Including Electron Correlation Research Articles

Evolution of the Electronic Properties of SrFe1 − x − y − zAlxMnyCozO3 Solid Solutions Depending on the Composition and the Degree of Localization of Electronic States

The genesis of the electronic spectrum in SrFe1 −xAlxO3, SrFe1 −xMnxO3, SrFe1 −xCoxO3, SrFe1 − 2xAlxCoxO3, and SrFe1−y−zMnyCozO3cubic solid solutions of strontium ferrite, where0⩽x⩽0.15and0⩽y,z⩽0.125, is studied in the coherent potential approximation. The inclusion of electron correlations on 3datoms makes it possible to reproduce the experimental content tendencies in the variation of the electronic and magnetic properties. It is shown that codoping of ferrite with cobalt and aluminum effectively increases the concentration of electron carriers and their degree of localization in SrFe1 −x− 0.15AlxCo0.15O3, which is of interest in the development of oxide thermoelectrics. Due to a relatively large contribution from delocalized states at the Fermi level, SrFe1 −y−zMnyCozO3solid solutions withy=z=0.1−0.12are promising electrode materials.

Theoretical level energies and transition data for 4p[formula omitted]4d[formula omitted], 4p[formula omitted]4d[formula omitted] and 4p[formula omitted]4d[formula omitted]4f configurations of W[formula omitted] ion

The ab initio quasirelativistic approach developed specifically for the calculation of spectral parameters of highly charged ions has been used to determine transition data for the Mo-like tungsten ion W32+. The configuration interaction method is utilized to include electron correlation effects. The relativistic effects are taken into account in the Breit–Pauli approximation. Level energies, radiative lifetimes τ, Landé g-factors are determined for the ground configuration 4p64d6 and two excited configurations 4p54d7 and 4p64d54f. The radiative transition wavelengths λ, emission transition probabilities A, weighted oscillator strengths gf, and transition line strengths S for the electric dipole, electric quadrupole, electric octupole, magnetic dipole, and magnetic quadrupole transitions among the fine-structure levels of these configurations are produced. The uncertainties of computed spectroscopic parameters are evaluated.

The symmetric quasi-classical model using on-the-fly time-dependent density functional theory within the Tamm–Dancoff approximation

The primary computational challenge when simulating nonadiabatic ab initio molecular dynamics is the unfavourable compute costs of electronic structure calculations with molecular size. Simple electronic structure theories, like time-dependent density functional theory within the Tamm–Dancoff approximation (TDDFT/TDA), alleviate this cost for moderately sized molecular systems simulated on realistic time scales. Although TDDFT/TDA does have some limitations in accuracy, an appealing feature is that, in addition to including electron correlation through the use of a density functional, the cost of calculating analytic nuclear gradients and nonadiabatic coupling vectors is often computationally feasible even for moderately sized basis sets. In this work, some of the benefits and limitations of TDDFT/TDA are discussed and analysed with regard to its applicability as a ‘back-end’ electronic structure method for the symmetric quasi-classical Meyer–Miller model (SQC/MM). In order to investigate the benefits and limitations of TDDFT/TDA, SQC/MM is employed to predict and analyse a prototypical example of excited-state hydrogen transfer in gas-phase malonaldehyde. Then, the ring-opening dynamics of selenophene are simulated, which highlight some of the deficiencies of TDDFT/TDA. Additionally, some new algorithms are proposed that speed up the calculation of analytic nuclear gradients and nonadiabatic coupling vectors for a set of excited electronic states.

Finding Valence Antibonding Levels while Avoiding Rydberg, Pseudo-continuum, and Dipole-Bound Orbitals.

Electronic structure methods are now widely used to assist in the interpretation of many varieties of experimental data. The energies and physical characteristics (e.g., sizes, shapes, and spatial localization) of valence antibonding π* and σ* orbitals play key roles in a variety of chemical processes including photochemical reactions and electron attachment reductions and are used in Woodward-Hoffmann-type analyses to probe reaction energy barriers and energy surface intersections leading to internal conversion or intersystem crossings. One's ability to properly populate such valence antibonding orbitals within electronic structure calculations is often hindered by the presence of other molecular orbitals having similar energies. These intruding orbitals can be of Rydberg, pseudo-continuum, or dipole-bound characteristic. This article shows how, within the most widely available electronic structure codes, one can avoid the pitfalls presented by these intruding orbitals to properly populate a valence π* or σ* orbital and how to subsequently use that orbital in a calculation that includes electron correlation effects and thereby offers the possibility of chemically useful precision. Special emphasis is given to cases in which the electronic state is metastable.

