Configuration Interaction Calculations Research Articles

Resonant x-ray emission across the L3 edge of uranium compounds

Abstract A narrow bandwidth x-ray beamline and a multi-crystal von Hamos spectrometer were used to record x-ray absorption and x-ray emission (XES) across the uranium L 3 edge of UO2, UO3, Cs2UO2Cl4, and Cs2UCl6. Measurements were made over 17150–17250 eV with an instrumental resolution of ∼2 eV. This resolution allowed the use of Lorentzian peak fits to the L 3 N 4 and L 3 N 5 characteristic x-ray fluorescence lines that have ∼12–13 eV lifetime widths. The fluorescence yields of the four compounds display strong white lines near threshold and multiple scattering features at higher energies. The XES spectra were fit with three Lorentzians, two for the L 3 N 4 and L 3 N 5 lines and one for inelastic scattering by 2p–6d excitations and 4d–6d final states. The intensities of the white lines were largely due to the inelastic scattering component. To support the measurements, relativistic equation-of-motion coupled-cluster and restricted active space configuration interaction calculations were performed for uranium core-excitation energies in Cs2UO2Cl4 and Cs2UCl6. These calculations with rigorous treatments of relativistic effects are shown to provide accurate uranium L 3-edge binding energies, L 3 N 5 emission energies, as well as the energy losses in the inelastic scattering process. The measured and calculated results show variations between compounds with U(IV) oxidation states that contain 6d05f2 electrons in their ground configurations (UO2 and Cs2UCl6) compared with U(VI) compounds (UO3 and Cs2UO2Cl4) with empty 5f and 6d configurations.

The low-lying electronic states of 4H-pyran-4-thione; a photoionization and vacuum ultraviolet absorption study, with interpretation by configuration interaction and density functional calculations for the ionic and singlet states.

Two synchrotron-based studies on 4H-pyran-4-thione, photoelectron spectroscopy and vacuum ultraviolet (VUV) absorption spectra were performed. A highly resolved structure was observed in the photoelectron spectrum (PES), in contrast to an earlier PES study, where little structure was observed. The sequence of ionic states was determined using configuration interaction and coupled cluster methods. The vibrational structure of the lowest three PES bands was analyzed by configuration interaction and density functional calculations, providing a detailed explanation of the observed profiles. Several vibrational bands in the VUV absorption spectrum showed a similar structure to the bands in the PES and were identified as Rydberg states.

The Properties of the Low-Lying Electronic States and Avoided Crossings of the SbP Molecule: A Theoretical Investigation Includes Spin-Orbit Coupling.

High-level multireference configuration interaction plus Davidson correction (MRCI + Q) calculation method was employed to determine the potential energy curves (PECs) of 10 Λ-S states, which come from the first and second dissociation channels of the SbP molecule, as well as 34 Ω states considering the spin-orbit coupling (SOC) effect. By solving the Schrödinger equation for nuclear motion, spectroscopic constants for the ground state X1Σ+ and low-lying excited states were obtained and compared with experimental data. The excellent agreement indicates the reliability of our calculations. Additionally, the calculated spin-orbit (SO) matrix elements of the 13Π and 15Π states with other Λ-S states were analyzed, and the majority of the values in the Franck-Condon region exceed 200 cm-1, indicating strong interactions between these states. What's more, the joint effects of spin-orbit coupling and avoided crossing were discussed in detail, leading to the complex potential energy curves and double-well phenomena observed in the Ω states. Taking forbidden transitions into account, transition dipole moments with the SOC effect are considered. The Franck-Condon factors, Einstein coefficients, and radiative lifetimes for the 13Σ+1 ↔ X1Σ+0+ transition were obtained. Analysis indicates that direct laser cooling of SbP is inappropriate.

Role of the Radical Character in Singlet Fission: An Ab Initio and Quantum Chemical Topology Analysis.

The radical character of molecules exhibiting singlet fission is related to the energy level matching relationships that facilitate this process. Using a linear H4 model molecule, we employ quantum chemical topology descriptors based on full configuration interaction calculations to rationalize singlet fission. In this context, the influence of the closed-shell to diradical and diradical to tetraradical character on the singlet fission energy matching conditions is analyzed. We find that in the diradical limit the singlet fission efficiency can be manipulated considering the active molecule coupled to an excited diradical, while in the diradical to tetraradical limit, the efficiency is dependent only on the gap between the lowest-lying excited singlet and triplet state. Furthermore, our results reveal possible design strategies for molecules with radical character exhibiting efficient singlet fission.

