Core-valence Correlation Effects Research Articles

Spectroscopy and molecule opacity investigation on excited states of SiS

The SiS molecule, which plays a significant role in space, has attracted a great deal of attention for many years. Due to complex interactions among its low-lying electronic states, precise information regarding the molecular structure of SiS is limited. To obtain accurate information about the structure of its excited states, the high-precision multireference configuration interaction (MRCI) method has been utilized. This method is used to calculate the potential energy curves (PECs) of the 18 Λ–S states corresponding to the lowest dissociation limit of SiS. The core–valence correlation effect, Davidson’s correction and the scalar relativistic effect are also included to guarantee the precision of the MRCI calculation. Based on the calculated PECs, the spectroscopic constants of quasi-bound and bound electronic states are calculated and they are in accordance with previous experimental results. The transition dipole moments (TDMs) and dipole moments (DMs) are determined by the MRCI method. In addition, the abrupt variations of the DMs for the 15Σ+ and 25Σ+ states at the avoided crossing point are attributed to the variation of the electronic configuration. The opacity of SiS at a pressure of 100 atms is presented across a series of temperatures. With increasing temperature, the expanding population of excited states blurs the band boundaries.

The p-block challenge: assessing quantum chemistry methods for inorganic heterocycle dimerizations.

The elements of the p-block of the periodic table are of high interest in various chemical and technical applications like frustrated Lewis-pairs (FLP) or opto-electronics. However, high-quality benchmark data to assess approximate density functional theory (DFT) for their theoretical description are sparse. In this work, we present a benchmark set of 604 dimerization energies of 302 "inorganic benzenes" composed of all non-carbon p-block elements of main groups III to VI up to polonium. This so-called IHD302 test set comprises two classes of structures formed by covalent bonding and by weaker donor-acceptor (WDA) interactions, respectively. Generating reliable reference data with ab initio methods is challenging due to large electron correlation contributions, core-valence correlation effects, and especially the slow basis set convergence. To compute reference values for these dimerization reactions, after thorough testing, we applied a computational protocol using state-of-the-art explicitly correlated local coupled cluster theory termed PNO-LCCSD(T)-F12/cc-VTZ-PP-F12(corr.). It includes a basis set correction at the PNO-LMP2-F12/aug-cc-pwCVTZ level. Based on these reference data, we assess 26 DFT methods in combination with three different dispersion corrections and the def2-QZVPP basis set, five composite DFT approaches, and five semi-empirical quantum mechanical methods. For the covalent dimerizations, the r2SCAN-D4 meta-GGA, the r2SCAN0-D4 and ωB97M-V hybrids, and the revDSD-PBEP86-D4 double-hybrid functional are found to be the best-performing methods among the evaluated functionals of the respective class. However, since def2 basis sets for the 4th period are not associated to relativistic pseudo-potentials, we obtained significant errors in the covalent dimerization energies (up to 6 kcal mol-1) for molecules containing p-block elements of the 4th period. Significant improvements were achieved for systems containing 4th row elements by using ECP10MDF pseudopotentials along with re-contracted aug-cc-pVQZ-PP-KS basis sets introduced in this work with the contraction coefficients taken from atomic DFT (PBE0) calculations. Overall, the IHD302 set represents a challenge to contemporary quantum chemical methods. This is due to a large number of spatially close p-element bonds which are underrepresented in other benchmark sets, and the partial covalent bonding character for the WDA interactions. The IHD302 set may be helpful to develop more robust and transferable approximate quantum chemical methods in the future.

Ab initio calculations of the spectra and lifetimes of the lead dimer.

