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One‐third of monolayer of Sn adatoms on a Ge(111) substrate forms a 2D triangular lattice with one unpaired electron per site. The system presents a metal–insulator transition when decreasing the temperature and it is known to exhibit strong electron–phonon coupling at 120–150 K. Herein, a study of the electronic band structure for α‐Sn/Ge(111) between 150 and 5 K is reported. Both the experimental Fermi surfaces and the energy dispersions along high symmetry directions as a function of the temperature are presented. At 5 K it is observed a weakly or low‐dispersing spectral feature, exhibiting an extended gap in the reciprocal space. This feature is derived from the topmost occupied band, which is metallic at high temperature and which develops a kink associated with the strong electron–phonon coupling. The spectral evolution is partially explained with an increase of the electron–phonon coupling when decreasing the temperature. The increase of the electron–phonon coupling at low temperatures gives light into the new physics of this 2D system. The bandwidth is progressively reduced when reducing the temperature, enhancing the electronic correlation effects, and triggering the Mott transition. [ABSTRACT FROM AUTHOR]

Here, the mechanism of charge‐disproportionation (CD) in BaBiO3 (BBO) using density functional theory under different crystal symmetries and by employing strain as an external perturbation is investigated. The competition between Bi 6sp–O 2p (s‐p) and O 2p–O 2p (p‐p) or Bi 6p–O 2p charge fluctuations decides the electronic ground state, CD, and bond‐disproportionation (BD) in BBO. An extended Hubbard Hamiltonian involving onsite (U) and long‐range (V) coulomb repulsion is also employed to ascertain the microscopic conditions for forming the lone pair on the bismuth site. A strong tensile strain increases p‐p fluctuation and drives the system into a strong negative‐CT character, while a strong compressive strain favors s‐p fluctuation leading to the enhanced positive‐CT character. This indicates that the change transfer energy of the bulk BBO can be tuned with external strain. [ABSTRACT FROM AUTHOR]

Here, the mechanism of charge‐disproportionation (CD) in BaBiO3 (BBO) using density functional theory under different crystal symmetries and by employing strain as an external perturbation is investigated. The competition between Bi 6sp–O 2p (s‐p) and O 2p–O 2p (p‐p) or Bi 6p–O 2p charge fluctuations decides the electronic ground state, CD, and bond‐disproportionation (BD) in BBO. An extended Hubbard Hamiltonian involving onsite (U) and long‐range (V) coulomb repulsion is also employed to ascertain the microscopic conditions for forming the lone pair on the bismuth site. A strong tensile strain increases p‐p fluctuation and drives the system into a strong negative‐CT character, while a strong compressive strain favors s‐p fluctuation leading to the enhanced positive‐CT character. This indicates that the change transfer energy of the bulk BBO can be tuned with external strain. [ABSTRACT FROM AUTHOR]

Copyright of Rare Metals is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Partial charges are a central concept in general chemistry and chemical biology, yet dozens of different computational definitions exist. In prior work [Cho et al., ChemPhysChem21, 688‐696 (2020)], we showed that these can be reduced to at most three 'principal components of ionicity'. The present study addressed the dependence of computed partial charges q on 1‐particle basis set and (for WFT methods) n‐particle correlation treatment or (for DFT methods) exchange‐correlation functional, for several representative partial charge definitions such as QTAIM, Hirshfeld, Hirshfeld‐I, HLY (electrostatic), NPA, and GAPT. Our findings show that semi‐empirical double hybrids can closely approach the CCSD(T) 'gold standard' for this property. In fact, owing to an error compensation in MP2, CCSD partial charges are further away from CCSD(T) than is MP2. The nonlocal correlation is important, especially when there is a substantial amount of nonlocal exchange. Employing range separation proves to be "mostly" not advantageous, while global hybrids perform optimally for 20%–30% Hartree‐Fock exchange across all charge types. Basis set convergence analysis shows that an augmented triple‐zeta heavy‐aug‐cc‐pV(T+d)Z basis set or a partially augmented jun‐cc‐pV(T+d)Z basis set is sufficient for Hirshfeld, Hirshfeld‐I, HLY, and GAPT charges. In contrast, QTAIM and NPA display slower basis set convergence. It is noteworthy that for both NPA and QTAIM, HF exhibits markedly slower basis set convergence than the correlation components of MP2 and CCSD. Triples corrections in CCSD(T), denoted as CCSD(T)‐CCSD, exhibit even faster basis set convergence. [ABSTRACT FROM AUTHOR]

We employ the four-component relativistic extended–coupled–cluster (ECC) method, a variational coupled–cluster (CC) approach, to compute the permanent electric dipole moment (PDM) of open-shell diatomic molecules (CaH, CaF, SrH and SrF) in their ground electronic state. The ECC results are compared with the PDM values estimated by the experiments as well as other single-reference CC-based approaches (the Z-vector technique, the expectation value method and the finite field approach) within the four-component relativistic framework to test the efficacy of the employed method. Our study reveals that the relativistic ECC method can yield reliable results for the PDMs of the considered molecular systems. We also observe that the computed results of the dipole moment improve upon the augmentation of diffused functions to the basis set. [ABSTRACT FROM AUTHOR]

