Search Results (21)

The potential energy curves and spectroscopic constants of the ground and several low-lying excited states of the Ca^q+-Kr (q = 0, 1, 2) van der Waals complexes were investigated using one- and two-electron pseudopotential approaches. This treatment effectively reduces the number of active electrons in Ca^q+-Kr to a single valence electron for q = 1 and two valence electrons for q = 0, allowing the use of large and flexible basis sets for both Ca and Kr atoms. Within this work, potential energy curves (PECs) were calculated at the SCF level for the Ca⁺-Kr system, while both SCF and full configuration interaction (FCI) calculations were performed for the neutral Ca-Kr. Spin–orbit coupling effects were explicitly included in all calculations to accurately describe the fine-structure splitting of the asymptotic atomic states. The short-range core–core interaction for Ca²⁺-Kr was obtained using high-level CCSD(T) calculations. Spectroscopic constants were derived from the computed PECs and compared with available theoretical and experimental results, showing consistent trends. Furthermore, the transition dipole moments (TDM) were evaluated as a function of internuclear distances, including spin–orbit effects, to provide a comprehensive description of the electronic structure and radiative properties of these weakly bound systems. Full article

The ab initio calculations of the electronic structure of the low-lying electronic states of the CdBr molecule are characterized in the ^2S+1Λ^(+/−) and Ω^(+/−) representations using the complete active-space self-consistent field (CASSCF) method, followed by the multireference configuration interaction (MRCI) method with Davidson correction (+Q). The potential energy curves are investigated, and spectroscopic parameters (T_e, R_e, ω_e, B_e, ${\alpha }_e$, ${\mu }_e$, and D_e) of the bound states are determined and analyzed. In addition, the rovibrational constants (E_v, B_v, D_v, R_min, and R_max) are reported for the investigated states with and without spin–orbit coupling. The electronic transition dipole moment curve (TDMC) is obtained for the C²Π_1/2 − X²Σ⁺_1/2 transition. Based on these data, Franck–Condon factors (FCFs), Einstein coefficient of spontaneous emission A_ν’ν, radiative lifetime τ, vibrational branching ratios, and the associated slowing distance are evaluated. The results indicated that CdBr is a promising candidate for direct laser cooling, and a feasible cooling scheme employing four pumping and repumping lasers in the ultraviolet region with suitable experimentally accessible parameters is presented. These findings provide practical guidance for experimental spectroscopists exploring ultracold diatomic molecules and their applications. Full article

Incited by the scant understanding of unsettled charmonia and newly observed $cc\bar{c}\bar{c}$ tetraquarks, this work aims to explore the canonical interpretations and spectroscopic properties of these fully hidden-charm states. In the framework of a relativistic flux tube model, the centroid masses of the low-lying $nL$-wave states with $1\le n+L\le 4$ are unraveled. In order to pin down the complete mass spectra, the hyperfine splittings induced by the spin-dependent interactions are incorporated into the final predictions. Accordingly, fourteen charmonia are well identified, including the ${\eta }_c\left(1S\right)$, $J/\psi \left(1S\right)$, ${\chi }_{c0}\left(1P\right)$, $h_c\left(1P\right)$, ${\chi }_{c1}\left(1P\right)$, ${\chi }_{c2}\left(1P\right)$, ${\eta }_c\left(2S\right)$, $\psi \left(2S\right)$, $\psi \left(3770\right)$, ${\psi }_2\left(3823\right)$, ${\psi }_3\left(3842\right)$, ${\chi }_{c0}\left(3915\right)$, ${\chi }_{c2}\left(3930\right)$, and $\psi \left(4040\right)$ states. Additionally, the exotic $T_{\psi \psi }\left(6400\right)$, $T_{\psi \psi }\left(6600\right)$, $T_{\psi \psi }\left(6900\right)$, and $T_{\psi \psi }\left(7300\right)$ states are interpreted as the $1S$-wave, $1P/2S$-wave, $1D/2P$-wave, and $2D/3P/4S$-wave $cc\bar{c}\bar{c}$ tetraquarks, respectively. Based on the achieved outcomes, the spin-parity quantum number is imperative to discriminate the nature of the $cc\bar{c}\bar{c}$ structures, pending further experimental measurement in the future. Full article

