A conference abstract from NIRItalia online 2021 in English and Italian. ; The authors gratefully acknowledge the European Commission for funding the InnoRenew project (grant agreement #739574) under the Horizon2020 Widespread-Teaming program and the Republic of Slovenia (investment funding of the Republic of Slovenia and the European Union European Regional Development Fund).
We investigate here the lowest‐energy (spin‐conserving) excitation energies for the set of He‐Ne atoms, with the family of nonempirical PBE, PBE0, PBE0‐1/3, PBE0‐DH, PBE‐CIDH, PBE‐QIDH, and PBE0‐2 functionals, after employing a wide variety of basis sets systematically approaching the basis set limit: def2‐nVP(D), cc‐pVnZ, aug‐cc‐pVnZ, and d‐aug‐cc‐pVnZ. We find that an accuracy (ie, mean unsigned error) of 0.3 to 0.4 eV for time‐dependent density functional theory (DFT) atomic excitation energies can be robustly achieved with modern double‐hybrid methods, which are also stable with respect to the addition of a double set of diffuse functions, contrarily to hybrid versions, in agreement with recent findings employing sophisticated multiconfigurational DFT methods. ; L.H.M. acknowledges the "Instituto Universitario de Materiales (IUMA)" for a research internship. The work in Alicante is supported by the projects AICO/2018/175 from the Regional Government (GVA/FSE) and FIS2015-64222-C2-2-P from the "Ministerio de Ciencia, Innovación y Universidades". E.B. thanks ANR ("Agence Nationale de la Recherche") and CGI ("Commissariat à l'Investissement d'Avenir") for financial support through Labex SEAM (Science and Engineering for Advanced Materials and devices) ANR 11 LABX 086, ANR 11 IDEX 05 02.
AbstractIn density estimation, the mean integrated squared error (MISE) is commonly used as a measure of performance. In that setting, the cross‐validation criterion provides an unbiased estimator of the MISE minus the integral of the squared density. Since the minimum MISE is known to converge to zero, this suggests that the minimum value of the cross‐validation criterion could be regarded as an estimator of minus the integrated squared density. This novel proposal presents the outstanding feature that, unlike all other existing estimators, it does not need the choice of any tuning parameter. Indeed, it is proved here that this approach results in a consistent and efficient estimator, with remarkable performance in practice. Moreover, apart from this base case, it is shown how several other problems on density functional estimation can be similarly handled using this new principle, thus demonstrating full potential for further applications.
The increasing demand for portable electronic devices and batteries has led to a growing interest in Li compounds. Lithium manganese oxides (LMO) are the most popular lithium-ion sieves (LIS) precursor materials due to their high lithium adsorption capacity and selectivity. The key step in forming LIS is the lithium desorption process from the crystalline lattice of the LMO. However, this process has been less researched than its counterpart, the lithium adsorption process. In this line, there are some studies describing the process of lithium desorption in acid media from spinel-type LMO. Nevertheless, there is no evidence of the lithium desorption process of layered-type lithium-rich LMO in acidic media. In the present work, we investigated the lithium desorption behavior of different LMO nanocomposites in HCl. LMOs with different Li/Mn ratios were synthesized by promoting the lithium-rich layered phase (Li2MnO3). The morphology, size, crystallinity, chemical composition, and surface properties of LMO nanocomposites and delithiated products were studied. In addition, density functional theory (DFT) calculations were carried out to understand the differential lithium desorption behavior, confirming its dependence on the Li/Mn ratio of the LMO nanocomposites. Herein, we demonstrate that the lithium diffusion energy barrier plays a major role during lithium desorption from LMO nanocomposites. Our results suggest that an exhaustive characterization of lithium precursor materials (LMO) is necessary to select a suitable desorption process ; This work was financially supported by CONICYT PFCHA/ DOCTORADO/2015-21151648 (Ruth Pulido), PFCHA/DOCTORADO/ 2017-21172001 (Nelson Naveas) and partially funded by project USAMPSA, PID-2020-112770-C22, from the Government of Spain. The simulations used in this paper have been performed in the Centro de Computacion ´ Científica-Universidad Autonoma ´ de Madrid (CCC-UAM); thanks to CPU time and other resources granted by the institution