Owing to the key role of the lead dimer (Pb2) as a heavy element benchmark for the Group IV-A dimers the assignment of its spectroscopic properties and chemical bonding is an important undertaking. To meet this demand, the present work provides comprehensive and detailed information on electronic structure and properties comprising a wide set of Pb2 states. Calculations are performed by a high-level ab initio approach. Firstly, the potential energy curves (PECs) of 19 Λ-S states as well as those of 24 ungerade Ω states are calculated by utilizing the multi-reference configuration interaction plus Davidson correction (MRCI + Q) method taking into account core-valence correlation (CV) and spin-orbit coupling (SOC) effect, where Ω is a quantum number of the total (Λ + S) angular momentum projection. Secondly, interactions between the bound F3Σ-u, 23Σ+u states and repulsive 15Πu state induced by strong SOC are discussed based on the PECs analysis and calculated SOC matrix, which also indicates that the F3Σ-u state is predissociative. Thirdly, based on the calculated electric dipole transition moments and energy gaps between the 0+u(III), F0+u(II), C0+u(I) and X0+g states, the intense absorption bands of Pb2 due to these transitions are interpreted. Our results indicate that the trends in intensity of absorption spectra (F0+u(II), C0+u(I) ← X0+g) in the range of 12 600-13 600 and 22 200-23 800 cm-1 are consistent with the previously observed spectra of Pb2 in the qualitatively similar regions (15 200-16 200 and 19 800-21 800 cm-1). Finally, the calculated intensity of the weak magnetic-dipole transitions from the singlet excited b1Σ+g and a1Δg states to the triplet ground X3Σ-g state and their electric quadrupole components are presented for the Pb2 molecule in terms of SOC perturbations for the calculated Ω states expressed in Λ-S state notation. Based on our theoretical assignment, we predict that the weak emission a1Δg2 → X3Σ-g1 bands could be observed experimentally. The present work provides comprehensive electronic structure information and sheds new light on the absorption and emission spectra of the Pb2 dimer.

Hunting for Complex Organic Molecules in the Interstellar Medium: The Role of Accurate Low-Cost Theoretical Geometries and Rotational Constants.

A new approach to computation at affordable cost of accurate geometrical structures and rotational constants for medium-sized molecules in the gas phase is further improved and applied to a large panel of interstellar complex organic molecules. The most distinctive feature of the new model is the effective inclusion of core-valence correlation and vibrational averaging effects in the framework of density functional theory (DFT). In particular, a double-hybrid functional in conjunction with a quadruple-ζ valence/triple-ζ polarization basis set is employed for geometry optimizations, whereas a cheaper hybrid functional in conjunction with a split-valence basis set is used for the evaluation of vibrational corrections. A thorough benchmark based on a wide range of prototypical systems shows that the new scheme approaches the accuracy of state-of-the-art wave function methods with the computational cost of the standard methods (DFT or MP2) routinely employed in the interpretation of microwave spectra. Since the whole computational workflow involves the postprocessing of the output of standard electronic structure codes by a new freely available web utility, the way is paved for the accurate yet not prohibitively expensive study of medium- to large-sized molecules also by nonspecialists.

The magnetic dipole transition in Rb-like ion and its core-valence correlation effect

This study presents both experimental and theoretical analysis of the 4d2D5/2→2D3/2 magnetic dipole transition in Rb-like ions. Spectral lines of Xe17+, La20+ and Pr22+ ions were recorded using an electron beam ion trap. The theoretical framework employed the fully relativistic Multi-configuration Dirac–Hartree–Fock method, accounting for the Breit interaction, QED effects and comprehensive core-valence correlation. An impressive correlation is observed between the theoretical and experimental findings. Notably, this study illustrates that the contribution of the core-valence correlation, especially concerning deep core subshells, is pivotal for the fine structure splittings.

Ab initio interaction potentials of alkali metal (M = Na and K)–rare gas (Rg = He, Ne, Ar, Kr, Xe and Rn) complexes

Spectroscopic properties and potential energy curves of alkali metal (M = Na and K) – rare gas (Rg = He, Ne, Ar, Kr, Xe and Rn) van der Waals molecules in their ground states have been studied in detail using spin-restricted open-shell coupled cluster with single and double excitations and perturbative contribution of connected triple excitations (RCCSD(T)) methods. The core-valence correlation (CV) effect was found to be crucial for M-RG molecules containing heavy rare gas atoms. The electronic energies were corrected for the basis set superposition error (BSSE) using the counterpoise method. Energies were extrapolated to the complete basis set (CBS) limit using a two-point scheme. The permanent electric dipole moments, static electric dipole polarizabilities and long-range dispersion coefficients were also calculated. The computed spectroscopic constants, vibrational levels and rotational constants were reported for M-RG and good agreement with the available experimental and theoretical values were found.

