Configuration Interaction Technique Research Articles

A Study of the Atomic Processes of Highly Charged Ions Embedded in Dense Plasma

The study of atomic spectroscopy and collision processes in a dense plasma environment has gained a considerable interest in the past few years due to its several applications in various branches of physics. The multiconfiguration Dirac-Fock (MCDF) method and relativistic configuration interaction (RCI) technique incorporating the uniform electron gas model (UEGM) and analytical plasma screening (APS) potentials have been employed for characterizing the interactions among the charged particles in plasma. The bound and continuum state wavefunctions are determined using the aforementioned potentials within a relativistic Dirac-Coulomb atomic structure framework. The present approach is applied for the calculation of electronic structures, radiative properties, electron impact excitation cross sections and photoionization cross sections of many electron systems confined in a plasma environment. The present study not only extends our knowledge of the plasma-screening effect but also opens the door for the modelling and diagnostics of astrophysical and laboratory plasmas.

Study of electron impact excitation of H-like Si13+ ion in dense plasma environment

The relativistic configuration interaction technique has been employed to study the electron impact excitation collision strengths of H-like Si13+ ion in strongly coupled plasma environment. The analytical plasma screening b-potential has been adopted to determine the physical effects of the screened Coulomb interaction due to the influence of plasma on the atomic structure and dynamics affecting atomic processes. The modified Dirac equations are solved to obtain the bound and continuum state wave functions. We examine the plasma screening effect by considering the plasma conditions at the electron temperatures of 200, 600 and 1000 eV over electron density range of 2.0 × 1023 - 1.0 × 1024 cm−3. The present results highlight that the inclusion of plasma environment induces significant effect on the binding energies, transition energies, radiative transition probabilities, oscillator strengths and collision strengths. It has been observed that the collision strength decreases with increase in plasma electron density for a fixed temperature. Our results will be beneficial for laser produced plasmas, high energy density physics, astrophysical applications and will open the door to modelling and advanced diagnostics of solar and stellar plasmas.

Relativistic atomic structure calculations of KIX with plasma parameters

Systematic calculations for energy levels, lifetimes, and radiative data for the KIX are reported, including oscillator strengths, transition wavelengths, line strengths, and radiative rates of electric dipole (E1) transition, electric quadrupole (E2) transition, magnetic dipole (M1) transition, and magnetic quadrupole (M2) transition, using GRASP. Quantum electrodynamics and Breit correction have been considered in our calculations. The importance and effect of valence valence and core valence correlations on the excitation energies have been discussed in graphical and tabular forms. Analogous calculations using flexible atomic code (FAC) and the large-scale configuration interaction technique have also been done to confirm the accuracy of energy levels. The calculated results are in close agreement with NIST compiled data and other available results. The influence of plasma temperature (2 × 106–1 × 1010 K) on the line intensity ratio with the number of electron density has been studied for the hot dense plasma (HDP) graph for KIX. Our reported results will be valuable or beneficial for the characterization of HDP, astrophysical plasmas, and plasma modeling.

Accessing ground-state and excited-state energies in a many-body system after symmetry restoration using quantum computers

We explore the possibility to perform symmetry restoration with the variation after projection technique on a quantum computer followed by additional postprocessing. The final goal is to develop configuration interaction techniques based on many-body trial states preoptimized on a quantum computer. We show how the projection method used for symmetry restoration can prepare optimized states that could then be employed as initial states for quantum or hybrid quantum-classical algorithms. We use the quantum phase estimation and the quantum Krylov approaches for the postprocessing. The latter method combined with the quantum variation after projection leads to very fast convergence toward the ground-state energy. The possibility to access excited states energies is also discussed. Illustrations of the different techniques are made using the pairing Hamiltonian.

Theoretical study of the electronic structure of hafnium (Hf,Z=72) and rutherfordium (Rf,Z=104) atoms and their ions: Energy levels and hyperfine-structure constants

Energy levels, magnetic dipole, and electric quadrupole hyperfine structure of the superheavy element rutherfordium (Rf, $Z$=104) and its first three ions are calculated using a combination of the configuration interaction, coupled-cluster single-doubles, and many-body perturbation theory techniques. The results are to be used in future interpretations of the measurements in terms of nuclear magnetic dipole and electric quadrupole momenta. To have a guide on the accuracy of the study, we perform similar calculations for hafnium (Hf, $Z$=72) and its ions. Hf is a lighter analog of Rf with a similar electronic structure. Good agreement with the experiment for Hf and with available previous calculations of the energy levels of Rf is demonstrated.

