Correlation Effects In States Research Articles

Consistent Second-Order Treatment of Spin-Orbit Coupling and Dynamic Correlation in Quasidegenerate N-Electron Valence Perturbation Theory.

We present a formulation and implementation of second-order quasidegenerate N-electron valence perturbation theory (QDNEVPT2) that provides a balanced and accurate description of spin-orbit coupling and dynamic correlation effects in multiconfigurational electronic states. In our approach, the energies and wave functions of electronic states are computed by treating electron repulsion and spin-orbit coupling operators as equal perturbations to the nonrelativistic complete active-space wave functions, and their contributions are incorporated fully up to the second order. The spin-orbit effects are described using the Breit-Pauli (BP) or exact two-component Douglas-Kroll-Hess (DKH) Hamiltonians within spin-orbit mean-field approximation. The resulting second-order methods (BP2- and DKH2-QDNEVPT2) are capable of treating spin-orbit coupling effects in nearly degenerate electronic states by diagonalizing an effective Hamiltonian expanded in a compact non-relativistic basis. For a variety of atoms and small molecules across the entire periodic table, we demonstrate that DKH2-QDNEVPT2 is competitive in accuracy with variational two-component relativistic theories. BP2-QDNEVPT2 shows high accuracy for the second- and third-period elements, but its performance deteriorates for heavier atoms and molecules. We also consider the first-order spin-orbit QDNEVPT2 approximations (BP1- and DKH1-QDNEVPT2), among which DKH1-QDNEVPT2 is reliable but less accurate than DKH2-QDNEVPT2. Both DKH1- and DKH2-QDNEVPT2 hold promise as efficient and accurate electronic structure methods for treating electron correlation and spin-orbit coupling in a variety of applications.

Observation of a long-lived triplet excited state and strong electron correlation effects in the copper oxide anion (CuO-) using cryogenic photoelectron imaging.

Views Icon Views Article contents Figures & tables Video Audio Supplementary Data Peer Review Share Icon Share Twitter Facebook Reddit LinkedIn Tools Icon Tools Reprints and Permissions Cite Icon Cite Search Site Citation G. Stephen Kocheril, Han-Wen Gao, Lai-Sheng Wang; Observation of a long-lived triplet excited state and strong electron correlation effects in the copper oxide anion (CuO−) using cryogenic photoelectron imaging. J. Chem. Phys. 21 June 2023; 158 (23): 236101. https://doi.org/10.1063/5.0151516 Download citation file: Ris (Zotero) Reference Manager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentAIP Publishing PortfolioThe Journal of Chemical Physics Search Advanced Search |Citation Search

Algebraic Diagrammatic Construction Theory for Simulating Charged Excited States and Photoelectron Spectra.

Charged excitations are electronic transitions that involve a change in the total charge of a molecule or material. Understanding the properties and reactivity of charged species requires insights from theoretical calculations that can accurately describe orbital relaxation and electron correlation effects in open-shell electronic states. In this Review, we describe the current state of algebraic diagrammatic construction (ADC) theory for simulating charged excitations and its recent developments. We start with a short overview of ADC formalism for the one-particle Green's function, including its single- and multireference formulations and extension to periodic systems. Next, we focus on the capabilities of ADC methods and discuss recent findings about their accuracy for calculating a wide range of excited-state properties. We conclude our Review by outlining possible directions for future developments of this theoretical approach.

CsV_{3}Sb_{5}: A Z_{2} Topological Kagome Metal with a Superconducting Ground State.

Recently discovered alongside its sister compounds KV_{3}Sb_{5} and RbV_{3}Sb_{5}, CsV_{3}Sb_{5} crystallizes with an ideal kagome network of vanadium and antimonene layers separated by alkali metal ions. This work presents the electronic properties of CsV_{3}Sb_{5}, demonstrating bulk superconductivity in single crystals with a T_{c}=2.5 K. The normal state electronic structure is studied via angle-resolved photoemission spectroscopy and density-functional theory, which categorize CsV_{3}Sb_{5} as a Z_{2} topological metal. Multiple protected Dirac crossings are predicted in close proximity to the Fermi level (E_{F}), and signatures of normal state correlation effects are also suggested by a high-temperature charge density wavelike instability. The implications for the formation of unconventional superconductivity in this material are discussed.