Theoretical studies of bound configuration levels and their radiative lifetimes in Pd− and Pt−

We performed multiconfiguration Dirac-Hartree-Fock and relativistic configuration-interaction calculations for states of the $4{d}^{9}5{s}^{2}$ and $4{d}^{10}5s$ configurations in ${\mathrm{Pd}}^{\ensuremath{-}}$ and for states of the $5{d}^{9}6{s}^{2}$ and $5{d}^{10}6s$ configurations in ${\mathrm{Pt}}^{\ensuremath{-}}$. Valence-valence, core-valence, and core-core electron correlation effects are accounted for in the present studies. The calculated excited energy levels agree within 2.5% with the available experimental values for both ${\mathrm{Pd}}^{\ensuremath{-}}$ and ${\mathrm{Pt}}^{\ensuremath{-}}$ and we confirm that ${\mathrm{Pd}}^{\ensuremath{-}}$ only has one bound excited state. We report a more precise value for the radiative lifetime of $5{d}^{9}6{s}^{2}{\phantom{\rule{4pt}{0ex}}}^{2}{D}_{3/2}$ in ${\mathrm{Pt}}^{\ensuremath{-}}$ ($67.0\ifmmode\pm\else\textpm\fi{}0.3$ ms), which agrees within the experimental range (50--200 ms), but the radiative lifetime of $5{d}^{10}6s{\phantom{\rule{4pt}{0ex}}}^{2}{S}_{1/2}$ ($4.00\ifmmode\pm\else\textpm\fi{}0.16$ s) in ${\mathrm{Pt}}^{\ensuremath{-}}$ is 50% longer than the experimental measurement ($2.54\ifmmode\pm\else\textpm\fi{}0.10$ s). The radiative lifetimes for $4{d}^{9}5{s}^{2}{\phantom{\rule{4pt}{0ex}}}^{2}{D}_{3/2,5/2}$ in ${\mathrm{Pd}}^{\ensuremath{-}}$ are also discussed in detail.

Influence of the complete basis set approximation, tight weighted-core, and diffuse functions on the DLPNO-CCSD(T1) atomization energies of neutral H,C,O-compounds.

The impact of complete basis set extrapolation schemes (CBS), diffuse functions, and tight weighted-core functions on enthalpies of formation predicted via the DLPNO-CCSD(T1) reduced Feller-Peterson-Dixon approach has been examined for neutral H,C,O-compounds. All tested three-point (TZ/QZ/5Z) extrapolation schemes result in mean unsigned deviation (MUD) below 2kJ mol-1 relative to the experiment. The two-point QZ/5Z and TZ/QZ CBS extrapolation schemes are inferior to their inverse power counterpart ( ) by 1.3 and 4.3kJ mol-1 . The CBS extrapolated frozen core atomization energies are insensitive (within 1kJ mol-1 ) to augmentation of the basis set with tight weighted core functions. The core-valence correlation effects converge already at triple-ζ, although double-ζ/triple-ζ CBS extrapolation performs better and is recommended. The effect of diffuse function augmentation converges slowly, and cannot be reproduced with double- ζ or triple- ζ calculations as these are plagued with basis set superposition and incompleteness errors.

Extended relativistic multiconfiguration calculations of energy levels and transition properties in singly ionized tin

Multiconfiguration Dirac-Hartree-Fock (MCDHF) and relativistic configuration interaction (RCI) calculations are performed for 22 states in singly ionized tin (Sn II) belonging to the 5s2ns(n=6,7), 5s2nd(n=5,6), 5s5p2 even parity configurations and the 5s2np(n=5,6,7), 5s24f odd parity configurations. Valence-valence and core-valence correlation effects are taken into account through configuration state function (CSF) expansions. Complete and consistent data sets of level energies, wavelengths, oscillator strengths, lifetimes and transition rates among all these states are given. The results are compared with existing theoretical and experimental results. There is an excellent agreement for calculated excitation energies with experimental data from the NIST database. Lifetimes and transition rates are also in agreement with the results from previous calculations and available measurements for most of the transitions.

