously considered as water-splitting photocatalysts.

cond-mat.mtrl-sci↗

arXiv2022

Accurate electronic properties and intercalation voltages of olivine-type Li-ion cathode materials from extended Hubbard functionals

The design of novel cathode materials for Li-ion batteries would greatly benefit from accurate first-principles predictions of structural, electronic, and magnetic properties as well as intercalation voltages in compounds containing transition-metal elements. For such systems, density-functional theory (DFT) with standard (semi-)local exchange-correlation functionals is of limited use as it often fails due to strong self-interaction errors that are especially relevant in the partially filled $d$ shells. Here, we perform a detailed comparative study of the phospho-olivine cathode materials Li$_x$MnPO$_4$, Li$_x$FePO$_4$, and the mixed transition metal Li$_x$Mn$_{1/2}$Fe$_{1/2}$PO$_4$ ($x=0, 1/4, 1/2, 3/4, 1$) using four electronic-structure methods: DFT, DFT+$U$, DFT+$U$+$V$, and HSE06. We show that DFT+$U$+$V$, with onsite $U$ and intersite $V$ Hubbard parameters determined from first principles and self-consistently with respect to the structural parameters by means of density-functional perturbation theory (linear response), provides the most accurate description of the electronic structure of these challenging compounds. In particular, we demonstrate that DFT+$U$+$V$ displays very clearly "digital" changes in oxidation states of the transition-metal ions in all compounds, including the mixed-valence phases occurring at intermediate Li concentrations, leading to voltages in remarkable agreement with experiments. We show that the inclusion of intersite Hubbard interactions is essential for the accurate prediction of thermodynamic quantities, balancing the drive for localization induced by the onsite $U$ with intersite $V$ orbital hybridizations.

cond-mat.mtrl-sci↗

arXiv2022

HP -- A code for the calculation of Hubbard parameters using density-functional perturbation theory

We introduce HP, an implementation of density-functional perturbation theory, designed to compute Hubbard parameters (on-site $U$ and inter-site $V$) in the framework of DFT+$U$ and DFT+$U$+$V$. The code does not require the use of computationally expensive supercells of the traditional linear-response approach; instead, unit cells are used with monochromatic perturbations that significantly reduce the computational cost of determining Hubbard parameters. HP is an open-source software distributed under the terms of the GPL as a component of Quantum ESPRESSO. As with other components, HP is optimized to run on a variety of different platforms, from laptops to massively parallel architectures, using native mathematical libraries (LAPACK and FFTW) and a hierarchy of custom parallelization layers built on top of MPI. The effectiveness of the code is showcased by computing Hubbard parameters self-consistently for the phospho-olivine Li$_x$Mn$_{1/2}$Fe$_{1/2}$PO$_4$ ($x=0, 1/2, 1$) and by highlighting the accuracy of predictions of the geometry and Li intercalation voltages.

cond-mat.mtrl-sci↗

arXiv2022

Pulay forces in density-functional theory with extended Hubbard functionals: From nonorthogonalized to orthogonalized manifolds

We present a derivation of the exact expression for Pulay forces in density-functional theory calculations augmented with extended Hubbard functionals, and arising from the use of orthogonalized atomic orbitals as projectors for the Hubbard manifold. The derivative of the inverse square root of the orbital overlap matrix is obtained as a closed-form solution of the associated Lyapunov (Sylvester) equation. The expression for the resulting contribution to the forces is presented in the framework of ultrasoft pseudopotentials and the projector-augmented-wave method, and using a plane wave basis set. We have benchmarked the present implementation with respect to finite differences of total energies for the case of NiO, finding excellent agreement. Owing to the accuracy of Hubbard-corrected density-functional theory calculations - provided the Hubbard parameters are computed for the manifold under consideration - the present work paves the way for systematic studies of solid-state and molecular transition-metal and rare-earth compounds.