This work has been performed within the framework of the EUROfusion Enabling Research project: ENR-MFE19.ISSP-UL-02 "Advanced experimental and theoretical analysis of defect evolution and structural disordering in optical and dielectric materials for fusion application". The views and opinions expressed herein do not necessarily reflect those of the European Commission. ; We have performed the density functional calculations (DFT) on the hole-type defects (V-centres) in magnesium aluminate spinel (MgAl2O4) following the results of recent paramagnetic resonance measurements (EPR) in Nucl. Inst. Methods Phys. Res. B 435 (2018) 31–37. The hybrid B3LYP functional calculations using large supercells of 448 atoms have demonstrated excellent results not only for bulk properties but also properties of the V-centres in MgAl2O4. Three types of V-centres have been considered and confirmed, namely V1, V2 and V22. The DFT calculations have revealed the atomic relaxation pattern and spin density distribution around the hole-type defects that is suggested as an important complement to the experiments. Moreover, the calculated hyperfine coupling constants (HCCs) have been analyzed and compared with those from the measured EPR spectra. A good agreement between the calculated and measured HCC values is observed and discussed. ; EUROfusion Enabling Research project: ENR-MFE19.ISSP-UL-02; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union's Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART²
Applied magnetic fields can couple to atomic displacements via generalized Lorentz forces, which are commonly expressed as gyromagnetic $g$ factors. We develop an efficient first-principles methodology based on density-functional perturbation theory to calculate this effect in both molecules and solids to linear order in the applied field. Our methodology is based on two linear-response quantities: the macroscopic polarization response to an atomic displacement (i.e., Born effective charge tensor), and the antisymmetric part of its first real-space moment (the symmetric part corresponding to the dynamical quadrupole tensor). The latter quantity is calculated via an analytical expansion of the current induced by a long-wavelength phonon perturbation, and compared to numerical derivatives of finite-wavevector calculations. We validate our methodology in finite systems by computing the gyromagnetic $g$ factor of several simple molecules, demonstrating excellent agreement with experiment and previous density-functional theory and quantum chemistry calculations. In addition, we demonstrate the utility of our method in extended systems by computing the energy splitting of the low-frequency transverse-optical phonon mode of cubic SrTiO$_3$ in the presence of a magnetic field. ; C.E.D. acknowledges support from the National Science Foundation under Grant No. DMR-1918455. The Flatiron Institute is a division of the Simons Foundation. A.Z. and M.S. acknowledge support from Ministerio de Economia, Industria y Competitividad (MINECO-Spain) through Grant No. PID2019-108573GB-C22; from Severo Ochoa FUNFUTURE center of excellence (CEX2019- 000917-S); from Generalitat de Catalunya (Grant No. 2017 SGR1506); and from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (Grant Agreement No. 724529). ; With funding from the Spanish government through the 'Severo Ochoa Centre of Excellence' accreditation (CEX2019-000917-S). ; Peer reviewed
Meso-aryl expanded porphyrins present a structural versatility that allows them to achieve different topologies with distinct aromaticities. Several studies appeared in the literature studying these topological switches from an experimental and theoretical point of view. Most of these publications include density functional theory calculations, being the B3LYP the most used methodology. In this work, we show that the selection of the functional has a critical role on the geometric, energetic, and magnetic results of these expanded porphyrins, and that the use of an inadequate methodology can even generate spurious stationary points on the potential energy surface. To illustrate these aspects, in this article we have studied different molecular distortions of two expanded porphyrins, [32]-heptaphyrin and [26]-hexaphyrin using 11 DFT functionals and performing single point energy calculations at the local pair natural orbital coupled cluster DLPNO-CCSD(T) method, which have been carried out for benchmarking purposes. For some selected functionals, the dispersion effects have also been evaluated using the D3-Grimme's dispersion correction with Becke-Johnson damping. Our results let us to conclude that the CAM-B3LYP, M05-2X, and M06-2X functionals are the methodologies that provide a more consistent description of these topological switches, while other methods, such as B3LYP, BPE, and BP86, show a biased description ; The calculations described in this work were carried out at the Consorci de Serveis Universitaris de Catalunya (CSUC), SGI/IZO-SGIker, and DIPC. Financial support was provided by the Ministerio de Economía y Competitividad (MINECO) of Spain and FEDER (projects CTQ2016-80375-P, CTQ2014-52525-P, and Red de Excelencia Consolider CTQ2014-51912-REDC), the UPV/EHU (UFI11/22 QOSYC), the Basque Government (GV/EJ, grant IT673-13), and the Generalitat de Catalunya (Grant 2014SGR139).