Excitation energies belonging to the (1s2)2s22p2, 2s22p3p, 2s2p3, 2s22p3s, and 2s22p3d configurations of carbon-like Na VI and Al VIII have been calculated with the multi-configuration Rayleigh–Ritz variation method and restricted variation method, as well as wavelengths, line strengths, oscillator strengths, transition rates for electric dipole transitions among these terms. High-accuracy calculations have been performed using a moderate scale of Slater basis function and accurate treatments of relativity, electron correlation, and quantum electrodynamic (QED) effects. The line strengths, transition oscillator strengths, and transition probabilities for the electric dipole transitions are determined. Deviations of line strengths between the length and velocity gauges are discussed, as well as with the experimentally compiled values from the National Institute for Standards and Technology (NIST) and other theoretical data wherever available. Furthermore, the accuracy of each electric dipole transition is assessed. The present results are accurate enough for identification of emission lines involving these terms and are also useful for precise spectral modeling and diagnosing in astrophysical and laboratory plasmas. [ABSTRACT FROM AUTHOR]

A self-consistent field method based on a density matrix functional theory scheme is presented to compute the potential energy curve of the hydrogen molecule. A functional to recover the so-called cumulant correlation energy is introduced. The form of such functional is very simple being dependent explicitly only on the square of the overlap matrix element between the unrestricted non-orthogonal occupied orbitals of the calculation. The functional is a sum of two contributions referring to non-dynamic and dynamic correlation. The agreement with the full-CI potential energy curve is within chemical accuracy. [ABSTRACT FROM AUTHOR]

Vanadium dioxide is a promising thermochromic material for novel applications to smart optoelectronic devices due to its reversible insulator-to-metal transition which is accompanied by rapid variation in its optical properties. In this work, rare-earth (La, Nd, Sm, Eu, Dy and Er) dopants substituting vanadium site were investigated in detail to control the phase transition temperature (T MIT) and optical properties of VO 2. It is found that T MIT is mainly related to the nature of 3d-4f hybridization in RE-doped VO 2 (M 1): the direct overlapping of 3d-4f orbitals contributes to V–V dimerization, but it is less pronounced to lower T MIT ; the indirect 3d-4f interaction via O-2p makes 4f electrons itinerant to e g π states to assist MIT from M 1 to R phase at lower temperatures. The dominating nature of 3d-4f hybridization varies with changing RE dopant's ionic radius and electronegativity. Er offered the lowest T M I T with a decreasing rate of 12.8 °C/at. % due to effective 4f-O2p-3d hybridization. RE dopants were also found effective in changing optical properties by scaling E g 1 and E g 2 bandgaps of VO 2. Hence, the fashion of 3d-4f interaction in RE-doped VO 2 is found to play important role in modulating thermochromic properties of VO 2 for smart window applications and smart optoelectronics devices. [ABSTRACT FROM AUTHOR]

Ab-initio quantum chemistry simulations are essential for understanding electronic structure of molecules and materials in almost all areas of chemistry. A broad variety of electronic structure theories and implementations has been developed in the past decades to hopefully solve the many-body Schrödinger equation in an approximate manner on modern computers. In this review, we present recent progress in advancing low-rank electronic structure methodologies that rely on the wavefunction sparsity and compressibility to select the important subset of electronic configurations for both weakly and strongly correlated molecules. Representative chemistry applications that require the many-body treatment beyond traditional density functional approximations are discussed. The low-rank electronic structure theories have further prompted us to highlight compressive and expressive principles that are useful to catalyze idea of quantum learning models. The intersection of the low-rank correlated feature design and the modern deep neural network learning provides new feasibilities to predict chemically accurate correlation energies of unknown molecules that are not represented in the training dataset. The results by others and us are discussed to reveal that the electronic feature sets from an extremely low-rank correlation representation, which is very poor for explicit energy computation, are however sufficiently expressive for capturing and transferring electron correlation patterns across distinct molecular compositions, bond types and geometries. This article is categorized under: Electronic Structure Theory > Ab Initio Electronic Structure Methods Software > Quantum Chemistry Software > Simulation Methods [ABSTRACT FROM AUTHOR]

The unrestricted Hartree‐Fock method is extended to correlation calculation within the density‐matrix functional theory. The method is derived from an entropic cumulant functional for the correlation energy. The eigenvalue equations for the spin‐orbitals are modified by the orbital occupation numbers. The Euler equation for the occupation numbers results in the Fermi‐Dirac distribution, which is very efficient to update as soon as the orbital eigenvalue equations are solved. The method is demonstrated on the ground state of O2. [ABSTRACT FROM AUTHOR]

The interacting quantum atoms (IQA) method offers a rigorous and minimal route to calculate atomic electron correlation energies from the two-particle density matrix (2PDM). The price paid is that this method is very time-consuming. However, employing CCSD and CCSD(T), we explore several approaches to speed up such calculations. We make the pivotal observation that the removal, from the true 2PDM, of both the Hartree–Fock part of the 2PDM and an approximate 2PDM (Müller) dramatically reduces the size of the quadrature grid needed to obtain accurate energies. [ABSTRACT FROM AUTHOR]