cond-mat.mtrl-sci↗

arXiv2021

Ab initio electron-phonon interactions in correlated electron systems

Electron-phonon ($e$-ph) interactions are pervasive in condensed matter, governing phenomena such as transport, superconductivity, charge-density waves, polarons and metal-insulator transitions. First-principles approaches enable accurate calculations of $e$-ph interactions in a wide range of solids. However, they remain an open challenge in correlated electron systems (CES), where density functional theory often fails to describe the ground state. Therefore reliable $e$-ph calculations remain out of reach for many transition metal oxides, high-temperature superconductors, Mott insulators, planetary materials and multiferroics. Here we show first-principles calculations of $e$-ph interactions in CES, using the framework of Hubbard-corrected density functional theory (DFT+$U$ ) and its linear response extension (DFPT+$U$), which can describe the electronic structure and lattice dynamics of many CES. We showcase the accuracy of this approach for a prototypical Mott system, CoO, carrying out a detailed investigation of its $e$-ph interactions and electron spectral functions. While standard DFPT gives unphysically divergent and short-ranged $e$-ph interactions, DFPT+$U$ is shown to remove the divergences and properly account for the long-range Fröhlich interaction, allowing us to model polaron effects in a Mott insulator. Our work establishes a broadly applicable and affordable approach for quantitative studies of e-ph interactions in CES, a novel theoretical tool to interpret experiments in this broad class of materials.

cond-mat.mtrl-sci↗

arXiv2021

Optimizing accuracy and efficacy in data-driven materials discovery for the solar production of hydrogen

The production of hydrogen fuels, via water splitting, is of practical relevance for meeting global energy needs and mitigating the environmental consequences of fossil-fuel-based transportation. Water photoelectrolysis has been proposed as a viable approach for generating hydrogen, provided that stable and inexpensive photocatalysts with conversion efficiencies over 10% can be discovered, synthesized at scale, and successfully deployed (Pinaud et al., Energy Environ. Sci., 2013, 6, 1983). While a number of first-principles studies have focused on the data-driven discovery of photocatalysts, in the absence of systematic experimental validation, the success rate of these predictions may be limited. We address this problem by developing a screening procedure with co-validation between experiment and theory to expedite the synthesis, characterization, and testing of the computationally predicted, most desirable materials. Starting with 70,150 compounds in the Materials Project database, the proposed protocol yielded 71 candidate photocatalysts, 11 of which were synthesized as single-phase materials. Experiments confirmed hydrogen generation and favorable band alignment for 6 of the 11 compounds, with the most promising ones belonging to the families of alkali and alkaline-earth indates and orthoplumbates. This study shows the accuracy of a nonempirical, Hubbard-corrected density-functional theory method to predict band gaps and band offsets at a fraction of the computational cost of hybrid functionals, and outlines an effective strategy to identify photocatalysts for solar hydrogen generation.

cond-mat.mtrl-sci↗

arXiv2021

Self-consistent Hubbard parameters from density-functional perturbation theory in the ultrasoft and projector-augmented wave formulations

The self-consistent evaluation of Hubbard parameters using linear-response theory is crucial for quantitatively predictive calculations based on Hubbard-corrected density-functional theory. Here, we extend a recently-introduced approach based on density-functional perturbation theory (DFPT) for the calculation of the on-site Hubbard $U$ to also compute the inter-site Hubbard $V$. DFPT allows to reduce significantly computational costs, improve numerical accuracy, and fully automate the calculation of the Hubbard parameters by recasting the linear response of a localized perturbation into an array of monochromatic perturbations that can be calculated in the primitive cell. In addition, here we generalize the entire formalism from norm-conserving to ultrasoft and projector-augmented wave formulations, and to metallic ground states. After benchmarking DFPT against the conventional real-space Hubbard linear response in a supercell, we demonstrate the effectiveness of the present extended Hubbard formulation in determining the equilibrium crystal structure of Li$_x$MnPO$_4$ (x=0,1) and the subtle energetics of Li intercalation.

cond-mat.mtrl-sci↗

arXiv2020

Electronic structure of pristine and Ni-substituted LaFeO$_3$ from near edge x-ray absorption fine structure experiments and first-principles simulations