Parametrisation of the optimised effective potential method based on the Coulson–Fischer wave function for excited states

ABSTRACT This article further develops the Coulson–Fischer formalism in conjunction with the optimised effective potential method [V.N. Glushkov and S. Wilson, Molec. Phys. 110, 149 (2012)] to incorporate both exchange and static correlation effects for excited states having the same symmetry as the ground state. By using this formalism, we compute complete potential energy curves for the ground and first excited states of the molecule and the isoelectronic heteronuclear ion. Effective potentials are generated in according to a novel density functional theory [A.K. Theophilou, J. Chem. Phys. 149, 074104 (2018)] in which the Kohn–Sham (KS) like potential has spherical symmetry around each nucleus. In this way, the inconsistencies of standard density functional theory concerning the asymmetry of the KS potential are remedied. In our implementation, an effective potential which satisfies this requirement is expressed as a direct mapping of the external potential. Different approximations for the effective potentials are investigated. It is shown that applications of the proposed formalism support good agreement with the exact Coulson–Fischer theory for both the ground and first excited state potential energy curves and thus a qualitatively correct description of dissociation.

Correlation effects in B1s core-excited states of boronic-acid derivatives: An experimental and computational study.

We performed a theoretical investigation on the influence of electronic correlation effects on the B1s NEXAFS spectrum of boronic acid derivatives, namely, boric acid [B(OH)3], phenyl boronic acid (PBA), and 1,4-phenyl diboronic acid (PDBA), employing different computational schemes of increasing complexity, ranging from the purely one-electron scheme based on the transition potential method of density functional theory (DFT-TP), time-dependent DFT (TDDFT), and multiconfigurational self-consistent field (MCSCF). We also report experimental measurements of the B1s NEXAFS spectra of the aforementioned molecules together with the high-resolution C1s NEXAFS spectrum of PBA. We demonstrate that due to the shallow B1s core energy levels compared to C, O, and N, the inclusion of static correlation effects, which can be incorporated by using multireference approaches to excited states, assumes a decisive role in reconciling experiment and theory on B1s core-electron excitation energies and oscillator strengths to valence states. This claim is corroborated by the good agreement that we find between the DFT-TP calculated C1s NEXAFS spectrum and that experimentally measured for PBA and by the failure of both DFT-TP and TDDFT approaches with a selection of xc functionals kernels to properly describe the B1s NEXAFS spectrum of PBA and PDBA, at variance with the good agreement with the experiment that is found by employing the MCSCF wave function approach.

Explicitly-correlated double ionization potentials and double electron attachment equation-of-motion coupled cluster methods

Double ionization and double electron attachment equation-of-motion methods, based on linearly approximated explicitly correlated coupled-cluster singles and doubles [CCSD(F12)] are formulated and implemented. An extension of double electron attachment operator is introduced for proper account of short-range correlation effects in states with two additional electrons. Numerical tests for set of doubly ionized and doubly electron attached states of several molecules have shown a good agreement between obtained explicitly-correlated results and the corresponding complete basis set limit values already at double-ζ level.

Explicitly correlated similarity transformed equation-of-motion coupled-cluster method.

Similarity transformed equation-of-motion method, based on linearly approximated explicitly correlated coupled-cluster singles and doubles [CCSD(F12)] model, has been formulated and implemented. An extension of similarity transformation operator is introduced in order to treat short-range correlation effects for excited states. Additionally, effective reduction of the number of active virtuals can be obtained by such modification. Numerical tests for sets of valence and Rydberg excited states of several molecules are conducted. Statistical measures of errors in excitation energies show that explicitly correlated results are accurate up to 0.1 e.V already at a double-ζ level compared to those in the complete basis set limit, if the excitation energy is not too close to an ionization threshold. An example of long-range charge transfer excitation is also considered and highly accurate results are obtained.

THE CHARACTERIZATION OF TOPOLOGICAL PROPERTIES IN QUANTUM MONTE CARLO SIMULATIONS OF THE KANE–MELE–HUBBARD MODEL

Topological insulators present a bulk gap, but allow for dissipationless spin transport along the edges. These exotic states are characterized by the Z2 topological invariant and are protected by time-reversal symmetry. The Kane–Mele model is one model to realize this topological class in two dimensions, also called the quantum spin Hall state. In this brief review article, we provide a pedagogical introduction to the influence of correlation effects in the quantum spin Hall states, with special focus on the half-filled Kane–Mele–Hubbard model, solved by means of unbiased determinant quantum Monte Carlo (QMC) simulations. We explain the idea of identifying the topological insulator via π-flux insertion, the Z2 invariant and the associated behavior of the zero-frequency Green's function, as well as the spin Chern number in parameter-driven topological phase transitions. The examples considered are two descendants of the Kane–Mele–Hubbard model, the generalized and dimerized Kane–Mele–Hubbard model. From the Z2 index, spin Chern numbers and the Green's function behavior, one can observe that correlation effects induce shifts of the topological phase boundaries. Although the implementation of these topological quantities has been successfully employed in QMC simulations to describe the topological phase transition, we also point out their limitations as well as suggest possible future directions in using numerical methods to characterize topological properties of strongly correlated condensed matter systems.