Multireference configuration interaction (MRCI), Davidson-corrected MRCI (MRCI+Q), coupled-cluster singles, doubles, and perturbative triples [CCSD(T)], and frozen-core full configuration interaction (fcFCI) calculations were carried out using large, correlation-consistent basis sets to investigate the excited states of the Sc atom and the spin–free and spin–orbit coupled potential energy profiles, energetics, spectroscopic constants, and electron populations of low-lying states of MH⁺ (M = Sc, Y, La). The core electron correlation effects, complete basis set effects, and spin–orbit coupling effects were also evaluated. The first four electronic states of all MH⁺ are 1²Δ, 1²Σ⁺, 1²Π, and 2²Σ⁺ with 1σ²1δ¹, 1σ²2σ¹, 1σ²1π¹, and 1σ²3σ¹ single-reference electron configurations, respectively. These states of MH⁺ can be represented by the M²⁺H^– ionic structure. The ground states of ScH⁺, YH⁺, and LaH⁺ are 1²Δ_3/2, 1²Σ⁺_1/2, and 1²Δ_3/2 with 55.45, 60.54, and 62.34 kcal/mol bond energies, respectively. The core electron correlation was found to be vital for gaining accurate predictions on the ground and excited state properties of MH⁺. The spin–orbit coupling effects are minor for ScH⁺ but become substantial moving to YH⁺ and LaH⁺. Overall, the results of this work are in good agreement with the limited set of experimental findings of MH⁺ available in the literature and will be of use for future investigations. Furthermore, the theoretical approaches, findings, and trends reported here are expected to aid studies of similar species. Full article

In this work, we have considered the family of indenofluorene (IF) and its longitudinally elongated variants fluorenofluorene and diindenoanthracene and investigated their low-lying excited states and optical properties via quantum-chemical studies at the density functional theory (DFT) level. Singlet ground-state diradicals exhibit distinct optical properties due to the presence of a low-lying state dominated by a doubly excited configuration (DE state), often below the lowest allowed singly excited state (SE state). IFs and their elongated derivatives, with tunable diradical character and both symmetric and nonsymmetric structures, provide an ideal platform for exploring DE state energy modulation and spectroscopic behavior. The study shows that absorption spectra simulated using time-dependent (TD) calculations based on unrestricted broken-symmetry antiparallel-spin reference configuration (TDUDFT) closely match the available experimental data. Additionally, it reveals distinct spectral behavior for symmetric and nonsymmetric derivatives, highlighting the role of lowest-lying weakly allowed excited states potentially promoting non-radiative deactivation pathways. Full article

Vibrational circular dichroism (VCD) enhancement by low-lying electronic states (LLESs) is a fascinating phenomenon, but accounting for it theoretically remains a challenge despite significant research efforts over the past 20 years. In this article, we synthesized two transition metal complexes using the tetradentate Schiff base ligands (R,R)- and (S,S)-N,N′-Bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediamine with Co(II) and Mn(III), referred to as Co(II)-salen-chxn and Mn(III)-Cl-salen-chxn, respectively. Their stereochemical properties were explored through a combined experimental chiroptical spectroscopic and theoretical approach, with a focus on Co(II)-salen-chxn. Extensive conformational searches in CDCl₃ for both high- and low-spin states were carried out and the associated infrared (IR), VCD, ultraviolet-visible (UV-Vis) absorption, and electronic circular dichroism (ECD) spectra were simulated. A good agreement between experimental and simulated data was achieved for IR, VCD, UV-Vis, and ECD, except in the case of VCD of Co(II)-salen-chxn which exhibits significant intensity enhancement and monosignate VCD bands, attributed to the LLESs. Interestingly, detailed comparisons with Mn(III)-Cl-salen-chxn and previously reported Ni(II)-salen-chxn and Cu(II)-salen-chxn complexes suggest that the enhancement factor is predicted by the current density functional theory simulations. However, the monosignate signatures observed in the experimental Co(II) VCD spectrum were not captured theoretically. Based on the experiment and theoretical VCD and ECD comparison, it is tentatively suggested that Co(II)-salen-chxn exists in both low- and high-spin states, with the former being dominant, while Mn(III)-Cl-salen-chxn in the high-spin state. The study indicates that VCD enhancement by LLESs is at least partially captured by the existing theoretical simulation, while the symmetry consideration in vibronic coupling provides further insight into the mechanisms behind the VCD sign-flip. Full article

This review is devoted to summarizing recent developments of the linear sigma model (LSM) in cold and dense two-color QCD (QC₂D), in which lattice simulations are straightforwardly applicable thanks to the disappearance of the sign problem. In QC₂D, both theoretical and numerical studies derive the presence of the so-called baryon superfluid phase at a sufficiently large chemical potential (${\mu }_q$), where diquark condensates govern the ground state. The hadron mass spectrum simulated in this phase shows that the mass of an iso-singlet ($I=0$) and $0^{-}$ state is remarkably reduced, but such a mode cannot be described by the chiral perturbation theory. Motivated by this fact, I have invented a LSM constructed upon the linear representation of chiral symmetry, more precisely Pauli–Gürsey symmetry. It is shown that my LSM successfully reproduces the low-lying hadron mass spectrum in a broad range of ${\mu }_q$ simulated on the lattice. As applications of the LSM, topological susceptibility and sound velocity in cold and dense QC₂D are evaluated to compare with the lattice results. Additionally, the generalized Gell–Mann–Oakes–Renner relation and hardon mass spectrum in the presence of a diquark source are analyzed. I also introduce an extended version of the LSM incorporating spin-1 hadrons. Full article