Theoretical study on spectroscopic properties of 8 Λ-S and 23 Ω states for BH molecule

In this work, the potential energy curves of eight low electronic states (X<sup>1</sup>Σ<sup>+</sup>, a<sup>3</sup>Π, A<sup>1</sup>Π, b<sup>3</sup>Σ<sup>-</sup>, 2<sup>3</sup>Π, 1<sup>3</sup>Σ<sup>+</sup>, 1<sup>5</sup>Σ<sup>-</sup>, and 1<sup>5</sup>Π) and twenty-three Ω states of BH molecule, and the transition dipole moments among the <inline-formula><tex-math id="M10">\begin{document}$ {\text{X}}{}^{\text{1}}{\Sigma}_{{{\text{0}}^ + }}^ + $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="10-20220038_M10.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="10-20220038_M10.png"/></alternatives></inline-formula>, <inline-formula><tex-math id="M11">\begin{document}$ {{\text{a}}^{\text{3}}}{\Pi_{{{\text{0}}^ + }}} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="10-20220038_M11.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="10-20220038_M11.png"/></alternatives></inline-formula>, a<sup>3</sup>Π<sub>1</sub>, a<sup>3</sup>Π<sub>2</sub>, and A<sup>1</sup>Π<sub>1</sub> states are calculated by using the internally contracted multireference configuration interaction (icMRCI) method. In order to obtain the accurate potential energy curve, the errors caused by single and double electron excitation, core-valence correlation effects, relativistic effects and basis set truncation are corrected. The spectral and transition data of BH molecule are in good agreement with the available theoretical and experimental data. The calculation results show that the A<sup>1</sup>Π<sub>1</sub>(<i>υ′</i> = 0-2, <i>J′</i> = 1, +) →<inline-formula><tex-math id="M12">\begin{document}$ {\text{X}}{}^{\text{1}}{\Sigma}_{{{\text{0}}^ + }}^ + $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="10-20220038_M12.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="10-20220038_M12.png"/></alternatives></inline-formula>(<i>υ′′</i> = 0-2, <i>J′′</i> = 1, –) transition has large Einstein <i>A</i>-coefficient, weighted absorption oscillator strength, and highly diagonal vibrational branching ratio<i> R</i><sub><i>υ′υ′′</i></sub>, and the excited state A<sup>1</sup>Π<sub>1</sub>(<i>υ′</i> = 0, 1) have short spontaneous radiation lifetimes. Moreover, the effects of <inline-formula><tex-math id="M13">\begin{document}$ {{\text{a}}^{\text{3}}}{\Pi_{{{\text{0}}^ + }}} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="10-20220038_M13.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="10-20220038_M13.png"/></alternatives></inline-formula>and a<sup>3</sup>Π<sub>1</sub> states on A<sup>1</sup>Π<sub>1</sub>(<i>υ′</i> = 0) ↔ <inline-formula><tex-math id="M14">\begin{document}$ {\text{X}}{}^{\text{1}}{\Sigma}_{{{\text{0}}^ + }}^ + $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="10-20220038_M14.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="10-20220038_M14.png"/></alternatives></inline-formula>(<i>υ′′</i> = 0) cycle transition can be ignored. Therefore, according to the A<sup>1</sup>Π<sub>1</sub>(<i>υ′</i><sub> </sub>= 0-1, <i>J′</i> = 1, +) ↔ <inline-formula><tex-math id="M15">\begin{document}$ {\text{X}}{}^{\text{1}}{\Sigma}_{{{\text{0}}^ + }}^ + $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="10-20220038_M15.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="10-20220038_M15.png"/></alternatives></inline-formula>(<i>υ′′</i><sub> </sub>= 0-3, <i>J</i>′′ = 1, –) cycle transition, we propose to apply one main cooling laser (<i>λ</i><sub>00</sub> = 432.45 nm) and two repumping lasers (<i>λ</i><sub>10</sub> = 479.67 nm and <i>λ</i><sub>21</sub> = 481.40 nm) to laser cooling BH molecules, and evaluation of the cooling effect.