Communication: Application of state-specific multireference coupled cluster methods to core-level excitations

The concept of the model space underlying multireference coupled-cluster (MRCC) formulations is a powerful tool to deal with complex correlation effects for various electronic states. Here, we demonstrate that iterative state-specific MRCC methods (SS-MRCC) based on properly defined model spaces can be used to describe core-level excited states even when Hartree-Fock orbitals are utilized. We show that the SS-MRCC models with single and double excitations are comparable in accuracy to high-level single reference equation-of-motion coupled cluster (EOMCC) formalism.

Origin of the light green color and electronic ground state ofLaCrO3

The electronic structure and optical properties of LaCrO3 have been calculated and the origin of the light green color of LaCrO3 has been identified. An optical gap of 1=3.4 eV was reproduced in agreement with experiment, which is a charge transfer gap. Another optical gap, 0=2.15 eV, which is not present in experiments has also been found and explains the light green color of this compound. The energy band gap between the top of the valence band and the bottom of the conduction band is =1.40 eV and has Mott character. Our calculated x-ray photoemission spectra XPS and x-ray absorption spectra at the oxygen K edge O-K XAS are in very good agreement with experiments. In contrast to previous reports, these results have been obtained using standard generalized gradient approximation calculations, while additional correlation effects for Cr 3d states due to an effective Hubbard U would result in a colorless LaCrO3 and a disagreement with XPS and XAS experiments. Thus, LaCrO3 should not be considered as highly correlated transition metal oxide, a conclusion also valid for many Cr3+ compounds.

Hybrid coupled cluster and molecular dynamics approach: Application to the excitation spectrum of cytosine in the native DNA environment

Evolution of the excited state energies of cytosine base in the native DNA environment was investigated using a hybrid coupled cluster and classical molecular dynamics approach. The time averaged excitation energies obtained with the variant of the completely renormalized equation-of-motion with singles, doubles, and non-iterative triples approach that includes a bulk of the correlation effects for excited states, are compared with the analogous calculations in the gas phase. Significant blue shifts for the two lowest singlet excitation energies can be observed as a result of the interaction of the quantum system with the surrounding environment.

Spin Flip Models in the Spin Coupling Method of Many-Particle Amplitudes

A special class of models is presented in which correlation effects for quasidegenerate electronic states are calculated without resorting to complete MRCI schemes. The high-spin reference determinant is transformed into the desired low-spin state by the spin flip procedure with concomitant electron elimination or addition. For the given spin flip process, variational equations that define the corresponding spin coupling matrix have been obtained. For the simplest spin flip model presented by A. Krylova as the SF-CIS method, a sequential spin projection procedure has been carried out. Illustrative calculations for the excited and dissociative states of small molecules have revealed the high quality of the suggested approximate schemes.

Nuclear quadrupole moment of 57Fe from microscopic nuclear and atomic calculations.

The nuclear quadrupole moment (NQM) of the Ipi = 3/2(-) excited nuclear state of 57Fe at 14.41 keV, important in Mössbauer spectroscopy, is determined from the large-scale nuclear shell-model calculations for 54Fe, 57Fe, and also from the electronic ab initio and density functional theory calculations including solid state and electron correlation effects for the molecules Fe(CO)(5) and Fe(C5H5)(2). Both independent methods yield very similar results. The recommended value is 0.15(2) e b. The NQM of the isomeric 10+ in 54Fe has also been calculated. The new NQM values for 54Fe and 57Fe are consistent with the perturbed angular distribution data.

Electron Propagator Theory of Guanine and Its Cations: Tautomerism and Photoelectron Spectra

Electron propagator methods are applied to the ionization energies of the five most stable tautomers of guanine. Excellent agreement with gas-phase photoelectron spectra is obtained for the amino-oxo form of 7H-guanine. According to ionization energy assignments, both 9H-guanine and its amino-oxy tautomers also may be present in the gas phase. The presence of amino-oxy 7H-guanine, however, is less certain, due to its higher total energy. In all cases, the lowest ionization occurs from a π level. There are strong correlation effects for higher cationic states. Energy orderings of π and σ hole states are different for each of the isomers.