The metal–insulator transition induced by the gate electric field in the charge order phase of the α-(BEDT-TTF)2I3 single-crystal field-effect transistor (FET) structure was clearly observed near the phase transition temperature. An abrupt increase in the electrical conductance induced by the applied gate electric field was evident, which corresponds to the partial dissolution of the charge order phase triggered by the gate electric field. The estimated nominal dissolved charge order region (i.e., the gate-induced metallic region) was overestimated in 130–150 K, suggesting additional effects such as Joule heating. On the other hand, in the lower temperature region below 120 K, the corresponding dissolved charge order was several monolayers of BEDT-TTF, suggesting that it is possible to dissolve the charge order phase within the bistable temperature region.

Magnetic properties of molecules such as magnetizabilities represent second order derivatives of the energy with respect to external perturbations. To avoid the need for analytic second derivatives and thereby permit evaluation of the performance of methods where they are not available, a new implementation of quantum chemistry calculations in finite applied magnetic fields is reported. This implementation is employed for a collection of small molecules with the aug-cc-pVTZ basis set to assess orbital optimized (OO) MP2 and a recently proposed regularized variant of OOMP2, called κ-OOMP2. κ-OOMP2 performs significantly better than conventional second order Møller-Plesset (MP2) theory, by reducing MP2's exaggeration of electron correlation effects. As a chemical application, we revisit an old aromaticity criterion called magnetizability exaltation. In lieu of empirical tables or increment systems to generate references, we instead use straight chain molecules with the same formal bond structure as the target cyclic planar conjugated molecules. This procedure is found to be useful for qualitative analysis, yielding exaltations that are typically negative for aromatic species and positive for antiaromatic molecules. One interesting species, N2S2, shows a positive exaltation despite having aromatic characteristics.

The metal–insulator transition induced by the gate electric field in the charge order phase of the  α -(BEDT-TTF) 2 I 3  single-crystal field-effect transistor (FET) structure was clearly observed near the phase transition temperature. An abrupt increase in the electrical conductance induced by the applied gate electric field was evident, which corresponds to the partial dissolution of the charge order phase triggered by the gate electric field. The estimated nominal dissolved charge order region (i.e., the gate-induced metallic region) was overestimated in 130–150 K, suggesting additional effects such as Joule heating. On the other hand, in the lower temperature region below 120 K, the corresponding dissolved charge order was several monolayers of BEDT-TTF, suggesting that it is possible to dissolve the charge order phase within the bistable temperature region. [ABSTRACT FROM AUTHOR]

Structure vis-à-vis the stability of mono- and poly-atomic buckminsterfullerene (C 60 ) complexes are capable of providing intrigue information about these systems. To obtain an insight of these complexes, geometrical parameters of fullerene encapsulated noble gas elements (He, Ne and Ar) and poly-atomic molecules (H 2 , H 2 O, NH 3 and CH 4 ) are computed at the restricted Hartree-Fock (RHF) as well as density functional (DFT) at the Becke, 3-parameter, Lee-Yang-Parr (B3LYP) level of theories. Ellipticity values estimated from mean maximal and mean minimal diameters of these endohedrals are found to be ∼ 0.2 which indicate that these complexes are of spheroidal shape. It is further observed that the fullerene ring is resilient to deformation and the structural parameters of these systems depend more on the method than the embedded system. Binding energies of these complexes are computed at the RHF, DFT, second order Möller-Plesset perturbation (MP2), spin-component scaled (SCS) MP2 and coupled-cluster with single and double excitation (CCSD) level of theories to assess electron correlation effects on the stability of these endohedrals. Resulted energies from the RHF procedure are found to be positive (energetically unstable), whereas those yielded by MP2 and SCS-MP2 procedures predict these endohedrals to be stable. CCSD calculations also exhibit similar trend except for H 2 O complex. Binding energies obtained using RHF, MP2, SCS-MP2 and CCSD procedures with correlation consistent polarized basis indicate a strong correlation between the basis set and the stability of these endohedral complexes. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Xinyang Normal University Natural Science Edition is the property of Journal of Xinyang Normal University Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

VO2 is one of the most studied vanadium oxides because it undergoes a reversible metal-insulator transition (MIT) upon heating with a critical temperature of around 340 K. One of the most overlooked aspects of VO2 is the band's anisotropy in the metallic phase when the Fermi level is crossed by two bands: π* and d||. They are oriented perpendicularly in one respect to the other, hence generating anisotropy. One of the parameters tuning MIT properties is the unbalance of the electron population of π* and d|| bands that arise from their different energy position with respect to the Fermi level. In systems with reduced dimensionality, the electron population disproportion is different with respect to the bulk leading to a different anisotropy. Investigating such a system with a band-selective spectroscopic tool is mandatory. In this manuscript, we show the results of the investigation of a single crystalline 8 nm VO2/TiO2(101) film. We report on the effectiveness of linearly polarized resonant photoemission (ResPES) as a band-selective technique probing the intrinsic anisotropy of VO2. [ABSTRACT FROM AUTHOR]

