Configuration Interaction Calculations Research Articles

Fine structure splitting cancellation in highly asymmetric InAs/InP droplet epitaxy quantum dots

We find the fine structure splitting (FSS) of a single exciton, which splits its degenerate ground-state manifold into singlets, nearly vanishes in highly asymmetric quantum dots (QDs) due to the cancellation of splitting effects with markedly different origin. The dots simulated are those that emerge on top of etch pits through the droplet epitaxy growth process; these etch pit dots break square (C4v) spatial symmetry, which has been previously associated with small FSS. Configuration interaction calculations predict a vanishing FSS at a specific finite etch pit displacement from the center of the dot, for a structure far from square symmetry. We thus predict that highly asymmetric QDs may still display negligible FSS, providing avenues for high-fidelity generation of indistinguishable, polarization entangled photon pairs on demand. Published by the American Physical Society 2024

Relativistic configuration-interaction calculations on K α X-ray satellites of medium-charge Be-like ions

This study puts forward a calculation of the transitions of 1s2s22p 1P1−1s22s2 1S0 and 1s2s22p 3P1−1s22s2 1S0 for Be-like ions (Z = 6–30) using the multiconfiguration Dirac-Hartree–Fock (MCDHF) and relativistic configuration interaction methods. The computational results include transition wavelengths, transition probabilities, line strengths, and weighted oscillator strengths. The calculations also consider the contributions of the Breit interaction and quantum electrodynamics corrections (QED) to these levels. The results show that the contribution of the Breit interaction, self-energy, and vacuum polarization increase rapidly with increasing nuclear charge for certain configurations. The present calculated values are in good agreement with previous theoretical and experimental results, with errors better than 0.3%. A V/A L comparisons demonstrate the reliability of the data.

Transition probabilities and radiative lifetimes of C[formula omitted]: A high level theoretical investigation

Dicarbon species are of particular interest within the physical chemistry community. However, contrary to neutral C2, the theory of electronic structure and spectra concerning C2− ions is not well-established. To address this gap in information, high-level multireference configuration interaction (MRCI) calculations have been performed, and new potential energy curves for fourteen low-lying states of C2− are reported. The best estimate for the ground dissociation energy is 64,720 cm−1 at the MRCI+Q/AV5Z level of theory. The first excited state, A2Πu, lies at 3,836 cm−1 (Re = 2.4829 a0 and ωe = 1649 cm−1). The infrared A2Πu − X2Σg+ transition has Einstein coefficients of the order of 103–104 s−1. The lifetime of the A2Πu state is given in microseconds. For the B2Σu+ state, it was calculated τ0≈ 78 ns in good accord with the experimental value of 77 ± 8 ns. The a4Σu+ state is placed at an excitation energy of 32,369 cm−1, much higher than expected by experimentalists (19,448 cm−1). The spin-forbidden transitions a4Σu+ − X2Σg+ are strong enough to be detected experimentally (A04≈ 3 × 106 s−1). The 14Πg − a4Σu+ transitions are more concentrated in the IR region. The radiative lifetime of 14Πg is computed to be 12μs. There is no doubt that the current data considerably enhance our understanding of the spectroscopy of C2−.

Multiexcitonic and optically bright states in subunits of pentacene crystals: A hybrid DFT/MRCI and molecular mechanics study.