ADAPT-QSCI: Adaptive Construction of an Input State for Quantum-Selected Configuration Interaction.

We present a quantum-classical hybrid algorithm for calculating the ground state and its energy of the quantum many-body Hamiltonian by proposing an adaptive construction of a quantum state for the quantum-selected configuration interaction (QSCI) method. QSCI allows us to select important electronic configurations in the system to perform configuration interaction (CI) calculation (subspace diagonalization of the Hamiltonian) by sampling measurement for a proper input quantum state on a quantum computer, but how we prepare a desirable input state remains a challenge. We propose an adaptive construction of the input state for QSCI in which we run QSCI repeatedly to grow the input state iteratively. We numerically illustrate that our method, dubbed ADAPT-QSCI, can yield accurate ground-state energies for small molecules, including a noisy situation for eight qubits where error rates of two-qubit gates and the measurement are both as large as 1%. ADAPT-QSCI serves as a promising method to take advantage of current noisy quantum devices and pushes forward its application to quantum chemistry.

Efficient and scalable wave function compression using corner hierarchical matrices.

The exponential scaling of complete active space and full configuration interaction (CI) calculations limits the ability of quantum chemists to simulate the electronic structures of strongly correlated systems. Herein, we present corner hierarchically approximated CI (CHACI), an approach to wave function compression based on corner hierarchical matrices (CH-matrices)-a new variant of hierarchical matrices based on block-wise low-rank decomposition. By application to dodecacene, a strongly correlated molecule, we demonstrate that CH matrix compression provides superior compression compared to truncated global singular value decomposition. The compression ratio is shown to improve with increasing active space size. By comparison of several alternative schemes, we demonstrate that superior compression is achieved by (a) using a blocking approach that emphasizes the upper-left corner of the CI vector, (b) sorting the CI vector prior to compression, and (c) optimizing the rank of each block to maximize information density.

First-Principles Analysis of the Influence of the Valence on the Optical Spectra of Uranium Ions in Oxide Crystals

Recently U6+-doped oxide green phosphors are attracting attentions for the white LED application. However, the absorption and emission mechanisms in some of these materials are still unclear. For the development of the novel uranium-doped phosphors, clarification of the origins of the peaks in the absorption and emission spectra of these materials is indispensable.In our previous research, we investigated the origin of the peaks in the optical spectra of uranium ions in some oxide crystals by comparing the experimental spectra with the theoretical ones obtained by first-principles calculations. The results indicated that there is a U-doped phosphor whose spectrum seems to originate from 5f-6d transitions of U3+ instead of LMCT transitions of U6+. In this work, in order to investigate the influence of the valence of uranium ions, we created some hypothetical model clusters representing uranium ions in oxides and performed first-principles calculations for the LMCT transitions of U6+ and the 5f-6d transitions of U3+ 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 effects of coordination numbers and bond distances on valence of uranium ions were also investigated in detail by performing calculations using model clusters with coordination numbers from 6 to 12.

Relativistic Prolapse-Free Gaussian Basis Sets of Double- and Triple-ζ Quality for s- and p-Block Elements: (aug-)RPF-2Z and (aug-)RPF-3Z.

This study presents two new relativistic Gaussian basis sets without variational prolapse of double- and triple-ζ quality, RPF-2Z and RPF-3Z, along with augmented versions including additional diffuse functions, aug-RPF-2Z and aug-RPF-3Z, which are available for all s and p block elements from Hydrogen to Oganesson. The exponents of the Correlation/Polarization (C/P) functions are obtained from a polynomial version of the generator coordinate Dirac-Fock method (p-GCDF). The choice of C/P functions was guided by multireference configuration interaction calculations with single and double excitations (MR-CISD) based on a valence active space. Finally, calculations of fundamental properties done for atomic and molecular systems (bond lengths, vibrational frequencies, dipole moments, and electron affinities) ensure the expected quality of these new basis sets, which may also exhibit some computational efficiency advantages. Additionally, the prolapse-free feature of these sets must provide a reliable description of properties more dependent on core electron distributions, as well.