In this work, it is presented an approach to the local electron energy of the first thirty-six atoms in their ground state using Rényi's entropy is defined in terms of the electron density in position and momentum spaces. To carry out our study, we calculated and compared the trends obtained from the local functionals of kinetic, exchange, and Coulomb energy, with Rényi's entropy. Our results suggest that Rényi's entropy could be a good option to perform studies on the local behavior of the energy in atoms and also in studies on the electron correlation phenomenon. [ABSTRACT FROM AUTHOR]

The effect of approximating electron correlation in few-electron systems is investigated using three wavefunctions: the many body wavefunction referred to as fully-correlated (FC), the Hartree Fock wavefunction (HF), and the Colle and Salvetti Jastrow-style wavefunction (CS) used in the derivation of the popular Lee, Yang and Parr (LYP) correlation functional. The electron correlation energy, static and dynamic Coulomb holes, various expectation values, and radial energy distributions (energy densities) are considered. It is shown that whilst the CS wavefunction can capture the behaviour of the wavefunction at the singularities of the Coulomb potential corresponding to the coalescence of two particles and decreases the area of the primary static Coulomb hole by nearly 30% compared to HF for He and Li + , it is unable to accurately model expectation values that depend on the whole of space and key features of the electron density behaviour in dynamic Coulomb holes and the energy density behaviour in energy distributions. These issues are amplified when considering the hydride ion and a system with the critical nuclear charge for binding. [ABSTRACT FROM AUTHOR]

Correlated states have emerged in low-dimensional systems owing to enhanced Coulomb interactions. Elucidating these states requires atomic-scale characterization and delicate control capabilities. Herein, spectroscopic imaging-scanning tunneling microscopy was employed to investigate the correlated states residing in 1D electrons of the monolayer and bilayer MoSe2 mirror twin boundary (MTB). The Coulomb energies, determined by the wire length, drive the MTB into two types of ground states with distinct respective out-of-phase and in-phase charge orders. The two ground states can be reversibly converted through a metastable zero-energy state with in situ voltage pulses, which tune the electron filling of the MTB via a polaronic process, substantiated by first-principles calculations. Our Hubbard model calculation with an exact diagonalization method reveals the ground states as correlated insulators from an on-site U-originated Coulomb interaction, dubbed the Hubbard-type Coulomb blockade effect. Our study lays a foundation for understanding and tailoring correlated physics in complex systems. [ABSTRACT FROM AUTHOR]

Dramatically sharp resonances manifesting stable negative ion formation characterize Regge pole-calculated low-energy electron elastic total cross sections (TCSs) of heavy multi-electron systems. The novelty of the Regge pole analysis is in the extraction of rigorous and unambiguous negative ion binding energies (BEs), corresponding to the measured electron affinities (EAs) of the investigated multi-electron systems. The measured EAs have engendered the crucial question: is the EA of multi-electron atoms and fullerene molecules identified with the BE of the attached electron in the ground, metastable or excited state of the formed negative ion during a collision? Inconsistencies in the meaning of the measured EAs are elucidated and new EA values for Bk, Cf, Fm, and Lr are presented. [ABSTRACT FROM AUTHOR]

Singlet and triplet spin state energies for three‐dimensional Hooke atoms, that is, electrons in a quadratic confinement, with even number of electrons (2, 4, 6, 8, 10) is discussed using Full‐CI and CASSCF type wavefunctions with a variety of basis sets and considering perturbative corrections up to second order. The effect of the screening of the electron–electron interaction is also discussed by using a Yukawa‐type potential with different values of the Yukawa screening parameter (λee = 0.2, 0.4, 0.6, 0.8, 1.0). Our results show that the singlet state is the ground state for two and eight electron Hooke atoms, whereas the triplet is the ground spin state for 4‐, 6‐, and 10‐electron systems. This suggests the following Aufbau structure 1s < 1p < 1d with singlet ground spin states for systems in which the generation of the triplet implies an inter‐shell one‐electron promotion, and triplet ground states in cases when there is a partial filling of electrons of a given shell. It is also observed that the screening of electron–electron interactions has a sizable quantitative effect on the relative energies of both spin states, specially in the case of two‐ and eight‐electron systems, favoring the singlet state over the triplet. However, the screening of the electron–electron interaction does not provoke a change in the nature of the ground spin state of these systems. By analyzing the different components of the energy, we have gained a deeper understanding of the effects of the kinetic, confinement and electron–electron interaction components of the energy. [ABSTRACT FROM AUTHOR]

We study effects of lattice imperfections on high-harmonic generation from correlated systems using the Fermi–Hubbard model. We simulate such imperfections by randomly modifying the chemical potential across the individual lattice sites. We control the degree of electron–electron interaction by varying the Hubbard U . In the limit of vanishing U , this approach results in Anderson localization. For nonvanishing U , we rationalize the spectral observations in terms of qualitative k -space and real-space pictures. When the interaction and imperfection terms are of comparable magnitude, they may balance each other out, causing Bloch-like transitions. If the terms differ significantly, each electron transition requires a relatively large amount of energy and the current is reduced. We find that imperfections result in increased high-harmonic gain. The spectral gain is mainly in high harmonic orders for low U and low orders for high U .