Accurate line strengths, transition rates and lifetimes of 2s2p2(4P)3s in N II

Abstract The accurate line strengths, transition rates, oscillator strengths, and lifetimes from the inner-shell 2s2p2(4P)3s states 5P and 3P of N II are reported within the framework of the multiconfiguration Dirac-Hartree-Fock (MCDHF) method. The effects of core-core, core-valence, and valence correlations; the Breit interaction; and QED effect are taken into account. Through investigating the influences of electron correlation effects on the quintet states, it is found that the core correlation effect has certain compensation effect in accurately optimizing the line strengths, transition rates and lifetimes of quintet states; the branching fractions of 2s2p3 5S$^\circ _{2}$–2s2p2(4P)3s 5P1, 2, 3 tend to 1, which is inferred as a peculiar intrinsic property that can be useful for plasma diagnostics; unlike 3P0, 1, 2 and 5P1, 3, the calculations of lifetime of 5P2 show some J-dependence, and the velocity form calculations for this J=2-dependence are much more sensitive to electron correlations of 1s22s2p2(4P)3l (l = p, d) than those in the length gauge, which will be helpful to deal with the calculations of lifetime with J=2-dependence in the carbon like isoelectronic series.

Thermochemical Trends in Carbon Chain Molecules HC2kH/HC2k-1H (k = 1-6) Studied by Explicitly Correlated CCSD(T)-F12b Composite Methods.

We present a composite procedure based on explicitly correlated CCSD(T)-F12 calculations for accurate energetic predictions for carbon chain molecules HCnH encompassing both the even (HC2kH) and odd series (HC2k-1H), with the shorter members playing a key role in the evolution of cosmic carbon compounds in both circumstellar envelopes and interstellar medium. This approach considers the contributions of core-valence correlation, scalar relativistic effect, spin-orbit coupling, and zero-point vibrational energy in an additive manner. The computed ionization energies demonstrate outstanding agreement (±0.07 eV) up to a chain size of k = 6 and the literature heats of formation for k ≤ 2 are reproduced with "chemical accuracy" of 1 kcal mol-1. Among the various corrections included, the importance of core-valence correlation effect has been highlighted in the thermochemical calculations for carbon chain growth. The thermochemical trend toward infinite length is also highlighted by extrapolation of ionization energy and triplet-singlet splitting at the CCSD(T) level for k up to 15. The correlation between the end-group effect and the even-odd parity effect observed for HCnH chains has been established with the aid of intrinsic bond orbital localization.

High-Level ab Initio Predictions for the Ionization Energies, Bond Dissociation Energies, and Heats of Formation of Vanadium Methylidene, Vanadium Methyl Species, and Their Cations (VCH2/VCH2+, VCH3/VCH3+).

The ionization energies of VCH2 and VCH3, the various 0 K bond dissociation energies (D0s) in their neutrals and cations, and their respective heats of formation at 0 and 298 K are computed by the single-reference, wave function-based CCSDTQ/CBS procedure. The core of the composite method is the approximation to the complete basis set (CBS) limit at the coupled cluster (CC) level which includes up to full quadruple excitations. The zero-point vibrational energy, core-valence correlation, spin-orbit coupling, and scalar relativistic effects have their contributions incorporated in an additive manner. For the species in the current study, this protocol requires geometry optimizations and harmonic frequency calculations practically no higher than the CCSD(T)/aug-cc-pwCVTZ and CCSD(T)/aug-cc-pVTZ levels, respectively. The present calculations successfully predict D0(V+-CH3) = 2.126 eV and D0(V+-CH2) = 3.298 eV in remarkable agreement with the data recently measured by a spin-orbit state selected V+ + CH4 collision experiment (Phys. Chem. Chem. Phys. 2021, 23, 273-286). The good accord encourages the use of CCSDTQ/CBS protocol in thermochemical predictions of various feasible product channels identified in methane activation by transition metal species.

Electronic states and spectroscopic parameters of the iodocarbyne cation, CI+

We present a theoretical characterization of the cation iodocarbyne, CI+, using a correlated theoretical approach (SA-CASSCF/MRCI) to describe a manifold of singlet, triplet, and quintet electronic states. Reliable potential energy curves were constructed for both Λ+S and relativistic Ω states. Spin–orbit couplings strongly affect the energetic profile of the Λ+S representation. Spectroscopic parameters were calculated, and effects of core-valence correlation investigated. The transition dipole moment functions for the 113Π0+→ X11Σ0++ and 133Π1→ X11Σ0++ states were evaluated and the corresponding radiative lifetimes obtained. Comparisons with other halocarbynes can now help understanding trends in properties in this class of molecules.

