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Excited-state absorption (ESA) corresponds to the transition between two electronic excited states and is a fundamental process for probing and understanding light-matter interactions. Accurate modeling of ESA is indeed often required to interpret time-resolved experiments. In this contribution, we present a dataset of 53 ESA oscillator strengths in three different gauges and the associated vertical transition energies between 71 excited states of 23 small- and medium-sized molecules from the QUEST database. The reference values were obtained within the quadratic-response (QR) CC3 formalism using eight different Dunning basis sets. We found that the d-aug-cc-pVTZ basis set is always adequate while its more compact double-$\zeta$ counterpart, d-aug-cc-pVDZ, performs well in most applications. These QR-CC3 data allow us to assess the performance of QR-TDDFT, with and without applying the Tamm-Dancoff approximation, using a panel of global and range-separated hybrids (B3LYP, BH{\&}HLYP, CAM-B3LYP, LC-BLYP33, and LC-BLYP47), as well as several lower-order wavefunction methods, i.e., QR-CCSD, QR-CC2, EOM-CCSD, ISR-ADC(2), and ISR-ADC(3). We show that QR-TDDFT delivers acceptable errors for ESA oscillator strengths, with CAM-B3LYP showing particular promise, especially for the largest molecules of our set. We also find that ISR-ADC(3) exhibits excellent performance

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Building on our recent study [https://doi.org/10.1021/acs.jpclett.3c02052, J. Phys. Chem. Lett. 14, 8780 (2023)], we explore the generalization of the ground-state Kohn-Sham (KS) formalism of density-functional theory (DFT) to the (singlet) excited states of the asymmetric Hubbard dimer at half-filling. While we found that the KS-DFT framework can be straightforwardly generalized to the highest-lying doubly-excited state, the treatment of the first excited state presents significant challenges. Specifically, using a density-fixed adiabatic connection, we show that the density of the first excited state lacks non-interacting $v$-representability. However, by employing an analytic continuation of the adiabatic path, we demonstrate that the density of the first excited state can be generated by a complex-valued external potential in the non-interacting case. More practically, by performing state-specific KS calculations with exact and approximate correlation functionals -- each state possessing a distinct correlation functional -- we observe that spurious stationary solutions of the KS equations may arise due to the approximate nature of the functional.

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Reduced density matrix functional theory (RDMFT) and coupled cluster theory restricted to paired double excitations (pCCD) are emerging as efficient methodologies for accounting for the so-called non-dynamic electronic correlation effects. Up to now, molecular calculations have been performed with real-valued orbitals. However, before extending the applicability of these methodologies to extended systems, where Bloch states are employed, the subtleties of working with complex-valued orbitals and the consequences of imposing time-reversal symmetry must be carefully addressed. In this work, we describe the theoretical and practical implications of adopting time-reversal symmetry in RDMFT and pCCD when allowing for complex-valued orbital coefficients. The theoretical considerations primarily affect the optimization algorithms, while the practical implications raise fundamental questions about the stability of solutions. Specifically, we find that complex solutions lower the energy when non-dynamic electronic correlation effects are pronounced. We present numerical examples to illustrate and discuss these instabilities and possible problems introduced by N-representability violations.

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The Bethe-Salpeter equation has been extensively employed to compute the two-body electron-hole propagator and its poles which correspond to the neutral excitation energies of the system. Through a different time-ordering, the two-body Green's function can also describe the propagation of two electrons or two holes. The corresponding poles are the double ionization potentials and double electron affinities of the system. In this work, a Bethe-Salpeter equation for the two-body particle-particle propagator is derived within the linear-response formalism using a pairing field and anomalous propagators. This framework allows us to compute kernels corresponding to different self-energy approximations ($GW$, $T$-matrix, and second-Born) as in the usual electron-hole case. The performance of these various kernels is gauged for singlet and triplet valence double ionization potentials using a set of 23 small molecules. The description of double core hole states is also analyzed.

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Sujets

Quantum Chemistry Atomic and molecular structure and dynamics CIPSI Relativistic quantum mechanics Atomic charges Parity violation New physics X-ray spectroscopy Molecular properties Acrolein 3115bw 3115vn A posteriori Localization Range separation Single-core optimization Electron electric moment 3315Fm Valence bond Carbon Nanotubes Atrazine-cations complexes Ab initio calculation Atrazine Argile Anharmonic oscillator Petascale Coupled cluster 3115aj Electron electric dipole moment Dipole 3115ag AB-INITIO ALGORITHM Anderson mechanism Polarizabilities Wave functions Corrélation électronique Atom Ground states 3115vj Path integral Chemical concepts Numerical calculations Coupled cluster calculations Pesticide Adiabatic connection CP violation Relativistic corrections Perturbation theory Auto-énergie Diffusion Monte Carlo Dispersion coefficients Configuration Interaction 3115ae Pesticides Metabolites Clustering Molecular modeling Environmental fate Partial least squares BENZENE MOLECULE 3470+e Configuration interactions Fonction de Green A priori Localization Atomic and molecular collisions Electron correlation États excités Rydberg states Aimantation Azide Anion Quantum chemistry Théorie des perturbations Spin-orbit interactions Approximation GW AB-INITIO CALCULATION Argon Analytic gradient Relativistic quantum chemistry Biodegradation Excited states Time reversal violation Quantum Monte Carlo QSAR Atomic processes Chimie quantique Diatomic molecules Parallel speedup BIOMOLECULAR HOMOCHIRALITY Abiotic degradation Line formation Ion 3115am Molecular descriptors Density functional theory Large systems Atomic data AROMATIC-MOLECULES Time-dependent density-functional theory Dirac equation Green's function Atoms Hyperfine structure Atomic charges chemical concepts maximum probability domain population Xenon Mécanique quantique relativiste

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