Single Excitation Configuration Interaction Research Articles

Optical, thermal and semiempirical study of Samarium bi-tartrate trihydrate

In this comprehensive study, we investigated the structure, optical, and thermal aspects of [Sm(C4H5O6)(C4H4O6)][3H2O], a samarium based metal-organic framework. Testing and exploring advanced semiempirical quantum models, we provide some insights into ground-state molecular properties within the solid metal-organic framework. The single-excitation configuration interaction approach within the spectroscopic method INDO/S-CIS was utilized in the computation of UV spectra, singlet, and triplet excited states for the uncoordinated organic component. We also exploited the established Judd-Ofelt theory to elucidate the spectroscopic properties, allowing us to predict various attributes, including absorption and luminescence traits. The inclusion of thermogravimetric analysis exposes a multifaceted thermal decomposition process. Moreover, this analysis was explored in the determination of activation energies associated with distinct stages of decomposition.

An approach of ontology and knowledge base for railway maintenance

<p>Maintenance methods have become automated and innovative, especially with the transition to maintenance 4.0. However, social issues such as coronavirus disease of 2019 (COVID-19) and the war in Ukraine have caused significant departures of maintenance experts, resulting in the loss of enormous know-how. As part of this work, we will propose a solution by exploring the knowledge and expertise of these experts for the purpose of sharing and conservation. In this perspective, we have built a knowledge base based on experience and feedback. The proposed method illustrates a case study based on the single excitation configuration interaction (SECI) method to optimally capture the explicit and tacit knowledge of each technician, as well as the theoretical basis, the model of Nonaka and Takeuchi.</p>

Electronic structures of zwitterionic and protonated forms of glycine betaine in water: Insights into solvent effects from ab initio simulations

Glycine betaine (GB), a naturally occurring osmolyte, has many biological functions and is used in food supplements, pharmaceuticals, and animal feed. GB usually exists as a zwitterion in aqueous solutions. Due to the hygroscopic property of GB, protonated form of GB, GBH+ (noted as GBH) is often used. In order to understand the hydration structures of GB and GBH at the atomic and electronic-structure level, a combination of ab initio molecular dynamics simulations and electronic structure calculations based on density functional theory (DFT), single-excitation configuration interaction method, and coupled-cluster ansatz was utilized. A hybrid explicit–implicit solvation model was used to elucidate solvent effects. Within the hybrid scheme, microsolvated structures of GB and GBH with up to 100 water molecules were obtained, showing that water molecules bind to GB and GBH in a dramatically different manner. By calculating the molecular electron-density distributions at PBE0-D4/def2-TZVP level, it shows that the variation of electron density for GB is more significant than GBH, suggesting that GB can interact with more water molecules than GBH. The interaction energies of GB–(H2O)n and GBH–(H2O)n systems (n = 1–6) at DLPNO-CCSD(T)/cc-pVQZ level highlight the difference from an energy point of view. Using the microsolvated structures of GB and GBH, the features of experimental K-edge spectra can be reproduced using the efficient PNO-ROCIS/B3LYP method, implying the reliability of the hybrid explicit–implicit solvation approach. The results suggest that complementary insights into the geometries and electronic structures of GB and GBH could be obtained through combined methods, which provides a methodological approach for understanding the hydration structures of zwitterionic and charged molecules.

The ground and ionized states of azulene: A combined study of the vibrational energy levels by photoionization, configuration interaction, and density functional calculations.

A synchrotron-based photoionization spectrum of azulene shows a significant additional vibrational fine structure when compared to previous studies. This spectrum was successfully analyzed by using Franck-Condon (FC) methods. Previously reported zero-kinetic-energy electron spectra for azulene have been reinterpreted in FC terms, leading to some alternative assignments to the earlier work. The sequence of ionic states has been determined by using ab initio configuration interaction (CI) methods, leading to reliable theoretical values for both the calculated adiabatic ionization energy (AIE) and vertical ionization energy (VIE). VIEs were calculated by both symmetry-adapted cluster (SAC-CI), together with Green's function (GF) and Tamm-Dancoff approximation (TDA), and single excitation CI methods; AIEs for highest states of each symmetry were determined by open-shell self-consistent field (SCF) methods at the restricted Hartree-Fock level. Complete active space SCF was used for the pairs of 12A2 + 22A2 and 12B1 + 22B1 states, each of which occurs as antisymmetric and symmetric (higher energy) combinations. The combined ionic state sequences (AIE and VIE) from these methods are 12A2 < 12B1 < 22A2 < 22B1. The photoelectron spectrum (PES) shows a series of broadbands above 11eV, each of which is attributed to more than one ionization. The calculated PES sequence of states of up to 19eV shows that the SAC-CI and GF results are in almost exact agreement. The internal spacing of the bands is best reproduced by the simpler GF and TDA methods. States involving simultaneous ionization and electronic excitation are considered by both SAC-CI and TDA methods.