Magnesium monofluoride is a polar molecule amenable to laser cooling which has caused renewed interest in its spectroscopy. In this work, we consider the case of three low-lying electronic excitations, namely $X^2{\mathrm{\Sigma }}^{+}$→$A^2\mathrm{\Pi }$, $X^2{\mathrm{\Sigma }}^{+}$→$B^2{\mathrm{\Sigma }}^{+}$, $X^2{\mathrm{\Sigma }}^{+}$→$C^2{\mathrm{\Sigma }}^{+}$, using well-developed quantum chemistry approaches, i.e., without reference to the spin-orbit splitting of the $A^2\mathrm{\Pi }$ states. Accurate experimental data for these transitions have been available for over 50 years. Here, we explore the linear response method at the level of CC2 theory, as well as equation of motion methods at the level of CCSD and CC3, using two families of basis sets. Excellent agreement is obtained for the first three transitions when using the correlation-consistent basis sets and extrapolation to the complete basis limit within EOM-CC3 (at a relative precision of ${10}^{-4}$), and qualitative agreement for the other two methods. The purpose of this paper is to serve as a guide on how to approach the accurate calculation of excitations in polar diatomic molecules. Full article

Tetroxane derivatives are interesting drugs for antileishmaniasis and antimalaric treatments. The gas-phase thermal decomposition of 3,6,-dimethyl-1,2,4,5-tetroxane (DMT) and 3,3,6,6,-tetramethyl-1,2,4,5-tetroxane (acetone diperoxide (ACDP)) was studied at 493–543 K by direct gas chromatography by means of a flow reactor. The reaction is produced in the injector chamber at different temperatures. The resulting kinetics Arrhenius equations were calculated for both tetroxanes. Including the parent compound of the series 1,2,4,5-tetroxane (formaldehyde diperoxide (FDP)), the activation energy and frequency factors decrease linearly with the number of methyl groups. The reaction mechanisms of ACDP and 3,6,6-trimethyl-1,2,4,5-tetroxane (TMT) decomposition have been studied by means of the DFT method with the BHANDHLYP functional. Our calculations confirm that the concerted mechanism should be discarded and that only the stepwise mechanism occurs. The critical points of the singlet and triplet state potential energy surfaces (S- and T-PES) of the thermolysis reaction of both compounds have been determined. The calculated activation energies of the different steps vary linearly with the number of methyl groups of the methyl-tetroxanes series. The mechanism for the S-PES leads to a diradical O···O open structure, which leads to a C···O dissociation in the second step and the production of the first acetaldehyde/acetone molecule. This last one yields a second C···O dissociation, producing O₂ and another acetone/acetaldehyde molecule. The O₂ molecule is in the singlet state. A quasi-parallel mechanism for the T-PES from the open diradical to products is also found. Most of the critical points of both PES are linear with the number of methyl groups. Reaction in the triplet state is much more exothermic than the singlet state mechanism. Transitions from the singlet ground state, S₀ and low-lying singlet states S_1–3, to the low-lying triplet excited states, T_1–4, (chemical excitation) in the family of methyl tetroxanes are also studied at the CASSCF/CASPT2 level. Two possible mechanisms are possible here: (i) from S₀ to T₃ by strong spin orbit coupling (SOC) and subsequent fast internal conversion to the excited T₁ state and (ii) from S₀ to S₂ from internal conversion and subsequent S₂ to T₁ by SOC. From these experimental and theoretical results, the additivity effect of the methyl groups in the thermolysis reaction of the methyl tetroxane derivatives is clearly highlighted. This information will have a great impact for controlling these processes in the laboratory and chemical industries. Full article

Phase and amplitude modes, also called polariton modes, are emergent phenomena that manifest across diverse physical systems, from condensed matter and particle physics to quantum optics. We study their behavior in an anisotropic Dicke model that includes collective matter interactions. We study the low-lying spectrum in the thermodynamic limit via the Holstein–Primakoff transformation and contrast the results with the semi-classical energy surface obtained via coherent states. We also explore the geometric phase for both boson and spin contours in the parameter space as a function of the phases in the system. We unveil novel phenomena due to the unique critical features provided by the interplay between the anisotropy and matter interactions. We expect our results to serve the observation of phase and amplitude modes in current quantum information platforms. Full article