A hybrid quantum mechanics/molecular mechanics setup was used to model electronically excited pentacene in the crystal phase. Particularly interesting in the context of singlet fission (SF) is the energetic location of the antiferromagnetically coupled multiexcitonic singlet state, 1(TT), and the ferromagnetically coupled analog in relation to the optically bright singlet state. To provide photophysical properties of the accessible spin manifold, combined density functional theory and multi-reference configuration interaction calculations were performed on pentacene dimers and a trimer, electrostatically embedded in the crystal. The likelihood of a quintet intermediate in the SF process was estimated by computing singlet-quintet electron spin-spin couplings employing the Breit-Pauli Hamiltonian. The performance of the applied methods was assessed on the pentacene monomer. The character of the optically bright state and the energetic location of the 1(TT) state depend strongly on the relative orientation of the pentacene units. In the V-shaped dimers and in the trimer, the optically bright state is dominated by local and charge transfer (CT) excitations, with admixtures of doubly excited configurations. The CT excitations gain weight upon geometry relaxation, thus supporting a CT-mediated SF mechanism as the primary step of the SF process. For the slip-stacked dimer, the energetic order of the bright and the 1(TT) states swaps upon geometry relaxation, indicating strong nonadiabatic coupling close to the Franck-Condon region-a prerequisite for a coherent SF process. The multiexcitonic singlet, triplet, and quintet states are energetically too far apart and their spin-spin couplings are too small to bring about a noteworthy multiplicity mixing.

Many-body theory calculations of positron binding to hydrogen cyanide

Positron bound state properties in hydrogen cyanide are studied via many-body theory calculations that account for strong positron-electron correlations including positron-induced polarization, screening of the electron–positron Coulomb interaction, virtual-positronium formation and positron–hole repulsion. Specifically, the Dyson equation is solved using a Gaussian basis, with the positron self-energy in the field of the molecule calculated using the Bethe–Salpeter equations for the two-particle and particle–hole propagators. The present results suggest near cancellation of screening corrections to the bare polarization, and the non-negligible role of the positron–hole interaction. There are no existing measurements to compare to for HCN. Previous configuration interaction (CI) and fixed-node diffusion Monte Carlo (FN-DMC) calculations give positron binding energies in the range 35–44 meV, most of which used a single even-tempered basis centred near the nitrogen atom. Using a similar single-centre positron basis we calculate a positron binding energy of 41 meV, in good agreement. However, we find that including additional basis centres gives an improved description of the positron wave function near the nuclei, and results in a converged binding energy in the range 63–73 meV (depending on geometry and approximation to the positron–molecule correlation potential used).Graphical

Benchmarking calculations of level energies, transition parameters and electron impact excitation cross sections of S-like Xe38+

Atomic structure parameters for levels corresponding to 2s22p63s23p4, 2s22p63s3p5, 2s22p63s23p23d2, 2s22p63s3p43d and 2s22p63s23p33d configurations of S-like Xe38+ are calculated using the fully relativistic multiconfiguration Dirac-Hatree-Fock (MCDHF) method followed by the subsequent relativistic configuration interaction (RCI) calculations. The parameters evaluated include energies of the lowest 75 levels and E1, M1, E2, and M2 transition parameters among these levels. The effect of the choice of virtual orbitals for generating the wavefunctions is discussed. The accuracy of our line strengths is established through the rigorous calculations of their associated uncertainties using three different methods. Another set of calculations using the many-body perturbation theory (MBPT) to validate the present MCDHF-RCI energies is carried out. Further, the electron impact excitation cross-sections using the relativistic distorted wave (RDW) theory are determined for all transitions from the ground and metastable states in the incident electron energy range from the excitation threshold to 10 keV. For plasma physics applications, the fitting parameters for these cross sections employing two distinct equations tailored for low and high-energy regimes are reported. Moreover, the rate coefficients are determined in the electron temperatures range of 15 eV to 100 eV by taking into account the Maxwellian electron energy distribution function and our computed cross-sections. In addition to addressing the atomic data gap in highly charged Xe ions, the present results are of good accuracy and can serve as benchmark tests for other theoretical evaluations. Moreover, they can also assist in line identification of the complex spectra of highly charged sulphur-like xenon ions.