Application of the Solute-Solvent Intermolecular Interactions as Indicator of Caffeine Solubility in Aqueous Binary Aprotic and Proton Acceptor Solvents: Measurements and Quantum Chemistry Computations.

The solubility of caffeine in aqueous binary mixtures was measured in five aprotic proton acceptor solvents (APAS) including dimethyl sulfoxide, dimethylformamide, 1,4-dioxane, acetonitrile, and acetone. The whole range of concentrations was studied in four temperatures between 25 °C and 40 °C. All systems exhibit a strong cosolvency effect resulting in non-monotonous solubility trends with changes of the mixture composition and showing the highest solubility at unimolar proportions of organic solvent and water. The observed solubility trends were interpreted based on the values of caffeine affinities toward homo- and hetero-molecular pairs formation, determined on an advanced quantum chemistry level including electron correlation and correction for vibrational zero-point energy. It was found that caffeine can act as a donor in pairs formation with all considered aprotic solvents using the hydrogen atom attached to the carbon in the imidazole ring. The computed values of Gibbs free energies of intermolecular pairs formation were further utilized for exploring the possibility of using them as potential solubility prognostics. A semi-quantitative relationship (R2 = 0.78) between caffeine affinities and the measured solubility values was found, which was used for screening for new greener solvents. Based on the values of the environmental index (EI), four morpholine analogs were considered and corresponding caffeine affinities were computed. It was found that the same solute–solvent structural motif stabilizes hetero-molecular pairs suggesting their potential applicability as greener replacers of traditional aprotic proton acceptor solvents. This hypothesis was confirmed by additional caffeine solubility measurements in 4-formylmorpholine. This solvent happened to be even more efficient compared to DMSO and the obtained solubility profile follows the cosolvency pattern observed for other aprotic proton acceptor solvents.

Born–Oppenheimer potential energy curves of NaK from the optimised atomic basis sets

The article presents adiabatic potential energy curves of the ground and excited electronic states for the diatomic NaK molecule. The calculations were made using the ab initio computational methods to include electron correlation. The studied molecule was calculated as the effective two-electron problem, in which only the valence electrons of the molecule are explicitly taken into account. The remaining electrons with atomic nuclei are described with appropriate, energy-consistent relativistic pseudopotentials. Additionally, a bespoke basis set, generated and optimised for both ground and excited electronic states of the NaK system was developed. The spectroscopic parameters of the calculated potential energy curves were determined and compared with the available experimental and theoretical results. The compliance of the obtained results, despite slight differences, is very satisfactory.

Theoretical level energies and transition data for 4p64d8, 4p54d9 and 4p64d74f configurations of W[formula omitted] ion

The ab initio quasirelativistic approach developed specifically for the calculation of spectral parameters of highly charged ions was used to derive transition data for the Ru-like tungsten ion W30+. The configuration interaction method was applied to include electron correlation effects. The relativistic effects were taken into account in the Breit–Pauli approximation. The level energies, radiative lifetimes τ, Landé g-factors were determined for the ground configuration 4p64d8 and two excited configurations 4p54d9 and 4p64d74f. The radiative transition wavelengths λ and emission transition probabilities A for the electric dipole, electric quadrupole, electric octupole, magnetic dipole, and magnetic quadrupole transitions among the levels of these configurations were generated.