On the nature of hydrogen bonding in the H2S dimer

Hydrogen bonding is a central concept in chemistry and biochemistry, and so it continues to attract intense study. Here, we examine hydrogen bonding in the H2S dimer, in comparison with the well-studied water dimer, in unprecedented detail. We record a mass-selected IR spectrum of the H2S dimer in superfluid helium nanodroplets. We are able to resolve a rotational substructure in each of the three distinct bands and, based on it, assign these to vibration-rotation-tunneling transitions of a single intramolecular vibration. With the use of high-level potential and dipole-moment surfaces we compute the vibration-rotation-tunneling dynamics and far-infrared spectrum with rigorous quantum methods. Intramolecular mode Vibrational Self-Consistent-Field and Configuration-Interaction calculations provide the frequencies and intensities of the four SH-stretch modes, with a focus on the most intense, the donor bound SH mode which yields the experimentally observed bands. We show that the intermolecular modes in the H2S dimer are substantially more delocalized and more strongly mixed than in the water dimer. The less directional nature of the hydrogen bonding can be quantified in terms of weaker electrostatic and more important dispersion interactions. The present study reconciles all previous spectroscopic data, and serves as a sensitive test for the potential and dipole-moment surfaces.

Small tensor product distributed active space (STP-DAS) framework for relativistic and non-relativistic multiconfiguration calculations: Scaling from 109 on a laptop to 1012 determinants on a supercomputer

Despite the power and flexibility of configuration interaction (CI) based methods in computational chemistry, their broader application is limited by an exponential increase in both computational and storage requirements, particularly due to the substantial memory needed for excitation lists that are crucial for scalable parallel computing. The objective of this work is to develop a new CI framework, namely, the small tensor product distributed active space (STP-DAS) framework, aimed at drastically reducing memory demands for extensive CI calculations on individual workstations or laptops, while simultaneously enhancing scalability for extensive parallel computing. Moreover, the STP-DAS framework can support various CI-based techniques, such as complete active space (CAS), restricted active space, generalized active space, multireference CI, and multireference perturbation theory, applicable to both relativistic (two- and four-component) and non-relativistic theories, thus extending the utility of CI methods in computational research. We conducted benchmark studies on a supercomputer to evaluate the storage needs, parallel scalability, and communication downtime using a realistic exact-two-component CASCI (X2C-CASCI) approach, covering a range of determinants from 109 to 1012. Additionally, we performed large X2C-CASCI calculations on a single laptop and examined how the STP-DAS partitioning affects performance.

Solving the Puzzle of the Carbonic Acid Vibrational Spectrum - an Anharmonic Story.

Against the general belief that carbonic acid is too unstable for synthesis, it was possible to synthesize the solid[1,2] as well as gas-phase carbonic acid.[3] It was suggested that solid carbonic acid might exist in Earth's upper troposphere and in the harsh environments of other solar bodies,[4] where it undergoes a cycle of synthesis, decomposition, and dimerization.[5] To provide spectroscopic data for probing the existence of extraterrestrial carbonic acid,[2,6] matrix-isolation infrared (MI-IR) spectroscopy has shown to be essential.[3,4,6-8] However, early assignments within the harmonic approximation using scaling factors impeded a full interpretation of the rather complex MI-IR spectrum of H2CO3. Recently, carbonic acid was detected in the Galactic center molecular cloud,[9] triggering new interest in the anharmonic spectrum.[10] In this regard, we substantially reassign our argon MI-IR spectra based on accurate anharmonic calculations. We calculate a four-mode potential energy surface (PES) at the explicitly correlated coupled-cluster theory using up to triple-zeta basis sets, i. e., CCSD(T)-F12/cc-pVTZ-F12. On this PES, we perform vibrational self-consistent field and configuration interaction (VSCF/VCI) calculations to obtain accurate vibrational transition frequencies and resonance analysis of the fundamentals, first overtones, and combination bands. In total, 12 new bands can be assigned, extending the spectral data for carbonic acid and thus simplifying detection in more complex environments. Furthermore, we clarify disputed assignments between the cc- and ct-conformer.