Ground state property calculations of LiHn complexes using IBM Qiskit’s quantum simulator

In this study, the variational quantum eigensolver (VQE) on a quantum simulator is used in calculating ground state electronic structure properties of the LiHn, n = 1–3, complexes including their singly charged ions. Results calculated using classical electronic structure algorithms are also included. We investigate the use of the unitary coupled cluster with singles and doubles (UCCSD) Ansatz using VQE within Qiskit and compare results to full configuration interaction (FCI) calculations. Computed ground state energies, electron affinities, ionization potentials, and dipole moments are considered. We report the first-of-its-kind simulated quantum computing results of selected LiHn species and use the parity orbital to qubit mapping scheme. We find that VQE/UCCSD results are comparable to classical coupled clusters with singles and doubles for all considered systems with respect to FCI. A VQE calculation cost evaluation is included in which we evaluate performance using both Jordan–Wigner and parity orbital to qubit mapping schemes. We also discuss some of the current limitations of utilizing VQE for the study of chemical systems.

The ultraviolet and vacuum ultraviolet absorption spectrum of gamma-pyrone; the singlet states studied by configuration interaction and density functional calculations

A synchrotron based vacuum ultraviolet absorption spectrum for γ-pyrone has been interpreted in terms of singlet excited electronic states using a variety of coupled cluster, configuration interaction, and density functional calculations. The extremely weak spectral onset at 3.557 eV shows eight vibrational peaks, which following previous analyses, are attributed to a forbidden 1A2 state. A contrasting broad peak with a maximum at 5.381 eV has a relatively high cross-section of 30 Mb; this arises from three overlapping states, where a 1A1 state dominates over progressively weaker 1B2 and 1B1 states. After fitting the second band to a polynomial Gaussian function and plotting the regular residuals over 20 vibrational peaks, we have had limited success in analyzing this fine structure. However, the small separation between these three states clearly shows that their vibrational satellites must overlap. Singlet valence and Rydberg state vibrational profiles were determined by configuration interaction using the CAM-B3LYP density functional. Vibrational analysis using both the Franck–Condon and Herzberg–Teller procedures showed that both procedures contributed to the profiles. Theoretical Rydberg states were evaluated by a highly focused CI procedure. The superposition of the lowest photoelectron spectral band on the vacuum ultraviolet spectrum near 6.4 eV shows that the 3s and 3p Rydberg states based on the 2B2 ionic state are present; those based on the other low-lying ionic state (X2B1) are destroyed by broadening; this is a dramatic extension of the broadening previously witnessed in our studies of halogenobenzenes.

Measuring the nuclear magnetic quadrupole moment of optically trapped ytterbium atoms in the metastable state

We propose a scheme to measure a nuclear magnetic quadrupole moment (MQM), a CP -violating electromagnetic moment that appears in the nuclear sector, using the long-lived 3P2 metastable state in neutral 173Yb atoms. Laser-cooling and trapping techniques enable us to prepare ultracold 173Yb atoms in the 3P2 state trapped in an optical lattice or an optical tweezer array, providing an ideal experimental platform with long spin coherence time. In addition, our relativistic configuration interaction calculation for the 3P2 electronic wavefunction reveals a large magnetic field gradient generated by the atomic electrons in this state, which amplifies the measurable effect of an MQM. Our scheme could lead to an improvement of more than one order of magnitude in MQM sensitivity compared to the best previous measurement (Murthy et al 1989 Phys. Rev. Lett. 63, 965).

The CuTe Molecule: Theoretical investigation of the electronic structure and dipole moments of the ground and lowest excited states with rovibrational calculations

The multireference configuration interaction calculation MRSDCI (with Davidson correction) has been performed to investigate the potential energy curves, the transition electronic energies Te, the spectroscopic constants (Re, ωe and Be), the transition and permanent dipole moments, and the vibrational levels corresponding to the ground and 18 excited electronic states of the molecule CuTe in the representation 2S+1Λ(+/-) (neglecting the spin–orbit effects), whereas the PECs and the spectroscopic constants have been calculated for 27 states in the representation Ω(+/-) [including the spin–orbit (SO) effects]. The ground state equilibrium bond length and the vibrational frequency have also been investigated using the density functional theory (DFT), where different density functionals with different basis sets have been tested. This study represents the first theoretical study of the excited states of the CuTe molecule. Comparison of the present results with the available experimental ones shows an overall satisfying agreement.