Effect of spin–orbit coupling on strong field ionization simulated with time-dependent configuration interaction

Time-dependent configuration interaction with a complex absorbing potential has been used to simulate strong field ionization by intense laser fields. Because spin-orbit coupling changes the energies of the ground and excited states, it can affect the strong field ionization rate for molecules containing heavy atoms. Configuration interaction with single excitations (CIS) has been employed for strong field ionization of closed shell systems. Single and double excitation configuration interaction with ionization (CISD-IP) has been used to treat ionization of degenerate states of cations on an equal footing. The CISD-IP wavefunction consists of ionizing single (one hole) and double (two hole/one particle) excitations from the neutral atom. Spin-orbit coupling has been implemented using an effective one electron spin-orbit coupling operator. The effective nuclear charge in the spin-orbit coupling operator has been optimized for Ar+, Kr+, Xe+, HX+ (X = Cl, Br, and I). Spin-orbit effects on angular dependence of the strong field ionization have been studied for HX and HX+. The effects of spin-orbit coupling are largest for ionization from the π orbitals of HX+. In a static field, oscillations are seen between the 2Π3/2 and 2Π1/2 states of HX+. For ionization of HX+ by a two cycle circularly polarized pulse, a single peak is seen when the maximum in the carrier envelope is perpendicular to the molecular axis and two peaks are seen when it is parallel to the axis. This is the result of the greater ionization rate for the π orbitals than for the σ orbitals.

Molecular Opacity Calculations for Lithium Hydride at Low TemperatureSupported by the National Key Research and Development Program of China (Grant No. 2017YFA0402300), the National Natural Science Foundation of China (Grant Nos. 11934004 and 11604052), and the University Nursing Program for Young Scholars with Creative Talents in Heilongjiang Province, China (Grant No. 135409230).

The opacities of the lithium hydride molecule are calculated for temperatures of 300 K, 1000 K, 1500 K, and 2000 K, at a pressure of 10 atm, in which the contributions from the five low-lying electronic states are considered. The ab initio multi-reference single and double excitation configuration interaction (MRDCI) method is applied to compute the potential energy curves (PECs) of the 7LiH, including four 1 Σ + states and one 1 Π state, as well as the corresponding transition dipole moments between these states. The ro-vibrational energy levels are calculated based on the PECs obtained, together with the spectroscopic constants. In addition, the partition functions are also computed, and are provided at temperatures ranging from 10 K to 2000 K for 7LiH, 7LiD, 6LiH, and 6LiD.

Angular dependence of strong field sequential double ionization for neon and acetylene simulated with time-dependent configuration interaction using CIS and CISD-IP.

Strong field ionization is fundamentally important for attosecond spectroscopy and coherence control. However, the modeling beyond the single active electron approximation is still difficult. Time-dependent configuration interaction with singly excited configurations and a complex absorbing potential (TDCIS-CAP), can be used to simulate single and double ionization by intense laser fields. When the monocation does not have degenerate states, TDCIS-CAP starting from a Hartree-Fock calculation of the cation is suitable for simulating the second ionization step. When the monocation has two or more degenerate states, the simulations should treat these degenerate states equivalently. CISD-IP (single and double excitation configuration interaction with ionization) can be used to treat degenerate states of the cation on an equal footing by representing the cation wavefunctions with ionizing single (1 hole) and double (2 holes/1 particle) excitations from the neutral molecule. Since CISD-IP includes single excitations for each of the monocation states, time dependent CISD-IP with a complex absorbing potential (TDCISDIP-CAP) can also be used to simulate ionization to the dications states. In this work, TDCIS-CAP and TDCISDIP-CAP have been used to simulate the angular dependence of ionization of the neon cation and acetylene cation. In both cases, the second electron is ionized predominantly from an orbital perpendicular to the orbital involved in the first ionization. The TDCISDIP-CAP simulations show some features involving interactions between the monocation states that are not seen in the TDCIS-CAP simulations.