Neural-network approach to running high-precision atomic computations

Modern applications of atomic physics, including the determination of frequency standards and the analysis of astrophysical spectra, require prediction of atomic properties with exquisite accuracy. For complex atomic systems, high-precision calculations are a major challenge due to the exponential scaling of the involved electronic configuration sets. This exacerbates the problem of required computational resources for these computations and makes indispensable the development of approaches to select the most important configurations out of otherwise intractably huge sets. We have developed a neural-network (NN) tool for running high-precision atomic configuration interaction (CI) computations with iterative selection of the most important configurations. Integrated with the established atomic codes, our approach results in computations with significantly reduced computational requirements in comparison with those without NN support. We showcase a number of NN-supported computations for the energy levels of Fe16+ and Ni12+ and demonstrate that our approach can be reliably used and automated for solving specific computational problems for a wide variety of systems. Published by the American Physical Society 2024

PyCI: A Python-scriptable library for arbitrary determinant CI.

PyCI is a free and open-source Python library for setting up and running arbitrary determinant-driven configuration interaction (CI) computations, as well as their generalizations to cases where the coefficients of the determinant are nonlinear functions of optimizable parameters. PyCI also includes functionality for computing the residual correlation energy, along with the ability to compute spin-polarized one- and two-electron (transition) reduced density matrices. PyCI was originally intended to replace the abinitio quantum chemistry functionality in the HORTON library but emerged as a standalone research tool, primarily intended to aid in method development, while maintaining high performance so that it is suitable for practical calculations. To this end, PyCI is written in Python, adopting principles of modern software development, including comprehensive documentation, extensive testing, continuous integration/delivery protocols, and package management. Computationally intensive steps, notably operations related to generating Slater determinants and computing their expectation values, are delegated to low-level C++ code. This article marks the official release of the PyCI library, showcasing its functionality and scope.

High-Precision Mass Measurements of Neutron Deficient Silver Isotopes Probe the Robustness of the N=50 Shell Closure.

High-precision mass measurements of exotic ^{95-97}Ag isotopes close to the N=Z line have been conducted with the JYFLTRAP double Penning trap mass spectrometer, with the silver ions produced using the recently commissioned inductively heated hot cavity catcher laser ion source at the Ion Guide Isotope Separator On-Line facility. The atomic mass of ^{95}Ag was directly determined for the first time. In addition, the atomic masses of β-decaying 2^{+} and 8^{+} states in ^{96}Ag have been identified and measured for the first time, and the precision of the ^{97}Ag mass has been improved. The newly measured masses, with a precision of ≈1 keV/c^{2}, have been used to investigate the N=50 neutron shell closure, confirming it to be robust. Empirical shell-gap and pairing energies determined with the new ground-state mass data are compared with the state-of-the-art abinitio calculations with various chiral effective field theory Hamiltonians. The precise determination of the excitation energy of the ^{96m}Ag isomer in particular serves as a benchmark for abinitio predictions of nuclear properties beyond the ground state, specifically for odd-odd nuclei situated in proximity to the proton dripline below ^{100}Sn. In addition, density functional theory calculations and configuration-interaction shell-model calculations are compared with the experimental results. All theoretical approaches face challenges to reproduce the trend of nuclear ground-state properties in the silver isotopic chain across the N=50 neutron shell and toward the proton dripline.

Capturing Correlation Effects in Positron Binding to Atoms and Molecules.

A major challenge in contemporary electronic structure theory involves the development of methods to describe in a balanced manner the contribution of correlation effects to energy differences. This challenge can be even greater for multicomponent systems containing more than one type of quantum particle. In the present work, we describe a flexible code for carrying out self-consistent field and configuration interaction (CI) calculations on multicomponent systems and use it to generate trial wave functions for use in diffusion Monte Carlo (DMC) calculations of the positron affinity of Be, Be2, Be4, Mg, CS2, and benzene. The resulting positron affinities (PAs) are in good agreement with the best values from the literature.

Singlet-Triplet Inversion in Triangular Boron Carbon Nitrides.

The discovery of singlet-triplet (ST) inversion in some π-conjugated triangle-shaped boron carbon nitrides is a remarkable breakthrough that defies Hund's first rule. Deeply rooted in strong electron-electron interactions, ST inversion has garnered significant interest due to its potential to revolutionize triplet harvesting in organic LEDs. Using the well-established Pariser-Parr-Pople model for correlated electrons in π-conjugated systems, we employ a combination of CISDT and restricted active space configuration interaction calculations to investigate the photophysics of several triangular boron carbon nitrides. Our findings reveal that ST inversion in these systems is primarily driven by a network of alternating electron-donor and electron-acceptor groups in the molecular rim, rather than by the triangular molecular structure itself.