Determination of atomic properties in the oxygen isoelectronic sequence

Atomic data including energy levels, excitation energies, line strengths, transition probabilities, and weighed oscillator strengths for 2s22p4, 2s2p5 and 2p6 levels are evaluated for O-like ions (Kr, Rb, Sr, Y, Nb, Zr, Mo, La, Ce) by using Multiconfiguration Dirac-Hartree-Fock (MCDHF) approach. Valence and core-valence correlation effects were accounted for through single-double multireference (SD − MR) expansions to increasing sets of active orbitals. To confirm the accuracy of present results, independent calculations have been performed by using fully relativistic Flexible Atomic Code (FAC). Comparisons are made between our two different set of results, as well as with the other available experiments measurements and theoretical calculations. The present data are accurate enough for identification and are also useful for modeling and diagnosis fusion plasmas. We have reported many new atomic data which is not available in the literature and we believe our presented data can be considered as a benchmark of other calculations.

Energies and E1, M1, E2, and M2 line strengths for Nb XXXIV

The multiconfiguration Dirac-Hartree-Fock (MCDHF) method is used to calculate energies, lifetimes, wavelengths, transition probabilities, weighted oscillator strengths, and line strengths for electric dipole (E1), magnetic dipole (M1), electric quadrupole (E2), and magnetic quadrupole (M2) transitions among the lowest 226 fine-structure levels arising from the 2s22p4, 2s2p5, 2p6, 2s22p33l, 2s2p43l, and 2p53l (l = s, p, d) configurations in O-like Nb XXXIV. The valence and core-valence correlation effects are included in the calculations. The Breit-interaction and the leading quantum electrodynamics contributions are taken into account in the relativistic configuration interaction calculations. The present energies, wavelengths, line strengths, and oscillator strengths are compared with the available experimental and other theoretical results. The accuracy of the present results is estimated using the comparison between the results from the two largest layers in the present MCDHF calculations.

Transition rates, branching ratio, and lifetime of the 2p33sS2∘5 metastable state in O I

Based on the multiconfiguration Dirac-Hartree-Fock method, the accurate line strengths, transition rates, branching ratio, branching fraction, and lifetime of the $2{p}^{3}3s\phantom{\rule{0.16em}{0ex}}^{5}S_{2}^{\ensuremath{\circ}}$ metastable state of neutral oxygen atom (O i), together with the uncertainties evaluated from the residual electron correlation effects, are determined by taking into account the effects of core-core, core-valence, and valence correlations, the Breit interaction, and QED effect. It is shown that the core correlation effect has a certain influence if the separation between $^{3}S^{\ensuremath{\circ}}$ and $^{5}S^{\ensuremath{\circ}}$ in the ${p}^{3}s$ configuration is accurately calculated. It is found that the calculations of branching ratio of $^{5}S_{2}^{\ensuremath{\circ}}$ remain stable in different electron correlation models. We infer that it is a peculiar intrinsic property leading to results useful for plasma diagnostics and other applications. The transition rates from $^{5}S_{2}^{\ensuremath{\circ}}$ change considerably as a result of the Breit interaction, because the line strengths of $^{3}P_{2}$--$^{5}S_{2}^{\ensuremath{\circ}}$ ($\ensuremath{\lambda}135.560$ nm) and $^{3}P_{1}$--$^{5}S_{2}^{\ensuremath{\circ}}$ ($\ensuremath{\lambda}135.851$ nm) are different in sensitivity to this effect. This affects the calculations of the branching ratio and lifetime. There exist large discrepancies among several calculations of lifetimes, such as the MCHF calculation by [C. Froese Fischer and G. Tachiev, At. Data Nucl. Data Tables 87, 1 (2004)]. The main reasons are ascribed to the neglected electron correlation and relativistic effects. In addition, by checking different calculations with various sets of multireference configurations, we find that the calculations of the electric dipole matrix elements for $^{5}S_{2}^{\ensuremath{\circ}}$ in the length gauge are much more sensitive to electron correlation effects than in the velocity gauge. This may also lead to some errors in the calculations of the lifetime of $^{5}S_{2}^{\ensuremath{\circ}}$ in the length gauge.