Electronic structure of the ferroelectromagnet YMnO 3

Using the self-consistent linear muffin-tin orbitals and atomic sphere approximation (LMTO-ASA) method, electronic structure calculations of the ferroelectromagnet YMnO 3 in the magnetic and nonmagnetic phases are performed in the local density approximation (LDA) of the density-functional theory. Total energy calculations reproduce the magnetic phase observed at low temperatures. Due to the Jahn–Teller distortion, antiferromagnetic insulating solution with a very small energy gap is also obtained. After including the strong electron–electron correlation effects in Mn 3d states by the on-site Coulomb interaction correction, the energy gap is increased to 1.1 eV and the density of states is greatly redistributed. The large component of oxygen 2p states at the top of the valence states shows that the insulator YMnO 3 has a charge-transfer character.

Simultaneous handling of dynamical and nondynamical correlation via reduced multireference coupled cluster method: Geometry and harmonic force field of ozone

The geometry and vibrational frequencies of the ozone molecule are studied using the reduced multireference coupled cluster method with singles and doubles (RMR CCSD) relying on a two-electron/two-orbital (2,2) active space. The role played by the choice of the reference space is analyzed in detail by considering both the spin adapted, two-configuration reference space and the complete three-configuration reference space based on the (2,2) active space. It is shown that while the two-reference RMR CCSD approach, using the C2v symmetry-adapted (2,2) active space, provides a better description of the geometry and harmonic frequencies of symmetric vibrational modes than the standard single-reference (SR) CCSD method, the three-reference approach is essential to achieve an accurate description of the harmonic frequency of the asymmetric stretching mode. A unique behavior of ozone in this regard is highlighted by a comparison with other symmetric triatomics. The comparison with results obtained by other methods, including the SR configuration interaction with singles and doubles (CISD), the standard SR CCSD, as well as two-reference and large reference space MR CISD methods, clearly demonstrates the superiority of the RMR CCSD approach. In fact, the performance of a small reference space RMR CCSD method is similar to that of a large reference space MR CISD, thus enabling a compact description of both dynamical and nondynamical correlation effects in states having a multireference character.

Superconducting phases in the presence of Coulomb interaction: From weak to strong correlations

We study the evolution of the superconducting phases in a model with competing short-range attractive (W) and on-site repulsive (U) interactions. The influence of the correlations on the phase diagram of $s\ensuremath{-}$ and d-wave superconductors is studied by means of various approximations. The knowledge of the normal state correlation effects allows the tracking of the influence of U on the superconductivity in correlated systems. We have found the qualitatively different response of the superconducting phases in systems with $W<{U}_{\mathrm{cr}},$ when the system ceases to superconduct for large enough U, and in the limit of strong $W>{U}_{\mathrm{cr}},$ when the system remains superconducting for arbitrarily large U.

Multireference self-consistent size-extensive state-selective configuration interaction

In this work, we propose a state-specific self-consistent ‘‘dressing’’ of the multireference configuration interaction (MRCI) space to include all single- and double-substituted determinants for the most important reference configurations. The aim of the method is to provide a size-extensive description of the dynamic electron correlation effects for states which mandate a multideterminantal reference wave function. Such states can represent electronic excited states or ground states of the molecular systems which are significantly deformed from their equilibrium structures. The proposed approach follows the concept introduced in our recently proposed quasilinear ansatz for the state-selective multireference coupled-cluster method. The purpose of the dressing procedure is to eliminate the contributions which introduce size-extensivity violating terms in the MRCI approach.

Weak correlation effects in doubly-excited circular states of barium

In the presence of weak external fields, we have investigated doubly-excited Ba atoms with one valence electron in a Rydberg circular state (n1=21, l1=n1-1), oriented with respect to the fields ( mu 1=l1), and the other electron in one of the low-lying states n2l2j2=6p1/2,3/2, 6d3/2,5/2 or 7s1/2. A theoretical model, including the e-e Coulomb interaction, predicts weak correlation effects leading to an angular-momentum mixing of the circular state when the inner electron is excited to anisotropic core states j2> 1/2 . Taking advantage of the field-ionization detection technique sensitive to the structure of the outer orbital, we have found evidence of such mixing. The implications and the limits of the model have been checked.