Machine learning (ML) provides access to fast and accurate quantum chemistry (QC) calculations for various properties of interest such as excitation energies. It is often the case that high accuracy in prediction using a ML model, demands a large and costly training set. Various solutions and procedures have been presented to reduce this cost. These include methods such as Δ-ML, hierarchical-ML, and multifidelity machine learning (MFML). MFML combines various Δ-ML like sub-models for various fidelities according to a fixed scheme derived from the sparse grid combination technique. In this work we implement an optimization procedure to combine multifidelity models in a flexible scheme resulting in optimized MFML (o-MFML) that provides superior prediction capabilities. This hyperparameter optimization is carried out on a holdout validation set of the property of interest. This work benchmarks the o-MFML method in predicting the atomization energies on the QM7b dataset, and again in the prediction of excitation energies for three molecules of growing size. The results indicate that o-MFML is a strong methodological improvement over MFML and provides lower error of prediction. Even in cases of poor data distributions and lack of clear hierarchies among the fidelities, which were previously identified as issues for multifidelity methods, the o-MFML is advantageous for the prediction of quantum chemical properties.

It has been suspected since the early days of the random-phase approximation (RPA) that corrections to RPA correlation energies result mostly from short-range correlation effects and are thus amenable to perturbation theory. Here we test this hypothesis by analyzing formal and numerical results for the most common beyond-RPA perturbative corrections, including the bare second-order exchange (SOX), second-order screened exchange (SOSEX), and approximate exchange kernel (AXK) methods. Our analysis is facilitated by efficient and robust algorithms based on the resolution-of-the-identity (RI) approximation and numerical frequency integration, which enable benchmark beyond-RPA calculations on medium- and large-size molecules with size-independent accuracy. The AXK method systematically improves upon RPA, SOX, and SOSEX for reaction barrier heights, reaction energies, and noncovalent interaction energies of main-group compounds. The improved accuracy of AXK compared with SOX and SOSEX is attributed to stronger screening of bare SOX in AXK. For reactions involving transition-metal compounds, particularly 3d transition-metal dimers, the AXK correction is too small and can even have the wrong sign. These observations are rationalized by a measure α̅ of the effective coupling strength for beyond-RPA correlation. When the effective coupling strength increases beyond a critical α̅ value of approximately 0.5, the RPA errors increase rapidly and perturbative corrections become unreliable. Thus, perturbation theory can systematically correct RPA but only for systems and properties qualitatively well captured by RPA, as indicated by small α̅ values.

Actinoid tetroxide molecules AnO4 (An = Ac – Cm) are investigated with the ab initio density matrix renormalization group (DMRG) approach. Natural orbital shapes are used to read out the oxidation state (OS) of the f‐elements, and the atomic orbital energies and radii are used to explain the trends. The highest OSs reveal a "volcano"‐type variation: For An = Ac – Np, the OSs are equal to the number of available valence electrons, that is, AcIII, ThIV, PaV, UVI, and NpVII. Starting with plutonium as the turning point, the highest OSs in the most stable AnO4 isomers then decrease as PuV, AmV, and CmIII, indicating that the 5f‐electrons are hard to be fully oxidized off from Pu onward. The variations are related to the actinoid contraction and to the 5f‐covalency characteristics. Combined with previous work on OSs, we review their general trends throughout the periodic table, providing fundamental understanding of OS‐relevant phenomena. [ABSTRACT FROM AUTHOR]

Enhancing electron correlation in a weakly interacting topological system has great potential to promote correlated topological states of matter with extraordinary quantum properties. Here, the enhancement of electron correlation in a prototypical topological metal, namely iridium dioxide (IrO2), via doping with 3d transition metal vanadium is demonstrated. Single‐crystalline vanadium‐doped IrO2 nanowires are synthesized through chemical vapor deposition where the nanowire yield and morphology are improved by creating rough surfaces on substrates. Vanadium doping leads to a dramatic decrease in Raman intensity without notable peak broadening, signifying the enhancement of electron correlation. The enhanced electron correlation is further evidenced by transport studies where the electrical resistivity is greatly increased and follows an unusual T$\sqrt T $ dependence on the temperature (T). The lattice thermal conductivity is suppressed by an order of magnitude via doping even at room temperature where phonon‐impurity scattering becomes less important. Density functional theory calculations suggest that the remarkable reduction of thermal conductivity arises from the complex phonon dispersion and reduced energy gap between phonon branches, which greatly enhances phase space for phonon–phonon Umklapp scattering. This work demonstrates a unique system combining 3d and 5d transition metals in isostructural materials to enrich the system with various types of interactions. [ABSTRACT FROM AUTHOR]