State-of-the-Art Quantum Chemistry Meets Variable Reaction Coordinate Transition State Theory to Solve the Puzzling Case of the H2S + Cl System.

The atmospheric reaction of H2S with Cl has been reinvestigated to check if, as previously suggested, only explicit dynamical computations can lead to an accurate evaluation of the reaction rate because of strong recrossing effects and the breakdown of the variational extension of transition state theory. For this reason, the corresponding potential energy surface has been thoroughly investigated, thus leading to an accurate characterization of all stationary points, whose energetics has been computed at the state of the art. To this end, coupled-cluster theory including up to quadruple excitations has been employed, together with the extrapolation to the complete basis set limit and also incorporating core–valence correlation, spin–orbit, and scalar relativistic effects as well as diagonal Born–Oppenheimer corrections. This highly accurate composite scheme has also been paralleled by less expensive yet promising computational approaches. Moving to kinetics, variational transition state theory and its variable reaction coordinate extension for barrierless steps have been exploited, thus obtaining a reaction rate constant (8.16 × 10–11 cm3 molecule–1 s–1 at 300 K and 1 atm) in remarkable agreement with the experimental counterpart. Therefore, contrary to previous claims, there is no need to invoke any failure of the transition state theory, provided that sufficiently accurate quantum-chemical computations are performed. The investigation of the puzzling case of the H2S + Cl system allowed us to present a robust approach for disclosing the thermochemistry and kinetics of reactions of atmospheric and astrophysical interest.

The relativistic all-electron ab initio calculations on the multiplet electronic structure of U3+ ion

The relativistic all-electron ab initio calculations have been performed on 5f3 manifold of free U3+ ion to explore the electronic structure and spectra. The spin-orbit-free low lying electronic states have been calculated at the level of theories of State-Averaged Complete Active Space Self-Consistent Field (SA-CASSCF) and Multi-State Complete Active Space Second-Order Perturbation (MS-CASPT2) using different sets of active orbitals. The corresponding spin-orbit states and the energy levels have been computed using Douglas-Kroll type of atomic mean-field integral (AMFI) approach. The ground state is 4I9/2 and the electron correlation has been found to be small in case of the excited 4I states. The low-lying energy levels obtained with the core-valence correlation effects in the treatment are in very good agreement with the experimental values.

Extension of the "Cheap" Composite Approach to Noncovalent Interactions: The jun-ChS Scheme.

A new variant of the so-called "cheap" composite scheme has been purposely developed for the evaluation of the interaction energy of noncovalent molecular complexes, with its various contributions being tested for a set of 15 systems using the accurate interaction energies reported as reference values in the following: [ Řezáč, J. et al. Phys. Chem. Chem. Phys. 2015 , 17 , 19268 - 19277 ]. The modified scheme, starting from the CCSD(T) method in conjunction with a triple-ζ-quality basis set augmented by diffuse functions, includes two additional terms computed at the MP2 level: (i) the extrapolation to the complete basis set (CBS) limit and (ii) the contribution of core-valence correlation effects (CV term). Various families of basis sets including diffuse functions have been tested for the CCSD(T) model as well as for the extrapolation to the CBS limit, with a mean absolute error of about 1% (below 0.2 kJ·mol-1 in absolute terms) obtained with the jun-cc-pVnZ and the jul-cc-pVnZ families. As far as the CV term is concerned, the cc-pCVTZ and cc-pwCVTZ basis sets provide comparable contributions, which are non-negligible in several cases. While the benchmark analysis has been carried out using accurate structures available in the literature, geometrical effects due to the use of reference B2PLYP(-D3BJ) geometries, optimized in conjunction with a triple-ζ-quality basis set, have been investigated, thus pointing out their suitability. Finally, the modified scheme has been applied to a number of test cases for which interaction energies were already available in the literature; among these, a number of molecular complexes bearing second-row atoms have been considered.