Rydberg electron stabilizes the charge localized state of the diamine cation

A previous controversial discussion regarding the interpretation of Rydberg spectra of gaseous dimethylpiperazine (DMP) as showing the co-existence of a localized and delocalized mixed-valent DMP+ radical cation is revisited. Here we show by high-level quantum-chemical calculations that an apparent barrier separating localized and delocalized DMP+ minima in previous multi-reference configuration-interaction (MRCI) calculations and in some other previous computations were due to unphysical curve crossings of the reference wave functions. These discontinuities on the surface are removed in state-averaged MRCI calculations and with some other, orthogonal high-level approaches, which do not provide a barrier and thus no localized minimum. We then proceed to show that in the actually observed Rydberg state of neutral DMP the 3s-type Rydberg electron binds more strongly to a localized positive charge distribution, generating a localized DMP* Rydberg-state minimum, which is absent for the DMP+ cation. This work presents a case where interactions of a Rydberg electron with the underlying cationic core alter molecular structure in a fundamental way.

First-Principles Calculations of the Optical Spectra of Uranium Ions in Oxide Crystals

Recently U6+-doped phosphors are attracting attentions as potential green phosphors for the white LED application. However, the origins of the peaks in the optical spectra in some U-doped phosphors have not been clarified yet. For the development of the novel uranium phosphors, detailed analysis of the absorption and emission mechanisms of these materials is indispensable.In this study, we have performed the first-principles electronic structure calculations of U in oxide crystals using the relativistic discrete-variational multi-electron (DVME) code which is a configuration-interaction calculation program based on the four-component fully relativistic wave functions obtained by the relativistic discrete-variational Xα (DV-Xα) molecular orbital method. The optical spectra due to the charge transfer transitions of U6+ in oxides were calculated using the explicitly obtained many-electron wave functions. For comparison, the 5f-6d transitions of U3+ in oxide crystals were also calculated and the influence of the valency was investigated. The theoretical spectra were compared with the experimental spectra and the origins of the peaks were clarified based on the configuration analysis of the corresponding relativistic many-electron wave functions.

Synthesis, Structural Characterization, and Phosphorescence Properties of Trigonal Zn(II) Carbene Complexes.

The sterically demanding N-heterocyclic carbene ITr (N,N'-bis(triphenylmethyl)imidazolylidene) was employed for the preparation of novel trigonal zinc(II) complexes of the type [ZnX2(ITr)] [X = Cl (1), Br (2), and I (3)], for which the low coordination mode was confirmed in both solution and solid state. Because of the atypical coordination geometry, the reactivity of 1-3 was studied in detail using partial or exhaustive halide exchange and halide abstraction reactions to access [ZnLCl(ITr)] [L = carbazolate (4), 3,6-di-tert-butyl-carbazolate (5), phenoxazine (6), and phenothiazine (7)], [Zn(bdt)(ITr)] (bdt = benzene-1,2-dithiolate) (8), and cationic [Zn(μ2-X)(ITr)]2[B(C6F5)4]2 [X = Cl (9), Br (10), and I (11)], all of which were isolated and structurally characterized. Importantly, for all complexes 4-11, the trigonal coordination environment of the ZnII ion is maintained, demonstrating a highly stabilizing effect due to the steric demand of the ITr ligand, which protects the metal center from further ligand association. In addition, complexes 1-3 and 8-11 show long-lived luminescence from triplet excited states in the solid state at room temperature, according to our photophysical studies. Our quantum chemical density functional theory/multireference configuration interaction (DFT/MRCI) calculations reveal that the phosphorescence of 8 originates from a locally excited triplet state on the bdt ligand. They further suggest that the phenyl substituents of ITr are photochemically not innocent but can coordinate to the electron-deficient metal center of this trigonal complex in the excited state.