Electronic excitations lie at the origin of most of the commonly measured spectra. However, the first-principles computation of excited states requires a larger effort than ground-state calculations, which can be very efficiently carried out within density-functional theory. On the other hand, two theoretical and computational tools have come to prominence for the description of electronic excitations. One of them, many-body perturbation theory, is based on a set of Green's-function equations, starting with a one-electron propagator and considering the electron-hole Green's function for the response. Key ingredients are the electron's self-energy σ and the electron-hole interaction. A good approximation for σ is obtained with Hedin's GW approach, using density-functional theory as a zero-order solution. First-principles G W calculations for real systems have been successfully carried out since the 1980s. Similarly, the electron-hole interaction is well described by the Bethe-Salpeter equation, via a functional derivative of σ. An alternative approach to calculating electronic excitations is the time-dependent density-functional theory (TDDFT), which offers the important practical advantage of a dependence on density rather than on multivariable Green's functions. This approach leads to a screening equation similar to the Bethe-Salpeter one, but with a two-point, rather than a four-point, interaction kernel. At present, the simple adiabatic local-density approximation has given promising results for finite systems, but has significant deficiencies in the description of absorption spectra in solids, leading to wrong excitation energies, the absence of bound excitonic states, and appreciable distortions of the spectral line shapes. The search for improved TDDFT potentials and kernels is hence a subject of increasing interest. It can be addressed within the framework of many-body perturbation theory: In fact, both the Green's functions and the TDDFT approaches profit from mutual insight. This review compares the theoretical and practical aspects of the two approaches and their specific numerical implementations, and presents an overview of accomplishments and work in progress. ; This work was supported by the RTN program of the European Union NANOPHASE (contract HPRN-CT-2000-00167). G.O. acknowledges INFM for financial support (project PRA 1MESS). Computer time has been granted by IDRIS (CNRS, France) on project 010544. ; Peer Reviewed
Financial support by the German–Israeli Foundation for Scientific Research and Development (grant I-1342-302.5/2016) and the Latvian Council of Science (grant lzp-2018/1-0147 (D.G., E.A.K.)) is gratefully acknowledged. The authors further thank Guntars Zvejnieks for help with CRYSTAL code calculations. ; Protonic-electronic mixed-conducting perovskites are relevant as cathode materials for protonic ceramic fuel cells (PCFCs). In the present study, the relation between the electronic structure and the thermodynamics of oxygen nonstoichiometry and hydration is investigated for BaFeO3-δ and Ba0.5Sr0.5FeO3-δ by means of density functional theory. The calculations are performed at the PBE + U level and yield ground-state electronic structures dominated by an oxygen-to-metal charge transfer with electron holes in the O 2p valence bands. Oxygen nonstoichiometry is modeled for 0 ≤ δ≤ 0.5 with oxygen vacancies in doubly positive charge states. The energy to form an oxygen vacancy is found to increase upon reduction, i.e., decreasing concentration of ligand holes. The higher vacancy formation energy in reduced (Ba,Sr)FeO3-δ is attributed to a higher Fermi level at which electrons remaining in the lattice from the removed oxide ions have to be accommodated. The energy for dissociative H2O absorption into oxygen vacancies is found to vary considerably with δ, ranging from ≈-0.2 to ≈-1.0 eV in BaFeO3-δ and from ≈0.2 to ≈-0.6 eV in Ba0.5Sr0.5FeO3-δ. This dependence is assigned to the annihilation of ligand holes during oxygen release, which leads to an increase in the ionic charge of the remaining lattice oxide ions. The present study provides sound evidence that p-type electronic conductivity and the susceptibility for H2O absorption are antagonistic properties since both depend in opposite directions on the concentration of ligand holes. The reported trends regarding oxygenation and hydration energies are in line with the experimental observations. ; Latvian Council of Science lzp-2018/1-0147; German–Israeli Foundation for Scientific Research and Development grant I-1342-302.5/2016; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union's Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART²