Influence of strongly coupled plasma environment on photoionization of H-like O7+ion

The effect of strongly coupled plasma environment on the photoionization cross sections of the H-like O7+ ion is studied within the framework of relativistic configuration interaction technique implemented in the flexible atomic code. The analytical b-potential of Li and Rosmej [Phys. Lett. A 384, 126478 (2020)] and uniform electron gas model potential of Saha and Fritzsche [J. Phys. B 40, 259 (2007)] have been employed for incorporating the plasma shielding effects. The level energies for unscreened H-like O7+ ion are in excellent agreement with the National Institute of Standards and Technology values. The bound and continuum state wavefunctions are determined by solving the modified Dirac equations. The photoionization cross sections evaluated in the dipole approximation are obtained from the bound-free transition matrix element. Cross section calculations are performed over an electron density range of 1.0 × 1019–2.0 × 1023 cm−3 and a temperature range of 200–1000 eV. The plasma environment is found to considerably influence the photoionization cross sections. It has been observed that the ionization thresholds move to the low energy region with the increase in free electron densities. The present results will be beneficial for the interpretation of spectra observed in a variety of astrophysical and laboratory plasmas.

Spectroscopic and theoretical optical properties of indoleninyl‐substituted dibenzotetraaza[14]annulenes

AbstractTwo new macrocyclic dibenzotetraaza[14]annulene (DBTAA) compounds with indolenine (5) and pyridoindolenine (6) moieties were synthesized and characterized by spectroscopy. Both DBTAAs exhibit strong UV‐Vis absorption properties in the Soret band region. The theoretical second‐order nonlinear optical property, electric dipole moment (μ), dispersion‐free dipole polarizability (α) and first hyper‐polarizability values were calculated by density functional theory and time dependent density functional theory. The ab‐initio quantum mechanical calculation by time‐dependent Hartree‐Fock method was utilized to investigate the dynamic dipole polarizabilities, dynamic second‐order, static, and dynamic third‐order (γ) hyper‐polarizabilities of the DBTAAs. The configuration interaction technique of all doubly occupied molecular orbitals possesses theoretically defined single‐photon absorption (OPA) specifications for the examined structures. The computed maximum OPA wavelengths on both macrocyclic compounds coincide with the preceding measurement outcomes.

Characterization of charge transfer excited states in [2Fe\u20132S] iron\u2013sulfur clusters using conventional configuration interaction techniques

The experimental UV–Vis spectra of the biologically relevant [2Fe–2S] iron–sulfur clusters feature typically three bands in the 300–800 nm range. Based on ground-state orbitals and using the one electron transition picture, these bands are said to be of charge transfer character. The key complication in the electronic structure calculations of these compounds are the antiferromagnetic coupling of the iron centers and high covalency of Fe–S bonds. Thus, the examples of the direct computations of electronically excited states of these systems are rare. Whereas low lying electronic excited states were subject of recent studies, higher energy states computed with many-body theories were never reported. In this work we present, for the first time, calculations of the electronic spectra of [Fe2S2](SMe)42− biomimetic compound. We demonstrate that spin-averaged restricted open-shell Hartree–Fock orbitals are superior to high-spin orbitals and are convenient reference for subsequent configuration interaction calculations. Moreover, the use of conventional configuration interaction methods enabled us to study the nature of the excited states in details with the difference density maps. By systematic extension of the donor orbital space we show that key excitations in the 300–800 nm range are of Fe 3d ← (μ-S) character.