The conceptual connections between [4n] Hückel antiaromaticity, disjoint orbitals, correlation energy, pro‐aromaticity and diradical character for a variety of extended π‐conjugated systems, including some salient recent examples of nanographenes and polycyclic aromatic radicals, are provided based on their [4n]annulene peripheries. The realization of such structure–property relationships has led to a beneficial pedagogic exercise establishing design guidelines for diradicaloids. The antiaromatic fingerprint of the [4n]annulene peripheries upon orbital interactions due to internal covalent connectors gives insights into the diradicaloid property of a diversity of π‐conjugated molecules that have fascinated chemists recently. [ABSTRACT FROM AUTHOR]

Density functional theory (DFT) is currently experiencing a golden age. The past two decades witnessed remarkable advances in the general applicability of density functionals in the top rungs of Jacob's Ladder. Nevertheless, Jacob's Ladder may have reached its highest rung in terms of dependencies on occupied (rung four) and unoccupied orbitals (rung five). Moreover, the fifth rung is associated with a computational cost far greater than the lower rungs. Another limitation is that each rung includes dozens of different functionals, and at present, there is no clear pathway for systematic improvements within each rung of the ladder. This highlight provides an overview of the exchange–correlation (XC) hole and how it could be used in developing new density functionals. We begin with a brief overview of the current status and challenges in developing better density functionals, followed by the intimate relationship between the XC functional and hole. We present a conceptually simple and computationally economical method for calculating the XC hole and how this method could offer new directions in developing better exchange–correlation functionals. In this highlight we briefly review the strategies used for developing better DFT methods and outline future directions based on a conceptually simple method for calculating the exchange–correlation hole. [ABSTRACT FROM AUTHOR]

Density functional theory (DFT) is currently experiencing a golden age. The past two decades witnessed remarkable advances in the general applicability of density functionals in the top rungs of Jacob's Ladder. Nevertheless, Jacob's Ladder may have reached its highest rung in terms of dependencies on occupied (rung four) and unoccupied orbitals (rung five). Moreover, the fifth rung is associated with a computational cost far greater than the lower rungs. Another limitation is that each rung includes dozens of different functionals, and at present, there is no clear pathway for systematic improvements within each rung of the ladder. This highlight provides an overview of the exchange–correlation (XC) hole and how it could be used in developing new density functionals. We begin with a brief overview of the current status and challenges in developing better density functionals, followed by the intimate relationship between the XC functional and hole. We present a conceptually simple and computationally economical method for calculating the XC hole and how this method could offer new directions in developing better exchange–correlation functionals. In this highlight we briefly review the strategies used for developing better DFT methods and outline future directions based on a conceptually simple method for calculating the exchange–correlation hole. [ABSTRACT FROM AUTHOR]

The emerging field of quantum simulation of many-body systems is widely recognized as a very important application of quantum computing. A crucial step towards its realization in the context of many-electron systems requires a rigorous quantum mechanical treatment of the different interactions. In this pilot study, we investigate the physical effects beyond the mean-field approximation, known as electron correlation, in the ground state energies of atomic systems using the classical-quantum hybrid variational quantum eigensolver algorithm. To this end, we consider three isoelectronic species, namely Be, Li−, and B+. This unique choice spans three classes—a neutral atom, an anion, and a cation. We have employed the unitary coupled-cluster ansätz to perform a rigorous analysis of two very important factors that could affect the precision of the simulations of electron correlation effects within a basis, namely mapping and backend simulator. We carry out our all-electron calculations with four such basis sets. The results obtained are compared with those calculated by using the full configuration interaction, traditional coupled-cluster and the unitary coupled-cluster methods, on a classical computer, to assess the precision of our results. A salient feature of the study involves a detailed analysis to find the number of shots (the number of times a variational quantum eigensolver algorithm is repeated to build statistics) required for calculations with IBM Qiskit’s QASM simulator backend, which mimics an ideal quantum computer. When more qubits become available, our study will serve as among the first steps taken towards computing other properties of interest to various applications such as new physics beyond the Standard Model of elementary particles and atomic clocks using the variational quantum eigensolver algorithm.

Non-covalent interactions play a primordial role in chemistry. Beyond their quantification, the detailed understanding of their physical processes is necessary to rationalize chemical trends and improve designs of chemical systems. Energy decomposition analyses allow detailed insight into non-covalent interactions by extracting electrostatics, Pauli repulsion, polarization, dispersion and charge transfer components from interaction energies. Recent work has demonstrated that electronic correlation influenced significantly all of these energy components, whereas previous decompositions only partitioned correlation between dispersion and charge transfer. The MP2 energy decomposition analysis with Absolutely Localized Molecular Orbitals (MP2 ALMO-EDA) takes these results fully into account and offers a correlation correction for each extracted component. A recent detailed investigation of the CCSD dispersion energy showed that a small number of virtual orbitals is sufficient to describe dispersion interactions accurately in the long-range, which potentially offers a basis-set independent definition of dispersion. Finally, we present an application of MP2 ALMO-EDA to a series of unusual halogen bonding complexes where charge transfer dominates over the electrostatic σ-hole interaction.