High-accuracy DMBE potential energy surface for CNO(A''4) and the rate coefficients for the C + NO reaction in the A'2, A''2, and A''4 states.

An accurate potential energy surface (PES) for the lowest lying A''4 state of the CNO system is presented based on explicitly correlated multi-reference configuration interaction calculations with quadruple zeta basis set (MRCI-F12/cc-pVQZ-F12). The abinitio energies are fitted using the double many-body expansion method, thus incorporating long-range energy terms that can accurately describe the electrostatic and dispersion interactions with physically motivated decaying functions. Together with the previously fitted lowest A'2 and A''2 states using the same theoretical framework, this constitutes a new set of PESs that are suitable to predict rate coefficients for all atom-diatom reactions of the CNO system. We use this set of PESs to calculate thermal rate coefficients for the C(P3) + NO(Π2) reaction and compare the temperature dependence and product branching ratios with experimental results. The comparison between theory and experiment is shown to be improved over previous theoretical studies. We highlight the importance of the long-range interactions for low-temperature rate coefficients.

Atomic structure calculations, and electron impact ionization cross-sections of W LXI

The present study provides a wide range of calculations of atomic structure data for the tungsten ion W LXI, that belongs to the silicon isoelectronic sequence. The fully relativistic Multi-configuration Dira-Hartree–Fock (MCDHF) method including configuration interaction (CI) is applied to the current ion. Calculated results of energy levels, oscillator strengths, and radiative rates of W LXI are presented. A set of two configuration state lists including the configurations of 3s23p2, 3s3p3, 3s23p3d, and 3p4 at different angular momenta (J) and parity (P) are considered in the configuration interaction calculations of level energies and oscillator strengths. To increase the accuracy of the results, the electronic correlations from higher orbitals, i.e., 7l orbitals are included in the calculations. Quantum electrodynamics effects such as vacuum polarization and self-energy are recalled in the CI calculations. Many more transitions in the soft x-ray region (between 20 and 200 angstroms) have been identified. The flexible atomic code package has been used to compute the electron impact ionization cross-sections of 3s23p2 3P0,1, 1D2 levels in silicon-like tungsten. The cross-sections of the transitions from the ground level in W LXI to the first levels in the 61st ionization of tungsten have been computed using FAC code. The total ionization cross-sections of the 3s23p2 at electron energy range from 5–35 keV are evaluated as well.

Lowest electronic states of neutral and ionic LiN

AbstractWe have investigated the potential energy curves (PECs) of the LiN heteronuclear diatomic molecule, including its ionic species LiN+ and LiN−, using explicitly correlated multi‐reference configuration interaction (MRCI‐F12) calculations in conjunction with the correlation consistent quintuple‐𝜁 basis set. The effect of core–valence correlation, scalar relativistic effects, and the size of the basis sets has been investigated. A comprehensive set of spectroscopic constants determined based on the above‐mentioned calculations are also reported for the lowest electronic states and all systems, including dissociation energies, harmonic and anharmonic vibrational frequencies, and rotational constants. Additional parameters, such as the dipole moments, equilibrium spin‐orbit constants, excitation energies, and rovibrational energy levels, are also documented. We found that the three triplet states of LiN, namely, X 3∑−, A 3Π, and 2 3∑−, exhibit substantial potential wells in the PEC diagrams, while the quintet states are repulsive in nature. The ground state of the anion also shows a deep potential well in the vicinity of its equilibrium geometry. In contrast, the ground and excited states of the cation are very loosely bound. Charge transfer properties of each of these states are also analyzed to obtain an in‐depth understanding of the interatomic interactions. We found that the core–valence correlation has a substantial effect on the calculated spectroscopic constants.

Deep-Learning Approach for the Atomic Configuration Interaction Problem on Large Basis Sets.