Quantifying Multireference Character in Multicomponent Systems with Heat-Bath Configuration Interaction

Multicomponent quantum chemical methods seek to include nuclear quantum effects of select nuclei in quantum chemistry calculations by not invoking the Born-Oppenheimer approximation for these nuclei. In multicomponent methods, the inclusion of electron-proton correlation is essential for obtaining even qualitatively accurate protonic densities. However, most of the recently developed multicomponent methods have either used or obtained molecular orbitals from a single-reference mean-field wave function that neglects all electron-proton correlation that is analogous to using Hartree-Fock orbitals in a single-component framework. We examine the consequences of using Hartree-Fock orbitals in multicomponent calculations by developing the multicomponent heat-bath configuration interaction (HCI) method. Multicomponent HCI is a multicomponent selected configuration interaction (CI) technique that enables an accurate approximation of a complete active space or truncated CI wave function for systems with large active spaces. The multicomponent HCI method is shown to reproduce the ground-state protonic density of the HeHHe+, HCN, and FHF- systems when compared to reference grid-based calculations. For all three systems, the coefficient of the leading configuration in the wave function expansion is less than 0.95, indicating that all systems have multireference character. This is highly noteworthy as none of the systems have multireference character in a single-component framework and suggests that multireference character appears inherent to or at least more commonly in a multicomponent framework than a single-component framework. Even when natural orbitals are used rather than Hartree-Fock orbitals for the multicomponent HCI calculations, aspects of the multireference character remain for FHF- and HCN. Consequences and implications of the multireference character of multicomponent quantum chemical systems are discussed.

Elastic and inelastic low-energy electron collisions with PH2, PF2 and PCl2 radicals

Ab initio scattering calculations for low-energy electron collisions with PH2, PF2, and PCl2 are performed using the R-matrix method in this study. The elastic integral, differential, and momentum transfer cross sections and the excitation cross sections from the ground state to the five low-lying electron excited states are presented in the energy region of 0.01–15 eV. The target states are represented by including correlations via a configuration interaction technique. The resonant states are detected and identified. The sensitivity of the results to changes in the theoretical model is checked by comparing predictions from a variety of approximations including static exchange, one state close-coupling (CC), and many-state CC approximations. The similarities and differences among the resonances of these three radicals are discussed.

Clustering in structure and reactions using configuration interaction techniques

Background: Atomic nuclei are remarkable quantum many-body systems where clustering properties develop naturally from underlying interactions between the constituent nucleons. Clustering degrees of freedom manifest themselves in multiple structure and reaction observables.Purpose: Our goal is to study nuclear clustering and its emergence in many-nucleon dynamics from nucleon-nucleon interactions. Clustering is a phenomenon that is known to emerge on the boundary between structure and reactions, therefore developing appropriate techniques that bridge the structure-reaction divide and establishing connections to observables is among our principal objectives. Showing consistency and how the new techniques can be reduced to well established other methods is an important part of this work.Methods: The configuration-interaction technique based on second quantization is used to treat the quantum many-body problem assuring that fermionic antisymmetry is fully satisfied. The use of the harmonic oscillator single-particle basis allows for the center-of-mass coordinate to be separated and prepared in a desired oscillator state for each cluster. The relative motion reaction basis channels are constructed by coupling clusters in different harmonic oscillator states with respect to their relative motion. Finally, using a resonating group method strategy we solve the generalized eigenvalue problem to obtain scattering channels. Structural clustering characteristics are discussed and the modified harmonic oscillator representation for scattering equations method is used to extract scattering observables.Results: New methods for treating clustering problems have been put forward. We demonstrate broad applicability of the developed techniques. Examples highlight connections with algebraic techniques, and the role of approximations leading to algebraic limits is assessed using realistic examples. Various types of clustering characteristics are used to study alpha clustering in light nuclei that are relevant to currently ongoing experimental efforts. We demonstrate the emergence of strongly clustered bands of states in beryllium, triple alpha channels in $^{12}\mathrm{C}$, and molecular type clustering in $^{21}\mathrm{Ne}$. Starting from nucleon-nucleon interactions without any additional assumptions scattering phase shifts for alpha-alpha scattering are determined and shown to be consistent with those observed.Conclusions: In this work we put forward a new configuration-interaction-based method that targets the physics of clustering, and further unifies nuclear structure and reactions. We provide detailed discussions and many examples highlighting features and advantages of the approach.