Photosynthetic water splitting, when synergized with hydrogen production catalyzed by hydrogenases, emerges as a promising avenue for clean and renewable energy. However, theoretical calculations have faced challenges in elucidating the low-lying spin states of iron–sulfur clusters, which are integral components of hydrogenases. To address this challenge, we employ the Extended Broken-Symmetry method for the computation of the cubane–[Fe₃S₄] cluster within the [FeNi] hydrogenase enzyme. This approach rectifies the error caused by spin contamination, allowing us to obtain the magnetic exchange coupling constant and the energy level of the low-lying state. We find that the Extended Broken-Symmetry method provides more accurate results for differences in bond length and the magnetic coupling constant. This accuracy assists in reconstructing the low-spin ground state force and determining the geometric structure of the ground state. By utilizing the Extended Broken-Symmetry method, we further highlight the significance of the geometric arrangement of metal centers in the cluster’s properties and gain deeper insights into the magnetic properties of transition metal iron–sulfur clusters at the reaction centers of hydrogenases. This research illuminates the untapped potential of hydrogenases and their promising role in the future of photosynthesis and sustainable energy production. Full article

The geometry and electronic structures of iron(II) complexes with porphyrin (FeP) and tetrabenzoporphyrin (FeTBP) in ground and low-lying excited electronic states are determined by DFT (PBE0/def2-TZVP) calculations and the complete active space self-consistent field (CASSCF) method, followed by the multiconfigurational quasi-degenerate second-order perturbation theory (MCQDPT2) approach to determine the dynamic electron correlation. The minima on the potential energy surfaces (PESs) of the ground (³A_2g) and low-lying, high-spin (⁵A_1g) electronic states correspond to the planar structures of FeP and FeTBP with D_4h symmetry. According to the results of the MCQDPT2 calculations, the wave functions of the ³A_2g and ⁵A_1g electronic states are single determinant. The electronic absorption (UV–Vis) spectra of FeP and FeTBP are simulated within the framework of the simplified time-dependent density functional theory (sTDDFT) approach with the use of the long-range corrected CAM-B3LYP function. The most intensive bands of the UV–Vis spectra of FeP and FeTBP occur in the Soret near-UV region of 370–390 nm. Full article

Charged excitons or trions are essential for optical spectra in low-dimensional doped monolayers (ML) of transitional metal dichalcogenides (TMDC). Using a direct diagonalization of the three-body Hamiltonian, we calculate the low-lying trion states in four types of TMDC MLs as a function of doping and dielectric environment. We show that the fine structure of the trion is the result of the interplay between the spin-valley fine structure of the single-particle bands and the exchange interaction. We demonstrate that by variations of the doping and dielectric environment, the fine structure of the trion energy can be tuned, leading to anticrossing of the bright and dark states, with substantial implications for the optical spectra of the TMDC ML. Full article

A diagonalization scheme for the shell model mean-field plus isovector pairing Hamiltonian in the O(5) tensor product basis of the quasi-spin SU${}_{\mathrm{\Lambda }}$(2) ⊗ SU${}_I$(2) chain is proposed. The advantage of the diagonalization scheme lies in the fact that not only can the isospin-conserved, charge-independent isovector pairing interaction be analyzed, but also the isospin symmetry breaking cases. More importantly, the number operator of the np-pairs can be realized in this neutron and proton quasi-spin basis, with which the np-pair occupation number and its fluctuation at the J = 0${}^{+}$ ground state of the model can be evaluated. As examples of the application, binding energies and low-lying J = $0^{+}$ excited states of the even–even and odd–odd N∼Z $ds$-shell nuclei are fit in the model with the charge-independent approximation, from which the neutron–proton pairing contribution to the binding energy in the $ds$-shell nuclei is estimated. It is observed that the decrease in the double binding-energy difference for the odd–odd nuclei is mainly due to the symmetry energy and Wigner energy contribution to the binding energy that alter the pairing staggering patten. The np-pair amplitudes in the np-pair stripping or picking-up process of these N = Z nuclei are also calculated. Full article

Conjugated singlet ground state diradicals have received remarkable attention owing to their potential applications in optoelectronic devices. A distinctive character of these systems is the location of the double-exciton state, a low lying excited state dominated by the doubly excited HOMO,HOMOLUMO,LUMO configuration, (where HOMO=highest occupied molecular orbital, LUMO=lowest unoccupied molecular orbital) which may influence optical and other photophysical properties. In this contribution we investigate this specific excited state, for a series of recently synthesized conjugated diradicals, employing time dependent density functional theory (TDDFT) based on the unrestricted parallel spin reference configuration in the spin-flip formulation (SF-TDDFT) and standard TD calculations based on the unrestricted antiparallel spin reference configuration (TDUDFT). The quality of computed results is assessed considering diradical and multiradical descriptors, and the excited state wavefunction composition. Full article