High-precision atomic structure calculations require accurate modeling of electronic correlations typically addressed via the configuration interaction (CI) problem on a multiconfiguration wave function expansion. The latter can easily become challenging or infeasibly large even for advanced supercomputers. Here, we develop a deep-learning approach which allows us to preselect the most relevant configurations out of large CI basis sets until the targeted energy precision is achieved. The large CI computation is thereby replaced by a series of smaller ones performed on an iteratively expanding basis subset managed by a neural network. While dense architectures as used in quantum chemistry fail, we show that a convolutional neural network naturally accounts for the physical structure of the basis set and allows for robust and accurate CI calculations. The method was benchmarked on basis sets of moderate size allowing for the direct CI calculation, and further demonstrated on prohibitively large sets where the direct computation is not possible.

Ligand-Induced Symmetry Breaking as the Origin of Multiexponential Photoluminescence Decay in CdSe Quantum Dots.

The bright photoluminescence (PL) of colloidal CdSe quantum dots (QDs) makes them interesting for optical applications. For most of them, well-defined PL properties, dominated by a single excitonic state, are required. However, in many PL experiments with QD ensembles, multiexponential decay was observed. On the basis of spin-orbit density functional theory and screened configuration interaction calculations, we show that highly symmetric and defect-free CdSe QDs with diameters of 1.7 and 2.0 nm possess a multiexponential low-temperature PL at the single-dot level. This is a consequence of ligand-induced symmetry breaking with a subsequent rearrangement of the lowest eight excitonic states in two sets of four singly degenerate excitonic states. For each set, the lowest state is dark and the other three are bright. We find that the splitting between the sets can be modified by the coverage and choice of the ligand, which facilitates the engineering of the PL properties of CdSe QDs.

Accurate and efficient calculations of Hellmann-Feynman forces for quantum computation.

First-order derivatives of energies with respect to atomic coordinates are widely computed and used in quantum chemistry simulations. The rapidly emerging technology of quantum computing offers a new paradigm for solving relevant quantum chemistry equations. In this work, we have achieved analytical calculations of atomic forces based on the Hellmann-Feynman theorem within the framework of the variational quantum eigensolver. The accuracy of the approach is demonstrated by calculating the atomic forces of H2, LiH, H2O, and NH3 molecules, which are in excellent agreement with values obtained from full configuration interaction calculations. In particular, for systems with degenerate molecular orbitals, the analytical approach has a significant accuracy advantage over finite-difference-based methods and will not involve additional computational effort on a quantum computer. The calculated forces are further used to optimize the geometries of NH3 and CH4 molecules and to perform abinitio molecular dynamics simulations for the umbrella inversion of NH3, demonstrating the feasibility of the approach in practical quantum chemistry simulations.

Benchmarking Time-dependent Density Functional Theory for the prediction of electronic absorption spectra of amorphous ices for astrochemical applications

ABSTRACT Time-dependent density functional theory (TDDFT) offers a tractable means to predict electronic excitation spectra of ices if sufficient care is taken in selecting the DFT functional, basis sets, and the number of excited states. In this work 48 functionals in conjunction with various correlation consistent basis sets were benchmarked for predicting electronic spectra. A training set of important astromolecules was chosen: carbon monoxide, water, hydrogen cyanide, ammonia, methanol, and the hydroxyl radical. Vertical excitation energies between the ground and first excited state and some higher excited states were calculated using multireference configuration interaction calculations to serve as benchmarks for gas phase TDDFT calculations. When used in conjunction with aug-cc-pVDZ quality correlation consistent basis sets, two functionals, MPW1K and ωB97, emerged as very good choices to model electronic excitations for most of the molecules in the training set. Cluster calculation predictions for the UV spectra of amorphous ices of water and ammonia were performed at the TDDFT-MPW1K/AVDZ and TDDFT-ωB97/AVDZ levels and then compared against experimental data. The calculations qualitatively reproduce the shapes of the spectra for the most part and quantitatively predict the position of the critical first absorption peak.