All-order calculations of the energy levels of heavy elements Indium (In) and Tin (Sn)

The energy levels of the heavy elements In, Sn+ and Sn are presented in this article. Dominating corrections beyond the relativistic Hartree-Fock method are included to all orders in the Coulomb interaction using the Feynman diagram technique and the correlation potential method. The configuration interaction technique is combined with the many-body perturbation theory to construct the many-electron wave function for valence electrons and to include core-valence correlations. The good agreement of the results of our calculation with experiment data illustrates the power of the method.

Coupled Cluster as an Impurity Solver for Green's Function Embedding Methods.

We investigate the performance of Green's function coupled cluster singles and doubles (CCSD) method as a solver for Green's function embedding methods. To develop an efficient CC solver, we construct the one-particle Green's function from the coupled cluster (CC) wave function based on the non-Hermitian Lanczos algorithm. The major advantage of this method is that its scaling does not depend on the number of frequency points. We have tested the applicability of the CC Green's function solver in the weakly to strongly correlated regimes by employing it for a half-filled one-dimensional (1D) Hubbard model projected onto a single-site impurity problem and a half-filled two-dimensional (2D) Hubbard model projected onto a four-site impurity problem. For the 1D Hubbard model, for all interaction strengths, we observe an excellent agreement with the full configuration interaction (FCI) technique, both for the self-energy and spectral function. For the 2D Hubbard, we have employed an open-shell version of the current implementation and observed some discrepancies from FCI in the strongly correlated regime. Finally, on an example of a small ammonia cluster, we analyze the performance of Green's function CCSD solver within the Self-Energy Embedding Theory (SEET) with Hartree-Fock (HF) and Green's Function second order (GF2) for the treatment of the environment.

Theoretical study of the electronic structure and bonding of LaNH

We present the results of a theoretical investigation on the geometries and energy levels of the experimentally relevant doublet spin states of LaNH which have been observed earlier by laser excitations from the ground state. These electronic states have been studied using state averaged MCSF calculations followed by multireference configuration-interaction (MRCI) techniques. The computed values for the state energy, bond length, vibrational frequency, and dipole moment of states which were characterized experimentally by previous authors are in excellent agreement. Physical insights of nature of bonding and electronic charge distributions on individual atoms are discussed.

Relativistic atomic structure calculations and study of plasma parameters for Na-like Se XXIV

Extensive calculations of energy levels and radiative data such as transition wavelengths, radiative rates, oscillator strengths, and line strengths for electric dipole (E1) transitions have been performed for Se XXIV using the multiconfiguration Dirac–Fock method. For other types of transitions, namely, magnetic dipole (M1), electric quadrupole (E2), and magnetic quadrupole (M2), only the radiative rates (A) are listed. The importance and effect of valence-valence and core-valence correlations on the excitation energies have been studied. For additional accuracy assessments, analogous calculations have been carried out by employing the flexible atomic code and configuration interaction technique. Comparisons are made with the available experimental and theoretical data in the literature. Close agreement has been found ensuring the accuracy and reliability of our results. The effect of plasma temperature on the line intensity ratio, electron density, plasma frequency, and skin depth has been analyzed for hot dense plasma (HDP) in local thermodynamic equilibrium. The present work might be beneficial in the modeling and characterization of HDP.

Relativistic R-matrix photoionization cross section calculations of Ne-like Co XVIII with resonance parameters

We report total photoionization cross sections of the ground state and the first three excited states () of Ne-like Co XVIII ion using the close-coupling Breit–Pauli R-matrix (BPRM) approximation. The target wavefunctions are constructed by configuration interaction technique (CIV3). Our calculated ionization threshold energies for the ground and excited states show a good agreement with the NIST ones. To further affirm the accuracy and reliability of our BPRM results, photoionization cross section calculations for the ground state have also been performed by employing the fully relativistic Dirac Atomic R-matrix Code. Our two independent calculations show a reasonable agreement both in magnitude and resonant photoionization cross sections. The resonance positions (Er), widths (Γ) and quantum defects (μ) due to ejection of a 2p or 2s electron from the ground state are given by adopting Quigley and Berrington method. We believe that our cross section data may be beneficial for the modelling and diagnostics of astrophysical plasmas.

