Neodymium and uranium mono-oxide (NdO and UO) and the corresponding anions and cations are of interest to both experimental and theoretical studies. These systems exhibit a high density of low-lying electronic states and strong correlation among valence f-type electrons. They thus emerge as challenging examples for electronic-structure calculations. In this presentation, we demonstrate the usefulness of coupled-cluster techniques in understanding many properties of low-lying electronic states including ionization energies, electron affinities, and geometrical constants. We show that the inclusion of spin-orbit coupling in orbitals plays an important role in the capability to treat dense electronic states using single reference methods. Possible extension to treat transuranium-containing small molecules is discussed.

We perform a computational study of the Stark effect for X-ray absorption spectra, and analyze the electric field response through the orbital and geometry variation with the electric field strength and orientation. We utilize a combination of Q-Chem and CFOUR, using the powerful CVS-EOM-CCSD/aug-cc-pCVTZ method for treating the vertical x-ray absorption energies and transition properties. External electric fields are applied collinear to the molecular dipole moment and the molecular geometry and orientation (when allowed by symmetry) are optimized in the presence of the field. We discuss how the symmetry of the molecule affects the X-ray spectra and identify characteristic features for the finite-field spectra. A rich structure is observed in the variation of X-ray spectra with varying electric field strength.

Dual fluorescence in dimethylaminobenzonitrile (DMABN) and its derivatives in polar solvents, has been studied extensively for the past several years. Intramolecular charge transfer (ICT), in addition to the localized low-energy (LE) valence minimum, has been proposed as a mechanism for this dual fluorescence, with large geometric relaxation and molecular orbital reorganization a key feature of the ICT pathway. Herein, we have used both equation-of-motion coupled-cluster singles and doubles (EOM-CCSD) and time-dependent density functional (TD-DFT) methods to investigate the landscape of excited state potential energy surfaces across the several geometric conformations proposed as ICT structures, including Franck-Condon (FC), twisted (TICT), partially twisted (PTICT), wagged (WICT), and rehybridized (RICT) structures. Initial geometries for each type of structure were selected for excited state PES optimizations, as well as a systematic exploration of the low-lying PESs starting from the FC geometry. We find a number of minimum-energy (near)-crossing points among the lowest three excited states leading to the eventual ICT minima. Finally, we have calculated the carbon and nitrogen K-edge transient absorption spectra for all important “signpost” structures in order to investigate the differential pump-probe features along the LE and ICT pathways.

r0pt Figure Thermally activated delayed fluorescence (TADF) is one of the most promising routes to enhance the luminescent efficiency of an organic light-emitting diode (OLED) device by converting a non-emissive triplet exciton (T₁) back to an emissive singlet configuration (S₁) through reverse intersystem crossing (RISC) before it fluoresces back to the ground state (S₀). However, the TADF rate is generally restricted if only the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are involved. This is due to the conflict between a fast RISC rate between S₁ and T₁ (which requires a small HOMO-LUMO overlap), and a large transition dipole moment (μ_\textT) between S₁ and S₀ (which requires a large HOMO-LUMO overlap).A. Endo et al., Appl. Phys. Lett. 2011, 98, 083302.n the present study, we proposed two solutions to enhance the overall fluorescent rate: an inclusion of higher-lying singlet and triplet states (S_{n ≥ 2} and T_{n ≥ 2}) in ISC-RISC routes to avoid the trade-off, and a fluxional molecular conformation to sample a broad range of HOMO-LUMO overlap. We provided a proof-of-concept for our solutions based on computational modeling of sample di-tert-butyl carbazole derivatives with the pyrazine or dipyrazine substituents (DTCz-Pz or DTCz-Pz), using a combination of density functional theory (DFT) and molecular dynamics (MD). Our study will provide a computational and quantitative strategy for the design of new TADF emitters with maximum luminescent efficiency. A. Endo et al., Appl. Phys. Lett. 2011, 98, 083302.I

The observation of ultrafast time-resolved molecular dynamics after electronic excitation often relies on the measurement and interpretation of nonlinear optical signals. These signals can be very challenging to interpret without the aid of a theoretical model. A common approach to understand these signals is by using parameterized semi-empirical models that describe the specific process under study. These methods can be very useful and are very flexible but finding appropriate parameter values can be challenging, and the physical interpretation of these parameters can be ambiguous. Ab initio calculations can reduce the number of free parameters. However, available quantum chemistry packages like Dalton, QChem, and others, typically report frequency domain information, and tracking the evolution of the target usually requires the mapping of time onto a nuclear reaction coordinate which may not be observable. Here we present an ab initio approach to modeling time domain ultrafast nonlinear optical signals that addresses these issues by using the Dalton quantum chemistry package to parameterize a general N-level model which is then evaluated using a Liouville space representation. We compare these results to recent Ultrafast Transient Polarization Spectroscopy measurements of nitrobenzene. This work was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division. Work at the Molecular Foundry was supported by the Office of Science, Office of Basic Energy Sciences of the U.S. Department of Energy under Contract no. DE-AC02-05CH11231.

We have reported on the vibrationally averaged structure and frequencies of (XeHXe)⁺ at ISMS2021^a: (a) (XeHXe)⁺ is a linear molecule having a bent vibrationally averaged structure, (b) the ultra-heavy Xe atoms keep almost standstill during vibration, (c) severe matrix-effect for the ν₁ symmetric stretching mode, (d) ν₃ > ω_{e ,3} for the antisymmetric stretching mode, and (e) the zero-point structure has non-equivalent two r(Xe-H) bond distances irrespective of the potential being symmetrical for the bond-distance of these two bonds. We proposed in the previous report^a that the unusual features (d) and (e) are characteristic for the [ultra-heavy]-[light]-[ultra-heavy] system. In this report, we disclose, from the viewpoint of computational molecular spectroscopy, why features (d) and (e) become characteristic for this system.