The interplay of electron correlations and topological phases gives rise to various exotic phenomena including fractionalization, excitonic instability and axionic excitation. Recently discovered transition-metal pentatellurides can reach the ultra-quantum limit in low magnetic fields and serve as good candidates for achieving such a combination. Here, we report evidence of density wave and metal-insulator transition in HfTe5 induced by intense magnetic fields. Using the non-linear transport technique, we detect a distinct non-linear conduction behavior in the longitudinal resistivity within the a–c plane, corresponding to the formation of a density wave induced by magnetic fields. In high fields, the onset of non-linear conduction in the Hall resistivity indicates an impurity-pinned magnetic freeze-out as the possible origin of the insulating behavior. These frozen electrons can be gradually reactivated into mobile states above a threshold of electric field. This experimental evidence calls for further investigation into the underlying mechanism of the bulk quantum Hall effect and field-induced phase transitions in pentatellurides. [ABSTRACT FROM AUTHOR]

In this work, we explored two possible generalizations of Shannon's entropy, the first one using the so-called q-algebra and the second one through the q-calculus. The expression obtained through the q-algebra cannot be used when the calculations are performed with probabilistic distributions because in several cases the results obtained are complex, while with the equation obtained with the q-calculus such problems were not detected. As a study case, we analyzed a simple chemical reaction and found that the generalization of Shannon's entropy through Jackson's integral can be linked to the electron correlation of the system. [ABSTRACT FROM AUTHOR]

Rayleigh-Schrödinger perturbation theory corrections are developed for an algebraic Bethe ansatz of individual electrons. Numerical results are ambiguous and would need either an orbital optimization or a configuration interaction singles solution to be satisfactory. Two different expressions are computed to be certain that results are correct. [ABSTRACT FROM AUTHOR]

The single-reference coupled-cluster (CC) method is one of the most efficient approaches for describing electron correlation effects in atoms and molecules but its successful application is limited to states dominated by a single Slater determinant. In cases where several determinants dominate the wave function expansion, the multi-reference (MR) CC formulations are required. They usually use the effective Hamiltonian formalism which allows dealing with two types of electron correlation appearing in the MR cases: nondynamic and dynamic. There are two MR-CC formulations of this type: the Hilbert space CC method and the Fock-space (FS) one. In the paper, a version of the FS-CC method designated for the description of systems with three-valence electrons is being considered. The idea of the intermediate Hamiltonian (IH) reformulation of the standard FS-CC method, which simplifies the calculations and avoids the intruder state problem, is discussed in this context. A new IH-FS-CC approach is presented which takes advantage of the redefinition of the normal ordering of the FS cluster expansion introduced by Lindgren. The redefinition significantly simplifies the IH-FS-CC formalism and lowers the computational cost of the calculations. The advantages of the new IH formulation are discussed in detail for the case of the FS-CC method in which the lowest-order cluster operators are included in all sectors of the Fock-space, including the three-valence one. [ABSTRACT FROM AUTHOR]

The bonding in metals is analysed within the framework of the PATMOS (Perturbed AToms in MOlecules and Solids) model. The electronic binding energy per atom is written as a sum of a distortion energy of the atoms and the partitioned interaction energy comprising Coulombic, exchange and correlation terms. The adopted physical model of the infinite system, is spherical embedding of the atoms of the reference unit cell. Correlation energies are calculated by second-order Møller-Plesset and second-order Epstein-Nesbet perturbation theory. The binding energy of lithium solid is calculated for 16 nearest neighbour distances from 4.0 to 10.0 Bohr. Electron correlation is of paramount importance for the binding energy. The calculated cohesive energy is (0.0571 ± 0.004) Hartree comparing with experimental value 0.0599 Hartree. The bonding picture is characterised by slightly expanded atomic orbitals. [ABSTRACT FROM AUTHOR]

The research progresses on the investigations of atomic structure and collision dynamics with highly charged ions based on the heavy ion storage rings and electron ion beam traps in recent 20 years are reviewed. The structure part covers test of quantum electrodynamics and electron correlation in strong Coulomb field studied through dielectronic recombination spectroscopy and VUV/x-ray spectroscopy. The collision dynamics part includes charge exchange dynamics in ionâ€"atom collisions mainly in Bohr velocity region, ion-induced fragmentation mechanisms of molecules, hydrogen-bound and van de Waals bound clusters, interference, and phase information observed in ionâ€"atom/molecule collisions. With this achievements, two aspects of theoretical studies related to low energy and relativistic energy collisions are presented. The applications of data relevant to key atomic processes like dielectronic recombination and charge exchanges involving highly charged ions are discussed. At the end of this review, some future prospects of research related to highly charged ions are proposed. [ABSTRACT FROM AUTHOR]

Version 5.0 of the ORCA quantum chemistry program suite was released in July 2021. ORCA 5.0 represents a major improvement over all previous versions of ORCA and features (1) highly improved performance, (2) increased numerical robustness, (3) a host of new functionality, and (4) greatly improved user friendliness. The article describes the most salient features of the program. This article is categorized under:Electronic Structure Theory > Ab Initio Electronic Structure MethodsData Science > Computer Algorithms and ProgrammingSoftware > Quantum Chemistry [ABSTRACT FROM AUTHOR]