Generalized single excitation configuration interaction: an investigation into the impact of the inclusion of non-orthogonality on the calculation of core-excited states.

In this paper, we investigate different non-orthogonal generalizations of the configuration interaction with single substitutions (CIS) method for the calculation of core-excited states. Fully non-orthogonal CIS (NOCIS) has been described previously for species with singlet and doublet ground states, and this paper reports the extension to molecules in their triplet ground state. In addition to NOCIS, we present a novel method, one-center NOCIS (1C-NOCIS), for open-shell molecules which is intermediate between NOCIS and the computationally less demanding static exchange approximation (STEX). We explore this hierarchy of spin-pure methods for core excitations of molecules with singlet, doublet, and triplet ground states. We conclude that, while NOCIS provides the best results and preserves the spatial symmetry of the wavefunction, 1C-NOCIS retains much of the accuracy of NOCIS at a dramatically reduced cost. For molecules with closed-shell ground states, STEX and 1C-NOCIS are identical.

Molecular Structures and Spectral Properties of Natural Indigo and Indirubin: Experimental and DFT Studies.

This paper presents a comparative study on natural indigo and indirubin in terms of molecular structures and spectral properties by using both computational and experimental methods. The spectral properties were analyzed with Fourier transform infrared (FTIR), Raman, UV-Visible, and fluorescence techniques. The density functional theory (DFT) method with B3LYP using 6-311G(d,p) basis set was utilized to obtain their optimized geometric structures and calculate the molecular electrostatic potential, frontier molecular orbitals, FTIR, and Raman spectra. The single-excitation configuration interaction (CIS), time-dependent density functional theory (TD-DFT), and polarization continuum model (PCM) were used to optimize the excited state structure and calculate the UV-Visible absorption and fluorescence spectra of the two molecules at B3LYP/6-311G(d,p) level. The results showed that all computational spectra agreed well with the experimental results. It was found that the same vibrational mode presents a lower frequency in indigo than that in indirubin. The frontier molecular orbital analysis demonstrated that the UV-Visible absorption and fluorescence bands of indigo and indirubin are mainly derived from π → π* transition. The results also implied that the indigo molecule is more conjugated and planar than indirubin, thereby exhibiting a longer maximum absorption wavelength and stronger fluorescence peak.

Exciton effects on the nonlinear optical properties of the endohedral La@C82 complex

Using the Hartree–Fock single-excitation configuration interaction (CI) model, we calculated the second-order and third-order nonlinear polarizability of the La@C82 molecule. Obtained results indicate that the third-order nonlinear polarizability $$ \gamma $$ is much larger than the second-order nonlinear polarizability $$ \beta $$ and linear polarizability. Also, our results show that the $$ \gamma $$ value of the La@C82 molecule is much larger than those of the C60 and C70 molecules. The spectral shapes of the third-order nonlinear polarizability tensor, the second-order nonlinear polarizability tensor and linear polarizability of the C82 molecule turn out to be determined mainly by the geometrical distributions of the pentagons in the fullerene structures.

The electronically excited states of cyclooctatetraene-An analysis of the vacuum ultraviolet absorption spectrum by ab initio configuration interaction methods.

A new synchrotron-based study of the vacuum ultraviolet (VUV) absorption spectrum for cyclooctatetraene (COT) shows a series of broad peaks. A significant sharp structure was extracted from the strongest band between 5.9 and 6.3 eV by fitting this range of the spectrum to a polynomial; the regular residuals show a set of sharp peaks. Comparison of this region of the VUV with the photoelectron spectrum demonstrates the presence of several Rydberg states, all based on the lowest observed ionization energy ionic state. The UV onset contains a broad band in the range 4.0 eV-5.3 eV. Theoretical vertical excitation energies, determined by configuration interaction (CI) studies at the multireference multiroot singles and doubles CI level, enabled interpretation of the principal absorption bands of the VUV spectrum. Adiabatic excitation energies (AEEs) for several singlet and triplet valence states (V) were evaluated by multiconfiguration self-consistent field methods. Theoretical Rydberg series AEEs were obtained by use of extremely diffuse Gaussian orbitals in highly correlated wave-functions. The second moments of the charge distribution identify which roots are valence or Rydberg states. A contrast was found between some density functional methods and Hartree-Fock (HF) wave-functions during single-excitation CI, when degenerate orbitals were involved in the leading configurations. The 7a16e* state contained the expected 8-membered ring in the density functional theory calculations. The HF wave-functions led to a 1,5-cross-ring interaction which converged on a singlet excited state of a bicyclo[3,3,0]octatriene; this is reminiscent of the photochemical conversion of COT to semibullvalene.