Based on the 3D vibrational potential energy surface (PES) calculated at the valence-CCSD(T)_DK3/[ANO-RCC 5ZP(Xe), cc-pV5Z-DK(H)] level, ro-vibrational wavefunctions (DVR3D wavefunction) were derived by the Discrete Variable Representation (DVR) method. In the antisymmetric stretching mode of a [ultra-heavy]-[light]-[ultra-heavy] system, the central light atom moves back and forth between the almost standstill ultra-heavy atoms just like a ball in catch-ball play. We will show this is the key for understanding unusual features (d) and (e), using the results of the PES and vibrational wavefunction analyses. We will also show why the symmetric stretching mode ν₁ is severely affected^b by the molecular mass of the matrix medium.

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Footnotes:

^aT. Hirano, U. Nagashima, M. Baba, ISMS2021, WA07.
^bM. Tsuge, J. Kalinowski,R.B. Gerber, Y-P. Lee, J. Phys. Chem. A 119, 2651 (2015).

Vanadium oxide (VO) is believed to play an important role in driving temperature inversion in the atmospheres of hot-Jupiters. It also characterises the spectra of late M and early L dwarfs and subdwarfs, where it is understood to be a significant opacity source. A MARVEL (measured active rotational-vibrational energy levels) analysis of the spectra of VO is performed, involving thirteen electronic states (6 quartets and 7 doublets). ⁵¹V¹⁶O data from 14 sources are used to form three networks: hyperfine-resolved quartets, hyperfine-unresolved quartets and hyperfine-unresolved doublets. A single quartet network is formed by deperturbing the hyperfine lines and 191 lines are assigned to an intercombination 2 ²Π-X ⁴Σ⁻ band system in the visible region previously recorded by Hopkins et al. (2009), allowing the doublet and quartet networks to be merged. As a result 6535/4393 and 8610/4641 validated transitions/final energies were obtained from analysis of the hyperfine-resolved/unresolved networks. T₀ energy values are determined for the 2 ²Π_1/2, ν=0,1 and 2 ²Π_3/2, ν=0 states.

MAJIS (Moons And Jupiter Imaging Spectrometer) is one of the key scientific instruments on board the Jupiter ICy Moons Explorer (JUICE), the next mission to the Jovian system. A reliable determination of H₂O and CH₄ densities in the vertical structure and distribution of Jupiter's atmosphere is one of our main goals. In order to achieve this, we implemented the current knowledge of physical and chemical properties of Jupiter in ASIMUT-ALVL to perform simulations with different viewing geometries of the MAJIS instrument from 0.5μm to 2.5μm. ASIMUT-ALVL is a Radiative Transfer (RT) code developed at BIRA-IASB that has been extensively used to characterize Mars and Venus atmospheres.Vandaele, A.C., et al., Optics Express. 2013, 21(18), 21148^, Vandaele, A.C., et al., Icarus. 2017, 295, 1-15.OLópez-Puertas, M., et al., The Astronomical Journal. 2018, 156.4, 169.TCisneros-González, M.E., et al., SPIE Astronomical Telescopes and Instrumentation. 2020, 114431L.tThis project acknowledges the funding provided by the Belgian National Scientific Research Fund (FNRS by its acronym in french) through the Aspirant-Renewal Grant 34828872 MAJIS detectors and Impact on Science".

We present the GPU Accelerated Absorption Simulation (GAAS). GAAS is an open-source software package for simulating broadband absorption spectra rapidly using Nvidia graphics processing units (GPUs). GAAS is intended to provide a fast alternative to HAPI [1], capable of simulating absorbance spectra given a pressure, temperature, and concentration. GAAS is written in C++ and C and comes with a python interface so that it can be easily integrated into existing codebases. GAAS supports Voigt lineshape profiles and primarily contains a python function to replace HAPI’s absorptionCoefficientVoigt. GAAS uses spectroscopic data in HITRAN’s “par” format in order to be compatible with existing codebases that use HAPI. The software realizes up to a 100x reduction in computation time by simulating each Voigt lineshape in the spectrum on its own GPU thread, achieving enough parallelization for full utilization of GPU resources for spectra containing a few thousand absorption lines. [1] R.V. Kochanov, I.E. Gordon, L.S. Rothman, P. Wcisło, C. Hill, J.S. Wilzewski, HITRAN Application Programming Interface (HAPI): A comprehensive approach to working with spectroscopic data, Journal of Quantitative Spectroscopy and Radiative Transfer, Volume 177, 2016, Pages 15-30, ISSN 0022-4073

In quantum systems, out of time order correlators (OTOCs) can be used to probe the sensitivity of the dynamics to perturbing the Hamiltonian or changing the initial conditions ordinarily associated with classical chaos or its quantum analog. The vibrations of polyatomic molecules are known to undergo a transition from regular dynamics at low energy to facile energy flow at sufficiently high energy. Molecules therefore represent ideal quantum systems to study the transition to chaos in many-body systems of moderate size (here 6 to 36 degrees of freedom). By computing quantum OTOCs and their classical counterparts we quantify how information becomes ‘scrambled’ quantum mechanically in molecular systems. I0pt Figure