Multihole models for deterministically placed acceptor arrays in silicon

We compute the electronic structure of acceptor clusters in silicon by using three methods to include electron correlations: the full configuration interaction, the Heitler-London approximation, and the unrestricted Hartree-Fock method. We show both the HL approach and the UHF method are good approximations to the ground state of the full CI calculation for a pair of acceptors and for finite linear chains. The total energies for finite linear chains show the formation of a 4-fold degenerate ground state when there is a weak bond at the end of the chain, which is shown to be a manifold of topological edge states. We identify a change in the angular momentum composition of the ground state at a critical pattern of bond lengths and show it is related to a crossing in the Fock matrix eigenvalues. We also test the symmetry of the UHF solution and compare it to the full CI; the symmetry is broken under almost all the arrangements by the formation of a magnetic state in UHF, and we find further broken symmetries for some particular arrangements related to crossings or potential crossings between the Fock-matrix eigenvalues. We also compute the charge distributions across the acceptors obtained from the eigenvectors of the Fock matrix: with weak bonds at the chain ends, two holes are localized at either end of the chain while the others have a nearly uniform distribution over the middle; this implies the existence of the non-trivial edge states. We apply the UHF method to treat an infinite linear chain with periodic boundary conditions. We find the band structures in the UHF approximation and compute the Zak phases for the occupied Fock-matrix eigenvalues; we find they do not correctly predict the topological edge states in this interacting system. We find direct study of the quantum numbers characterising the edge states provides a better insight into their topological nature.

Photoionization of aligned excited states in neon by attosecond laser pulses

We numerically describe the ionization process induced by linearly and circularly polarized extreme ultraviolet (XUV) attosecond laser pulses on an aligned atomic target, specifically the excited state Ne*(1s 22s 22p 5[]3s[]). We compute the excited atomic state by applying the time-dependent restricted-active-space self-consistent field method to fully account for the electronic correlation. We find that correlation-assisted ionization channels can dominate channels accessible without correlation. We also observe that the rotation of the photoelectron momentum distribution by circularly polarized laser pulses compared to the case of linear polarization can be explained in terms of differences in accessible ionization channels. This study shows that it is essential to include electron correlation effects to obtain an accurate description of the photoelectron emission dynamics from aligned excited states.

Modeling (Prediction) of the Structure of Possible Transition States of Aromatic Hydrocarbons

Based on the properties of a conjugate p-electron system, a method is proposed for predicting possible spatial structures of aromatic hydrocarbons during their skeletal carbon transformation. The existence of predicted spatial structures is explained by the stabilizing effect of π-electron conjugation. The proposed method for predicting possible spatial structures was verified by DFT B3LYP/6-31G* and RHF/6-31G calculations including electron correlation at the MR4-SDTQ level on the example of benzene and hexafluorobenzene molecules. The proposed method gives stable results regardless of the methods used for calculating the electronic structure of molecules. It is found that of all the predicted spatial structures, one corresponds to the ground state, and the others correspond to transition states. The schemes of thermal isomerization of benzene and hexafluorobenzene, constructed with the potential barriers calculated by the Gonzales–Schlegel method, are consistent with the experimental data.

Statistical and quantum photoionization cross sections in plasmas: Analytical approaches for any configurations including inner shells

Statistical models combined with the local plasma frequency approach applied to the atomic electron density are employed to study the photoionization cross-section for complex atoms. It is demonstrated that the Thomas–Fermi atom provides surprisingly good overall agreement even for complex outer-shell configurations, where quantum mechanical approaches that include electron correlations are exceedingly difficult. Quantum mechanical photoionization calculations are studied with respect to energy and nl quantum number for hydrogen-like and non-hydrogen-like atoms and ions. A generalized scaled photoionization model (GSPM) based on the simultaneous introduction of effective charges for non-H-like energies and scaling charges for the reduced energy scale allows the development of analytical formulas for all states nl. Explicit expressions for nl = 1s, 2s, 2p, 3s, 3p, 3d, 4s, 4p, 4d, 4f, and 5s are obtained. Application to H-like and non-H-like atoms and ions and to neutral atoms demonstrates the universality of the scaled analytical approach including inner-shell photoionization. Likewise, GSPM describes the near-threshold behavior and high-energy asymptotes well. Finally, we discuss the various models and the correspondence principle along with experimental data and with respect to a good compromise between generality and precision. The results are also relevant to large-scale integrated light–matter interaction simulations, e.g., X-ray free-electron laser interactions with matter or photoionization driven by a broadband radiation field such as Planckian radiation.