A scaled CIS(D) based method for the calculation of valence and core electron ionization energies.

The calculation of electron ionization energies is a key component for the simulation of photoelectron spectroscopy. CIS(D) is a perturbative doubles correction for the single excitation configuration interaction (CIS) method which provides a new approach for computing excitation energies. It is shown that by introducing a virtual orbital subspace that consists of a single "ghost" orbital, valence electron ionization energies can be computed using a scaled CIS(D) approach with an accuracy comparable with considerably more computationally intensive methods, such as ionization-potential equation of motion coupled cluster theory, and simulated spectra show a significant improvement relative to spectra based upon Koopmans' theorem. When the model is applied to the ionization energies for core orbitals, there is an increase in the error, particularly for the heavier nuclei considered (silicon to chlorine), although the relative energy of the ionization energies are predicted accurately. In addition to its inherent computational efficiency relative to other wavefunction based approaches, a significant advantage of this approach is that the ionization energies for all electrons can be obtained in a single calculation, in contrast to Δself-consistent field based methods.

Effect of Coulomb interaction on the second-order nonlinear optical and magneto-optical properties of the endohedral La@C82 crystal with spatial dispersion

Using the Hartree-Fock single-excitation configuration interaction (CI) model, the second- order nonlinear optical dielectric tensor, refractive index, reflectivity, circular dichroism (CD), birefringence coefficient (θ) and the effects of spatial dispersion of the simple cubic (sc) phase of the La@C82 crystal are calculated. It is shown that the intermolecular polarization interaction has significant effects on the nonlinear properties of the endohedral fullerenes, and that, in some contrast to other organic species, nondipolar contributions to this interaction are important. The spectral shapes of the second-order nonlinear dielectric tensor of the La@C82 crystal turn out to be determined mainly by the geometrical distributions of the pentagons in the fullerene structures. Our results on the effect of spatial dispersion show that the nonlinear refractive index and reflectivity of the La@C82 crystal in the energy range 0.3–6 eV have sharp structures in each band.

Determining whether diabolical singularities limit the accuracy of molecular property based diabatic representations: The 1,21A states of methylamine.

An efficient, easily implemented method for locating singularities attributable to the failure of the defining equations in a molecular property based diabatization, termed diabolical singular points, is reported. For two state diabatizations, the singular points form a seam of dimension N int - 2, where N int is the number of internal degrees of freedom. The dynamical outcomes of nuclear trajectories that reach the region of this seam are flawed. The algorithm easily identifies these otherwise hard to anticipate regions of fallaciously large derivative coupling. The fact that the algorithm is easily incorporated into a two state diabatization code based on molecular properties makes it a practical tool for determining whether the existence of diabolical singularities is relevant to the problem being considered. The algorithm is illustrated using a multireference single and double excitation configuration interaction description of the 1,21A states of CH3NH2.

Development of the Fragment Molecular Orbital Method for Calculating Nonlocal Excitations in Large Molecular Systems.

We developed the fragment-based method for calculating nonlocal excitations in large molecular systems. This method is based on the multilayer fragment molecular orbital method and the configuration interaction single (CIS) wave function using localized molecular orbitals. The excited-state wave function for the whole system is described as a superposition of configuration state functions (CSFs) for intrafragment excitations and for interfragment charge-transfer excitations. The formulation and calculations of singlet excited-state Hamiltonian matrix elements in the fragment CSFs are presented in detail. The efficient approximation schemes for calculating the matrix elements are also presented. The computational efficiency and the accuracy were evaluated using the molecular dimers and molecular aggregates. We confirmed that absolute errors of 50 meV (relative to the conventional calculations) are achievable for the molecular systems in their equilibrium geometries. The perturbative electron correlation correction to the CIS excitation energies is also demonstrated. The present theory can compute a large number of excited states in large molecular systems; in addition, it allows for the systematic derivation of a model exciton Hamiltonian. These features are useful for studying excited-state dynamics in condensed molecular systems based on the ab initio electronic structure theory.