We present a joint theoretical and experimental study of the oxygen $K$-edge spectra for LaFeO$_3$ and homovalent Ni-substituted LaFeO$_3$ (LaFe$_{0.75}$Ni$_{0.25}$O$_3$), using first-principles simulations based on density-functional theory with extended Hubbard functionals and x-ray absorption near edge structure (XANES) measurements. Ground-state and excited-state XANES calculations employ Hubbard on-site $U$ and inter-site $V$ parameters determined from first principles and the Lanczos recursive method to obtain absorption cross sections, which allows for a reliable description of XANES spectra in transition-metal compounds in a very broad energy range, with an accuracy comparable to that of hybrid functionals but at a substantially lower cost. We show that standard gradient-corrected exchange-correlation functionals fail in capturing accurately the electronic properties of both materials. In particular, for LaFe$_{0.75}$Ni$_{0.25}$O$_3$ they do not reproduce its semiconducting behaviour and provide a poor description of the pre-edge features at the O $K$ edge. The inclusion of Hubbard interactions leads to a drastic improvement, accounting for the semiconducting ground state of LaFe$_{0.75}$Ni$_{0.25}$O$_3$ and for a good agreement between calculated and measured XANES spectra. We show that the partial substitution of Fe for Ni affects the conduction-band bottom by generating a strongly hybridized O($2p$)-Ni($3d$) minority-spin empty electronic state. The present work, based on a consistent correction of self-interaction errors, outlines the crucial role of extended Hubbard functionals to describe the electronic structure of complex transition-metal oxides such as LaFeO$_3$ and LaFe$_{0.75}$Ni$_{0.25}$O$_3$ and paves the way to future studies on similar systems.

cond-mat.str-el↗

arXiv2020

LDA+Usc calculations of phase relations in FeO

Using the LDA+$\textit{U}_\text{sc}$ method, we present calculations phase relations of iron monoxides involving five polytypes in multiple spin-state configurations. The Hubbard parameter $U$ is determined self-consistently simultaneously with the occupation matrix and structures at arbitrary pressures. The Hubbard parameter strongly depends on pressure, structure, and spin state. Comparison with experimental structural data indicates the LDA+$\textit{U}_\text{sc}$ can predict structure, compression curves, phase relations, and transition pressures very well for the insulating B1 and iB8 states. However, it requires additional calculations using the Mermin functional that includes the electronic entropic contribution to the free energy to obtain an nB8 metallic state and a consistent iB8 to nB8 insulator-to-metal transition pressure.

cond-mat.mtrl-sci↗

arXiv2020

Hubbard-corrected density functional perturbation theory with ultrasoft pseudopotentials

We present in full detail a newly developed formalism enabling density functional perturbation theory (DFPT) calculations from a DFT+$U$ ground state. The implementation includes ultrasoft pseudopotentials and is valid for both insulating and metallic systems. It aims at fully exploiting the versatility of DFPT combined with the low-cost DFT+$U$ functional. This allows to avoid computationally intensive frozen-phonon calculations when DFT+$U$ is used to eliminate the residual electronic self-interaction from approximate functionals and to capture the localization of valence electrons e.g. on $d$ or $f$ states. In this way, the effects of electronic localization (possibly due to correlations) are consistently taken into account in the calculation of specific phonon modes, Born effective charges, dielectric tensors and in quantities requiring well converged sums over many phonon frequencies, as phonon density of states and free energies. The new computational tool is applied to two representative systems, namely CoO, a prototypical transition metal monoxide and LiCoO$_2$, a material employed for the cathode of Li-ion batteries. The results show the effectiveness of our formalism to capture in a quantitatively reliable way the vibrational properties of systems with localized valence electrons.