Theoretical level energies and transition data for ion W[formula omitted

The quasirelativistic approach was used to derive transition data for the Rh-like W29+ tungsten ion. The quasirelativistic approach was developed specifically for the calculation of spectral parameters of highly charged ions. The relativistic effects were taken into account in the Breit–Pauli approximation. The configuration interaction method was applied to include electron correlation effects. The ground W29+ configuration 4p64d9 and two odd-parity configurations 4p54d10 and 4p64d84f were investigated. Different sets of configuration-interaction extensions were applied for assessment of calculation accuracy. The level energies, radiative lifetimes τ, and Landé g-factors were determined for levels of the excited configurations 4p54d10 and 4p64d84f and the ground configuration 4p64d9. The radiative transition wavelengths λ and emission transition probabilities A for the electric and magnetic transitions of high multipole order among the levels of these configurations were produced. The uncertainties of computed spectroscopic parameters were evaluated. Such a complete and inclusive data set for the levels of two excited configurations and their radiative properties in W29+ is reported for the first time.

Solubility of sulfanilamide in binary solvents containing water: Measurements and prediction using Buchowski-Ksiazczak solubility model

The solubility of Sulfanilamide in binary mixtures comprising water and five organic solvents was studied at different temperatures using a spectrophotometric method. Additionally, the differential scanning calorimetry (DSC) measurements were performed to characterize the non-dissolved portion of Sulfanilamide obtained after shake-flask experiments. The Buchowski-Ksiazczak solubility model was applied to interpret the obtained experimental results. The model was successful in reproducing experimental data, as it was evidenced by several statistical measures, including the Akaike Information Criterion. The obtained parameters of the λh-equation were used for calculating the values of enthalpy of mixing. It was observed that the minimum value of enthalpy of mixing for each system is associated with those binary solvent compositions which result in the highest solubility of Sulfanilamide. Also the maximum value of the λ parameter corresponds to the composition of the mixture yielding the highest solubility. The application of COSMO-RS methodology revealed the sensitivity of solute activities and solvation Gibbs free energies to solvent-cosolvent composition. Also, there was noticed a strong correlation of these thermodynamic properties. It happened that low values of Sulfanilamide activities and their monotonous variations with the composition of considered binary solutions are associated with monotonous solubility profiles and also the reverse conclusion holds. Besides, differences in solvation of monomers and intermolecular complexes are the very probable origins of the observed trends in excess enthalpies of saturated solutions. The advanced quantum chemistry computations, including electron correlation and extended basis set, were used for quantification of affinities of systems components toward pairs formation. From the obtained values of activity equilibrium constants it was concluded that the highest affinity in all studied conditions should be attributed to Sulfanilamide dimers. It was also observed that pairs formed by Sulfanilamide with water are more thermodynamically stable than the ones with cosolvent molecules with only one exception, namely 1,4-dioxane.

Assessment of the proposed pseudo-potential theoretical model for the static and dynamic Raman scattering intensities: Multivariate statistical approach to quantum-chemistry protocols.

Accurate calculation of molecular polarizabilities and Raman intensities required high-level correlated wave functions (CCSD) and large basis set with the inclusion of electronic correlation within experimental precision. These requirements, in terms of time and computation, are economically costly. Polarized Gaussian basis sets adapted to effective core potentials (ECPs) for the static and frequency dependent Raman intensities is presented. The results of the proposed basis sets at CCSD and DFT levels in comparison with Sadlej-pVTZ, as reference basis set, show quite a good quantitative agreement in the properties with a valuable reduction in the computational time and resources. Multivariate principal component analysis (PCA) was performed to study the assessment of the efficiency of proposed methodology and diagnose the inherent information related to the kind of normal vibrational mode of each molecule, based on the variations in the computed Raman intensities. The results, in the form of score-plots, explored a clear segregation and classification among the Raman intensities data, revealing its dependence on the excitation frequencies of laser and nature of the vibrational mode of each molecule of interest. Moreover, the projection of the loadings-plots of the PCs successfully enabled to classify the most correlated computational methods in to the same groups, and made isolations of the less efficient basis functions at the corresponding theoretical method.