Comparison of curvilinear coordinates within vibrational structure calculations based on automatically generated potential energy surfaces.

For floppy molecules showing internal rotations and/or large amplitude motions, curvilinear internal coordinates are known to be superior to rectilinear normal coordinates within vibrational structure calculations. Due to the myriad definitions of internal coordinates, automated and efficient potential energy surface generators necessitate a high degree of flexibility, supporting the properties arising from these coordinates. Within this work, an approach to deal with these challenges is presented, including key elements, such as the selection of appropriate fit functions, the exploitation of symmetry, the positioning of grid points, or elongation limits for different coordinates. These elements are tested for five definitions of curvilinear coordinates, with three of them being generated in an automated manner. Calculations for semi-rigid molecules, namely H2O, H2CO, CH2F2, and H2CNH, demonstrate the general functionality of the implemented algorithms. Additional calculations for the HOPO molecule highlight the benefits of these curvilinear coordinates for systems with large amplitude motions. This new implementation allowed us to compare the performance of these different coordinate systems with respect to the convergence of the underlying expansion of the potential energy surface and subsequent vibrational configuration interaction calculations.

Long-Range Configuration Interaction with an Ab Initio Short-Range Correction and an Asymptotic Lower Bound†.

Short-range corrections to long-range selected configuration interaction calculations are derived from perturbation theory considerations and applied to harmonium (with two to six electrons for some low-lying states). No fitting to reference data is used, and the method is applicable to ground and excited states. The formulas derived are rigorous when the physical interaction is approached. In this regime, the second-order expression provides a lower bound to the long-range full configuration interaction energy. A long-range/short-range separation of the interaction between electrons at a distance of the order of one atomic unit provides total energies within chemical accuracy, and, for the systems studied, provide better results than short-range density functional approximations.

Tracking ultrafast non-adiabatic dissociation dynamics of the deuterated water dication molecule.

We applied reaction microscopy to elucidate fast non-adiabatic dissociation dynamics of deuterated water molecules after direct photo-double ionization at 61eV with synchrotron radiation. For the very rare D+ + O+ + D breakup channel, the particle momenta, angular, and energy distributions of electrons and ions, measured in coincidence, reveal distinct electronic dication states and their dissociation pathways via spin-orbit coupling and charge transfer at crossings and seams on the potential energy surfaces. Notably, we could distinguish between direct and fast sequential dissociation scenarios. For the latter case, our measurements reveal the geometry and orientation of the deuterated water molecule with respect to the polarization vector that leads to this rare 3-body molecular breakup channel. Aided by multi-reference configuration-interaction calculations, the dissociation dynamics could be traced on the relevant potential energy surfaces and particularly their crossings and seams. This approach also unraveled the ultrafast time scales governing these processes.

Theoretical Modeling of Sr(q+) He (q = 0, 1, 2) van der Waals Systems Including Spin-Orbit Coupling.

Using an ab initio methodology that incorporates pseudopotential technique in conjunction with pair potential approaches, core polarization potentials (CPP), large basis sets of Gaussian type, and full configuration interaction calculations, we investigate interaction of neutral and charged Srq+(q = 0,1,2) with helium atom. In this context, the core-core interaction of Sr2+-He is included using an accurately performed potential for the ground state at CCSD(T) level of calculation. Also, the potential energy curves and permanent and transition dipole moments of the ground state and numerous excited states have been performed respectively for Sr+He and SrHe systems. Subsequently, the spin-orbit effect is considered by utilizing a semiempirical method for states dissociating into Sr+(5p) + He, Sr+(6p) + He, Sr+(4d) + He, Sr+(5d) + He, Sr(5s5p) + He, and Sr(5s4d) + He. The spectroscopic constants of the Srq+(q = 0, 1, 2) He states, with and without spin-orbit interaction, are derived and assessed in comparison to the existing theoretical and experimental studies. Such comparison has revealed good agreement, especially, for the Sr+He ionic system. Additionally, the spin-orbit effect is considered for the X2Σ+ → 22Π1/2,3/2 and X2Σ+ → 32Σ1/2 + transition dipole moments for Sr+He.