cond-mat.str-el↗

arXiv2020

Self-consistent DFT+$U$+$V$ study of oxygen vacancies in SrTiO$_3$

Contradictory theoretical results for oxygen vacancies in SrTiO$_3$ (STO) were often related to the peculiar properties of STO, which is a $d^0$ transition metal oxide with mixed ionic-covalent bonding. Here, we apply, for the first time, density functional theory (DFT) within the extended Hubbard DFT+$U$+$V$ approach, including on-site as well as inter-site electronic interactions, to study oxygen-deficient STO with Hubbard $U$ and $V$ parameters computed self-consistently via density-functional perturbation theory. Our results demonstrate that the extended Hubbard functional is a promising approach to study defects in materials with electronic properties similar to STO. Indeed, DFT+$U$+$V$ provides a better description of stoichiometric STO compared to standard DFT or DFT+$U$, the band gap and crystal field splitting being in good agreement with experiments. In turn, also the description of the electronic properties of oxygen vacancies in STO is improved, with formation energies in excellent agreement with experiments as well as results obtained with the most frequently used hybrid functionals, however at a fraction of the computational cost. While our results do not fully resolve the contradictory findings reported in literature, our systematic approach leads to a deeper understanding of their origin, which stems from different cell sizes, STO phases, the exchange-correlation functional, and the treatment of structural relaxations and spin-polarization.

cond-mat.mtrl-sci↗

arXiv2019

Self-consistent site-dependent DFT+$U$ study of stoichiometric and defective SrMnO$_3$

We propose a self-consistent site-dependent Hubbard $U$ approach for DFT+$U$ calculations of defects in complex transition-metal oxides, using Hubbard parameters computed via linear-response theory. The formation of a defect locally perturbs the chemical environment of Hubbard sites in its vicinity, resulting in different Hubbard $U$ parameters for different sites. Using oxygen vacancies in SrMnO$_3$ as a model system, we investigate the dependence of $U$ on the chemical environment and study its influence on the structural, electronic, and magnetic properties of defective bulk and strained thin-film structures. Our results show that a self-consistent $U$ improves the description of stoichiometric bulk SrMnO$_3$ with respect to GGA or GGA+$U$ calculations using an empirical $U$. For defective systems, $U$ changes as a function of the distance of the Hubbard site from the defect, its oxidation state and the magnetic phase of the bulk structure. Taking into account this dependence, in turn, affects the computed defect formation energies and the predicted strain- and/or defect-induced magnetic phase transitions, especially when occupied localized states appear in the band gap of the material upon defect creation.

cond-mat.mtrl-sci↗

arXiv2019

Energetics and cathode voltages of LiMPO$_4$ olivines (M = Fe, Mn) from extended Hubbard functionals

Transition-metal compounds pose serious challenges to first-principles calculations based on density-functional theory (DFT), due to the inability of most approximate exchange-correlation functionals to capture the localization of valence electrons on their $d$ states, essential for a predictive modeling of their properties. In this work we focus on two representatives of a well known family of cathode materials for Li-ion batteries, namely the orthorhombic LiMPO$_4$ olivines (M = Fe, Mn). We show that extended Hubbard functionals with on-site ($U$) and inter-site ($V$) interactions (so called DFT+U+V) can predict the electronic structure of the mixed-valence phases, the formation energy of the materials with intermediate Li contents, and the overall average voltage of the battery with remarkable accuracy. We find, in particular, that the inclusion of inter-site interactions in the corrective Hamiltonian improves considerably the prediction of thermodynamic quantities when electronic localization occurs in the presence of significant interatomic hybridization (as is the case for the Mn compound), and that the self-consistent evaluation of the effective interaction parameters as material- and ground-state-dependent quantities allows the prediction of energy differences between different phases and concentrations.

cond-mat.mtrl-sci↗

arXiv2018

Hubbard parameters from density-functional perturbation theory

We present a transparent and computationally efficient approach for the first-principles calculation of Hubbard parameters from linear-response theory. This approach is based on density-functional perturbation theory and the use of monochromatic perturbations. In addition to delivering much improved efficiency, the present approach makes it straightforward to calculate automatically these Hubbard parameters for any given system, with tight numerical control on convergence and precision. The effectiveness of the method is showcased in three case studies - Cu$_2$O, NiO, and LiCoO$_2$ - and by the direct comparison with finite differences in supercell calculations.

cond-mat.str-el↗