Ab initio interaction potentials of the Ba, Ba+ complexes with Ar, Kr, and Xe in the lowest excited states.

The complexes of the Ba atom and Ba+ cation with the rare gas atoms Ar, Kr, and Xe in the states associated with the 6s → 5d, 6p excitations are investigated by means of the multireference configuration interaction techniques. Scalar relativistic potentials are obtained by the complete basis limit extrapolation through the sequence of aug-cc-pwCVnZ basis sets with the cardinal numbers n = Q, T, 5, combined with the suitable effective core potentials and benchmarked against the coupled cluster with singles, doubles, and non-iterative triples calculations and the literature data available for selected electronic states. Spin-orbit coupling is taken into account by means of the state-interacting multireference configuration interaction calculations performed for the Breit-Pauli spin-orbit Hamiltonian. The results show weak spin-orbit coupling between the states belonging to distinct atomic multiplets. General trends in the interaction strength and long-range anisotropy along the rare gas series are discussed. Vibronic spectra of the Ba and Ba+ complexes in the vicinity of the 1S → 1P° and 2S → 2P° atomic transitions and diffusion cross sections of the Ba(1S0, 3DJ) atom in high-temperature rare gases are calculated. Comparison with available experimental data shows that multireference calculations tend to underestimate the interaction strength for excited complexes.

Spectroscopic study of EUV and SXR transitions of Cu XIX with plasma parameters

Extensive and elaborate theoretical computations of Cu XIX have been performed by employing multi-configuration Dirac-Fock (MCDF) method. We have presented energy levels and radiative data such as, transition wavelengths, radiative rates, oscillator strengths and line strengths for electric dipole (E1) transitions in Na-like Cu. For other types of transitions, namely magnetic dipole (M1), electric quadrupole (E2), and magnetic quadrupole (M2), only the A-values are listed. We have also discussed the importance and the effect of valence-valence (VV) and core-valence (CV) correlations on the excitation energies in graphical as well as in tabular form. The accuracies of the presented levels, wavelengths, transition rates and lifetimes are assessed by comparing them to available experimental and other theoretical results. Analogous calculations have been carried out by using Flexible Atomic Code (FAC) based on self-consistent Dirac-Fock-Slater iteration method. QED corrections due to vacuum polarization and self-energy effects, and Breit correction due to the exchange of virtual photons between two electrons are fully considered and their effects on the energy levels are studied graphically. The fine-structure energy levels have also been reported by using the Configuration Interaction (CIV3) technique. An intercomparison among three independent calculations helps us in assessing the credibility and integrity of our reported data. Additionally, we have shown the variation of the line intensity ratios and electron density with plasma temperature graphically for Cu XIX. We have also identified Extreme Ultraviolet (EUV) and Soft X-ray (SXR) transitions and believe that our reported results may be beneficial in space and laboratory plasma research.

Two-electron states of a group-V donor in silicon from atomistic full configuration interactions

Two-electron states bound to donors in silicon are important for both two-qubit gates and spin readout. We present a full configuration interaction technique in the atomistic tight-binding basis to capture multielectron exchange and correlation effects taking into account the full band structure of silicon and the atomic-scale granularity of a nanoscale device. Excited $s$-like states of ${A}_{1}$ symmetry are found to strongly influence the charging energy of a negative donor center. We apply the technique on subsurface dopants subjected to gate electric fields and show that bound triplet states appear in the spectrum as a result of decreased charging energy. The exchange energy, obtained for the two-electron states in various confinement regimes, may enable engineering electrical control of spins in donor-dot hybrid qubits.

Accurate ab initio investigation of the ground and some low-lying electronic states of boron monoiodide, BI, and its ions BI+ and BI−

The lowest electronic states of boron monoiodide, BI, and its ions BI± have been theoretically studied by the multireference configuration interaction technique employing basis sets of quadruple and quintuple-ζ quality. Scalar relativistic effects and spin-orbit coupling are taken into account as well. For all states we report potential energy curves, binding energies, and spectroscopic constants. Some quantities such as electron affinity, ionization potential, and dipole moment of BI are evaluated for the first time. The nature of the bonding in the systems under scrutiny is also discussed in some detail.