5 páginas. ; Time-dependent current–density-functional theory (TDCDFT) provides an, in principle, exact scheme to calculate efficiently response functions for a very broad range of applications. However, the lack of approximations valid for a range of parameters met in experimental conditions has so far delayed its extensive use in inhomogeneous systems. On the other side, in many-body perturbation theory accurate approximations are available, but at a price of a higher computational cost. In the present work, the possibility of combining the advantages of both approaches is exploited. In this way, an exact equation for the exchange-correlation kernel of TDCDFT is obtained, which opens the way for a systematic improvement of the approximations adopted in practical applications. Finally, an approximate kernel for an efficient calculation of spectra of solids and molecular conductances is suggested and its validity is discussed. ; Acknowledge the support from the European Union's (EU's) 7th framework program through the ETSF e-I3 infrastructure project (Grant No. 211956), the Spanish MEC (FIS2007-65702-C02-01), ACI Promociona (ACI2009-1036), Grupos Consolidados UPV/EHU del Gobierno Vasco (IT-319-07), and ETORTEK projects. ; Peer reviewed
Density functional theory (DFT) emerged in the last two decades as the most reliable tool for the description and prediction of properties of molecular systems and extended materials, coupling in an unprecedented way high accuracy and reasonable computational cost. This success rests also on the development of more and more performing density functional approximations (DFAs). Indeed, the Achilles' heel of DFT is represented by the exchange-correlation contribution to the total energy, which, being unknown, must be approximated. Since the beginning of the 1990s, global hybrids (GH) functionals, where an explicit dependence of the exchange-correlation energy on occupied Kohn–Sham orbitals is introduced thanks to a fraction of Hartree–Fock-like exchange, imposed themselves as the most reliable DFAs for chemical applications. However, if these functionals normally provide results of sufficient accuracy for most of the cases analyzed, some properties, such as thermochemistry or dispersive interactions, can still be significantly improved. A possible way out is represented by the inclusion, into the exchange-correlation functional, of an explicit dependence on virtual Kohn–Sham orbitals via perturbation theory. This leads to a new class of functionals, called double-hybrids (DHs). In this Account, we describe our nonempirical approach to DHs, which, following the line traced by the Perdew–Burke–Ernzerhof approach, allows for the definition of a GH (PBE0) and a DH (QIDH) model. In such a way, a whole family of nonempirical functionals, spanning on the highest rungs of the Perdew's quality scale, is now available and competitive with other—more empirical—DFAs. Discussion of selected cases, ranging from thermochemistry and reactions to weak interactions and excitation energies, not only show the large range of applicability of nonempirical DFAs, but also underline how increasing the number of theoretical constraints parallels with an improvement of the DFA's numerical performances. This fact further consolidates the strong theoretical framework of nonempirical DFAs. Finally, even if nonempirical DH approaches are still computationally expensive, relying on the fact that they can benefit of all technical enhancements developed for speeding up post-Hartree–Fock methods, there is substantial hope for their near future routine application to the description and prediction of complex chemical systems and reactions. ; This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (Grant Agreement No. 648558).