Tuning the Kondo effect via gating-controlled orbital selection in the LaAlO3/SrTiO3 interfacial d -electron system

The orbital degree of freedom plays a critical role in contemporary condensed-matter physics, with recent discoveries of orbital-dependent many-body effects including electron correlation and superconductivity. Nevertheless, these orbital-dependent many-body effects have mainly been observed in bulk materials, wherein the electrons' orbital attribute is challenging to tailor selectively. Here, by planar tunneling into the two-dimensional electron system (2DES) at the ${\mathrm{LaAlO}}_{3}/{\mathrm{SrTiO}}_{3}$ interface, we observed a Kondo resonance, a model many-body phenomenon. The observation of the Kondo resonance provides an ideal opportunity to quantitatively exploit the emerging interfacial Kondo physics. Furthermore, it was found that the Kondo coupling strength and resonance line shape can be effectively regulated by electrostatic gating and show a sharp transition at the Lifshitz point, where the orbital occupancy of the 2DES changes from only ${d}_{\mathrm{xy}}$ to both ${d}_{\mathrm{xy}}$ and ${d}_{\mathrm{xz}/\mathrm{yz}}$. This unusual behavior is attributed to the orbital selectivity effect in this unique multiple-$d$-band system, wherein the itinerant ${d}_{\mathrm{xy}}$ and ${d}_{\mathrm{xz}/\mathrm{yz}}$ electrons have different orbital symmetries and therefore different Kondo exchange coupling with the localized ${d}_{\mathrm{xy}}$ electrons. The present study may pave the way for manipulating many-body effects in 2D multiband materials via orbital engineering of itinerant electrons.

Theoretical level energies and transition data for ion W[formula omitted

The ab initio quasirelativistic approach was used to derive transition data for Pd-like tungsten W28+ ion. This approach was developed specifically for the calculation of spectral parameters for highly charged ions. The relativistic effects were taken into account in the Breit–Pauli approximation. The configuration interaction method was applied to include electron correlation effects. The ground configuration 4p64d10 and the lowest excited 4p64d94f were considered. The calculations for W28+ were performed with different sets of interacting configurations in order to evaluate reliability and accuracy of produced results. Level energies, radiative lifetimes τ, Landé g-factors were determined for the excited configuration 4p64d94f of W28+ levels. The radiative transition wavelengths λ and emission transition probabilities A for the electric and magnetic transitions of various multipole order (up to E5 and M4) among the levels of these two configurations were produced. The uncertainties of computed spectroscopic parameters were evaluated.

Chirality-Induced Spin Selectivity: The Role of Electron Correlations.

Chirality-induced spin selectivity, discovered about two decades ago in helical molecules, is a nonequilibrium effect that emerges from the interplay between geometrical helicity and spin-orbit interactions. Several model Hamiltonians building on this interplay have been proposed, and while these can yield spin-polarized transport properties that agree with experimental observations, they simultaneously depend on unrealistic values of the spin-orbit interaction parameters. It is likely, however, that a common deficit originates from the fact that all these models are uncorrelated or single-electron theories. Therefore, chirality-induced spin selectivity is here addressed using a many-body approach, which allows for nonequilibrium conditions and a systematic treatment of the correlated state. The intrinsic molecular spin polarization increases by 2 orders of magnitude, or more, compared to the corresponding result in the uncorrelated model. In addition, the electronic structure responds to varying external magnetic conditions which, therefore, enables comparisons of the currents provided for different spin polarizations in one or both of the leads between which the molecule is mounted. Using experimentally feasible parameters and room temperature, the obtained normalized difference between such currents may be as large as 5-10% for short molecular chains, clearly suggesting the vital importance of including electron correlations when searching for explanations of the phenomenon.

Convergence patterns and rates in two-state perturbation expansions.