Molecular electric moments calculated by using natural orbital functional theory.

The molecular electric dipole, quadrupole, and octupole moments of a selected set of 21 spin-compensated molecules are determined employing the extended version of the Piris natural orbital functional 6 (PNOF6), using the triple-ζ Gaussian basis set with polarization functions developed by Sadlej, at the experimental geometries. The performance of the PNOF6 is established by carrying out a statistical analysis of the mean absolute errors with respect to the experiment. The calculated PNOF6 electric moments agree satisfactorily with the corresponding experimental data and are in good agreement with the values obtained by accurate ab initio methods, namely, the coupled-cluster single and doubles and multi-reference single and double excitation configuration interaction methods.

Rydberg, valence, and ion-pair quintet states of O2.

We report an ab initio study of the quintet states of molecular oxygen. The calculations are carried out employing the multireference single and double excitation configuration interaction package. Potential energy curves of the six quintet valence states dissociating into ground state atoms and of the four quintet states dissociating to ion-pair atoms were computed. A number of bound quintet Rydberg series converging to the a(4)Πu and b(4)Σg(-) states of the O2(+) cation have been identified.

An improved quasi-diabatic representation of the 1, 2, 3(1)A coupled adiabatic potential energy surfaces of phenol in the full 33 internal coordinates.

In a recent work we constructed a quasi-diabatic representation, H(d), of the 1, 2, 3(1)A adiabatic states of phenol from high level multireference single and double excitation configuration interaction electronic structure data, energies, energy gradients, and derivative couplings. That H(d) accurately describes surface minima, saddle points, and also regions of strong nonadiabatic interactions, reproducing the locus of conical intersection seams and the coordinate dependence of the derivative couplings. The present work determines the accuracy of H(d) for describing phenol photodissociation. Additionally, we demonstrate that a modest energetic shift of two diabats yields a quantifiably more accurate H(d) compared with experimental energetics. The analysis shows that in favorable circumstances it is possible to use single point energies obtained from the most reliable electronic structure methods available, including methods for which the energy gradients and derivative couplings are not available, to improve the quality of a global representation of several coupled potential energy surfaces. Our data suggest an alternative interpretation of kinetic energy release measurements near λphot ∼ 248 nm.

An experimental and theoretical study of core-valence double ionisation of acetaldehyde (ethanal).

Core-valence double ionisation spectra of acetaldehyde (ethanal) are presented at photon energies above the carbon and oxygen 1s ionisation edges, measured by a versatile multi-electron coincidence spectroscopy technique. We use this molecule as a testbed for analyzing core-valence spectra by means of quantum chemical calculations of transition energies. These theoretical approaches range from two simple models, one based on orbital energies corrected by core valence interaction and one based on the equivalent core approximation, to a systematic series of quantum chemical electronic structure methods of increasing sophistication. The two simple models are found to provide a fast orbital interpretation of the spectra, in particular in the low energy parts, while the coverage of the full spectrum is best fulfilled by correlated models. CASPT2 is the most sophisticated model applied, but considering precision as well as computational costs, the single and double excitation configuration interaction model seems to provide the best option to analyze core-valence double hole spectra.

The first and second hypersusceptibility of the La@C82 crystal: Effects of disorder

Using the Hartree–Fock single-excitation configuration interaction (HF-CI) model in conjunction with the local field method, the linear susceptibility tensor, first hypersusceptibility tensor and second hypersusceptibility tensor of the simple cubic (sc) phase of the La@C82 crystal are calculated. The HF-CI-type electron–phonon model for the La@C82 crystal is solved with site disorder. The calculations show that the second hypersusceptibility tensor of the La@C82 crystal is large. The Coulomb interactions and effects of disorder cause increasing hypersusceptibility tensor. Our results show that the optical excitation in the region where the energy is lower than 4eV has the strongest nonlinear optical absorption bands in the La@C82 crystal.