In this work, we analyze a simple chemical reaction, H 2 + H - ⟶ H 2 + H - , employing several entropies definitions which were written using several probability distributions, such as joint probability, mutual probability, and conditional probability. We show, that the equations obtained permit the introduction of the concept of information transfer in a chemical process, in this regard, we also show that such information transfer is related to the bond-forming and bond-breaking process and to the electron correlation phenomenon. [ABSTRACT FROM AUTHOR]

VO2 is one of the most studied vanadium oxides because it undergoes a reversible metal-insulator transition (MIT) upon heating with a critical temperature of around 340 K. One of the most overlooked aspects of VO2 is the band’s anisotropy in the metallic phase when the Fermi level is crossed by two bands: π* and d||. They are oriented perpendicularly in one respect to the other, hence generating anisotropy. One of the parameters tuning MIT properties is the unbalance of the electron population of π* and d|| bands that arise from their different energy position with respect to the Fermi level. In systems with reduced dimensionality, the electron population disproportion is different with respect to the bulk leading to a different anisotropy. Investigating such a system with a band-selective spectroscopic tool is mandatory. In this manuscript, we show the results of the investigation of a single crystalline 8 nm VO2/TiO2(101) film. We report on the effectiveness of linearly polarized resonant photoemission (ResPES) as a band-selective technique probing the intrinsic anisotropy of VO2.

Quantum multireference effects are associated with degeneracies and near-degeneracies of the ground state and are critical to a variety of systems. Most approximate functionals of density functional theory (DFT) fail to properly describe such effects. A number of diagnostics have been proposed to estimate in advance the reliability of a given single-reference solution in this respect. Some of these diagnostics, however, lack size-consistency while remaining computationally expensive. In this work, we propose the DFT method of atomic populations of effectively localized electrons (APELE) as a novel diagnostic in this vein. It is compared with existing diagnostics of nondynamic correlation on select exemplary systems. The APELE method is on average in good agreement with the popular T1 index, while being size-consistent and less costly. It becomes particularly informative in cases involving bond stretching or bond breaking. The APELE method is applied next to organic diradicals like the bis-acridine dimer and the p-quinodimethane molecule which possess unusually high nonlinear optical response, and to the reaction of ethylene addition to Ni dithiolene. Our results for this reaction are consistent with the T1 diagnostics and in addition, shed some light on the degree of d-electron localization at the Ni center. [ABSTRACT FROM AUTHOR]

Using the MP2/aug-cc-pVTZ method, quantum chemical calculations of binary complexes with intermolecular C(sp3)...Hal−M (Hal = F, Cl, Br; M = Li, Na, Cu, Mg) bonds formed by the methide anion and its derivatives with metal halides were carried out. The calculations showed that the binding energy in the carbanion complex is higher, the greater the distance between the monomers. The least stable complexes form the strongest partially covalent intermolecular bond with the largest values of electron density at the bond (3, − 1) critical point of the C...Hal contact. The unusual properties of carbanion complexes analyzed in this study are determined by an increase in the energy of electron correlation Ecorr with increasing the intermolecular distance; Ecorr can exceed 50 kcal/mol. The MP2 and CCSD(T) levels of the theory predict a linear relationship between Ecorr values and binding energies. Synopsis: Dramatic changes in the stability of carbanion complexes with the replacement of a hydrogen atom by lithium. [ABSTRACT FROM AUTHOR]

Double photoionization events provide a direct evaluation of electron correlation. The recent focus on few-electron targets continues to reveal the consequences of electron correlation for targets that possess several electrons. We consider the double photoionization of the 2 p 2 valence electrons of atomic carbon and focus on the first energetically accessible final-state symmetries that originate from coupling the active electrons in 3 P configurations, which are doubly ionized by a single photon. Comparison of this process in carbon with neon provides an analogous case for the resulting final-state symmetries within the framework where the ejected electrons are influenced by the remaining bound electrons in a frozen-core approximation. Choosing this symmetry allows for comparison with previous theoretical results for total and energy sharing cross-sections of carbon. Fully differential cross-sections for both carbon and neon are also compared. [ABSTRACT FROM AUTHOR]

It can be argued that electron correlation, as a concept, deserves the same prominence in general chemistry as molecular orbital theory. We show how it acts as Nature's "chemical glue" at both the molecular and supramolecular levels. Electron correlation can be presented in a general chemistry course in an at least somewhat intuitive manner. We also propose a simple classification of correlation effects based on their length scales and the size of the orbital gap (relative to the two‐electron integrals). In the discussion, we also show how DFT can shed light on wavefunction theory, and conversely. We discuss two types of "honorary valence orbitals", one related to small core‐valence gaps, the other to the ability of empty 3d orbitals in 2nd row elements to act as backbonding acceptors. Finally, we show why the pursuit of absolute total energies for their own sake becomes a sterile exercise, and why atomization energies are a more realistic "fix point". [ABSTRACT FROM AUTHOR]