A simple two-state model has previously been shown to be able to describe and rationalize the convergence of the most common perturbation method for including electron correlation, the Møller-Plesset expansion. In particular, this simple model has been able to predict the convergence rate and the form of the higher-order corrections for typical Møller-Plesset expansions of the correlation energy. In this paper, the convergence of nondegenerate perturbation expansions in the two-state model is analyzed in detail for a general form of two-state perturbation expansion by examining the analytic expressions of the corrections and series of the values of the corrections for various choices of the perturbation. The previous analysis that covered only a single form of the perturbation is thereby generalized to arbitrary forms of the perturbation. It is shown that the convergence may be described in terms of four characteristics: archetype, rate of convergence, length of recurring period, and sign pattern. The archetype defines the overall form of a plot of the energy-corrections, and the remaining characteristics specify details of the archetype. For symmetric (Hermitian) perturbations, five archetypes are observed: zigzag, interspersed zigzag, triadic, ripples, and geometric. Two additional archetypes are obtained for an asymmetric perturbation: zigzag-geometric and convex-geometric. For symmetric perturbations, each archetype has a distinctive pattern that recurs with a period which depends on the perturbation parameters, whereas no such recurrence exists for asymmetric perturbations from a series of numerical corrections. The obtained relations between the form of a two-state perturbation and the energy corrections allow us to obtain additional insights into the convergence behavior of the Møller-Plesset and other forms of perturbation expansions. This is demonstrated by analyzing several diverging or slowly converging perturbation expansions of ground state and excitation energies. It is demonstrated that the higher-order corrections of these expansions can be described using the two-state model and each expansion can therefore be described in terms of an archetype and the other three characteristics. Examples of all archetypes except the zigzag and convex-geometric archetypes are given. For each example, it is shown how the characteristics may be extracted from the higher-order corrections and used to identify the term in the perturbation that is the cause of the observed slow convergence or divergence.

Performance of Delta-Coupled-Cluster Methods for Calculations of Core-Ionization Energies of First-Row Elements.

A thorough study of the performance of delta-coupled-cluster (ΔCC) methods for calculations of core-ionization energies for elements of the first long row of the periodic table is reported. Inspired by the core-valence separation (CVS) scheme in response theories, a simple CVS scheme of excluding the vacant core orbital from the CC treatment has been adopted to solve the convergence problem of the CC equations for core-ionized states. Dynamic correlation effects have been shown to make important contributions to the computed core-ionization energies, especially to chemical shifts of these quantities. The maximum absolute error (MaxAE) and standard deviation (SD) of delta-Hartree-Fock results for chemical shifts of core-ionization energies with respect to the corresponding experimental values amount to more than 1.7 and 0.6 eV, respectively. In contrast, the inclusion of electron correlation in ΔCC singles and doubles augmented with a noniterative triples correction [ΔCCSD(T)] method significantly reduces the corresponding deviations to around 0.3 and 0.1 eV. With the consideration of basis set effects and the corrections to the CVS approximation, ΔCCSD(T) has been shown to provide highly accurate results for absolute values of core-ionization energies, with a MaxAE of 0.22 eV and SD of 0.13 eV. To further demonstrate the usefulness of ΔCCSD(T), calculations of carbon K-edge ionization energies of ethyl trifluoroacetate, a molecule of significant interest to the study of X-ray spectroscopy and dynamics, are reported.

Structural properties and isomerisation of simple S-nitrosothiols: ab initio studies with a simplified treatment of correlation effects

Despite the enormous biological significance, the structure-stability relationship in S-Nitrosothiols (RSNOs) that govern their activity in vivo is not well understood. We provide useful structural properties (bond length, vibrational frequency, dissociation energy, and interconversion barrier) of the -SNO group in RSNOs ( and ) using the recently suggested computationally cost-ffective multireference ab initio method which includes electron correlation effects by means of the perturbation theory using a multiconfigurational zero-order wave function. A sizable rotation barrier of cis-trans RSNO interconversion imply a partial S-N double-bond like nature. The low stretching frequency, elongated length, and small dissociation energy indicate that this bond is relatively weak and labile. Although the study of S– bond in RSNOs is a challenging case even for the current generation computational approaches, IVO-SSMRPT method (having several appealing features) employed here is competent to reproduce the structure and stability of RSNO isomers in close agreement with the reference high-level estimates.